EFFECTS OF WATER STRESS ON PHOTOSYNTHESIS AND EVAPOTRANSPIRATION IN ST. AUGUSTINEGRASS, Stenotaphrum secundatum (WALT . ) KUNTZE BY CHARLES HARRIS PEACOCK A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 1981 No family is complete without the support of its individuals. To my wife, Madeline, I extend my love and appreciation for constant support, encouragement, and physical help (even while she was pregnant with Daniel Adam) during long, hot summer days. To my mother and father I add a word of special appreciation. I'm sure at times they all wondered if my collegiate career would ever end. It is to them I dedicate this work. ACKNOWLEDGEMENTS I should like to thank Dr. A.E. Dudeck, Associate Professor, Department of Ornamental Horticulture, for his advice and guidance during all stages of planning, organizing, executing, and interpre- tation of results of the research project. His leadership was invaluable. My committee members. Dr. Bruce Augustin, Assistant Professor, Department of Ornamental Horticulture; Dr. Jerry Bennett, Assistant Professor, Department of Agronomy; Dr. Ken Boote, Associate Professor, Department of Agronomy; Dr. Charles Johnson, Associate Professor, Department of Ornamental Horticulture; and Dr. Allen Smaj stria. Assistant Professor, Agricultural Engineering Department, made the project possible by providing technical expertise, construc- tive advice, and loans of equipment, and by expressing genuine in- terests in both the research project and my educational goals. I sincerely appreciate their efforts. I should like to acknowledge financial support from the Golf Course Superintendents' Association of America, The Toro Company, and Mr. Bill Speelman. Their assistance provided funds needed for conducting the project. To the personnel at the Horticultural Unit, Gainesville, I am indebted for their patience and assistance. Especially helpful in body and spirit were B.J. Williams, Rob Polk, and Tonya Mikell. iii TABLE OF CONTENTS PAGE ACKNOWLEDGEMENTS iii ABSTRACT v INTRODUCTION 1 LITERATURE REVIEW 4 Plant Response to Water Stress 4 Water Stress Studies on Grasses 10 METHODS AND MATERIALS 18 Preparation and Maintenance of Field Plots 18 Experimental Design 19 Turfgrass Measurement 21 WTater Use Estimates 25 Data Analysis 27 RESULTS AND DISCUSSION 28 1980 Stress Study 28 1981 Stress Study 47 CONCLUSIONS 88 APPENDIX 95 LITERATURE CITED 95 BIOGRAPHICAL SKETCH 102 iv Abstract of Dissertation Presented to the Graduate Council of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy EFFECTS OF WATER STRESS ON PHOTOSYNTHESIS AND EVAPOTRANSPIRATION IN ST. AUGUSTINEGRASS Stenotaphrum secundatum (Walt.) Kuntze By Charles Harris Peacock December 1981 Chairman: A.E. Dudeck Major Department: Horticultural Science The response of St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze] to irrigation scheduling was studied during the summer of 1980 and 1981. Research plots were constructed to main- tain a natural soil profile that vertically isolated adjacent plots and prevented lateral water movement. Water stress treatments were implemented by scheduling irrigation with equal volumes of water per day (0.64 cm in 1980 and 0.38 cm in 1981) at either 2-, 3-, 4-, or 6-day intervals. Turf grass swards were evaluated on 12-day cycles under stress and after 24 hours of recovery following irrigation. Measured parameters included turf grass quality, carbon exchange rates (CER) , evapotranspiration rates (ET), plant water potential component changes, leaf diffusive resistance and transpiration, and rooting densities. During 1980, the 6-day treatment plots had the greatest reduc- tion in CER under stress (57%) and increase during recovery (77%) v with little change in ET or leaf water potential for the first stress cycle. Two additional stress periods were studied but no differences in leaf water potentials were found among treatments and only moderate reductions in CER and ET were observed while leaf water potentials remained high. Depth of rooting and perching of the water table into the rooting zone precluded stress based on irrigation scheduling. Irrigation scheduling did affect many of the parameters studied during 1981. The 6-day treatment caused a reduction in CER and ET associated with stomatal closure and loss of turgor pressure during each stress cycle. During recovery following irrigation, CER and ET increased (by as much as 353% and 209%, respectively), turgor pressure increased, and diffusive resistance decreased during each 12-day stress cycle _ Averaged over the en^re study period CER, ET, leaf water potentials, and transpiration were lowest and diffusive resistance highest for the 6-day treatment plots. All parameters increased or decreased significantly during recovery following irrigation. Turf grass quality and percent cover were not affected by treatments. Turf grass water use rates decreased with increasing days between irrigations. Scheduling irrigation once a week for St. August inegrass with 2.7 cm of water will likely have no affect on turf grass quality if adequate mowing height is maintained. vi INTRODUCTION Turfgrass science involves the interaction of environmental factors with management practices to produce an acceptable quality turfgrass. Irrigation is an integral part of a turfgrass management program where the major objective has been to create an environment which is aesthetically suitable for recreation or relaxation. The intensity of management is directly related to cost and availability of energy input. Falk (1976) noted that on a yearly basis of energy input for maintenance, irrigation was the greatest user of external energy resources accounting for 68.6% of the management input into the lawn system on a Kcal/year basis. However, management objectives must change as potable water supplies decrease (Smerdon, 1974), and subsequent use of limited water will most probably impose stress on turfgrass areas which will result in reduced quality. Surveys in Florida have projected that water stress research on turfgrass is needed to help better manage and conserve water. Previous research in Florida has focused on water requirements for turf grasses (McCloud, 1954; Weaver and Stephens, 1963; Stewart and Mills, 1967; Stewart et al., 1969). These findings indicate an imbalance during peak growth months between turf water use and precipitation. Irrigation is commonly practiced to offset this deficit even under common home lawn and golf course situations. 1 2 Plant water stress can be manifested by a number of physiological responses. Water stress can be a short term effect during a diurnal cycle whereby root absorption lags behind transpiration during the day, but the water deficit is relieved overnight. More severe cases are noted when soil water becomes limiting and complete recovery overnight is impossible (Kramer, 1969). General effects of water stress on plant growth include reductions in cell growth, stomatal opening, carbon dioxide assimilation, respiration, sugar accumulation and translocation, etc. (Hsiao, 1973). The results of water stress may be subtle or dramatic, varying from small changes in leaf water potential to a complete inactivity of the photosynthetic apparatus. Plant water stress results in a lowered rate of photosynthesis, thus reducing plant growth by limiting production of new leaf area (Boyer, 1976). Turf grass resistance to water stress is highly variable among grass species. This is dependent not only on genetic characters but also on cultural and management practices (Beard, 1973). Turfgrass can tolerate severe stress for periods during the growth season and completely recover following maintenance of adequate irrigation during the remainder of the season (Jensen, 1968). Hsiao and Acevedo (1974) note that almost any parameter or plant process can be altered by a water stress that is severe and long enough. The objective of this study was to assess the responses of St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze] to short-term water stress implemented by scheduling irrigation at either 2-, 3-, 4-, or 6-day intervals. The resulting water stress treatments approximate the lack of rainfall or various irrigation scheduling programs which 3 are presently used. In the assessment of the treatment effects, turf- grass swards were evaluated for a number of parameters including turf grass quality, carbon exchange rates, evapotranspiration rates, plant water potential component changes, and rooting densities. LITERATURE REVIEW Plant Response to Water Stress Plant responses to water stress can be manifested in a variety of morphological and physiological changes. Morphological modifi- cations to plant water deficits include increased rooting depth and root-shoot ratio, decreased leaf number, size and total leaf area, and reduced shoot elongation. Physiological changes are more subtle but involve lower leaf water potentials, lower turgor and osmotic potentials, decreased carbon exchange rates, increased soluble carbohydrate concentration, decreased protein concentration and increased amounts of bound water (Beard, 1973). Plant Water Potentials and Stomatal Control Plant water status as measured by water potentials influences both physical and physiological characteristics of the plant. Water movement from the soil through the roots and xylem to the leaf cells must replace water which is transpired from the stomata in the leaf or {^e-Licits occur. Water deficits cause stress and can be measured as reductions in the leaf water potential (Levitt, 1980). The most obvious visual effect of water deficits is leaf wilting. Many physiological changes occur prior to wilting (Kramer, 1969). The most sensitive process as affected by water stress is cell expan- sion (Hsiao and Acevedo, 1974) which is inhibited earlier and more severely than photosynthesis (Boyer, 1970). Leaf water potentials varying from -1 to -50 bars can inhibit growth (Levitt, 1980). 4 5 Perennial ryegrass (Lolium perenne L.) leaves continue to expand fully under stressed conditions in the field although at a slower rate throughout the duration of the stress (Jones et al., 1980a). Turgor potential is centrally important as the parameter which links the water balance of a crop to its growth processes including carbon dioxide assimilation and cell elongation (Zur and Jones, 1981). Growth is physically dependent on maintaining turgor pressure not only with newly dividing cells in the leaf but also with the guard cells of the stomata. Stomatal opening is directly controlled by the turgor of the guard cells of the stomata and of other epidermal cells (Levitt, 1980). Leaf water potentials at which the stomata begin to close from loss of turgor is variable with species and growth conditions. Kanemasu and Tanner (1969) reported threshold leaf water potential values for tomato (Lycopersicon esculentum Mill.) of -7 to —9 bars and —10 and —12 bars for beans (Phaseolus vulgaris L.). Jordan and Ritchie (1971) measured values as low as -12 to -16 bars in cotton (Gossypium hirsutum L.) before stomatal opening was affected. Hansen (1971) found that sugar beet (Beta vulgaris L.) has little change in diffusive resistance of the leaf until leaf water potentials reached -18 bars, corresponding to initial stomatal closure. O’Toole and Cruz (1980) saw increased stomatal resistance and leaf rolling at the same leaf water potential range (-8 to -12 bars) in rice (Oryza sativa L.). Once the threshold leaf water potential is reached and the stomata begin to close, there is increased resistance to carbon dioxide movement into the leaf and water vapor movement out. A general assumption would be that once the stomata closed, the diffusion processes 6 would be severely inhibited and both photosynthesis and evapotrans- piration would be quite low. Ackerson et al. (1977) in studying the effects of water stress on field grown cotton noted that as leaf water potentials decreased there were increases in leaf diffusive resistance although complete stomatal closure was not observed even at zero turgor potentials. Sorghum [Sorghum bicolor L. (Moench)] plants exposed to water stress for the first time closed their stomata com- pletely, but stomata remained partially open during subsequent drought periods. One study indicated that the stomata remained slightly open all day during a severe stress (Sullivan and Eastin, 1974). Water Stress Effects on Photosynthesis Changes in photosynthesis during water stress have been well documented. Boyer (1976) noted that decreases in photosynthesis resulted from decreased stomatal opening and non-stomatal effects on chloroplast activity. Kramer (1969) explains the reduction in photo- synthetic rates as a coordination of resistances at the stomata and the mesophyll, limiting carbon dioxide flux into the plant and fixation into sugars at the cellular level. Most data indicate that the primary effect of water stress on photosynthesis is through the reduction of carbon dioxide diffusion caused by increased stomatal resistance (Hsiao, 1973). Numerous studies link the beginning of stomatal closure at some critical threshold leaf water potential level to the beginning of a decrease in photosynthetic rates (Hsiao and Acevedo, 1974). In some species during brief stress, reduction in carbon dioxide assimi- lation is totally accounted for by stomatal closure (Troughton and Slatyer , 1969). Beadle et al. (1973) found that photosynthetic rates 7 of corn (Zea mays L.) and sorghum were different at similar leaf water potentials. Sorghum at a leaf water potential of -11.5 bars had a photosynthetic rate of 25% of maximum while corn was severely wilted and photosynthesis had ceased. Recent evidence suggests a more complicated hypothesis. Photosynthesis or substomatal carbon dioxide concentrations have been shown to affect stomatal conductance. Therefore a water stress that reduces photosynthesis may thus reduce stomatal conductance (Wong et al., 1979). Fock and Lawlor (1979) found that true and apparent photosynthesis decreased linearly in sunflower (Helianthus annuus L.) as a consequence of stomatal and possibly non— stomatal effects of severe stress. Parsons et al. (1979) found similar results in cotton where apparent photosynthesis was reduced under stress conditions in a drying soil. During development of water stress, the non-stomatal changes are probably directly related to biochemical alterations owing to changes in the hydration of the protein molecules (Hansen, 1971). Hsiao (1973) concluded that non-stomatal effects on net carbon dioxide assimilation do occur under conditions of mild or moderate water stress. He notes that there apparently are differences in the magnitude of these responses among species. Stomatal and Non-Stomatal Aspects of Photosynthetic Recovery Researchers (Hansen, 1971; Ackerson et al., 1977) have shown closure of stomata during water stress does not totally control photo- synthesis and Jones (1973) concluded that water stressed cotton plants showed greater intracellular resistances (non-stomatal) than controls. Wardlaw (1966) suggested that water stress acted directly on the leaf and that some of the effects were non-stomatal since increasing 8 ambient carbon dioxide concentrations under stressed conditions did not alter photosynthetic rates. Recovery of photosynthesis following rewatering of stressed plants is dependent on the duration and severity of the stressed period. Recov- ery time can range from a few hours to a day or more (Kramer, 1969). These responses are generally linked to renewal of cell turgor poten- tials necessary for stomatal opening, although non-stomatal mechanisms ar5 also involved. Ceulemans et al. (1979) noted recovery of photo- synthesis in water stressed Rhododendron sinsii (Planch.) plants 24 hours following rewatering. Stonatal Control of Evapotranspiration Evapotranspiration is the sum of two unique yet similar processes, evaporation from the soil surface and transpiration from the plant. Stonatal transpiration accounts for more than 90% of the loss of water from the plant (Beard, 1981). Stomatal closure is therefore the major cause of decline in transpiration during water stress (Hsiao, 1973). Hansen (1971) showed that stomata of severely dehydrated plants still regulated transpiration. Shimshi (1963) working with corn found that stonatal closure markedly reduced transpiration, but photosynthesis was reduced to a lesser extent. Changes in the ratio of dry matter produced to water use (water use efficiency) under stress do not conform strictly to a stonatal effect. Hsiao and Acevedo (1974) cite evidence that the accitional effect of water stress on photosynthesis (non-stomatal) negates any gain in water use efficiency when water deficits develop. Consequently a one to one relation between transpiration and dry matter production is often observed under conditions of varying water supplies. 9 Soil Moisture Influence on Plant Water Status Plant moisture stress may develop any time water uptake is less than transpiration. The extent of any imbalance is limited by the water storage capacity of the plant (Turner and Begg, 1981). Plants can extract water from the soil only when their water potential is lower than the soil matrix potential. These differences can result in plant water deficits through inadequate absorption, excessive transpiration or a combination of the two. Plant roots absorb soil water in response to these potential gradients. As the potential differences become greater, the water flow rates become proportional to the potentials. However, as soil moisture decreases, less water is available for root absorption and further increases in potential differences will not increase water flow and eliminate plant water deficits. The effects of depletion of soil., moisture on the plant are well documented in the literature. More important is the soil matrix potential at which physiological responses of individual plant species are noted. Beadle et al. (1973) found differences in the rate of stomatal closure between corn (drought intolerant) and sorghum (drought tolerant) . Stomatal closure of both species started at a similar soil water tension of —0.3 bars. Corn stomata were completely closed by —0.7 to —0.8 bars while sorghum did not exhibit complete closure until soil water potentials reached -6 to -7 bars in a loam-peat soil. Al— Ani and Bierhuizen (1971) working with beans, cucumber (Cucumis sativas L.) and tomato saw transpiration rates rise and then reduce steadily as soil moisture decreased. Shimshi (1963) suggested that as the soil dries, resistances to water movement at the evaporating surfaces of the mesophyll cells increase. However, 10 plant water potentials at these surfaces may reach -80 bars without wilting of the leaves. Ritchie (1974) concluded that one of the most important findings concerning plant water relations is that plant growth is controlled directly by water deficits in plants and only indirectly by soil water deficits and atmospheric stresses. Further, soil moisture availability is a function of rooting densities and depth, which effectively controls plant and soil water relationships. Water Stress Studies on Grasses Water stress studies on grasses have focused mainly on those species and cultivars grown for forage or pasture with little research conducted on grass under true turf grass management situations. Beard (1981) has noted that the effects of water stress include increased rooting depth, decreased photosynthesis, and reduced shoot growth rate, all of which are interrelated. He also stated that inherent physiolog- ical hardiness to water stress can be affected by cultural practices such as fertilization and irrigation, and noted that mowing will enhance drought resistance. These inherent properties are evident in studies on range and forage grasses. Dyer and Trlica (1972) in studies on blue grama [Bouteloua gracilis (H.B.K. ) Lag. ex Steud.] found that photosynthetic rates were not influenced by leaf water potentials as low as -44 bars. Assimilation was reduced by 50% as the leaf water potential reached -60 bars. Hutcheson and Knight (1972) noted that leaf transpiration in blue grama was maximum at -30 bars and leaf area expansion was rapid. Only at -70 bars did leaf expansion stop. 11 Water Stress Effects on Physiological Parameters in Grasses Water stress effects on photosynthesis, evapotranspiration and other physiological parameters have been studied in cool-season grasses. Sheehy et al. (1975) observed that canopy photosynthesis of ryegrass declined rapidly only after several days of water stress and that all swards showed a rapid increase in photosynthesis during the first 24 hours after rewatering. Leaf water potential in their study became more negative preceding the decline in photosynthesis. The decline in canopy photosynthesis was associated with decreased single leaf values for photosynthesis, leaf water potential, and conductance. In their more severely stressed plots, loss of leaf tissue limited the recovery of canopy photosynthesis. Jones et al. (1980a) studied water stress effects on perennial ryegrass and found that canopy photosyn- thesis was reduced by about 50% in stressed field swards and more than 80% in stressed simulated swards (boxes with soil of low water holding capacity). This resulted in a marked reduction in dry matter produc- tion. They noted an increased stomatal resistance accounted for these results, although leaf water potentials were not drastically lowered compared to other crops (-16.0 to -20.0 bars). It was found (Jones et al., 1980b) that in the rapidly stressed simulated swards turgor fell to zero for a significant part of the day, and that the critical threshold for stomatal closure was at a relatively high leaf water potential (-13 bars). They concluded that the response of the grass crop to water stress began slowly in the field and was a combination of drought avoidance (reduction of leaf area) and drought tolerance (continued photosynthesis) allowing for the plant to maintain positive 12 Carbon balance. This enabled the crop to resume growth at a vigorous r2te once the stress was relieved. The studies on perennial ryegrass were conducted under forage management conditions, not true turf grass situations, since mowing was not done to maintain uniformity. Sheffer (1979) in greenhouse studies on turfgrass species under turfgrass management conditions found that Kentucky bluegrass (Poa pratensis L.), perennial ryegrass, and tall fescuegrass (Festuca arundinacea Schreb.) used less water when soil water was limiting. Leaj. bud extension and water use were more sensitive to water stress than was leaf water potential. Under moisture stress, less dry matter was produced indicating a limiting of growth processes and photosynthe- sis. In addition, he noted that species differences were apparent for most: parameters studied and that species by treatment interactions were also observed for some factors. - — Rooting Effects on Plant Water Status Rooting can affect plant water status by limiting soil moisture availability. Under non-stress conditions, 49% of the ryegrass assimilate fixed during a normal photoperiod is translocated to plant parts underground (Sheehy and Peacock, 1975). Creation of plant water deficits by reduced rooting can result under turfgrass situations due to low mowing heights (Beard, 1973). Soil moisture stress can also affect root distribution but in a positive manner. Doss et al. (I960) observed that the effective rooting depth of five warm-season forage species decreased as soil moisture increased, while Bennett and Doss (l^oO) noted similar results on cool-season forage species. This implies that greater rooting depth during soil moisture stress may be a drought avoidance mechanism. Water stress has been shown to alter shoot: root 13 ratios (Beard, 1973) and this presumably is due to lack of turgor for leaf expansion and redistribution of assimilates to root growth. Sneffer (1979) did not detect changes in root mass due to soil moisture stress, but Ritchie (1974) noted that container-grown plants do not always respond to soil water deficits in the same manner as field plants. Turf grass Water Studies Until recently, water for irrigation of turf grass was considered plentiful. Even 11 years ago Florida was described as being wealthy m water resources, both in ground water and surface water (Singley, 1970). Smerdon (1974) pointed out that the Water Resources Council concluded that there is enough water for future needs, but there would be regional shortages. Zachariah (1976) warned that while current water needs could be met, problems of water management were going to become mote acute as the competition for water increased. Drought prcDlems in 1981 have focused more attention on what now is considered a critical problem in water supplies and consequently turfgrass management for Florida in the coming decade (Warren, 1981). With an estimated one million acres of turfgrass in Florida (A.E. Smaj stria, 1931 personal communication) public awareness has prompted investi- gations into the area of turfgrass water use. A Florida Turfgrass Association survey noted that 86% of the respondents thought more research was needed in the area of irrigation techniques and 92% felt that research was needed in the area of plant water relations, includ- ing work on water use rates, drought resistance, moisture stress, water uptake, and transpiration (Augustin, 1979). 14 Turfgrass water studies in Florida have spanned some 28 years with work being conducted at Gainesville and Fort Lauderdale field stations. McCloud (19o4) measured water use in turfgrasses from s°il tanks and developed an empirical formula expressing the following relationship of evapotranspiration rate to mean temperature: T— 12 Evapotranspiration = K • W where K — 0.01 and W = 1.07 are constants and T is the mean temperature in degrees Fahrenheit. McCloud calculated evapotranspiration (ET) rates for three Florida cities based on average temperatures and noted that during the peax growing season of April through August for turfgrasses in -lorida, a deficit normally exists between water requirements and rainfall. To prevent water stress this deficit must be corrected by irrigation. McCloud (1970) calculated average daily water use rates of turrgrasses using monthly temperatures for north, central, and south l’l°ricia. He concluded that during the peak growing season (April through SepLember) ET would range between 0.30 and 0.87 cm of water per day. While Florida rainfall averages 135 cm per year, there are large seasonal variations and estimates indicate ET will greatly exceed rainfall during the early summer periods. Turfgrass Water Use Rates Evapotranspiration from St. August inegrass was studied in a series ot experiments at Fort Lauderdale, Florida. Weaver and Stephens (1963) found that Ei varied from 0.36 to 0.33 cm per day as depth to the water table increased. Evapotranspiration rates for St. Augustinegrass and bemudagrass (Cynodon dactylon (L.) Pers.) were determined in further studies by Stewart and Mills (1967) over a 3-year period in Fort Lauder- dale. Although exact data were not given they concluded that the ET 15 for St. Augustinegrass was slightly higher than for bermudagrass. As an average of five years over the two studies they found turfgrass water use rates, during the peak growing period of April through September, ranged from 0.36 to 0.44 cm of water per day. On a yearly basis turfgrass water use averages 109 cm. Estimated daily evapo- transpiration rates for north Florida correlate well with these mea- surements. Potential ET from long-term weather data at Jacksonville using the Penman method show values during the peak growing season from April to September that range from 0.31 to 0.43 cm per day (L.C. Hammond, personal communication). Values for ET measured by Weaver and Stephens (1963) are lower than those calculated by McCloud at Gainesville. In McCloud's studies ET rates may have been exaggerated by advective heat transfer inflating the ET values (Allen et al., 1978). Kneebone and Pepper (1978) measured water loss under arid conditions from St. Augustinegrass in Arizona and found ET values that ranged from 0.49 to 0.56 cm per day during the summer growing period and a yearly use rate of 118 cm, slightly higher than those values observed in south Florida. Youngner (1979) studied turfgrass water use in bermudagrass and St. Augus— tinegrass in California and measured daily use rates for the summer months between 0.25 and 0.41 cm. These data correspond well with studies on bermudagrass in the southeastern United States by van Bavel (1961) and in Hawaii by Ekem (1966) where ET values ranged between 0.27 to 0.44 cm per day and between 0.32 to 0.47 cm per day, respec- tively. Allen et al. (1978) calculated potential ET based on Fort Lau- derdale climatological data by the Penman formula and compared these results with the measured ET values for the studies at Fort Lauderdale. They found that in most cases the daily potential ET averaged over 16 five days exceeded the measured ET values. Further, the yearly water budget was less than annual rainfall, but values during the study period showed maximum monthly deficits ranging from -8.9 cm for 1959 data to -35.6 cm for 1965. It is apparent from ET data that irrigation is considered a requirement to provide enough water for high quality turfgrass in Florida. Irrigation techniques are based on these three main considerations: (1) water needs of the turfgrass, (2) available moisture capacity of the root zone, and (3) availablility and quality of irrigation water (Harrison, 1973). Irrigation interval is a function of soil water holding capacity and rooting depth. General- ized values for Florida indicate a 3-day irrigation interval is necessary to maintain adequate soil moisture for turfgrass growth (Bartholic, 1977). Tovey et al. (1969) found that irrigation fre- quency could influence turfgrass ET and they measured slight de- creases with decreasing frequency of irrigation. Many studies on water use by grasses have indicated that ET is controlled to a large extent by soil water availability and that as the soil dries, less water is used (Garwood and Williams, 1967; Makkink and van Heemst, 1956; Doss et al., 1964 Doss et al., 1962). Slabbers (1980) reviewing irrigation frequency of field crops, notes that irrigation should be applied when 40 to 60% of available soil moisture is exhausted. Youngner (1979) successfully utilized ten- siometers to control irrigation on St. Augustinegrass turf. Three regimes were utilized which provided irrigation when soil moisture tensions reached -15 cb (wet), -40 cb (medium) or -65 cb (dry). 17 at a depth of either 15 or 30 cm in the soil profile. Acceptable quality turf was maintained with as little as 57.9 cm per year of irrigation (dry) while even the wet treatment required only 77.5 cm of irrigation water per year. Rainfall was 45.2 cm indicating a minimum water application of 103.1 and a maximum of 122.7 cm. These values are consistent with the previously mentioned studies, in- volving more efficient utilization and lower irrigation amounts by allowing stress to occur, without lowering turf quality. On the basis of Florida data, Bartholic (1977) using computer simulation, found that if effective rooting depth allowed for an increase in available soil water from 1.0 to 1.8 cm, irrigation frequency could be reduced from between 60 and 76 irrigations per year to between 30 and 36. Annual irrigation volumes would approach Youngner's values of between 81.3 and 96.5 cm per year -at a non-stress ET rate (Youngner, 1979). METHODS AND MATERIALS Preparation and Maintenance of Field Plots An experimental area 15 X 75 m at the Turf grass Field Laboratory, located at the IFAS Horticultural Unit, Gainesville, Florida was pre- pared in March 1980 by stripping the existing vegetation with a sod cutter and power raking the sod before removal. This allowed removal of the vegetation with little loss of topsoil. The area was graded and surveyed for installation of three blocks of four field plots 2 X 2 m with 25 cm between plots and 2 m between blocks. Elevations were determined with a transit so that a 5 cm crown could be placed on each block to allow for surface drainage. Installation of individual isolated research plots was accomplished by trenching the circumference of each plot to a depth of 81 cm and wrapping the resultant soil mono- litas with two layers of 4 mil polyethylene to act as a moisture barrier. The entire area was regraded, lightly rototilled, and fumigated with methyl bromide at a rate of 68 kg/ha. An application of a complete fertilizer in a 16-1.6-6.4 elemental N-P-K ratio (50% water insoluble nitrogen) with minor elements was made to the entire area at a rate of 50 r^g h/ha. The area was sodded with * Floratam' St. Augustinegrass, watered, and mowed as necessary. The surrounding area was seeded to Pensacola bahiagrass (Paspalum notatum Flugge) to provide adequate fetch. The sod was allowed a 4-week establishment period before experimentation began. Because of problems in 1980 with a fluctuating water table, a 10 cm diameter drainage pipe was installed 1.22 m deep around the perimeter of the area in January 1981. 18 19 Plots were mowed at 7.5 cm with a rotary mower and clippings returned. Frequency varied from one to two times in a 6-day period, and not less than every six days. In April of 1981, the area was verticut in two directions with a vertical mower spaced at 7.5 cm. A complete fertilizer in a 16-1.6-6.4 elemental N-P-K ratio (50% water insoluble nitrogen) was applied at a rate of 50 kg N/ha and the area watered and mowed at a height of 7.5 cm as needed. Approximately two weeks prior to initiation of the experimental period an additional application of 25 kg N/ha of ammonium nitrate was made. Experimenta- tion was delayed until May 1981 to allow optimum rooting and full turf cover to occur before initiation of treatments. During the 1981 treatment period, plots were mowed every six days at 7.5 cm with a rotary mower and clippings returned. At no time during the 1980 or 1981 experimental period was a herbicide, insecticide, or fungicide used since this might affect some of the physiological processes studied. Data was collected on selected environmental parameters. Solar radiation was monitored by a Weather Measure Corporation Model R401 Pyranograph on a 24-hour basis. Soil temperatures at 2.5 cm were recorded daily using Science Associates Model 120 Maximum-Minimum soil thermometers under two treatment plots. Daily air temperatures, relative humidity, wind run, rainfall, and pan evaporation were measured at the Field Crops Department Weather Station located east of the Turf grass Field Laboratory. Experimental Design The experimental design was a randomized complete block of four treatments in three replications. Treatments consisted of irrigation 20 on a 2-, 3-, 4-, or 6-day interval with equal volumes of water on a per oay basis. During the 1980 experimental period the grass was provided ^xth the equivalent of 0.64 cm/day of water (0.25 inches/day). This vas in excess of the potential ET rate as calculated from Penman’s equation based on Jacksonville, Florida data of 0.40 cm/day (0.16 inches/ cay) (L.C. Hammond, personal communication) so that stress would be in- duced by irrigation scheduling not quantity. On the basis of the 1980 results, during the 1981 experimental period the turf grass was irri- gated with the equivalent of 0.38 cm of water per day (0.15 inches/day) so that stress would be more easily induced. All plots were hand watered using a rosette nozzle on a hose and a gasoline powered portable sprayer. Care was taken at the high volumes on the 4- and 6-day treatments to match application with infil- tration rates so that no run-off occurred.— Each plot was watered in two directions to provide uniform coverage with a measured amount of water to impose the specified treatments. Watering was done as late in the day as possible to minimize evaporation losses. To insure that the only water the plots received was that applied during scheduled irrigations, three protective rain shelters were ini- tially constructed using PVC pipe as a framework and Monsanto® 602 poly- ethylene plastic as a cover. After the shelters were destroyed in a se- vere storm in 1981, new ones were constructed using 1.8 cm electrical conauit as a framework. Use of this polyethylene material provided greater than 90% light passage as photosynthetically active radiation. Shelters were installed over the plots at night and during periods of rainfall. They were designed with a 46 cm overlap to protect the plots 21 from a blowing rain and were placed on supports 46 cm above the ground to provide for wind movement to prevent excessive heat build-up. Turf grass Measurement Measurements of physiological parameters were taken at the beginning of the study period (initial) to insure that all swards were equivalent prior to initiation of treatments. Measurements were then taken at 12-day intervals (corresponding to a cycle) on the afternoon prior to irrigation. After 24 hours of recovery following irrigation, measurements were reported so comparisons on pre-irrigation and recovery data could be made. Physiological measurements of carbon dioxide exchange rate (CER) and evapotranspiration (ET) were made using a portable quick draw- down closed chamber system (Boote et al., 1980). Plots were mowed at 7.5 cm late on the preceding day to allow recovery from mowing shock and provide uniform turf height for measurement. Aluminum frame bases 76 X 109 cm covered with a non-permeable Ifylar plastic were sunk into the center of the turf grass plots approximately 2.0 cm deep to ensure a closed system. A 1.03 m aluminum frame chamber covered with >fylar plastic was seated to the previously installed chamber base for determinations of CER and ET. The chamber was secured to the base with clamps for 2-4 minutes and measurements recorded. This short time period precluded problems with heat build-up and carbon dioxide depletion. The chamber was equipped with thermocouples to monitor internal air temperatures and a quantum sensor to measure radiation levels as photosynthetically active radiation (PAR) . Air was circulated by a permanently mounted electric fan in the chamber to 22 insure adequate mixing. Tygon® tubing connected to the chamber allowed for air to be circulated by a pump for sampling purposes. Instrumentation for measurements was housed in a mobile van which allowed portability. Air at 0.6 liters/minute was extracted from the circulating chamber air and analyzed for changes in carbon dioxide content with a Beckman Model 865 Infra Red Gas Analyzer. The gas analyzer was calibrated to a carbon dioxide differential of 100 vpm using carbon dioxide gas in a nitrogen carrier certified as to concentration. The air was first passed through a double column of ® - Drierite (Anhydrous Calcium Sulfate) to remove water vapor. Carbon dioxide exchange rates as apparent photosynthesis were recorded for 60 seconds after a linear response was observed. A light-proof white cover was then placed over the chamber and an additional 60 seconds of data recorded once carbon dioxide evolution was observed to be linear. This allowed calculation of CER as dark respiration plus soil respiration. Gross photosynthesis was determined as the sum of apparent photosynthesis and dark respiration. Changes in water vapor content for the initial 30 seconds of sampling were recorded by simultaneously passing undried air from the 8 liter/minute circulation through a General Eastern System 1100 DP Dewpoint Hygrom- eter for changes in the dewpoint temperature. Dewpoint values were converted to absolute humidity and recorded as mg water vapor change per unit time. Chamber temperature was monitored with a Wescor TH-65 Thermocouple Thermometer using handmade copper— Constantine thermo- couples and PAR was measured with a Li-Cor Model 190 S Quantum Radiom- eter. All instrumentation was connected to Gould Brush 105 Dual Pen 23 Recorders for recording data. Changes in concentrations of carbon di- oxide and water vapor could then be calculated per unit time and per unit land area. Leaf water potentials were measured in 1980 with a Soil Moisture Equipment Company Model 3005 pressure chamber apparatus (Scholander et al., 1965). Fully expanded leaves subtending the leaf bud were ex- cised with a razor blade and mounted in the chamber head with approx- imately 5 mm of the cut end exposed. Air within the chamber was kept moistened by lining the chamber with paper towels to minimize evap- orative drying. Leaf water potentials in 1981 were measured using thermocouple psychrometers. Approximately 25 mm of fully expanded leaf tissue subtending the leaf bud were excised, rolled, care- fully placed in the psychrometer cap and sealed. Psychrometers were maintained at ambient temperatures and quickly transported to the laboratory. They were place in a 30°C water bath and allowed to equilibrate for four hours. Psychrometric determinations of water vapor content were measured with a Li-Cor Model HR-33 T Microvolt- meter and recorded by a strip chart recorder. Calibration curves for each psychrometer were pre-determined with a series of potassium chloride solutions. For osmotic potentials, the psychrometers were placed in a freezer overnight, thawed at room temperature, equili- brated in the water bath at 30°C, and determinations made as de- scribed above. Turgor pressure potential was calculated as the difference in leaf water potential of the fresh leaf minus the os- motic potential determined after freezing. Diffusive resistances of the last fully expanded leaf blade were determined for the 1981 24 period using a Li-Cor Model LI-1600 Steady State Porometer. Diffusive resistance, leaf temperature, and transpiration rates were determined on the adaxial and abaxial leaf surfaces. Individual leaf values were determined as the sum of conductance (expressed as resistance) and transpiration for adaxial and abaxial leaf surfaces. Soil samples were taken at the beginning and end of the study period each year to determine the treatment effects on rooting density. Rooting cores 5 cm in diameter by 15 cm in length were collected at 15 cm increments to a depth of 75 cm. Two cores were sampled per treatment plot. Samples were washed, screened free of sand and organic material, and the root counts were made on a 1 X 1 cm grid system. An estimate of the total length of root in a sample was determined by the following formula: R = — — where N equals the number of intersects on a one cm line grid (Newman, 1966). Roots were then dried in an advection oven at 80 C overnight, weighed, and ashed at 550°C for 8 hours and reweighed. Root mass on a dry weight basis was then calculated as the difference in dry weight and ash weight to the nearest tenth milligram on a Mettler H64 analytical balance. Clipping weights from a 2 m long by 56 cm wide swath were collected from each plot for dry weight determinations at varying intervals during the study period. Clippings represented a 3- to 6-day period of growth. Clippings were dried in an advection oven at 80°C for 24 hours and then reweighed to the nearest hundreth of a gram on a Mettler PL4000 pan balance. Clipping weight data was normalized on a per day basis. 25 Visual estimates of turfgrass quality and percent cover were made weekly during the study period. Turfgrass quality was assessed on a 1 to 9 scale with a value of 9 representing the highest quality. Percent cover was estimated on a one to one hundred basis as the amount of soil with vegetative cover. Water Use Estimates Soil moisture flux, estimated turfgrass water use, and effective rooting depth during stress conditions was monitored by instrumentation installed in each plot. Soil moisture tensiometers were constructed of one bar ceramic cups (Soil Moisture Equipment Company catalog # 655X1 -B1M1) glued with a waterproof epoxy (Armstrong Products Corp. A-34 Adhesive) to 2.22 cm diameter extruded acrylic tubing. These were placed at depths of 15, 30, 45, and 60 cm in each plot. Nylon tubing in- serted into rubber stoppers connected the tensiometers to a mercury well. Soil moisture tension recorded in millibars from a scale allowed evaluation of soil moisture flux on a periodic basis. Lengths of aluminum irrigation pipe (5.08 cm outside diameter with a 0.127 cm wall thickness) were placed into the center of each plot to a depth of 1.8 m. A Troxler Model 55A Neutron Depth Moisture Gauge and 2600 Scaler (Troxler Electronics, Research Triangle Park, NC) were used to monitor soil water content on a periodic basis. Gravimetric determinations of soil moisture content were taken at varying intervals to establish a soil water content calibration curve for the neutron probe and secondarily the samples were analyzed for nutrient status. Soil cores 1.9 cm in diameter X 15 cm in depth) were taken to a depth of 45 cm, placed in metal weighing cans, weighed to the 26 nearest hundreth of a gram, dried at 105°C for 24 hours, and reweighed to determine water loss. Conversion of gravimetric water content to volumetric water content was made on the basis of soil bulk, density as derived from the following procedure. In order to evaluate soil moisture changes from tensiometer data, a soil moisture release curve was constructed. In situ soil samples fron topsoil and subsoil were collected in brass rings (9 cm in 'diameter by 3 cm in height) as four replications by use of a Soil Moisture Equip- ment Co. Model 200-A Soil Core Sampler. These samples were placed in Tem pe Pressure Cells (Soil Moisture Equipment Co. Model 1450), saturated in a water bath, and placed on a pressure regulation system. Air pressure was applied in varying increments from zero to one bar and quan- with a mercury manometer. After each increment of pressure change and after equilibrium had occurred, the cells were weighed to the nearest hundreth of a gram on an analytical balance. Upon termination of the experiment, soil bulk densities for both topsoil and subsoil samples were determined based on oven dry weights of the rings which contained mea- sured volumes of soil. The soil moisture release curve was plotted as so-11 volumetric moisture content as a function of tension (pressure) . Water use estimates were made using a water balance method (lanner, 1968). Soil moisture flux within the soil profile was used to determine volumetric changes as they occurred. The sum of these depletions plus the irrigation applied were taken as estimates of turf- water use. Soil moisture contents were evaluated from tensiometer data in the top 60 cm of the plots and to a depth of 1.5 m from the neutron probe data twice weekly. 27 Data Analysis Data analysis was performed with Statistical Analysis System programs. Analysis of variance was performed with a randomized complete block design for most parameters. Student's t-Test or the Waller-Duncan k— ratio t— Test was employed for comparison of means (Waller and Duncan, 1969). RESULTS AND DISCUSSION 1980 Stress Study iurfgrass stress responses to irrigation scheduling during the 19c0 study were limited to two 12-day cycles and one final stress period of 19 days beginning 22 June and ending 6 August. Irri- gation treatments consisted of equal volumes of water (0.64 cm/day) applied at intervals of 2, 3, 4, or 6 days. Within a 12-day cycle all plots received 7.6 cm of water. Only the time between irri- gations was varied. .ipparent photosynthesis (AP) is the net carbon exchange ob- served from the turfgrass canopy inclusive-of carbon dioxide efflux from soil respiration. Dark respiration (Rd) is the carbon dioxide excaange irom tne canopy as observed under zero radiation flux. This would include plant respiration from above and below ground parts and microbiological activity from the mat and soil not di- rectly related to the plant respiration. Gross photosynthesis (Pg) is the sum of AP and Rd. Evapotranspiration while calculated on a unit land area basis can be converted to mm/hr by a 1/1000 factor. Carbon Exchange Rates, Evapotranspiration Rates, and Leaf Water Potentials — ' Physiological data for the 1980 experimental period indicated t'n“- tur^ grass carbon exchange rates (CER) of apparent photosynthesis (AP) , dark respiration (Rd) , and gross photosynthesis and evapo- transpiration rates (ET) were very similar in the treatment swards 28 29 Pri°r to initiation of treatments (Table 1) . Water stress as imposed by irrigation scheduling reduced the CER for 12-day cycle 1 by 4 to 57% with AP being more affected than Rd or Pg. The 6-day treatment showed the greatest reduction in AP and Pg. Leaf water potentials over all treatments were unaffected over the 12-day cycle (Table 2). Overnight recovery following irrigation showed that the greatest increase in CER occurred with the 6-day irrigation treatment with an increase in AP of 77% (Table 3). All treatments showed an increase in Rd (26 to 49%) and Pg (23 to 57%) and only the 2-day treatment exhibited no change in AP. Increases in ET ranged from 9 to 36%. After irrigation, leaf water potentials remained unaffected by the irrigation scheduling treatments (Table 4) . Problems with equipment prohibited collection of data under stress during the second 12-day cycle. Comparisons of recovery data for the first and second cycles indicated that week to week changes were evident as AP increased following irrigation, Rd decreased, and Pg was variable among treat- ments (Table 5). Small changes in ET among treatments were also noted. However, leaf water potentials after rewatering were not different between the study periods (Table 6) . Lack of an induced stress as indicated by high leaf water poten- tials for the first and second cycles and no indication of wilting of the plants prompted a change in the experimental design. At the end of the second cycle all irrigation was withheld until obvious visual plant water stress was observed by wTilting or folding of the leaves. After 19 days without surface irrigation and lack of visual evidence of water stress a decision was made to take final physiological Table 1. Carbon exchange rates (CER) us apparent (AP) and gross (Pg) photosynthesis, dark respiration (Rd), and evapotransplration rates (ET) of St. Augustinegrass at initiation and prior to irrigation on day 12 as affected by irrigation scheduling (12-day cycle 1, 1980). 30 H W G o *H r— H 1 P I cu cd co vO P P0 jc <3* CM CO CM Ph *H CO o CM P CM 1 m m vO m M I e O rH CM cd « CO P 60 o CM •H e OA OA ten G m M G 0 •H P cd to •H U U M 1 CU U Ph PS w o cd •H P •H G M PC PH G Ph < P0 Ph G Ph < G a 6 4-> cd G cu G P O O CU p CO G o •H P cd > p a) co rQ o o S3 31 Table 2. Leaf water potentials of St. Augustinegrass at initiation and prior to irrigation on day 12 as affected by irrigation scheduling (12-day cycle 1, 1980). Leaf Water Potentials Pre- Treatment Initial Irrigation -days between irrigations- 2 -13.9 -12.7 3 -12.8 -12.1 4 -12.6 -10.7 6 -12.9 -11.0 All differences are unpaired t-test. non-significant at the 5% level using an 32 Ml c 6 •H o S •H O 44 i — 1 cO i — 1 54 o *H 144 Pi- co CO O 54 54 G O y r- 54 CO 44 •H i-4 c 4-» >> cO - o Vj o r~_ •H cu Po O CO •u d CO 1 Ph J-l CM Vw/ O r— H •H V*/ CO CO a. PO o G 54 to *H CO to r— 1 a P) d s- 'TO g M H d H a cd V— ^ 4-J w a u CO a v-/ a •H O Pi- o o co CD X G 54 a cO 54 PO c 4J G o o *H -O Ci- 54 54 cO G co > d CJ r-4 CM 00 o i 00 CM 00 o vo vO m 0) rC Pd CM 1 e c T3 o •H O 1 4J CM a) cd EC CO vO M M O O > cO TO I vO vO m cm CM o o CM co o o cm vO CO cn o\ m vO CM cn 5-i rC CM I & 'd CM O O PC e CM 33 Table 4. Leaf water potentials of St. Augustinegrass prior to irrigation and 24 hours following irrigation during recovery as affected by irrigation scheduling (12-day cycle 1, 1980). Treatment Leaf Water Potentials Pre- Irrigation Recovery -days between irrigations- bars 2 -12.7 -10.5 3 -12.1 -10.1 4 -10.7 - 9.0 6 -11.0 -10.1 * ^ differences are non-significant at the 5% level using an unpaired t-test. Table 5. Carbon exchange rates (CER) as apparent (AP) and gross (Pg) photosynthesis, dark respiration (Kd) , and evapotranspiration rates (EX') of St. Augustinegrass during recovery from 12-day cycle 1 and recovery from 12-day cycle 2 ns affected by irrigation scheduling (1980). 34 H W >v o > o a Q) CM o o CM I £ rj CM o CJ to B o VO vO o CM co co CM CO a jo CO So cd "O 1 03 G O 4J G to •H >x Jx CM CO vO 03 0) G rH Cti > +■ eported as single observations 35 Table 6. Leaf water potentials of St. Augustinegrass during recovery from 12-day cycle 1 and recovery from 12-day cycle 2 as affected by irrigation scheduling (1980 data). Treatment Leaf Water Potentials Recovery Cycle 1 Cycle 2 -days between bars — — — irrigations- 2 * -10.5 -11.5 3 -10.1 - 9.1 4 - 9.0 -10.3 6 -10.1 -12.0 * All differences are non-significant at the 5% level using an unpaired t-test. 36 measurements and terminate this portion of the experiment at this point. Final stress period CER and ET as compared to the recovery data for the second cycle indicated only a moderate reduction in AP (20 to 45%) , Rd (10 to 35%), and Pg (16 to 29%) (Table 7.). Even smaller reduc- tions in ET had occurred (9 to 15%) . Leaf water potentials still in- dicated a lack of water stress since no differences among treatments or between study periods were noted (Table 8). Physiological data for the first two cycles indicated that CER was affected most by the 6-day irrigation treatment. The 6-day treatment effected reductions prior to irrigation in AP (57%), Rd (16%), and Pg (34%) from initial values and while CER was lowest for this treatment a corresponding decrease in ET was not observed compared with values for other treatments. Following irrigation the 6-cay treatment had increased rates of AP (77%), Rd (49%), and Pg (57%) with only a small increase in ET (9%). While this increase in ET was the smallest among all treatments from pre-irrigation to recovery, it was lowest among treatments during recovery. Jones (1930) noted that there is a peak transpiration rate above which no further increase is paralleled by increases in CER. Any increase xn CER at this point without an increase in ET would imply non-stomatal inhibitions in photosynthesis. Leaf water potentials, which ranged among stressed treatments from -10.7 to -12.7 bars, indicated minimum water stress in response to these four different irrigation schedules. Crafts (1968) indicated thaL until leaf water potentials of many field crops reach -16 bars theo. e is little affect from lowered values. 37 a 3 o c ■h a •u 3 cm • S-i '—v rl II a CO rH 4-1 CO CJ 3 Ps 3 0 o >> CO ccj 04 3 iH 1 ^ CM 2) So So 3 3 cn •H co G •C 4J c CO o 4J O JZ CO 60 E -H O r-l So 3 14-4 '3 G ►..C SO O CO 60 Po So 3 CO -3 CO O CO So CO 60 3 M 4J 3 60 60 G -H So So rO ■3 c 3 •3 G 60 4J G O c T-l 4-4 Co « 4-1 4-1 3 3 ■3 O •H >0 G CO >> 3 3 PS W U m G 4-1 3 So G 60 3 3 O X G a o .3 3 3 U 3 3 I ON co G 4- > 3 5- i c o t4 4- > 3 5- i •H CO G CD JC G 4-1 3 H E 4-1 O O S-i Co 44 3 CO CO G S-4 4-4 CO > G 3 d 3 « G / — ■— 3 3 3 PS d ^ 3 H W PS W CJ CD tH CD d GJ d p *H 4J Ph CO CN Jn P o o >> a CJ GJ PS CD «H CD d gj d p CO CN >» S-i o CM GJ G o vO VO O « — 1 > CN cn NT cn O O 60 CN 00 o >* CJ G G PS E vO m VO vO 00 p^ TJ PS P4 < CO PL. TJ ps d GJ E 4J d GJ p H v *H d P TJ Pi I *H co vO cn VO vO CO cn — < 00 cn cn co vO o P GJ CD O co d CD d TJ GJ P P O P, GJ P GJ P d CD GJ d rH d > r— 1 < 38 xable 3. Leaf water potentials of St. Augustinegrass during recovery from 12-day cycle 2 and under stress from the 19-day final study period as affected by irrigation scheduling (1980 data). Leaf Water Potentials Treatment Cycle 2 Recovery Final Stress -days between irrigations- 2 * -11.5 -12.2 3 - 9.1 -10.0 4 -10.3 -12.8 6 -12.0 -12.1 * All differences are unpaired t-test. non-significant at the 5% level using an 39 Altnough leaf water potential values were unaffected by irrigation scheduling, overnight recovery for all treatments for AP, Rd, and Pg showed recovery values being greater in most cases than initial rates. An increase in ET was noted for all treatments which corresponds with more available soil water following irrigation. The highly variable response among CER parameters for cycle 2 recovery, small changes in ET, and lack of an effect of irrigation scheduling on leaf water potentials indicated stress was very low. Lower CER values without a lowering of ET or leaf water potentials might be inidcative of some non— stomatal mechanisms involved in control of photosynthesis under water stress. Pre-dawn leaf water potentials prior to irrigation on day 12 of cycle 2 averaged -2.8 bars over all plots with no differences among treatments. At the end of the final stress period CER was sligntly affected after 19 days without irrigation. Moderate reductions in ?g (24 to 35%) and ET (5 to 15%) had occurred. Leaf water potentials were unchanged from initial (pre-treatment) values. Soil matric potentials were rarely below -90 millibars during the entire study period. Sub-irrigation was probably occurring due to a fluctuating high water table 'preventing any induction of water stress based on irrigation scheduling. Turfgrass Quality and Clipping Weights Turfgrass quality as judged by visual ratings was unaffected by irrigation treatments averaged over the study period (Table 9). Clipping weights for two harvest ( 10 July and 1 August ) 40 Table 9. Quality ratings and clipping weights of St. August inegrass as affected by irrigation scheduling. Quality ratings are reported as the mean over the study period (1980 data) . Treatment Quality^ Clipping Weights Harvest 1 Harvest 2 Total -days between -1 -1 irrigations- kg ha day - 2 8.4 *§ 35 23 58 3 8.6 30 22 52 4 8.6 25 20 45 6 8.7 37 23 60 Mean 8.6 32 22 i Quality scores 1 to 9 with 9 representing the highest quality. *3 Means in columns are not significantly different at the 5% level using the Waller-Duncan K-ratio t-test. *% Mfans in row for harvest data are significantly different at the 5% level using an unpaired t-test. 41 indicated no treatment differences at either harvest. However, the overall means for each harvest are significantly different with the second harvest which was taken during the final stress period indi- cating a reduction in clipping weights. This reduction in growth correlates with a reduction in leaf area (Sheffer, 1979) and is therefore directly linked to the decreases observed in CER. However, the reduction in leaf area was not visually apparent. Additionally no visual response of the turfgrass to water stress such as color change or wilting was observed. This points out that the turfgrass was really not under severe stress since leaf area was reduced but photosynthetic rates were high. Turfgrass Water Use Rates Water use rates for St. Augustinegrass were estimated for six periods during the first year of the study based on irrigation volumes and changes in soil water storage. Estimates were based on two replications for all periods. Volumes of water were carefully controlled by the method of irrigation. Estimates of ET within a given period were based on the sum of irrigation volumes plus changes in soil water content within the soil profile from ten- siometer data to a depth of 60 cm. Water use rates for St. Augus- tinegrass were highly variable among periods (Table 10). Signifi- cant time X treatment interactions indicated no pattern to this variability. While differences exist among treatments within study periods, they are probably related to the method of calculation and the high water table indicating a greater change in water storage not entirely attributed to ET. Rates of water use were in general Table 10. Water use rates of St. Augustlnegrass as affected by irrigation scheduling for six treatment periods (1980 data). 42 rQ o 1 cd cd 3 1 1 3 CN o CO 1 i 3 m m 'd' <■ 1 3 • • • • i S o o o O 1 1 1 1 l 1 1 I l 1 rH 60 rQ cd O 1 3 >“3 < *— 1 co o 1 m i rH CN CN CN rH | CN Oh VO • • • CN o o O O 1 1 1 1 O 1 >h >h 1 1 1 1 | rH i-H cd cd cd I 1 *0 *0 rH CN o n- | 00 CO CO CN CN I CN CO 00 • • • • i CN CN o o O O 1 1 1 1 o >h 1 1 1 1 1 rH rH H H *— ( ■ rC cd 3 1 *0 1 m CO m 1 r-~ i CN o i vO m vD vo 1 VO -H CO CN cd • • • • i- 1 e 3 cd CN CO NT vO | cd CU CO to 1 0) m 0) .n 3 o d* a On CM r>- M vO vo vo d d d rQ to ON ON ON a o PM vO vO vo CM •H p d o rQ to d d d rO •H U "d 00 o r-H ON U CM CO co rH M 1 <1) U PM d d d rO r-H ON 00 00 <3 'd- CO co 1—i 1 M ^ ,r JC CM Pi 1 w o HO CM o d d d d u DO VO CM fH CM PM 60 E 00 CO 00 oo d d d d d •H P *0 co CO ON co •H C M Ptf (1) 6^ Q> (0 5 CO a> X 4-1 4-) .O CO (1) 13 4J 0) I 6 4J O I o o -u 4-4 G G CO I G H e 3 ! — I o o G G CJ G 3 c a •H I G to v u >N s "O I rM 3 ■U PH c o Cd CO *H U u w I 0 u PH o p-i CO t— I cd c 0 4-) o pH u a AJ cd <4-1 cd 0 3 4J Ph C M O P^ u p-i c 0 e ■u a a *H H co >h cU cd O + Cj vO m r—i I 3 O * 3 0) B-5 m 0) X, u c Cl) 3 3 cfl O •H 4-1 •H 3 to ■H CO 3 CD 3 3 i — I 3 O O CJ 3 3 3 Q •H I 3 CO CD 3 r— l 3 tH 3 3 £ |2 50 were equivalent in diffusive resistance (DR) and transpiration, on day 12 the 6-day treatment DR was highest followed by the 4-day treatment (Taole 15) . Transpiration rates for all treatments were lowest for the 6— day treatment followed by the 4— day and 3— day treatments. Loss of turgor pressure prompted stomatal closure in the 4-day and 6-day treatments while CER was affected for all the treatments. Reductions in AP and Pg with the 6-day treatment were probably related to stomatal and non-stomatal effects since partial stomatal closure is noted with the 4-day treatment yet AP and Pg rates were not different from the 2-day and 3-day treatments. The 6-day treat- ment had the lowest ET value among treatments prior to irrigation. Changes observed in ET from beginning to end of cycle 1 were different for both components since transpiration was reduced yet ET slightly increased from initial values for all but the 6-day treatment. During cycle 1, irrigation at 2-, 3-, or 4-day intervals may have allowed enough water for a slight increase in evaporation. The plants apparently were under stress since CER was reduced. Irrigation scheduling every six days resulted in a loss of turgor, partial stomatal closure, and lowered CER due to stomatal and non- stomatal mechanisms. Hsiao and Acevedo (1974) noted that complete loss of turgor pressure is not necessary to initiate stomatal closure. Apparently most of the reduction in photosynthesis was due to stomatal closure from loss of turgor pressure, while reductions in ET were due to reductions in evaporation and transpiration. Crafts (1968) observed that stress was not entirely dependent on loss of turgor since wilting can occur even when turgor potentials are at +2 to +3 bars. 51 Table 15. Single leaf diffusive resistance and transpiration rates of St. Augustinegrass at initiation and prior to irrigation on day 12 as affected by irrigation scheduling (12-day cycle 1, 1981). Treatment Diffusive Resistance Transpiration Initial Pre- Irrigation Initial Pre- Irrigation -days between -1 ha u n -2 -1 irrigations- Ug H2U cm s k 2 0.77 a 0.62 c 36.9 a 34.8 a 3 0.67 a 0.67 c 35.9 a 27.4 b 4 0.66 a 1.07 b 38.6 a 22.5 b 6 0.76 a 2.61 a 33.8 a 9.0 c * Means in columns followed by the same letter are not significantly different at the 5% level using the Waller-Duncan K-ratio t-test. 52 Snail changes in CER were noted during recovery following irri- gation (Table 16) for the 2-day, 3-day, and 4-day treatments. The 6-day treatment exhibited the greatest increase in AP (352%), Rd (62%) , Pg (217%), and ET (209%) among treatments following irriga- tion without a concomitant change in total leaf water potential i (Table 17). Values for CER and ET were equivalent among treatments following irrigation. Turgor pressure potentials indicated a re- hydration of the cells and no treatment differences were observed. An associated decrease in DR and increase in transpiration was ob- served in all treatments (Table 18). While statistically CER and ET data indicate complete recovery for the 6-day treatment following irrigation, higher DR and lower transpiration values for the 6-day treatment would correlate well with the lower AP and Pg values. Lower transpiration values with equivalenf ’ET values would support, the concept that some of the increase in ET is related to the evapor- ation component. Physiological Responses During Cycles 2 and 3 Carbon exchange rates from cycle 2 were different among treat- ments with significant reductions in Rd and Pg for the 6-day treat- ment, although Pg was not different from the 2-day and 4-day treat- ments (Table 19) . Variability in the data precluded noting treat- ment differences for AP , but the 6-day treatment value was reduced by 228% from the smallest value among the other treatments. The 6-day treatment did not have CER recovery levels following irrigation comparable to the other treatments as it did in cycle 1. Recovery ET was increased most for the 6-day treatment (86%), although the 4-day and 6-day treatments were not statistically different in ET rates. Table 16. Carbon exchange rates (CER) as apparent (AP) and gross (Pg) photosynthesis, dark respiration (Rd), and evapotranspiration rates (ET) of St. Augustinegrass prior to irrigation on day 12 and 24 hours 53 d cd cd cd 0 CM m vO > O vO m cn o T— 1 vO vO co 0 1 Gh o Oh o 0 34 »— 4 Pi rP to e 00 0 rH o o CM c o nd i 0 •H 4J O CM cd «d cd J0> 34 cd K vD m CM vO 34 to crs t-H o t—4 *H to vO cn r-H Jh V4 M B r-* CO Oh sr CO 3 0) > (!) LT) Q) ■U cd "O i CM t-H W to c •H » — i 3 (1) x: o m 3 O d to n •H >. X3 *3 0) •U o 0 0-1 d m d 34 0 > o o a u to c ■H 34 3 3 O d to H 3-4 34 to 3 H 5 O pi w u u o > o o 0 3 0 •H •U d to •H U 34 M 1 0 34 (X, to P-i 33 Pi Pi < cd cd cd cd 4-J c a o CM cn u v rH r— 4 co r-l 4J C m cn cn cn cd cd cd cd cd o •H <4-4 •H C fcC Oh CO £ c 4-> o CD rO 4-J cd CO to cd Sh no J-4 1 •H CO Mj- \D 0 3-1 d 34 0 0 g d cn 0 rC P> >4 XI 33 0 s o r~H > — I o <4-1 CO <1) u I -U 0 •H ■P cd u 1 W c cd 0 c 2 Q 1 U PH O AJ Cd *2 «-» a o cd rQ o rQ Hi o m o VO ■U • • • • Ph . — i f— 1 o + + 1 1 c o 4J Oj cd cd cd cd tc vD cr» m •H O • • • Jh Pi m m 4-1 >4-1 •H -a 2 r-4 4-1 CO CJ •tH G 60 O c QJ G cO n CJ 4-1 o g CO a) X 4J >. 4J jo co o to -u 0) I 5 4-1 o *— 4 O r—4 *H O 4-1 >4-1 CO s-i to I C 64 6 3 G co CJ G 3 G Q •H | G co tu C r-l CO r-4 CJ CO 2 S o CJ 55 Table 18. Single leaf diffusive resistance and transpiration rates of St. Augustinegrass prior to irrigation on day 12 and 24 hours following irrigation during recovery as affected by irrigation scheduling (12-day cycle 1, 1981). Treatment Diffusive Resistance Transpiration Pre- Irrigation Recovery Pre- Irrigation Recovery -days between -1 -2 -1 irrigations- sec cm yg h2o cm s * 2 0.62 c 0.52 b 34.8 a 45.3 a 3 0.67 c 0.54 b 27.4 b 46.4 a 4 1.07 b 0.52 b 22.5 b 45.1 a 6 2.61 a 0.75 a 9.0 c 34.3 b * Means in columns followed by the same letter are not significantly different at the 5% level using the Waller-Duncan K-ratio t-test. 56 to j-i 3 O J3 V}- 3 CM CA CO 3 3 C d 3 £ CM - k H N a 3 CD 3 i — l •>'3 0 CO pn 3 O 00 ai co 0) £ d d o 3 C 3 >. U N SI !J H O fcO^x 3 3 O d 00 £ d d a -h th r— I ^03 to 4J d Pi CD d £ o o co 3 co co d o p d d o oo co th CO 3 3 d d d d oo d co 3 CD d ^ d d d iH iH < 3 W >» 3 £ 3 00 d 3 3 CD CD d o J3 d d u • 3 on e 3 d 00 d 3 s O OJxnH 3 3 3 3 d O d 3 H >> G G rQ X d CD O vO CM > r— H co in CM CM O 1 t— 4 co CM m CD co CO vO (D £ PS CM 1 H G3 W d o o 3 CM G a 1 3 Cd cd d co ON m d oo oo iH vO pH PC 3 3 LO o co m d UO r^* m co d 3 G G G X 00 CM *■— 4 CM C\ PC CO c> CO Pi CO <• CO CM O Cl CD PS G G G X PC vO O m < t—A i -1 <■ m . vO CO d o 3 3 d G G G rO 00 3 3 vO O 00 d PS CO CO CO •H d 3 1 1 CD d * P-. G G G G Pi CM co vO < G CO o e CD m CD ,n AJ d d CD d (D 3 3 3 3 d d o 3 3 3 d 00 3 CO 3 O d CD d d d CD 3 3 ID CD 1 d co CD £ tn 3 £ CO CD 3 3 CD I S 3 o 3 O 0 3 3 d d CO I d td 1 d d CJ o d 3 d a 3 | d CO CD d 3 d i — i > G So P I CM to p u CO G co U U o G G 4_i CO G G G G G 3 CO CO CM p— i VI • • • • Ph m PH co T— 1 On CM O *— < *— 1 rH CJ 1 1 1 1 0) erf G G G G VI O vO O CD Ph • • • • H vO vO m vO G 1 1 1 1 •H P £ G P O Ph CO >-i -i G G P rQ G 3 4-1 G G ►3 G G G rP D d* o + + + + c o •H V* G G G G ca 60 CO m CTv •H O • • • Vi PH co m V r— < »— l rH M 1 1 1 1 1 0) Vi CV G G G G CO CM vO VI • • • • Ph oo *-— l O 1 r—1 1 r— 1 t— 1 a a; 1 VI 0) CO e 3 G > -H H G V "G V 1 •H G rP CO P •H CO 3 G > G m G rP ■U ■u p G U G 4-1 4-1 P G G •H •H C CO •H CO 4J o 3 G 5-i G 5-i G G 6 G co G ■p • P So CO P G P G £ O o cd 4-1 5-1 I CO 6 P cd G G 3 P 3 I •H 5-1 G CO rH G «H G G G ^ s 59 Table 21. Single leaf diffusive resistance and transpiration rates of St. Augustinegrass prior to irrigation on day 12 and 24 hours following irrigation during recovery as affected by irrigation scheduling (12-day cycle 2, 1981). Diffusive Resistance Transpiration Treatment Pre- Irrigation Recovery Pre- Irrigation Recovery -days between irrigations- sec ( -1 cm yg h2o -2 -1 cm s — 2 0.67 b* 0.57 a 28.4 b 44.9 a 3 0.57 b 0.46 a 34.3 a 48.6 a 4 0.58 b 0.64 a 36.3 a 40.5 a 6 1.79 a 0.49 a 13.6 c 48.6 a * Means in columns followed by the same letter are not significantly d^fferent at the 5/, level using the Waller— Duncan K-ratio t-test. 60 c o 3 3 iJ d d U CM *H & G 03 i — 1 a a >. o j*j» T! d a 3 3 | * CM • 03 i— ■ 1 d 03 £ 4-» a o d r* 5-4 "3 4J 4-4 £ i — { P>h >> 00 03 5-1 Gh O G r— 1 4_i > o O O to o- G 3 M •H /^N tH CO CO 3 E "3 •H G G r; 03 3 O 03 E3 03 O U cn 3 CO 03 O a *H ”3 i-i 4J 3 to d c G to E •H 5-i P* XJ 5-i < 03 *H M/ 3 CO Po 4-1 3 rQ c < G •3 J-i • G G 4-1 4J Q- CO a cu G d 4-4 4-4 O 4-4 03 d d H 03 W d PS v-/ w CO o 03 V-/ 0) G 4-1 5 1 03 d G a 5-4 Po -U G VJ 3 s-l O P^ •H d G 4-1 "3 to d 1 E M CM a r— H ^2 3- O 03 £ X 3 o G d V4 5-1 4-4 C 4-1 o O Po -a Cu 5-4 n d G a > > u o O G 3 G • 3 U CM d CM to •k 3 G /~N •H f — i "3 5-1 S3 Pi 3 G Nw/ *3 H H H 5-4 a > o o a E cn g rH o >> u Pi w c_> n G > o o G CM G fH o >, u CO rQ d d o G G r— 4 i— 1 1 vO co CM co O M CM > 3 <■ 00 CO Oh o o o Oh vO CM t— 4 iH 1 = 3 O n CM CM d d d O E G r— 1 vO CO i — i to co m CM CM O 3 t— i co CM m >> CO CO r^ vO u d d d d to - pi E G 4-1 o & G •H 4-1 E 4-1 G G CM CO vO G CO to 54 >, -H H d 5-4 3 5-4 1 ■X Means in columns followed by the same letter are not significantly different at the 5% level using the Waller-Duncan K-ratio t-test. 61 indicated cells probably were fully hydrated (Table 23). Unfortu- nately, due to rapidly changing climatic conditions on the data collection day, no values for diffusive resistance and transpi- ration were recorded for cycle 3. Physiological Responses During Cycle 4 Photosynthetic rates of turf prior to irrigation on day 12 at the end of cycle 4 were reduced in most treatments from initial rates and were lower than those from cycles 1 and 2 (Table 24). The 6-day treatment caused the lowest AP value, although it was not different from the 3-day treatment. Similarly while Rd and Pg were considerably lower for the 6-day treatment they too were not shown to be significantly different. Values of ET prior to irrigation were the lowest for the 6-day treatment although they were not different from the 3-day and 4-day treatments. All treatments showed increases in CER following irrigation. Values of AP and Pg were not significantly different among treat- ments. Recovery Rd was lowest for the 6-day treatment although it was not different from the 2-day or 4-day treatments. It should be noted that the lowest values for all CER parameters were recorded by the 6-day treatment. All treatments showed an increase in ET during recovery and while values were equivalent statistically, the 3-day, 4-day, and 6-day treatments were lower than the 2-day treatment. Leaf water potentials were high and failed to statistically show treatment differences for total, osmotic, or turgor potentials (Table 25) . While turgor pressure was positive prior to irrigation the values were low and all treatments indicated an increase in turgor pressure following irrigation. This supports the concept Table 23. Leaf water potentials as total (Pt), osmotic (Po) , and turgor (Ptu) potentials of St. Augustinegrass during recovery from 12-day cycle 2 and during recovery from 12-day cycle 3 as 1... J f . ...» 1 1 1 l n A. v J * ■' 62 d u d ■d 3D to c d O a co d o d to :o n D J J D -» -4 3 o n d d o 3 co m CM CM +J • • • Pm co vO n CO + + + + CO d d d d Q) CM VO O • • i — 1 Pm CM CM CM o O i— -4 i-— l T— ) Po | i l O d o .3 d CO 4-t CM n d o ■U o P-I u o CO u > -4 o o 3 4J d u x> d a) OT to CM CO ■, -H H d !m "d |M i H Means in columns followed by the same letter are not significantly different at the 5% level using the Waller-Duncan K-ratio t-t‘est. 63 >> a a a a 3 cn 3 vo oo m >4 > rH td O rH Cl 3 3 O 1 *C CM CNI VO o o 3 3 o vo •H 3= 3 r3 3 Pi 3 . Ph rH B o oi cn cnj •o S Cl 3 m cn cn cni Xd * — I 3 • o M m 3 >> •h o a CO a s >> HO O 3 u -ri •a C 3 1 Pn 3 cs CO to rH a a a a O tH — 4-> 3 to O CO rP VO OHM Ph CO O CO VC -C rH 3 CU -H O rH r-N 3 3 to X3 JO JO CH H N d d d jo ^03 3 •H 3 3 xd vo CT\ CO CO CO 3 CO > Pi <■ O 3 to CNI CO CO CN CO Ph to vo m m cn cd co 3 e H cd O r v H -H Pi n-t >, 3 w 3 3 U CO 0) to a a a a ^ o > tH 4J O 3 xd co Mf o co cd o 3 Pi co CO CO CM 333 M 4-* 3 1 3 3 3 3 O tO 3 rH 3 Ph * JO O *J rl a a a b tO 3 3 3 3 3 Ph P '<: 3 o 3 a *h 3 4H 3 4-1 Cd o o to 3 C.H 3 Cd 3 3 > 3 s U OJ rl 3 1 3 3 c n Xd to 3 & d • 3 3 3 3 o ■3- a rH e 3 *H cni 3 3 HP P « O 3 3 cni cn N rH 0 3 0 H d 3 ^ IH T3 H 1 rH CD rC P to c CO d (D > 0 m a rC p P d p d CD P CD ip

-> p rO CO 0 P I nD 0 S O t — I o o •H P ip d u CO I d & S d d rH d 0 d d d p •H I CO 0 d rH d » — i 0 d S ^ o a 64 u 3 > o a co 3 cn 4-1 O 3 a to c 3 3 3 O 3) 0< 3 ^-s o 3 -H 3 3 a. 3 to •H 3 3 O 3 co -w 3 CO 3 3 -H ■3 > 3 O 3 tH i — i - O • * 4 4-4 s~\ O r— i 04 CO CO 3 CTS 3 i— I o o •H ,3 * ■u sr o Sf £: cs 3 0) rH O 3) o 3 Jo » 3 3 u N >, H !J 3J fo I rH 3 CS 3 3 H 3 o 3 3 *H 3 3 3 O s o •H CO 3 3 33 3 3 CO .3 3 co 3 -h a o O -H o 04 4-1 J-l o 4-1 3 s 3 to 3 O •H 3 3 3 04 -H 4h CO >% 3 CO ,0 a 3 >-0 3 33 CO 3 3 4-1 • s a in -h 3 CS 3 4-1 CO 4-4 3 3 3 iH CO .3 3 3 3 < 3 H 3 •H 4-1 3 3 3 O P-4 3 3 4-1 3 & 4-1 3 3 hJ So 3 3 > O 3 3 P d- + + + + cd cd cd cd i — i CO co vO &-? • • • • m CO iH o Ov iH rH 3 i l 1 1 .3 3 CO o I co 3 3 HP 3 3 I 3 co & 3 3 O 3 tH -Q 3 3 cn co >1 iH 3 3 43 3 I -H CS 3 sf si- I 3 3 O r-» I co 3 CO CO I 3 CTi C\ L O I Cj cd cd cd CN CN co • • • • CO rH O rH + + + + cd cd cd cd rH CN • • • • o 00 o CN rH r-H 3 CT> o fH i vD 3 33 CO 3 •H CO 3 3 > 3 3 3 3 3 44 4-1 •H •3 3 3 3 O •H 4-4 •H 3 CO 3 O 3 3 3 3 3 3 3 3 3 3 CO 3 r3 3 to 3 JP CO 3 3 I 33 3 I rH -H O 3 4H 3 3 CO I 3 E 3 3 i — I 3 O 3 3 3 3 3 « •H I 3 CO 3 3 rH 3 rH 3 3 S & 65 that irrigation every other day is more effective for keeping ade- quate turgor pressure in the turf grass. Unfortunately due to rapidly changing climatic conditions, no diffusive resistance data was avail- able to document changes in stomatal activity. However, the dra- matic increases in ET would indicate that all plots are using the water made available by irrigation and this probably resulted in stomatal opening from increased turgor pressure. Physiological Responses During Cycle 5 Photosynthetic rates appeared lowest during the entire study period for all stressed treatments during cycle 5. While no sta- tistical differences among treatments were found for AP or Pg the 6— day treatment had the lowest values (Table 26) . While values are not as low as those observed prior to irrigation at the end of cycle 4, ET was reduced . The 6-day treatment following irrigation had in- creases in AP (353%), Rd (57%), and Pg (204%) among the highest re- corded during the entire study period. The 6-day treatment ET value was lowest among treatments prior to irrigation, and recovery values following irrigation were equivalent for all treatments. Variability in measurements from plots within the same treatment was noted. While plots were handled the same within treatments, small sample sizes may have masked distinct treatment differences. No treatment differences in leaf water potentials were noted on day 12 prior to irrigation, although the 3-day, 4-day, and 6-day treatments had lower values than the 2-day treatment (Table 27). All plots exhibited lower turgor pressures prior to irrigation which accompanied the lower CER noted previously. Turgor pressure was regained following irrigation for all treatments. Diffusive 66 CO 5-4 g O o o •H XJ G 5-i CN • •H /“N C-» 53 r— ! CO C CO o G ON 5-4 r— J CN r— i r\ 5-i to G "a G G 53 rH a a CO rj •H O G CO a £ r— ■ 5 G 4J •H "G c xj 1 c3 CN CO to »—* o •H v_/ XJ 5-i o 5-i to r“ •H rj O « — i XJ 3 to 53 P-. 5-i G V-/ O i — •H 13 CO M CO CO Ct o £ 5-* CD O to CO *H g XJ wr3 5-i G G to to G a •H 5-i •H 5-4 P-4 < CO V-/ Z3 tO p£J 4J 3 G < 53 a a 5-4 • XJ G 4-) G G- C/0 G £* 4-1 G 4-1 4-1 O G CO C CO H G /^v H v-/ K 5-i U CO G G > 4» O CO C G o 5-i G i-i 5-4 g C 5-i o to £ o XJ to G 5-4 c 5-» 3 a •H 53 r- C- a CO G x G O o 53 5-i XJ £ xj G o O to Cu *H 5-i G 5-i S3 > 5-i U G *H to • c G vO G *H CN S •» O O ✓“N r— ( H 53 r— 1 £3 o G W t4_j H >> G G G G Ph r-H »— H O O' cn o | CN m O' m o M r^- vO vO 0) & po CN B H 53 W c o o rn •H CN G G G pO 1 4-1 a) id o m cn m M 60 60 G> CO cn cn cn o o a) m v 4J pD CO a) u 53 0 1 4-1 to M a to 0 c 3 « 1 Pi cn o) a ih tO i — t CD to S & o o e tH 67 c/d ij-i o >% 54 3 > O O to 3 rH 5-4 3 •H co •u 3 3 iH 3 5j •u 3 O "3 (0- 3 H—' O 3 *H 44 44 Ph 3 to •H 54 54 O 54 CO -H 54 3 CO 4J 3 •H "2 & 3 o 3 rH rH r» O • /-“N MH /^N O rH Ph CO 00 ^ ^ o\ 3 iH o o •H HO - •U m o <* 6 cm 3 to rH o x> o 3 5n « d o •u CM tn P-l H Cd ' — 3d >> I rH Cd CM Id 3 rH O 3 •u o co 3 CO 3 *H d OH •H 3 CO 4J td -H 3 3 cono o •H iH 4J !-l CO 3 54 3 iH c w o O O vH a u w cd M M M 3 O -rl U *H Ih cd u 54 S h 3d co 3 3 4J • 3 a I — *H 3 CM 4J <4H CO HH 3 3 3 rH CO HO 3 CO 3 ■< 3 H 3 4-1 P-4 >> 5-1 3 > O 3 3 Pi O P-H 4-1 P-4 3 •H 4-> 3 3 4-1 O Ph 54 3 4-1 3 & 4H 3 3 rd 3 4-> Ph 3 O 3 CO •rl 54 54 M I 3 54 Ph O Ph 44 Ph d d d d o co • • • • m vO vO m + + + + ,3 3 O co t— I I w u d ^o d VO CM r— < I 3 1 4-4 3 CO 3 S 3 3 4J O e 3 vH 44 HO 4—1 3 3 3 CO CO 54 >4 -H H 3 54 3d 54 1 -H rQ d d o CM rH I d o co T— H I 3 en; in 3 n: 44 4-1 3 4-1 3 3 54 3 144 144 vH 3) fn 3 3 o •rH 144 •H 3 CO o 3 3 54 3 54 3 4-1 4-1 3 3 6 3 CO 3 r3 44 44 HO CO 3 3 3 rH 3 O 3 O 3 3 3 n •H | 54 CO 3 3 rH 3 rH 3 3 S & 68 resistance and transpiration measurements indicate significant vari- ability within an experiment (Table 28). The 2-day, 4-day, and 6-day treatments had high diffusive resistance values although no treatment differences existed. Transpiration rates, when compared to cycles 1 and 2, were similar. For the 2-day, 4-day, and 6-day treatments irrigation resulted in lower diffusive resistance and increased trans- piration. The 3-day treatment did not respond the same way. The low <^*^us-*-ve resistance and high transpiration values prior to irri- gation for this treatment cannot be explained and may have masked other distinct treatment effects present. Overall Physiological Responses of St. Augustinegrass Photosynthesis as measured via three parameters AP, Rd, and Pg was significantly lower over the 1981 study period for the 6-day treatment (Table 29). Values were reduced_for AP by 35%, Rd by 22%, and Pg by 31%. Evapotranspiration was also significantly reduced by the 6-day irrigation schedule by 20%. Response of St. Augustinegrass during recovery following irrigation from pre-irrigation values was well documented. Recovery values of CER as AP and Pg increased significantly from pre-irrigation measurements and usually recovered to pre-treatment values (Table 30). Only Rd showed no significant increases. This is not unexpected since changes in soil respiration and plant respiration (including roots) are not dependent on stomatal control and only in extreme cases of very low soil moisture levels would this parameter be greatly reduced. Recovery values for ET were also increased significantly over pre-irrigation values for each 12-day cycle, and were similar or greater than initial pre-treatment values. 69 Table 28. Single leaf diffusive resistance and transpiration rates of St. Augustinegrass prior to irrigation on day 12 and 24 hours following irrigation during recovery as affected by irrigation scheduling (12-day cycle 5, 1981). Treatment Diffusive Resistance Transpiration Pre- Irrigation Recovery Pre- Irrigation Recovery -days between -1 -2 -1 irrigations- yg h2o cm s * 2 0.87 a 0.47 a 35.9 b 49.7 a 3 0.49 a 0.45 a 54.5 a 57.8 a 4 0.90 a 0.44 a 33.7 b 56.9 a 6 1.00 a 0.49 a 33.7 b 53.5 a * Means in columns followed by the same letter are not significantly different at the 5% level using the Waller-Duncan K-ratio t-test. 70 Table 29. Carbon exchange rates as apparent (AP) and gross (Pg) photosynthesis, dark respiration (Rd), and evapotranspiration rates (ET) of St. Augustinegrass averaged over all cycles as affected by irrigation scheduling (1981 data). CER Treatment AP Rd Pg ET -days between irrigations- — mg C02 dm-2 hr"1 — mg H^O dm 2 hr ^ 2 34 a 38 a 72 a 7560 a 3 37 a 40 a 77 a 7807 a 4 36 a 36 a 72 a 7142 a 6 22 b 28 b 50 b 5752 b * Means in columns followed by the same letter are not significantly different at the 5% level using the Waller-Duncan K-ratio t-Eest. 71 Table 30. Carbon exchange rates as apparent (AP) and gross (Pg) pnotosynthesis , dark respiration (Rd) , and evapotranspiration rates (ET) of St. August inegrass averaged over all irrigation treatments for pre-irrigation (PI) and recovery (R) during all cycles (1981 data) . Stress Cycle AP CER Rd Pg ET -2 — mg CO^ dm hr -1 mg H20 dm -2 v, "I hr Initial J. 41 a 42 a 83 a 6915 b PI1 32 be 27 d 58 cd 7457 b R1 39 ab 34 be 73 ab 9894 a PI2 31 be 30 cd 61 cd 5493 c R2 42 a 34 be 76 ab 7559 b R3 37 ab 42 a 79 a 9396 a PI4 20 cd 32 cd 52 cd ' 3640 d R4 37 ab 43 a 80 a 7639 b PI5 16 e 33 be 50 d 5596 c R5 26 cd 39 ab 64 be 7063 b * Means in columns followed by the same letter are not significantly dirferent at the 5% level using the Waller— Duncan K-ratio t-test. 72 While total and osmotic leaf water potentials were not different among treatments, the turgor pressure decreased as days between irri- gations increased (Table 31). Zur and Jones (1981) observed that tur- gor pressure is the key point in a model simulating the control of photosynthesis by plant water status. Total leaf water potentials increased significantly from pre-irrigation to recovery for all but the first cycle (Table 32). More importantly, turgor potential in- creased indicating a relief from stress by increasing turgor pressure. Osmotic adjustment was not evident, possibly due to variability among leaves of different age at the same position on the plant; and/or the method used in determining osmotic potential; or that osmotic adjustment does not occur in St. Augustinegrass . Most of the impact of reduced turgor pressure is mediated by changes in leaf diffusive resistance to carbon dioxide and water vapor. Diffusive resistance was highest over the study period for the 6-day treatment, while transpiration was lowest (Table 33). Diffusive re- sistance and transpiration fit a similar pattern to CER, ET, and leaf water potential components. Each pre-irrigation to recovery measure- ment showed significant decreases in diffusive resistance and increases in transpiration (Table 34) . General Responses of St. Augustinegrass to Water Stress Turfgrass quality as judged by visual ratings initially was not different among treatments (Table 35). With the 6-day treatment, turfgrass quality was reduced at the end of the first cycle and re- mained low, but it was not different from the other treatments for the remainder of the study. No treatment differences were noted for any other rating period or for the average over the study period. 73 Table 31. Leaf water potentials as total (Pt), osmotic (Po) , and turgor (Ptu) potentials of St. Augustinegrass averaged over all cycles as affected by irrigation scheduling (1981 data). Treatment Leaf Water Potentials Pt Po Ptu -days between bars irrigations- * 2 - 9.3 a -13.4 a +4.1 a 3 - 9.2 a -13.0 a +3.8 a 4 -10.4 a -13.5 a +3.1 ab 6 -10.8 a -13.4 a +2.5 b * Means in columns followed by the same letter are not significantly different at the 5% level using the Ualler-Duncan K-ratio t-test. 4 74 Table 32. Leaf water potentials as total (Pt), osmotic (Po), and turgor (Ptu) potentials of St. Augustinegrass averaged over all ^rr^-Sat:ion treatments for pre— irrigation (PI) and recovery (R) during all cycles (1981 data). Stress Cycle Leaf Water Potentials Pt Po Ptu bars — - — Initial -13.0 ab* -15.5 b +2.5 de PI1 -14.0 a -15.0 b +0.9 e R1 -13.2 ab -17.2 a +4.0 c PI2 -11.3 b -14.6 be +3.3 cd R2 - 6.0 de -11.7 def +5.7 ab R3 - 7.1 cd -11.8 def +4.7 be PI4 - 8.9 c _ -10.5 f +1.6 e R4 - 4. 9 e -11.2 ef +6 ,3a PI5 -14.2 a -13.1 cd -1.1 f R5 - 6.8 de -12.6 de +5.8 ab Means in columns followed by the same letter are not significantly erent the 5 A level using the Waller-Duncan K— ratio t— test. 75 Table 33. Single leaf diffusive resistance and transpiration rates of St. Augustinegrass averaged over three cycles as affected by irrigation scheduling (1981 data). Treatment Diffusive Resistance Transpiration -days between irrigations- -1 sec cm ii r. -2-1 yg ^0 cm s 2 0.63 be* 39.1 b 3 0.55 c 43.6 a 4 0.69 b 39.1 b 6 1.13 a 32.4 c * Means in columns followed by the same letter are not significantly dif f erent at the 5% level using the Waller-Duncan K-ratio t-test. 76 Table 34. Single leaf diffusive resistance and transpiration rates of St. Augustinegrass averaged over all treatments for pre-irrigation (PI) and recovery (R) from three cycles (1981 data) . Stress Cycle Diffusive Resistance Transpiration -1 sec cm u n -2-1 yg H„0 cm s Initial 0.69 X cd 36.3 d PI1 1.24 a 23.4 f R1 0.58 de 42.8 be PI2 0.90 b 28.1 e R2 0.54 e 45.7 b PI5 0.81 be 39.5 cd R5 0.46 e 54.5 a * Means in columns followed by the same letter are not significantly different at the 5% level using the Waller-Duncan K-ratio t-test. $ Table 35. Quality of St. Augustinegrass as affected by irrigation scheduling during the 1981 study period. 77 a d d d d d co m (D • • • • s r^ vO a d d d d ,o i — i CO o • • • CM r>. r^ r^. a d d d d fO co »H o co co CO — * 3 • • • • • *"3 co vO d d d d rO >> CM ^ o m • • • • • •f- >> r^ co vO r^ P r— i a d d d d d rO 2 CD O' «% d d d rO rO o d CO CO • • • • vO r^. ten 6= •JC Po d d d d d oo d •p XI CO o co o CM • • • • • CO co CO CO CO d Q) 1 p CD W a £ d CD P o B CD -H p rO P d d d CM co MT vO d CD w to CD P Po *H H d P u P 1 *H •u 3 3 cr u CO 3 60 •H X 0) .3 4J to c c 0) CO o 3 n. 3 u o\ 4J •H CTi I CO to 3 3 pi a x: 4-1 to 3 •H CO 3 3 > 3 ?— t S'? in 3 .3 4-4 s 3 H 3 4-1 44 •H TJ r-4 4-) 3 3 O •H 44 3 to •H CO O 3 3 3 3 V4 3 3 i — I 3 6 3 CO 3 X 4J >> 4-> ,3 W 3 •0 4-1 3 I & 4J o 0 4-) 44 3 V4 CO I a « 1 3 r-4 3 O O 3 3 3 3 « •H I V4 W 3 3 i — 1 d r— I CD d s s ten *K *11 Irrigation means in row followed by the same letter are not significantly different at the 5% level using the Waller-Duncan K-ratlo t-test. 78 Quality ratings while reduced from initial values were never con- sidered to be unacceptable. Cover estimates were lower for the 6-day treatment plots during the fourth cycle but ground cover increased by the end of the study (Table 36) . The small visual changes throughout the entire study period were not a major factor in turf quality although these diff- erences were statistically significant. Changes in cover during cycle 4 apparently were a factor with the 6-day treatment in cycle 5. Changes in leaf area as estimated by percent cover may have con- founded moisture stress and contributed to lower CER and ET values ob- served during cycle 5. Clipping weights indicated that the turfgrass was under severe moisture stress from a growth standpoint and was not producing leaf area as rapidly as it would in a well watered condition. Clipping weights for harvest 1 and 3 and as a total of harvests 1, 2, and 4 showed that the 6-day treatment caused the lowest values, although the 6-day treatment was not different from the 2-day and 4-day treatments (Table 37). Harvests 1, 2, and 4 were taken under water stress during deficit irrigation and were significantly lower than harvest 3, which was taken during recovery after a severe storm destroyed the protective shelters and allowed excessive rainfall on the plots. Harvest data corresponded well with CER and ET in that the 6-day treat- ment had the greatest affect on all parameters. Decreases in CER can be attributed to not only physiological changes from stomatal and non-stoma- tal effects, but also from a reduction in leaf area in the canopy. Decreased ET can be due to stomatal closure and/or a reduction in leaf area. Table 36. Cover estimates of St. Augustinegrass as affected by irrigation scheduling during the 1981 study period. 79 to 3 4-1 •H AJ CO a S-i 3 > o CJ co co 3 c»0 4-4 M 3 H C 3 13 S vd- iH cm 3 *3 >4 CO rH ^ 3 *3 to •3 3 Mr 3 CM 3 *3 3 -4 e —I 3 *3 c 3 E 4-1 3 <3 H 3 (3 I 3 CO 5 3 ■u o 3 -H .o -u 3 co to to -H 3 M 3) 3 I iH cd cd cd cd oo g *— i oo •H • • • • CO Oh 00 Oh >d- 0 Oh Oh Oh Oh rH (1) > QJ rH cd cd cd cd cd m Oh 00 Oh vO m 0 • • • • • Oh Oh Oh Oh P o\ Oh Oh Oh Oh P cd 4-) G 0 P cd 0 cd cd rQ rO IH 4-1 •H Oh Oh CO o O • • • • • Oh Oh Oh h r— 1 P G cd u •H IH cd jo J3 *H cd cd X* rO G OO •H Oh CO CO CO 00 • • • • • CO Oh Oh Oh CO Oh Oh Oh Oh Oh P O G 0 ' P G cd P cd cd cd a 0 P m vo co co P • • • • • 0 Oh CO rH Oh Oh Oh Oh Oh 0 i G CO • 3 4-1 ,3 W ■T— tcr> 4-1 3 5c * 4-1 cd cd cd cd 1 31 4-1 CM o r^. ^r • • • • • 33 O oo vO r^» CM VO 3 iH Oh Oh Oh Oh Oh & 4-1 O 3 r-l V4 rH 1 o a <4-1 3 CO 3 3 3 E 3 3 3 rH Q O 1 3 3 3 3 rH •H rH 3 M S CM G 3 CO vO cd 3 3 CJ 3 X S S 4-1 4- 4C *§ Irrigation means in row followed by the same letter are not significantly different at the 5% level using the Waller-Duncan K-ratio t-test. harvests during the 1981 study period. 80 u 3 O w o 50 3 *3 3 O 35 3 50 U J- 3 3 3 3 3 3 3 J-i 50 3 i i rH 1-3 cd rQ JO i i cd a cd cd a 34 1 4J •* 1 O CM 60 4J CO cd cd d 3 1 35 | (1) CO CO o 4" 1 O • • • * 1 * H CM r^ CM r* I - — 1 1 l l l CO l l l l 1 ■U CO 1 rC _D 1 to •H 4-J CO cd cd d 34 1 3 | o o CO n i o • | • m o o 1 | CL cd | CL -a | •H j r— 1 rH | o i | 3 | 30 60 CM 34 1 JJ 1 | CO (U cd cd d d l o ■ £ o o co CO 1 CO H • • • • 1 • cd W CTv i oo 1 1 l \ rH l l l l 1 I +- 1 4J * 1 ten CO rO JO 1 -JC Q) > cd cd d jo i a CO o co 1 o cd • • • • i • w v£> CM o CM I ON rH rH 1 » 1 1 1 G 1 1 1 1 1 | 3 1 | 4-J 3 CO | c & d | o 44 o | e 3 •rH | 4J 30 4J 1 c cd cd CM CO vo i cd a) 3 00 1 H fn *H 1 s H 3 U 1 ■3 U 1 1 •H 1 50 G i-t 3 3 I — I 3 3 B'-S m 3 35 44 3 C 3 G 3 M— I <44 •H 13 44 c 3 O •H <4-1 -H c 60 •H 3 44 O C 3 G 3 3 4-> 3 I m 3 44 ,3 3 4-1 3 44 >. I 30 44 •a o 3 i-l S 44 O 3 rH M l—l I O 64 >4-1 c 3 3 S 3 3 S r4 O O I O 44 3 C T-4 *H i — | 3 3 |3 c 3 3 3 30 S 44 -I- 4C tea •* Means in row followed by the same letter are not significantly different at the 5% level using the Waller-Duncan K- ratio t-test. 81 Estimates of Turfgrass Stress Percent estimates of the extent of wilting of the plots were taken at various times during the first three cycles. Extensive wilting was evident one week into the experimental period (Table 38). Decreases in wilting usually occurred during recovery from stress after irrigation for all but the 6-day treatment. Over the entire study period more wilting occurred in the 6-day treatment plots than in the other treatments. Variation in wilting was highly evident among treatments and between days in the stress period. The ability of the turfgrass to tolerate reduced soil water content was low and St. Augustinegrass was extremely sensitive to environmental conditions early in the study period. When soil metric potentials in the top 15 cm exceeded -200 millibars ex- tensive wilt occurred (40-50%). Only in the 2-day, 3-day, and 4-day treatments did irrigation relieve mid-day wilting. However, days during late May and early June 1981 were extremely hot and dry with daily temperatures reaching 40°C and low relative humidity (40-60%). Once the high evaporative demand decreased in mid to late June and July with increased relative humidity mid-day wilting decreased even though soil matric potentials exceeded -200 milli- bars in the top 15 cm of the 6-day treatment plots. Turfgrass Water Use Turfgrass water use rates were variable among treatments when cal- culated during the 1981 study. Over the entire study period the 4-day treatment plots had the lowest water use rate followed by the 6-day and 2-day treatment plots and the 3-day treatment plots had the highest rate (Table 39). Higher water use rates for the early periods Table 38. Estimates of percent of total plot area wilted during afternoon hours from the 1981 study. 82 6-S a e •u o 0) -H X -U d CO 60 >,-H d V4 'O Vi I -H o X "G G G CM rH m rH O H T—\ •— 1 CM m X 43 G 43 cd G G 43 CM CO r* 4-J CM rH 4J G ■U X X c cd cd G G m OV J-i G VO vO o> UH MH *H X cd 43 X G iH r— 1 CM CM CM 4-> LO MT 00 G G O •H 43 » X I 4H cd cd >> 43 i G *H i cd >, i G vO >> m x r-H cO av >% to CM d > CO rH >1 ’’G d >V X d rO 43 G d G vO -a a *3 B H O fH u vO G r— 1 o cm yt CM 3 X W s x. O •H •u 4-J >4-1 CO G cd 43 >> X G >> X 4J V4 H • o\ cu CO G CO 0) m CU 4J r 1 > *-< > r— 1 > vO > 43 w a) G CU 0) G ■U CO CO CO CO G 1 43 c 43 G X 3 G C 4-J o o o o O vO •H 00 •H m •H rH O 4J CM 4H 4J m •U rH *H d G d d O 4-J 60 to 60 60 iw G 5C •H *H •H •H U cd Vi G M G Vl G Vi w 1 CM M V4 C *-H *H cr\ •H in •H B CO 1 CO 1 m 1 vO 1 G G rH G O a a G G G P •H 1 u W G CM G rH co j-j o 44 tO i — : 3 "3 a "u cn c •H AJ to •H >> JO a aj a a U-i 4-1 3 co 3 co CO 3 J-J to a 3 •H AJ CO 3 to < • AJ CO 4-t C3 O AJ 3 CO 'a o J-J iH 3 CC Jj a rH a co * 3 CO ro u o G *H AJ J-i 3 a ^ a AJ • j'-’ o o co s AJ G 3 rH G .O *-» 3 AJ H c rO 3 a i -a i 3 CM a> , 3 rH 3 3 3 3 rO 3 1 JO *-) H) as 3 *3 CO <- CO 1 co • vo co r— 1 o o o O 1 o 1 G >v 3 3 3 3 3 a *o | 3 E *3 CO vO r-J VO 1 VO m m m • co MT 1 >d- CM o o o O 1 O H- ten >•» 3 3 ■fc -O 3 JO 3 JO 1 * 3 S S r- m i co O vO co • m •rH CD CO r — I 3 6^ » — 1 40 G > 0 G xt T 1 4-J 4J m 3 G •u JO C AJ a J-4 4-J 0 3 44 44 4-J •H 3 •3 G U G rH 4-4 4-J 4-J 3 v 44 rH •H AJ C 3 to 3 •H a CO •H 4-J 4J *H o 3 c to •H 0 CO u 0 AJ O M 3 0 4-J G 4-J U 0 3 i — I M 0 G B 4J 3 AJ CO G rH 0 • -C 4-J G 4J CO i 0 3 >T 4-J CO P 1 • 4-J 0 4-1 •3 -C CO 0 O 4-J 0 £ -H 4-J O 4-* 1 rH 0 -O 4-1 rH >4 O 1 •3 O 44 ^ 0) -H & 4-J S 0 O ct) O 0 rH V4 ^ 0 rH 1 0 O txi C 3 44 •H p C 1 CO CCJ CO }4 g O 0 0 e 0 0 rH 3 3 0 rH ■H p B 0 O 1 & O M 0 3 O 0 0 rH •H -0 •H rH 4-J 4-J 3 0 CO 13 to to c •H 0 0 0 )H -H OJ J0 ^ W S 4-J M 3 +- un * H< 84 correspond well with the increased evaporative demands for these periods. The overall water use rate for the 1981 study period was 0.40 cm/day vmic’n is comparable to the irrigation amount of 0.38 cm/day. Once initial soil moisture levels were reduced, water use rates were identi- cal to the irrigation rates during the final three periods. Plant water use in excess of irrigation volumes must have come from water stored within the soil profile. Deep rooting allowed the plant to utilize this stored water. This would substantiate that deficit irrigation occurred. The lack of stress as noted by lack of severe wilt except in the 6— day treatment plots indicates that 0.38 cm of water per day was adequate to maintain St. Augustinegrass. Irrigation on a 6-day basis with 2.3 cm of water was not detrimental to overall turfgrass quality. Rooting Characteristics of St. Augustinegrass Root mass and length were not different among treatments at the five depths in the profile at initiation of the study. Differences were noted among depths in the profile with the greatest root mass and length from initial and final samples in the first 15 cm, followed by the depth to 30 cm and no differences in the remaining depths in the Pro^-^e (Taole 40). Irrigation scheduling had no affect on root distri- bution over the study period in that root length and mass were unaffected (Table 41). Root mass increased from initial to final samples for all but the 75 cm depth while root length increased only at the 45 cm and 60 cm depth. In contrast to the 1980 study, initially only 9% of the root length was found below 30 cm in the soil profile. At the final sam- pling date a small shift in mass (3%) and length (7%) below 30 cm in the soil profile was found. 85 Table 40. Rooting characteristics at 15 cm increments in the soil profile of St. Augustinegrass at initiation and end of study period. Values are reported as means over all treatments (1981 data). Profile Depth Root Mass Density Initial Final Root Length Density Initial Final cm -3 * mg rm — -3 15 1.34 * a 1.81 a 2.56 a cm cm — 2.20 a 30 0.28 b 0.53 b 0.64 b 0.77 b 45 0.07 c 0.17 c 0.18 c 0.30 c 60 0.06 c 0.12 c 0.16 c 0.26 c 75 0.03 c 0.05 c 0.10 c 0.14 c ’-u-'-uuuii, uuxowea Dy tne same letter are not significantly different at the 5% level using the Waller-Duncan K-ratio t-test. Means within mass or length connected by lines are not significantly different at the 5% level using the Waller-Duncan K-ratio t-test. 86 Table 41. Rooting characteristics as a total in the soil profile on a land area basis of St. Augustinegrass as affected by irrigation sched- uling at initiation and end of study period (1981 data) . Treatment Total Root Mass Length Initial Final Initial Final -days between -2 -2 irrigations- cm i cm * 2 27.3 38.2 54.6 50.7 3 23.7 41.4 54.6 56.9 4 31.2 43.5 56.7 60.0 6 24.7 38.2 52.6 54.0 All differences between treatments within the same sample period are non-significant at the 5% level using- the Waller-Duncan K-ratio t-test. Means within mass or length connected by lines are not significantly different at the 5% level using the Waller-Duncan K-ratio t-test. 87 Rooting characteristics probably enhance the induction of water stress since limited use can be made of water deeper in the soil pro- file. As with other studies, an increase in rooting did occur under these deficit irrigation schedules. Apparently the roots were acting as a sink for photosynthates produced by the turfgrass while under positive carbon balance during water stress. Lack of an increase in root length results in an inability of the plant to avoid drought. Because rooting did occur deep into the soil profile, there was an inherent ability of the turfgrass to maintain a certain level of photosynthesis and ET even when soil water content in the top 30 cm where the roots were most concentrated was at a minimum. CONCLUSIONS Responses of St. Augustinegrass to water stress resulting from xrrxgatxon scheduling indicated that while CER was reduced dra- matically, recovery overnight following irrigation was essentially complete. Turf grass response from initiation to end of the study period indicated a reduction in CER under each pre-irrigation mea- surement from initial values. This agrees well with research on other plants in that only when the stress was prolonged did re- covery after re-watering require several days (Sandhu and Horton, 1977). Jones (1973) working with cotton and Ceulemans et al. (1979) with Rhododendron simsii Planch, noted recovery 24 hours after re- watering. Reductions in CER under stress are linked directly to photosynthesis which is extremely sensitive to plant water deficits (Sandnu and Horton, 1977). Photosynthesis is more sensitive to water stress than translocation of photosynthates (Sung and Krieg, 1979). Errects on photosynthesis later into the study period may represent a general reduction in physiological response throughout the grow- xng period. Evapotranspiration rates were variable under stress from initial measurements probably in direct response to daily fluctuations xn environmental parameters and to undetectable changes in the turf- grass canopy. Changes in ET occurred from cycle to cycle and with similar values of AP ET varied by 87%. Evapotranspiration was more closely correlated to AP (r=0.69) than Pg (r=0.50). 88 89 Leaf water potentials under stress were essentially unchanged from initial values. Fock and Lawton (1979) observed linear de- creases in photosynthesis with decreasing leaf water potentials. Crafts (1968) assumed that decreased leaf water potentials control photosynthesis by stomatal closure, mesophyll permeability changes to carbon dioxide, and by enzymatic changes in the cytoplasm. He indicated that correlation of leaf water potentials with photosyn- tnetic activity is difficult. This study indicated that changes m CER did occur without a corresponding lowering of leaf water poten- tials. However, the most important changes in leaf water potential values were the increases from pre-irrigation to recovery. This implies that absolute value of leaf water potential is not necessarily a good indicator of plant water stress. Turgor pressure increased following eyery irrigation. Thus low turgor potential (not necessary zero turgor) affects the CER and ET more than leaf water potential. Crafts (1968) indicated that posi- tive turgor of 2 to 3 bars is necessary to prevent wilting. Levitt (1980) cites evidence that growth is inhibited even under posi- tive turgor of several bars. He noted that turgor is regained in most plants after irrigation. This was well documented for every cycle with the 6-day treatment. However, cycle 5 indicated a lack of turgor with all treatments and recovery following irrigation. This is extremely perplexing since it would indicate that the most fre- quenty irrigated treatment is undergoing loss of turgor every other day suggesting a decrease in general physiological status of the plant as the growing season progresses. This may mean that irrigation to match potential ET could have long-term effects not well documented 90 xn this study. With the plant in positive carbon balance partitioning of the photosvnthate may be very important. This could account for the increase observed in root mass with roots acting as a carbon sink. During each cycle, diffusive resistance was increased in the turf grass with the more severely stressed 6-day treatment. A decrease m diffusive resistance always accompanied recovery from stress for all cycles. Stomata responded to decreased turgor potential by closure. However, stomata are not closed even at zero turgor pressure in some plants (Hsiao and Acevedo, 1974). Similar results were noted m this study. At low or zero turgor pressure potentials diffusive resistance values indicated the stomata were still partly open and transpiration continued. Hsiao and Acevedo (1974) cited studies where photosynthesis was reduced by stomatal and non-stomatal effects. Sullivan and Eastin (1974) presented evidence supporting this in sorghum where stomata were closed when plants were exposed to water stress for the first time but failed to completely close stomata when subsequently droughted by soil drying and re-watering. Reduc- tions in CER in this study were accompanied by an increase in diffu- sive resistance. Diffusive resistance values during subsequent cycles always increased, but never to the level observed during cycle 1, even though CER was reduced during cycles 4 and 5 to levels near those observed for cycle 1. This implies that some form of adapta- tion occurred that is dependent on previous growth conditions. However, it was not related to osmotic adjustment since osmotic po- tentials did not indicate such an adaptation. Furthermore, this may indeed be a form of non-stomatal influence on photosynthesis 91 similar to that observed in sorghum. Regardless of the non-stomatal effects, the rapid recovery of St. Augustinegrass CER in cost instances to initial levels would lend support that stomatal closure and opening upon rehydration is the primary mechanism involved, other scientists have noted that stomatal changes occur primarily in association with low water potentials. Reicosky et al. (1975) reported complete clo- sure of stomata only when leaves were severely wilted in corn. Jordan and Ritchie (1971) measured leaf water potentials do™ to -27 bars in field grown cotton without stomatal closure. Ackerson et al. (1977) found that in cotton stomatal closure was not dependent on leaf water potentials. There was a significant correlation (r«0.79) found in this study between leaf turgor potential and stomatal conductance. There were also significant correlations (r-0.59) between turgor potential and diffusive resistance and between turgor potential and Pg (r-0.70). Two parameters that are used to aesthetically judge a turfgrass are tur. grass quality and cover. Both were maintained at accept- able levels and were essentially unaffected by stress, clipping weights indicated a reduction in leaf area (mass). This is a desirable effect since this would increase days between mowing. Johns et al. (1980) indicated this would also influence water use since they found an in- crease xn ET with an increase in leaf area. Turfgrass water use for 1980 and 1981 paralleled irrigation volumes. Acceptaole turfgrass quality under stress as judged by decreased CER was maintained in 1981 by a 0.38 cm/day irrigation rate. Even under the most severe stress conditions in 1981, the turfgrass remained in positive carbon balance with 0.38 cm/day of irrigation every six days. Continued growth through leaf area production was probably limited to 92 tines immediately following irrigation and when stress was relieved diurnally. Leaf expansion is much more sensitive to water deficits than photosynthesis and relief of stress can result in leaf ex- pansion rates similar to those in well-watered plants (Turner and Begg, 1981). Regaining turgor pressure following re-watering supports this concept. Turf grass water use as affected by irrigation fre- quency showed a general trend for rates to decline as days between irrigation increased. Water use data for the two years showed that the lowest rates were observed with the 4-day and 6-day treatments. Differences in water use rates among treatments from these estimates are small and while statistically significant may not be physiologically significant. Turf grass irrigation for St. Augustinegrass can be allowed on a 6-day frequency without significant loss of turfgrass quality. This affords an opportunity for -a natural rainfall event to provide water in lieu of irrigation. Transpiration rates tended to decrease as soil matric potentials decreased. Ackerson et al. (1977) noted corn ET dropped as soil matric potentials fell. Beadle et al. (1973) observed the stomata of corn close when soil matric potentials were below -0.3 bars. Therefore soil moisture tension has a pronounced influence upon water use. This was confirmed in this study since the 4-day and 6-day treatments had the lowest soil matric potentials and lowest water use rates. Turfgrass rooting characteristics indicate that at the proper height of cut, enough roots are maintained deep in the soil profile to provide some drought tolerance to the grass. While no affects of long-term stress were involved in this study, an increase in root mass was evident. Under more severe stress, this mechanism may result in 93 core rooting deeper in the profile. Although there was greater rooting length during the 1981 study root mass was not increased. This was not the consequence of irrigation treatment since initial and final root lengths were similar. Boote (1977) noted that drought intolerance ° i carpetgrass (Axonopus af finis Chase) is probably related to a very shallow root system making the grass sensitive to moisture stress. St. Augustinegrass exhibited a very similar root system in the 1981 study in that a great proportion of the roots were in the top 30 cm of the soil profile. St. Augustinegrass responded well to the longest ittigation scheduling and some rooting to greater depths in the pro- file may be a key factor. The general response of St. Augustinegrass during this study was one linked to stress induced by irrigation scheduling and the ability to recover upon re-watering. Irrigation rates equivalent to potential evapotranspiration based on climatological data were provided. Water use matched these irrigation rates ideally. Carbon balance changed from day to day under the most frequent irrigation regime. No infor- mation should be extrapolated from this study for extreme stress con- ditions of lower irrigation rates and longer periods between irrigations. APPENDIX Dates of Study Cycles 1980 Study Period 22 June - 2 July (PI) & 3 July (R) — Cycle 1 3 July - 14 July (PI) & 15 July (R) — Cycle 2 2 July - Pre-dawn leaf water potentials taken with mean for all plots -2.8 bars. 16 July - 6 August — Final Stress Cycle 1981 Study Period 18 May - 29 May (PI) & 30 May (R) — Cycle 1 30 May - 10 June (PI) & 11 June (R) — Cycle 2 11 June - 23 June (R) — Cycle 3 23 June - 4 July (PI) & 5 July (R) — Cycle 4 5 July - 16 July (PI) & 17 July (R) — Cycle 5 95 LITERATURE CITED Ackerson, R.C D.R. Krieg, C.L. Haring, and N. Chang. 1977. Effects of plant water status on stomatal activity, photosynthesis, and aCtivity of field grown cotton. Crop Science -L / l Ol— • Al-ani , T.A., and J.F. Bierhuizen. 1971. Stomatal resistance, trans- piration, and relative water content as influenced by soil mois- ture stress. Acta Bot. Neerl. 20:318-326. Allen, Jr. , L.H., J.S. Rogers, and E.H. Stewart. 1978. Evapotrans- piration as a benchmark for turfgrass irrigation, p. 85-97. Proc. of the Twenty-Sixth Annual Florida Turf-Grass Manage- ment . Conference Vol . XXVI, Orlando, Florida. 15-18 Oct. 1978. Florida Turf-Grass Association, Orlando, FL. shton, F. 1956. Effects of a series of cycles of alternating low and high soil water contents on the rate of apparent photo- synthesis in sugar cane. Plant Physiol. 31:266-286. Augustin, B.J. 1979. Research update: Turfgrass water use. p. 60-68. vwtt °£ the Florida Turf-Grass Management Conference Vol. XXVII, Tampa, Florida. 14-17 Oct. 1979. Florida Turf-Grass Association, Orlando, FL. Bartholic, J.F. 1977. Evapotranspiration. p. 116-120. Proc. of the lorida Turf-Grass Management Conference Vol XXV, Orlando londa. 16-19 Oct. 1977. Florida Turf-Grass Association,’ Orlando, FL. Beadle, C.L. ,K.R. Stevenson, H.H. Neumann, G.W. Thurtell and K.M. King. 1973. Diffusive resistance, transpiration, and photo- synthesis in single leaves of corn and sorghum in relation o leaf water potential. Can. J. Plant Science 53:537-544. Beard J.B. 1973. Turfgrass Science and Culture. Prentice-Hall, Englewood Cliffs, NJ. Inc. , Beard, J.B. 1981. Water use rates and turf Maintenance June: 10-18, 82. breeding programs. Grounds Bennett, O.L. and B.D root distribution 52:204-207. Doss. 1960. Effect of soil moisture level of cool-season forage species. Agron. J. on 96 97 Boote, K.J. 1977. Root: shoot relationships. Soil and Crop Science Society of Florida Proceedings 36:15-23. Boote, K.J., J.W. Jones, G.H. Smerage, C.S. Barfield and R.D. Berger. 1980. Photosynthesis of peanut canopies as affected by leaf spot and artificial defoliation. Agron. J. 72(2) :247-252. Boyer, J.S. 1970. Leaf enlargement and metabolic rates in corn, sorghum, soybean, and sunflower at various leaf water poten- tials. Plant Physiol. 46:233-235. Boyer, J.S. 1976. Photosynthesis at low water potentials. Phil. Trans. R. Soc. London B. 273:501-512. Ceulemans, R. , I. Impens, and R. Gabriels. 1979. Comparative study of leaf water potential, diffusion resistance, and transpi- ration of Azalea cultivars subjected to water stress. Hort- Science 14(4) :507-509. Crafts, A.S. 1968. Water deficits and physiological processes, p. 85-124. In T.T Kozlowski (ed.) Water Deficits and Plant Growth, Vol. II. Academic Press Inc., New York. Doss, B.D., D.A. Ashley and O.L. Bennett. 1960. Effect of soil moisture regime on root distribution of warm season forage species. Agron. J. 52:569-572. Doss, B.D., O.L. Bennett, D.A. Ashley, and H.A. W7eaver. 1962. Soil moisture regime effect on yield and evapotranspiration from warm season perennial forage species. Agron. J. 54:239-242. Doss, B.D., O.L. Bennett, and D.A. Ashley. 1964. Moisture use by forage species as related to pan evaporation and net radiation. Soil Science 98:322-327. Dyer, A.J. and M.J. Trlica. 1972. Carbon dioxide exchange of short grass sods. Colorado-Wyoming Acad. Sci. J. 7:28-29. Ekern, P.C. 1966. Evapotranspiration by bermudagrass sod, Cynodon dactylon L. Pers., in Hawaii. Agron. J. 58:387-390. Falk, J.H. 1976. Energetics of a lawn ecosystem. Ecology 57:141-150. Fock, H. and D.W. Lawlor. 1979. Photosynthesis, photorespiration, and carbon assimilation in water-stressed sunflower and maize leaves at two oxygen concentrations. Ber. Deutsch. Ges. Bd. 92:145-152. Garwood, E.A. and T.E. Williams. 1967. Soil water use and growth of a grass sward. J. Agric. Sci. 68:281-292. 98 Hansen, G.K. 1971. Photosynthesis, transpiration, and diffusion resistance in relation to water potential in leaves during water stress. Acta Agriculture Scaninavica 21:163-171. Harrison, D.S. 1973. Basic principles of irrigating Florida turf- grass. p. 33-38. Proc. of the Florida Turf-Grass Management Conference Vol. XXI, Ft. Lauderdale, Florida. 21-23 Oct. 1973. Florida Turf-Grass Association, Orlando, FL. Hsiao, T.C. 1973. Plant responses to water stress, p. 520-563. In W.R. Briggs (ed.) Annual Review of Plant Physiology 24:519- 570. Annual Reviews Inc., Palo Alto, CA. Hsiao, T.C. and E. Acevedo. 1974. Plant responses to water defi- cits, water-use efficiency, and drought resistance. Agricul- tural Meteorology 14:59-84. Hutcheson, J.L. and D.H. Knight. 1972. Seasonal and diurnal re- sponses of Bouteloua gracilis leaf water potential. Colorado- Wyoming Acad. Sci. J. 7:28-29. Jensen, M.E. 1968. Water consumption by agricultural plants. P* 1-19. In T.T. Kozlowski (ed.) Water Deficits and Plant Growth, Vol. II. Academic Press Inc., New York. Johns, D., J.B. Beard and C.H.M. van Bavel»~ 1980. Resistances to evapotranspiration from St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze] turf. Turf grass Research Report- Southern Turf grass Workgroup. Jones, H.G. 1973. Moderate-term water stresses and associated changes in some photosynthetic parameters in cotton. New Phytol . 72:1095-1105. Jones, M.B., E.L. Leafe, and W. Stiles. 1980a. Water stress in field-grown perennial ryegrass. I. Its effect on growth, canopy photosynthesis and transpiration. Ann. Appl. Biol. 96:87-101. Jones, M.B., E.L. Leafe, and W. Stiles. 1980b. Water stress in field-grown perennial ryegrass. II. Its effect on leaf water status, stomatal resistance and leaf morphology. Ann. Appl. Biol. 96:103-110. Jordan, W.R. and J.T. Ritchie. 1971. The influence of soil water stress on evaporation, root absorption and internal water status of cotton. Plant Physiol. 48:783-788. Kanemasu, E.T. and C.B. Tanner. 1969. Stomatal diffusion resistance of snap beans. Plant Physiol. 44:1547-1552. 99 Kneebone, W.R. and I.L. Pepper. 1978. Consumptive water use by grasses in Tucson 1977-78. p. 14-17. Proceedings 1978 Arizona Turf grass Conference, 10-11 May 1978, Tucson, Arizona. University of Arizona, Tucson, AZ. Kramer, P.J. 1969. Plant and Soil Water Relations. McGraw-Hill Book Co., New York. Levitt, J. 1980. Responses of Plants to Environmental Stresses, Vol. II. Academic Press Inc., New York. Makkink, G.F. and H.D.J. van Heemst. 1956. The actual evapotrans- piration as a function of the potential evapotranspiration and the soil moisture tension. Neth. J. Ag. Sci. 4(l):67-72. McCloud, D.E. 1954. Evapotranspiration as a guide to turf irri- gation. p. 25-29. Proc. of the Second Annual University of Florida Turf Conference, Gainesville, Florida. 3-5 Aug. 1954. Florida Turf Association and the Agricultural Division of the University of Florida, Gainesville, FL. McCloud, D.E. 1970. Water requirements for turf. p. 88-90. Proc. of the Florida Turf-Grass Management Conference Vol. XVIII, Ft. Lauderdale, Florida. 6-8 Oct. 1970. Florida Turf-Grass Association, Orlando, FL. Newman, E.I. 1966. A method of estimating the total length of root in a sample. J. Appl. Ecol. 3:139-145. O'Toole, J.C. and R.T. Cruz. 1980. Response of leaf water poten- tial, stomatal resistance, and leaf rolling to water stress. Plant Physiol. 65:428-432. Parsons, J.E., C.J. Phene, D.N. Baker, J.R. Lambert and J.M. Mc- Kinion. 1979. Soil water stress and photosynthesis in cotton. Physiol. Plant. 47:185-189. Reicosky, D.C., R.B. Campbell and W.C. Doty. 1975. Diurnal fluc- tuation of leaf-water potential of corn as influenced by soil ma- trix potential and microclimate. Agron. J. 67:380-385. Ritchie, J.T. 1974. Atmospheric and soil water influences on the plant water balance. Agricultural Meteorology 14:183-198. Sandhu, B.S. and M.L. Horton. 1977. Response of oats to water de- ficit. I. Physiological characteristics. Agron. J. 69:357-360. Scholander, P.F., H.T. Hammel, E.D. Bradstreet, and E.A. Hemmingsen. 1965. Sap pressure in vascular plants. Science 148:339-346. 100 Sheehy , J.E., R.M. Green, and M.J. Robson. 1975. The influence of water stress on the photosynthesis of a simulated sward of perennial ryegrass. Annals of Botany 39:387-401. Sneehy, J.E. and J.M. Peacock. 1975. Canopy photosynthesis and crop growth rate of eight temperate forage grasses. J. Exp. Botany 26:679-691. Snerfer, K.M. 1979. Response of three cool season turf grass species to heat and moisture stress. Ph.D. Thesis. University of Missouri-Columbia. Snimshi, D. 1963. Effect of soil moisture and phenylmercuric acetate upon stomatal aperture, transpiration, and photo- synthesis. Plant Physiol. 38:713-721. Singley, J.E. 1970. Water quality and quantity, p. 19-23. Proc. of the Florida Turf-Grass Management Conference Vol. XVIII, Ft. Lauderdale, Florida. 6-8 Oct. 1970. Florida Turf-Grass Association, Orlando, FL. Slaobers, P.J. 1980. Practical prediction of actual evapotrans- piration. Irrigation Science 1:185-196. Snerdon, E.T. 1974. An essential ingredient — water, p. 69-77. Proc. of the Florida Turf-Grass Management Conference Vol. XXII, Tampa, Florida. 16-19 Sep. 1974. Florida Turf-Grass Association, Orlando, FL. Stewart, E.H. , J.E. Browning and E.O. Burt. 1969. Evapotrans— piration as affected by plant density and water-table depth. Trans. ASAE 12:646-647. Stewart, E.H. and W.C. Mills. 1967. Effect of depth to water table and plant density on evapotranspiration rate in southern Florida. Trans. ASAE 10:746-747. Surlivan, C.Y. and J.D. Eastin. 1974. Plant physiological responses to water stress. Agricultural Meteorology 14:113-127. Sung, F.J.M. and D.R. Krieg. 1979. Relative sensitivity of photo- synthetic assimilation and translocation of 14-carbon to water stress. Plant Physiol. 64:852-856. Tanner, C.B. 1968. Evaporation of water from plants and soil. P* 74-104. In T.T. Kozlowski (ed.) Plant Water Deficits, Vol. II, Academic Press Inc., New York. Tovey , R. , J.S. Spencer and D.C. Muckel. 1969. Water requirements of lawngrass. Trans. ASAE 12:356-358. 101 Troughton, J.H. and R.O. Slatyer. 1969. Plant water status, leaf temperature, and the calculated mesophyll resistance to carbon dioxide of cotton leaves. Aust. J. Biol. Sci. 22:815-827. Turner, N.C. and J.E. Begg. 1981. Plant-water relations and adaptation to stress. Plant and Soil 58:97-131. van Bavel, C.H.M. 1961. Lysimetric measurements of evapotrans- piration rates in the eastern United States. Soil Science Society of America Proceedings 25:138-141. Waller, R.A. and D.B. Duncan. 1969. A Bayes rule for the sym- metric multiple comparison problem. Jour. Amer. Statis. Assoc. 64:1484-1503. Ward law, I.F. 1966. The effect of water stress on translocation in relation to photosynthesis and growth. I. Effect during grain development in wheat. Aust. J. Biol. Sci. 20:25-39. Warren, A. 1981. Talking about water. Golf Course Manage- ment 49(7) :14-17. Weaver, H.A. and J.C. Stephens. 1963. Relation of evaporation to potential evapotranspiration . Trans. ASAE 6:55-56. Wong, S.C., I . R. Cowan and G.D. Farquhar. 1979. Stomatal con- ductance correlates with photosynthetic capacity. Nature (London) 282:424-426. Youngner, V.B. 1979. Turfgrass water use — University of Cali- fornia studies, p. 8-17. Proceedings 1979 Arizona Turf- grass Conference, 9-10 May 1979. University of Arizona, Tucson, AZ. Zachariah, G.L. 1976. Water update, p. 51-57. Proc. of the Florida Turf-Grass Management Conference Vol. XXIV. Orlando, Florida. 10-13 Oct. 1976. Florida Turf-Grass Association, Orlando, FL. Zur, B. and J.W. Jones. 1981. A model for the water relations, photosynthesis, and expansive growth of crops. Water Re- sources Research 17 (2) : 311-320. BIOGRAPHICAL SKETCH Charles Harris Peacock was born in Salisbury, North Carolina on September 2, 1946. He graduated from A.R. Willingham High School, Macon, Georgia in June 1964. From September 1964 to December 1967 he attended the University of Georgia in Athens. In January 1968 he entered the US Army and served in the Republic of Vietnam from May to December 1970. After discharge from the army, he completed his BS degree in 1972 with a major in Biology at Columbus College, Columbus, Georgia. Following a year of graduate study at Auburn University and a year as a research associate at the University of Tennessee Center for the Health Sciences, Memphis, he completed his MS degree in Zoology at Clems on University, Clemson, South Carolina in 1976. After work- ing as an agricultural science associate in turfgrass research at Clemson University, he entered the University of Florida in January 1979 and began work toward a PhD in Turfgrass Science in the Horti- cultural Science curriculum. He and his wife Madeline, have one son, Daniel Adam, born February 20, 1981. They enjoy outdoor activities, especially gardening, camping, canoeing, and golf. 102 I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. Horticultural Science I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. Jerry m/ Bennett Assistant Professor of Agronomy I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. Keir-jT~Boot^7,_ Associate Professor of Agronomy I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. L Charles R. Johnson Associate Professor of Horticultural Science I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholary presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. AjUtwG. Allen G. Smaj stria I Assistant Professor of Agricultural Engineering This dissertation was submitted to the Graduate Faculty of the College O-1- Agriculture and to the Graduate Council, and was accepted as partial fulfillment of the requirements for the degree of Doctor of Philosophy. December 1981 ( J ot. Dean, Co^ege of Agriculgjfre Dean for Graduate Studies and Research