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AGRICULTURAL SCIENCES Vol. 3, No. 3,’pp. 37-54, plate 12 September 29, 1917

SOME ABNORMAL WATER RELATIONS IN CITRUS TREES OF THE ARID SOUTH- WEST AND THEIR POSSIBLE SIGNIFICANCE

BY ~ ROBERT W. HODGSON "

UNIVERSITY OF CALIFORNIA PRESS BERKELEY

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UNIVERSITY OF CALIFORNIA PUBLICATIONS

AGRICULTURAL SCIENCES

Vol. 3, No. 3, pp. 37-54, plate 12 September 29, 1917

SOME ABNORMAL WATER RELATIONS IN CITRUS TREES OF THE ARID SOUTH- WEST AND THEIR POSSIBLE SIGNIFICANCE

BY ROBERT W. HODGSON

INTRODUCTION

The progress of the development of the citrus industry, in general, and that of California in particular, has frequently been retarded or temporarily stopped by serious obstacles in the form of insect pests or plant diseases. Some of the most baffling of these troubles fall naturally into a group which for want of a better name has come to be known as that of ‘‘ physiological diseases,’’ which are thought to be caused by various obscure derangements of nutrition or other vital functions. This group includes mottled-leaf, die-back, chlorosis, June drop, puffing of the fruit, and others of less importance. Knowledge of the true nature of this class of diseases is extremely meager in spite of the fact that they have received much earnest attention from scien- tific investigators; and little can be accomplished in the way of devis- ing control measures until much more is known in regard to them. Nor can we hope to progress far beyond the realm of speculation with- out greatly augmenting our knowledge of the physiology and anatomy of the normal citrus tree when grown under any one of a series of very widely varying environmental complexes which obtain in differ- ent parts of the arid southwest.

It is, therefore, proposed to attempt by means of a series of sys- tematic experimental studies to obtain some definite and accurate information on the physiology of the genus Citrus. It is hoped that the results may serve as a basis for the elucidation of some, at least,

3 University of California Publications in Agricultural Sciences [Vol.3

of the important problems referred to above. The studies in ques- tion will attempt to shed light on transpiration problems, nutrition problems, and others equally important. The paper which is sub- mitted herewith forms an introductory contribution to the subject under investigation.

The writer is not unaware of the essential similarity between the physiological problems presented by citrus and other fruit trees. He has chosen, however, to study the physiology of the citrus tree as a separate entity because of the reasons given above, and the further one that the peculiar climatie conditions under which this tree is frequently placed in the arid southwest, demand a special treatment. Doubtless much may be gained from these studies which will apply to physiological problems connected with other trees,

The data here presented were obtained during an investigation of one of the so-called physiological diseases above mentioned, namely, the June drop.t Ever since the Washington Navel orange has been grown in the dry interior valleys of Arizona and California, this variety has been subject to excessive dropping of the young fruits. This has came to be known popularly as the June drop although the fall of the fruits is by no means confined to June but may oceur at any time from petal fall, in April, until the fruit reaches several inches in diameter in August. The prevalence and amount of this dropping seems to be influenced to a marked degree by certain environmental factors to which the trees are subject. The regular annual shedding of the young fruits is most serious in regions where the annual precipitation is lowest, the mean summer temperature highest, atmospheric humidity lowest, solar radiation most intense, and air movement greatest during the growing season. That the excessive drop of young fruit is in some way intimately connected with extreme climatic conditions is indicated by the fact that in some parts of southern California, where the drop is ordinarily not excess- ive, the hot wave of June 15-17, 1917, durimg which a temperature of 118° F was experienced in the Riverside and Redlands districts, was immediately followed by a drop so severe that practically the entire young crop of navel oranges was lost.

The experimental work from which the data were obtained was carried on at Edison, Kern County, California. Edison comprises "1 This investigation, which is now in progress, was carried on in collabora- tion with Professor J. Eliot Coit who planned the first series of experiments and began the work in February, 1916. A joint-authorship paper correlating

this and other aspects of the June drop phenomenon is in course of prepara- tion.

1917] Hodgson: Abnormal Water Relations in Citrus Trees 39

a small colony of about seven hundred aeres of orange orchard located eight miles southeast of Bakersfield and surrounded on two sides by typical desert of the southern San Joaquin valley, with its characteristic semixerophytiec flora. Extreme climatie conditions, as above mentioned, are operative there but the Washington Navel orange matures early and is of excellent quality, although crops are small because the drop referred to is excessive.

Water RELATIONS AND ABSCISSION

It has long been recognized that abnormalities or irregularities in the water relations of plants are often associated with the abscission of various plant parts. Balls? was able to cause complete shedding of leaves, flower buds, and bolls of the cotton plant Gossypium her- bacewm within four days by pruning the roots and so limiting the ability of the plant to take up water. Lloyd* in his investigation of the cause of abscission in the same plant came to the conclusion that the causative factor lay in a steady decrease in the moisture content of the soil in contact with the roots of the plant. This reduction causes a severe tax on the power of the plant to maintain normal water relations and results in fluctuations in the water content of the aerial parts which, in turn, leads to abscission.

Although the work of Lloyd was performed in the humid southern states, he makes the statement that ‘‘there seldom oceurs a day on which there is no minus water fluctuation in the plant.’’ He based this conclusion not only on data derived from shedding reeords but also on a study of transpiration rates, and water deficit in the leaves. In connection with his observations on the effect of temperature in causing acceleration of abscission, he came to the conclusion that ‘‘the water deficit is the cause of the rise of temperature in the tissues and that this constitutes the stimulus which directly leads to abscission.’’

Other evidence of the occurrence of marked deficits in the water content of plant organs is not lacking. Livingston and Brown,* work- ing with a number of plants growing near Tuscon, Arizona, found that (with the exception of the true xerophytes as Covillea and Prosopis) during the afternoon the leaves suffered a marked decrease in water content which was made up during the night. This periodic

2 Cairo Sci. Jour., vol. 5, p. 221, 1911.

3 Trans. Royal Soc. Can., ser. 3, vol. 10, p. 55, 1916, see also Bull. Torr. Bot.

Club, vol. 40, p. 1-26, Jan., 1913. 4 Bot. Gaz, vol. 53, p. 319, April, 1912.

40 University of California Publications in Agricultural Sciences [Vol.3

diurnal condition of dessication has been found by Livingston and Brown to serve as a check on the absolute transpiration and has been termed ‘‘incipient drying.’’ Lloyd’ independently obtained similar results in his investigations on Fouquieria splendens and Mrs. Shreve® established the same phenomonon in 1913 with Parkinsonia micro- phylla.

Inasmuch as the genus Citrus is undoubtedly a mesophyte of tropical origin and therefore grown in the interior valleys of Cali- fornia under purely artificial conditions,’ it would naturally be expected that the abnormal water relations above discussed might obtain to an unusual degree, especially during the hot growing period, when the ability of the plant to make up for excessive transpiration is taxed to the limit. Citrus fruits are borne on wood of the current season’s growth which ordinarily bears six to eight leaves on the same fruiting shoot. Therefore, it seemed reasonable that under conditions of excessive transpiration the leaves might draw on the water supply of the fruits and thus bring about an abnormal water relation. With the above considerations in mind it occurred to the writer that this premature fall of the fruits might be due to irregularities or abnormal- ities in the water relations between the fruits and foliage, resulting in abscission in some way analagous to the shedding of cotton bolls under the stimulus of a water deficit.

The method used in obtaining the data here presented consisted in the main of simple moisture determinations of leaves and fruits of various kinds taken at different hours of the day. The material was gathered and quickly placed in weighing cups fitted with ground glass covers. After weighing, the material was thoroughly dried and then reweighed. For convenience in the case of fruits and large leaves, the material was cut into small pieces. The calculations are based on the dry weight of the material, except as otherwise stated. The data obtained are shown in condensed form in table 1. The figures shown represent averages of at least ten duplicate determina- tions, and in most instances of more.

The data presented in table 1 show some very interesting condi- tions. It is quite clear that, with the exception of the new succulent growth, the young fruits are at all times higher in water content than the leaves situated near them. These data also seem to leave no doubt

5 Plant World, vol. 15, p. 11, 1912. 6 Ann. Rpt. Dir. Bot. Res. C. I. W., Feb. 12, 1913, p. 81.

7 For a more complete discussion see Livingston, B. E., ‘‘A single index to represent both moisture and temperature conditions as related to plants.’’ Physiological Researches, vol. I, No. 9, April, 1916.

1917] Hodgson: Abnormal Water Relations in Citrus Trees 41

TABLE 1

AVERAGE MOISTURE CONTENT Average water content

in per cent Kind of material (Dry weight) New leaves about two weeks old 242.0 Full grown leaves of current season’s growth 162.2

Leaves of one season’s growth—about one year old ............ 132.7

Leaves of two season’s growth—about two years old _ 126.1 Leaves of three or more season ’s growth. Over two years

OL Cae eens aati ase nec ecs ost cecrpeencesaracsenensaee=s 117.6 Leaves of current season’s growth. Gathered between

OURAC Maw 20 pile pea Mien eee meee me ere ons c seco Luster scencasaeucececseresaseas 164.9 Same gathered between 1 P.M. and 4 P.M 157.2 Leaves of current season’s growth gathered from eelinindl

LUULES MD Ete CW OM Aci ANG G2) Mie) cee nccereceneececeemnstennsnacncnasnasureac= 166.8 Same gathered between 1 P.M. and 4 P.M. —.........-.-.------eeeee- 160.4 Fruits destined to subsequent abscission, one-third to three-

PF OMI HAS me @ aT OMAN GOT ceressaseccactencsaxwecccc ne secenncecacnaxewse-rerenees 191.5 Fruits apparently normal gathered between 9 A.M. and 12 M.8 260.2 Same gathered between 1 P.M. and 4 P.M. .....--.----.---------c-eeceo-- 247.7 Fruits destined to subsequent abscission gathered between

OB) DRAIN, mL IN epee ener aoe see Sor See EEE SE ES SE EEE 201.4 Same gathered between 1 P.M. and 4 P.M. ~.......--...----.--cseeeeeee+ 179.2

of the fact that as the leaves grow older there is a progressive decrease in water content.

It is also quite evident that a regular diurnal decrease in the water content of leaves of the current season’s growth is manifest during the afternoon. Such leaves averaged 164.9% in water content for the period between 9 a.m. and 12 m. and only 157.2% for the period between 1 p.m. and 4 p.m. This difference does not appear significant when viewed in the light of the large differences obtained by Living- ston and Brown with some of their material. However, it should be borne in mind that those authors were dealing, for the most part, with much more succulent plants containing a large amount of water storage tissue. Further, it should be noted that these figures are averages, since the determinations on which they are based were not made at the same hours. Individual pairs of determinations fre- quently showed differences of as much as 25% to 30% in as short a period as six hours. On June 5 at 2:30 p.m., with the temperature at 95° F and the relative humidity at 19%, the water content of leaves of the current season’s growth was 144.3%. At 4 a.m. the next morn- 8 The fruits used for these determinations averaged a little larger than

those gathered in the forenoon and therefore would normally be somewhat higher in water content.

42 University of California Publications in Agricultural Sciences [Vol. 3

ing, with the temperature at 62° F. and the humidity 54%, the water content of similar leaves was found to be 172.6%, showing a differ- ence of 28.3%. This phenomenon is taken to indicate the presence of incipient drying in citrus and is in full accord with the results of the writers above mentioned as well as with those obtained by Lloyd.

Since the young fruits have a higher water content than adjoin- ing leaves which, in turn, exhibit a diurnal decrease in relative water content, the conclusion, a priori, that the leaves might possibly draw on the water supply of the fruit during periods of excessive transpira- tion seemed entirely plausible. If such is the case it would seem that leaves so favorably situated should not show this daily variation, at least to the degree shown in the leaves not so favorably situated. The data in table 1 show, however, that the average difference in water content of the two sorts of leaves gathered in the forenoon and after- noon is quite small. This is taken to indicate that if such leaves do utilize the water supply of the fruits, the evaporating power of the atmosphere is so strong that as fast as they receive this surplus water, it is lost and thus causes no appreciable difference in their relative water content.

The next step was to ascertain the water content of different kinds of fruits, those destined to remain and mature, and those showing indications of subsequent abscission. It is quite easy to distinguish between the two, from a week to ten days before abscission occurs, by the difference in their appearance. Exposed fruits destined to drop exhibit a small yellow spot about the navel end several weeks before the actual drop occurs. This spot gradually extends and spreads until at abscission it usually occupies at least half the area of the fruit. In the ease of well-shaded fruits, the yellow color is evenly distributed over the entire surface. A large number of mois- ture determinations were made which showed that those fruits destined to subsequent abscission averaged 59% less water than those fruits destined to remain and mature. (See table 1.) The presence of this condition in the fruits, especially when considered in connection with the daily inerease at certain hours in the water deficit of the leaves immediately behind them, seems to point to the possibility of the leaves depriving the fruit of a part of their normal water supply. It cer- tainly indicates an abnormal water relation.

Lemon growers prune their trees at all seasons of the year, even while the fruit is still on the trees. It is a well established practice to gather the good fruit from the excised branches immediately, in order to prevent it from becoming flaccid. Inasmuch as the fruit, as ordi-

1917] Hodgson: Abnormal Water Relations in Citrus Trees 43

narily picked from the tree, remains turgid for several months, it is the common belief that the leaves draw the water out of the fruit when the branch is severed from the tree. That this is exactly what does occur, when the leaves are deprived of their normal water supply, is shown by the following experiments:

Experiment 1— Two shoots bearing small terminal oranges of approximately the same size and having the same number of leaves and approximately the same leaf area, were taken to the laboratory, placed on the table and allowed to dry under similar conditions except that in one case the fruit was severed from the stem. All cut surfaces were sealed with vaseline.

Within twelve hours a marked difference in appearance was observed. The leaves on the shoot from which the orange was detached were considerably shriveled while those on the other shoot remained turgid and fresh. This difference became more pronounced as time elapsed and in thirty hours a distinct difference in the appearance of the fruits as well as leaves was visible. The detached fruit remained firm and retained its dark green color and lustre while the attached fruit was soft and flaccid and exhibited a dull green color without lustre. This experiment was performed repeatedly with both oranges and lemons with the same results. (See plate 12, fig. 1.)

As all the eut surfaces were sealed, it seems clear that the leaves on the shoot with fruit attached actually drew on its water content and that it was this supply of water which enabled them to remain alive and fresh long after the leaves on the other shoot had withered and died.

Heperiment 2—Quantitative data on water content were desired to substantiate the visible indications described in Experiment 1. Therefore the latter was repeated several times and moisture determin- ations on leaves and fruits were made at various periods. <A repre- sentative set of such determinations is given in table 2:

TABLE 2

MOoIstuRE CONTENT DETERMINATIONS, TWENTY-FOUR Hours AFTER BEGINNING OF EXPERIMENT 2

Weight of Weight of Weight of Water con- container and same when material in tent per Kind of material fresh material dry grams cent in grams Orange detached from branch...... 23.40 21.670 2.665 185.0 Orange attached to branch... 23.585 22.367 2.075 142.1

Leaves from branch {21.831 21.805 aul eee with fruit removed ] 22.045 22.010 170 21.4 avg. Leaves from branch {21.845 21.275 SLO peers

with fruit attached ) 20.604 20.477 .284 73.7 avg.

44 University of California Publications in Agricultural Sciences [.Vol.3

These data show that after twenty-four hours the leaves on the shoot with orange attached contained an average of 52.3% more water than those on the other shoot. They further show that the detached fruit contained 42.9% more water than the attached fruit from which the leaves had been drawing their supply. This is considered to be conclusive evidence that in the case of excised branches the leaves can draw water from the fruit.

Experiment 3—Two shoots in every respect similar to those used in the previous experiments were treated in the same manner as those of Experiment 1 and 2. These were then weighed at irregular inter- vals until they had reached a constant weight. During the interim they were kept on the laboratory table. The data obtained are found summarized in table 3:

TABLE 3

WATER CONTENT DETERMINATIONS MaAprE AT IRREGULAR INTERVALS BASED ON THE WHOLE WEIGHT

Shoot with orange attached Shoot with orange detached

Number Weight Loss in Lossin Difference Gyeigit ~ Geasia’| Ghossan Dinerenee

of hours in grams grams per cent in in grams in grams__ per cent in

elapsed per cent per cent

0 AS TI2i ii) accra recs sees ATT. ey ae ee 3 4.436 438 8.9 6 4.380 397 8.3 aes

19 3.957 -915 18.7 ae 3.830 947 19.8 iat 21 3.895 977 20.0 ie 3.742 1.035 21.7 ila? 24 8.803 1.069 21.9 s 3.607 1.170 24.4 2.5 26 3.683 1.189 24.4 = 3.442 1.835 27.9 3.5 27 3.610 1.262 25.9 aa 3.342 1.435 30.0 4.1 44 3.263 1.609 33.0 meee 2.911 1.866 39.1 6.0 49 3.047 1.825 37.4 Ses 2.682 2.095 34.8 6.4 51 2.920 1.952 40.0 Aon 2.575 2.202 36.0 6.0 91 2.125 2.747 56.3 eee 2.008 2.769 57.9 1.6 96 2.053 2.819 57.8 = 1.960 2.817 58.9 ail 99 2.000 2.872 58.9 ae 1.9385 2.842 59.5 6

116 1.921 2.951 60.5 vce 1.873 2.904 60.8 3)

119 1.894 2.978 Gel a 1.852 2.925 61.2 alll

121 1.881 2.991 61.3 ease 1.844 2.933 61.4 1

140 1.825 3.041 62.5 4 1.807 2.970 62.1

146 OM 3.075 63.1 o 1.783 2.994 62.6

162 1.774 3.098 63.5 6 1.770 3.007 62.9

186 1.736 3.136 64.3 8 1.743 3.084 63.5

195 1.717 3.155 64.7 9 1.726 3.051 63.8

211 1.705 3.167 65.0 1.1 1.720 3.057 63.9

218 1.695 3.177 65.2 1.0 1.710 3.067 64.2

260 1.666 3.206 65.8 Alea 1.686 3.091 64.7

285 1.652 3.220 66.0 a ll 1.675 3.102 64.9

306 1.642 3.230 66.2 atl 1.664 3.113 65.1

330 1.631 3.241 66.5 alsa 1.652 3.125 65.4

525 1.613 3.259 66.8 1.0 1.631 3.146 65.8

1917] Hodgson: Abnormal Water Relations in Citrus Trees 45

The data in this table indicate that the amount of water in the fruit available for use by the leaves was sufficient to maintain the latter alive for approximately 50 hours after the shoot was cut from the tree. It is further evident that when three hours had passed the leaves on the shoot with fruit attached had not yet begun to take water from the fruit to any appreciable extent because the shoot with fruit detached shows less water loss than the shoot with fruit attached. However, this condition was soon reversed and the leaves began to

20 40 60 80 100 120 140 160 180 200 220

Fig. 1. Showing the difference in per cent of water loss of shoot with orange attached and shoot with orange detached. The water loss curve of the shoot with fruit detached is considered as normal. .Ordinates represent differ- ences in per cent of water loss, abscissae, the time elapsed in hours. Water content calculated on basis of fresh weight.

draw on the water in the fruit while the leaves to which no water was available from the fruit showed indications of wilting.

That shortly after 50 hours had passed death oecurred in the leaves of the shoot with fruit attached is shown by the rapid increase in the amount of water loss. This was undoubtedly due to increased permeability of the cytoplasmic cell membranes after death. After 50 hours the difference in water content of the two was 18.3% in favor of the shoot with fruit attached. However, from this time on until both had reached a constant rate of water loss (after about 200

46 University of California Publications in Agricultural Sciences [Vol.3 hours), this shoot lost water more rapidly than the shoot with fruit detached. These relations are very clearly shown in figure 1. The normal water loss curve is illustrated in figure 2.

Experiment 4—A forked twig bearing a small terminal fruit on each branch was selected and cut. The fruits were immediately im- mersed in water and the shoot tied to a support in such a fashion that all the leaves were exposed to the air, the fruits alone being immersed. One orange was now removed by cutting it under water and all cut surfaces were sealed. The two fruits remained under

64 56 48

40

16

50 100 150 200 250 300 350 400 450 500

Fig. 2. Showing the general type of water loss curve of a shoot detached from the tree, including detached orange. Ordinates represent water loss in per cent and abscissae, the time elapsed in hours.

water. The container and support were then placed on a bench in the shade in the open air and left for fifteen hours, at the end of which time moisture determinations were made on the fruits.

TABLE 4 MoIstuRE DETERMINATIONS AFTER FIFTEEN Hours Weight of Weight of Weight of Water con- container and same when material in tent per Kind of material fresh material dry in grams cent

in grams grams Detached orange ..... seccseeeeeces- 24,680 21.435 3.330 206.9 Attached orange: 2-222 -2n. 23.210 21.773 2.570 126.8

1917] Hodgson: Abnormal Water Relations in Citrus Trees 47

The data in table 4 show that at the end of fifteen hours there was a difference in water content between the two fruits of 80.1%. There seems to be no way of accounting for this large difference other than that the leaves had actually drawn the major part of it at least, from the attached fruit.

Water TRANSPORT Stupres BY Means or Dyk SturF SoLutions

Experiment 5—Bearing the foregoing findings in mind, it seemed desirable to determine something of the nature of this reversal of normal water flow by means of dye solutions. Accordingly a shoot bearing a terminal fruit was cut from the tree and the orange pared away at the apical end to open the tracheal elements and admit the dye.’ This paring was done under the solution to prevent the entrance of air bubbles. Water soluble eosin was used. The orange was immersed in the liquid for a half hour, after which the shoot was split open. The tracheal tubes throughout all parts of the leaves, stems and fruits were found to be strongly stained.

Experiment 6—It seemed desirable to simulate the actual situation on the tree as nearly as possible and the following experiment was designed to accomplish this. A crooked fruiting branch bearing a number of small lateral shoots and leaves, and one terminal orange was cut under water. The eut end was kept under water and the branch so supported that the fruit was immersed in an eosin solution. The apex of the orange was then pared as described above. The branch then rested with its basal end in water and the vascular bundles of the fruit open to eosin at the other end of the branch. (See pl. 12, fig. 2.) If we substitute for the water container the con- ducting system of the tree, and for the watery solution of eosin the developing fruit high in water content, we have very similar conditions to those existing in the experiment, save for the fact that the fruit is not open to the air and the conducting system bears a certain relation to the rest of the tree.

The experiment was begun late in the afternoon and the branch left outdoors over night. At 8 o’clock the next morning the leaves were examined and found to be very fresh and turgid. Indeed they were noticeably much fresher in appearance than they had been the evening before. On careful examination absolutely no trace of eosin

9Tt should be stated here that the Washington Navel orange is in reality a double fruit, with a small secondary orange within a large primary fruit. This interior fruit constitutes what is known as the navel and it possesses an independent vascular system of its own which traverses the central pith of

the primary fruit before ramifying through the secondary orange. This central pith thus acts as the stem to the small fruit.

50 University of California Publications in Agricultural Sciences [Vol.3

ing between 50-100 per square millimeter as compared to 300-450 per square millimeter on the leaves. Measurements of the leaves situated within six inches of the fruit showed that, in addition the leaf area immediately behind the young growing fruit is larger than the area of the fruit until it reaches approximately two inches in diameter, after which falling of the fruit is comparatively rare. Therefore, it seems highly probable that the transpiration of the fruit as compared to that of the leaves situated immediately behind it is an almost negligible factor and it appears reasonably certain that either water is actually drawn back or that the normal supply is decreased.

Considering these two possibilities, the first merits more considera- tion as it is supported by proof which, though not absolute, is at least presumptive evidence of a strong enough character ; while on the other hand the second possibility, agreeing though it does with the most recent theory on sap movements in plants as put forth by Dixon, is still a theoretical consideration. According to this theory, which pos- tulates strong tensions existing in the ascending water columns, no assumption of an actual reversal of the current is necessary in order to explain a decrease in moisture content. During normal conditions the relation between the tensions existing in the water columns leading to the fruits and those leading to the leaves is such that both organs receive an adequate water supply. The tension existing in any one of these water columns is a function of the transpiring force existing in the transpiring plant organ as modified by atmospheric conditions. Therefore, as these transpiring forees vary, the tensions vary. Trans- piration from the leaves, for reasons pointed out above, is subject to much greater variation than that from the fruits. Therefore during periods when evaporation is greatly accelerated the tensions in those water columns leading to the leaves are greatly increased and as a consequence more water is drawn to them. As the source of supply in the conducting system is practically constant, the amount in the fruits is thereby reduced and this results in a decrease of relative water content of a magnitude conditioned by the transpiration of the fruit.

However, it should be noted that the data in table 1 show a decrease in absolute water content of the fruit of 15% to 20%, a loss of considerable magnitude. There are only two ways in which such a decrease in absolute water content can take place: (1) the water is lost by transpiration from the fruit, or (2) it is drawn back by the leaves. But since the fruit possesses a very small stomatal area

1917] Hodgson: Abnormal Water Relations in Citrus Trees 51

as compared with the leaves and, moreover, it is highly probable that a large percentage of this area is non-functional, being obstructed by accumulations of a resinous nature, there is small likelihood for absolute loss of water in this manner to the extent noted. Hence there seems but one way to explain it and that is by movement back from the fruits.

Evidence of an indirect nature pointing to the same conclusion lies in the fact that there are some indications that abscission of a certain proportion of the young fruits is directly due to the influence of hydrolysing enzymes secreted by certain saprophytie or facultative parasitic fungi always found present on the shriveled style and fre- quently in the proliferations of the navel. Such enzymes in order to act on the abscission layer must be drawn back through the vascular systems of the fruit imto the pedicel where this layer is located. Investigations on this point are now in progress.

Experiment 7—Three similar fruits were selected on different parts of a tree; on one of the lower branches in the shade, at a height of four feet, and in the top of the tree in full sunlight. At noon each fruit was pared so as to admit entrance of a solution and then plunged quickly into a small vial containing a watery solution of eosin. These vials were securely tied to the shoot and left suspended for two hours. At the end of that time, on cutting leaves from these shoots, eosin staining was found in the vascular systems of all. On examining back- ward toward the tree, eosin was found as far back as thirty centi- meters. This experiment was repeated a number of times both at Edison and at Riverside and uniformly gave the same results, although much less marked at the latter place. In every ease the backward movement of the eosin solution was at its maximum during the afternoon.

Cutting the ends of branches in situ under a watery solution of eosin was tried at different times of day and gave similar results. This experiment was performed at Edison, Riverside and Indio. At the latter place, with the temperature at 116° F and the humidity at 8% the eosin solution traveled backward at the astonishing rate of 30cm. per minute at 6 p.m. Similar results were obtained using Eucalyptus rudis as material. In fact with long slender poles of Eucalyptus tereticornis at Edison, such a remarkably rapid backward flow of eosin was observed (105 em. in one minute) in the afternoon as to compel the conclusion that after all, the force responsible for this movement under such conditions must be negative pressure pro-

PLATE 12 Fig. 1. Showing extent to which the leaves can draw on the water in the fruit. Both shoots were cut at the same time and had approximately the same leaf area. All cuts were sealed with vaseline. The fresh-appearing leaves on the shoot at the left have maintained themselves at the expense of water in the fruit. Note the difference in reflection of light from the two fruits. See Experiment 1.

Fig. 2. Photograph illustrating an orange shoot so arranged as to be able to draw water from one end and eosin solution through the pared apex of a small fruit at the other. In spite of this double supply a large water deficit occurred, and eosin was drawn back from the container on the right to the leaf next to the water container on the left. See Experiment 6.

[54]

UNIV. CALIF. PUBL. AGR. SCI. VOL. 3 [HODGSON] PLATE 12

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