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Bulletin 781 November, 1979
Split-night Temperatures in a Greenhouse:
The Effects on the Physiology and Growth of Plants
By Martin P. N. Gent, John H. Thorne, and Donald E. Aylor
The Connecticut Agricultural Experiment Station
Ne^Hayen
SUMMARY
The split-night regime refers to lowering the minimum
temperature of a greenhouse from 60 F (15.5 C) to 45 F
(7.2 C) for 8 hours after 10 pm. We calculate that this
scheme saves about 20% on fuel in the winter in Connecti-
cut. The temperature reduction does not appreciably slow
either the growth rate or the development of tomatoes or
Easter lilies. Our physiological studies of tomato plants
suggest two reasons why plant growth is so little affected
by this energy saving technique. First, plants subjected to
a repeated nightly drop in temperature do not show lin-
gering inhibition of photosynthesis, translocation, or car-
bohydrate metabolism the following morning. Second, to
compensate for the inhibition of physiological processes
during the cool part of the night, plants subjected to
split-night temperatures move sugars more quickly out of
the leaves and stems during the day by degrading their
starch reserves faster. This second phenomenon becomes
especially evident during fruit production when more
efficient translocation from the leaves is necessary for
rapid fruit growth. These physiological studies suggest
that economic production of many crops will benefit from
the split-night regime.
Split-night Temperatures in a Greenhouse:
The Effects on the Physiology and Growth of Plants
By Martin P. N. Gent, John H. Thorne, and Donald E. Aylor
During the winter in Connecticut, greenhouses are usu-
ally heated to 60 F or 65 F to insure proper plant develop-
ment and timeliness of flowers or fruit. In 1978, about
25% of the cost of producing greenhouse crops was for
heating fuel.
Although turning down the thermostats in greenhouses
will save fuel, the plants may grow poorly when tempera-
tures are kept low for an entire night. We reasoned how-
ever, that the limited amount of sugar produced in the
dim sunlight of winter might not require an entire night to
be translocated and metabolized. Therefore, we split the
night into two parts for purposes of temperature control;
starting at 10 pm (EST) we reduced the thermostat from
60 F (15.5 C) to 45 F (7.2 C) for 8 hours each night. We
call this the split-night temperature regime and calculate
that it would save about 20% of the fuel normally used to
maintain a greenhouse in New Haven, CT at 60 F for the
entire night (see Appendix 1).
In this report we compare the growth and physiology of
tomato, lily, and tobacco grown in split-night tempera-
tures, with plants grown at 60 F for the entire night. An
understanding of the physiological response of plants to
split-night temperatures should allow growers to choose a
management scheme that will save fuel without sacrific-
ing growth.
METHODS
Experimental design and temperature control
The experimental greenhouse, located in New Haven,
CT, has single-pane clear glass and an east-west ridge line.
The house is separated into an east and west half by a glass
partition covered with translucent polyethylene for insu-
lation. Each side was heated by a double row of steam
radiators on the side walls which were controlled by a
centrally-located thermostat. The top vents opened auto-
matically when temperatures exceeded 80 F(27 C). In the
east half, the temperature was lowered to 45 F (7.2 C) for
part of each night, i.e., the night was split into two parts.
The temperature in the west half was maintained at 60 F
(15.5 C) throughout the night. Air temperature was mea-
sured by shaded hygrothermographs located in each half
near the thermostats. Outdoor temperatures were
recorded by a thermograph in a weather shelter located 40
feet north of the greenhouse. Although both halves of the
greenhouse received almost equal amounts of sunlight,
the split-night side tended to get more in the morning and
less in the afternoon.
Fifteen Easter lily, 75 tomato, and 5 tobacco plants in
individual pots were arrayed in blocks five rows deep on
benches above the height of the radiators and near the
south wall of the greenhouse. The lilies were closest and
the tobacco farthest from the partition. Each week, the
plants were randomly rearranged. Nellie White lilies were
supplied (Long's Greenhouse, East Haven, CT) in 3 /pots
in soil-peat-perlite mix (1:1:1), pH 5.9. Tomato seeds
(Patio Hybrid, Comstock Co., Wethersfield, CT and
Fireball 861, Harris Seed Co., Rochester, NY) were ger-
minated on December 18,1 978 and grown at 80 F day and
65 F night in Promix until they were transplanted on
January 7 into 3 / pots containing equal parts soil, sand,
peat and vermiculite, at a pH of 6.5. Tobacco seedlings
(var. Havana seed), supplied in the 4 or 5 leaf stage by
Dr. I. Zelitch were grown in a similar soil mix.
The tomato and tobacco plants were watered daily at
7 am with 50 to 250 ml of water at 70 F (21 C) to raise the
temperature of the soil. The lilies were watered once a
week. The volume of water was adjusted to the size of the
plants to prevent waterlogging and root rot. Fertilizer was
applied once a week starting on February 1 as 100 ml per
pot of "Miracle-Gro" (15-15-15) at 2.64g-l_1. Starting
April 1 , fertilizer was increased by 50%. Soil temperatures
were measured in three pots on each side of the
greenhouse.
On January 8, 1979 the plants were divided into two
groups. Half were put in one side and half in the other side
Connecticut Agricultural Experiment Station Bulletin 781
of the greenhouse. Growth dates are calculated from
January 8, 1979 as day 1. Tomatoes were 21-days-old at
the start of the experiment.
Growth
The individual growth and development of 20 indicator
tomato plants (ten each, selected at random from the
control and split-night environments) was monitored
throughout the experiment. The growth of each tobacco
and lily plant was measured. Plant height and number of
leaves was recorded during the vegetative growth. The
length and width of tobacco leaves were measured during
the period of fastest growth.
To convert the height of the 20 indicator tomato plants
to dry weights, we used the heights and dry weights of
other plants that were harvested and dissected for sugar
determination or radioactive translocation analysis.
Since the height of the main stem indicates the weight
of tomato plants (Went, 1944), we obtained a second-
order regression of plant height versus dry weight.
Dry weight = -1.2114 + 0.2511 -height
+ 0.002618 -height2
(1)
with a correlation coefficient of 0.949 and a 3.0 g standard
error about the mean. This relationship fits the data well
throughout the growth of the plants (see Fig. 1).
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Fig. 1 The relationship between the dry weight and height of
control (*) and split-night (o) tomato plants. The regression
line calculated from equation 1 Is shown by the solid line.
The number of flower buds and flowers on each Easter
lily and tomato were recorded twice a week.
The same 20 tomato plants were observed during fruit
growth to maturity. When a fruit grew larger than 1 cm in
diameter it was identified with a tag, and its height and
diameter were recorded twice a week. A fruit's volume
was calculated using the geometric relationship for the
volume of an oblate spheroid. The volumes were summed
to give a growth curve for the total fruit of each plant
which was fitted to a logarithmic growth function.
As fruits turned red, they were picked, weighed while
fresh, then dried at 60 C for 2-3 days, and then reweighed.
Fruit that were still green when the experiment was termi-
nated on May 21 were weighed and included in the final
harvest.
The rates of vegetative and fruit growth were analyzed
separately for each of the 20 tomato plants measured
throughout the experiment. For each plant, the growth
curves were fitted to a logarithmic growth function:
G(0:
Final Weight
1 + exp [-(/_
Mid-growth Date)]
Duration
(2)
where G(t) is the weight in grams at time t; Final Weight is
the predicted harvest weight; Mid-growth Date is the time
in days when G(t) = half of the Final Weight; and Dura-
tion is the time required to reach the Final Weight if the
growth were linear and equal to the fastest growth rate
(given by Final Weight/ Duration). This procedure gives
four parameters (Final Weight, Mid-growth Date, Maxi-
mum Rate, and Duration) describing the growth of each
tomato plant. The parameters were varied independently
to minimize the mean squared difference between the
ideal and actual growth curves. These growth parameters
were subjected to an analysis of variance to find signifi-
cant differences between the control and split-night
populations.
Photosynthesis and transpiration
Usually starting at noon, a continuous, diurnal record
of water vapor and carbon dioxide (CO2) fluxes was
obtained for individual plants isolated in a chamber adja-
cent to the bench. Different plants were measured on
different days and plants from the split-night and control
sides were sampled on alternate days. Gas exchange with
the soil was prevented by a polyethylene bag surrounding
the pot, which was tied about the stem. The chamber was
a wooden frame 1 meter -0.6 meter -0.3 meter covered
with Propafilm-C 1 10 plastic (ICI, Wilmington, Del.); the
door and the base were sealed with foam rubber. A fan
and a baffle kept the air in the chamber well stirred.
Air was supplied to the chamber at 10 to 15/- min-1 and
sampled at 5 / • min-1 via 6 mm diameter tygon tubing. The
air was drawn from outdoors to keep the CO2 concentra-
tion relatively constant and the initial humidity low. After
passing through the chamber, the water vapor in the air
was measured with a dew point hygrometer. The air was
then dried, and the CO2 concentration was measured with
Split-night Temperatures in a Greenhouse
a differential infrared gas analyzer. These measurements,
as well as temperature and photosynthetically active radi-
ation, were recorded by a Fluke Datalogger every 15 min-
utes.
The net photosynthesis of tobacco was also measured
on isolated leaf discs. At 9 am on one day, four leaf discs,
6 mm in diameter, were sampled from tobacco plants
from both environments. They were allowed to photosyn-
thetically assimilate 14C02 at 600 ppm, 30 C, and
450 /^Einsteins • m~2 • sec-1 radiation for 5 minutes (Oliver
and Zelitch, 1977), and were then digested in hydrox-
lyamine before the radioactivity was assayed by liquid
scintillation.
Stomatal resistance of the lower leaf surface of the
tobacco was measured directly on several days using an
aspirated diffusion porometer (Turner et al., 1969; Turner
and Parlange, 1970). The same three leaves on each of five
plants were measured three or four times during the day.
The time to measure all leaves in both halves of the
greenhouse was about 20 minutes.
During early flowering (day 52) of the tomatoes and
again during fruit development (day 86) diurnal trends of
14C-labelled photosynthate distribution and carbohy-
drate levels were determined to evaluate the effects of
night-time temperatures on the production, distribution,
and metabolism of carbohydrates. Both day 52 and 86
were clear and sunny but were preceded by at least two
cloudy, overcast days.
Translocation
Six plants randomly selected from the control and six
from the split-night populations were allowed to assimi-
late radioactive CO2. Plants were labelled at 1 1 am and at
5 pm inside a Propafilm chamber. During labelling, l4CC)2
(0.06 millicurie • f1, 290 ppm CO2 in nitrogen) was sup-
plied at a rate of 0. 1 1 1 • min-1 for 5 minutes. The gas was
stirred by a fan. The light intensity was 500 ^Einstein • m"2
• sec-1 and the chamber remained lighted for 1 minute
after the gas was turned off. Immediately after, the plants
were returned to the greenhouse.
Six and 12 hours after labelling (5 pm, 11pm, 5 am)
three of the six plants from each treatment were imme-
diately dissected. Leaves, stems, roots, and immature
fruit were quickly frozen at -20 C and freeze dried. Roots
were severed at the cotyledonary node and washed in cold
water prior to freezing. The pericarp of immature fruit
and stems were sliced to speed drying. The dried samples
were weighed and ground. A subsample of 100 mg of the
ground material from each plant part was digested in a
solution of 0.5 ml 70% HC104 and 0.5 ml 30% H2O2 for
24 hours at 60 C; 5 ml water and 10 ml Aquasol II scintil-
lation fluid were added, and radioactivity was measured
by liquid scintillation spectroscopy using external stand-
ard correction for quenching.
Carbohydrate
Carbohydrate levels were also determined in these
tissues to evaluate the effects of reduced night tempera-
ture on the production and distribution of photosynthate.
Three plants from each regime were harvested at 5 pm,
1 1 pm, 5 am, and 1 1 am to provide samples just before and
just after the cold period and correspond to harvests for
the study of translocation. The plants were harvested,
dissected, and frozen as described above. Dried tissues
were weighed and ground to pass a Wiley 40-mesh screen.
Subsamples of 50 mg fruit, 100 mg leaves or stems, and
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Connecticut Agricultural Experiment Station Bulletin 781
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Fig. 3 The average height of control (*) and split-night (o)
tomato plants. The I bar indicates the standard deviation from
the sample mean.
200 mg root were repeatedly extracted with boiling
80%(v/v) ethanol. They were centrifuged, and the super-
natants were combined to a final volume of 25 ml. The
sucrose concentrations in the extracts were determined
with a resorcinol procedure (Ashwell, 1957), which assays
the fructose moiety of sucrose after free fructose is des-
troyed by NaOH(0.5 N final concentration). The concen-
trations of reducing sugars in extracts of immature fruit
were found, using Clark's modification of Nelson's test
(Clark, 1964).
Starch in the residue following ethanol extraction was
solubilized in 15 ml of boiling water for 30 minutes. After
cooling, starch was digested to glucose with 500 units of
glucoamylase in 0.2 M sodium acetate buffer (pH 4.5) at
40 C . After 44 hours, samples were filtered and the filtrate
brought to 100 ml with H2O. The glucose concentration
was determined colorimetrically (Clark, 1964), and the
starch equivalent was found by multiplying the result by
0.9.
RESULTS
Diurnal Trends in Temperature
Since the temperature of the greenhouse was controlled
by the minimum temperature settings of the thermostats,
temperature control was not precise. Figure 2 illustrates
the temperature variation over a 3-day period. On sunny
days in January and February the control side was
warmer than the split-night side by several degrees during
the day. During March and April, however, this trend was
reversed. After the thermostat was reduced to 45 F
(7.2 C), the split-night greenhouse slowly cooled to a min-
imum temperature in 4 to 6 hours. Whenever the outside
temperature was above 32 F (0 C), the split-night temper-
ature never reached 45 F, but it usually went below 50 F.
On very cold nights, the control greenhouse cooled below
the set point to 55 F ( 1 2.8 C). The temperature differential
of the two night environments was greater than 10 F
(5.6 C) and less than 15 F (8.3 C) throughout the experi-
ment.
Soil temperatures lagged behind the air temperatures
about 2 hours. To speed the heating of roots in the morn-
ing, plants received 70 F (21 . 1 C) water at 7 am. Since the
volume of warm water was limited to 50 to 250 ml per pot
per day, the soil temperature was only raised 3.5 F (2C).
The soil temperatures of the split-night plants did not
reach the temperatures of the controls until 10 am, but
from then until nearly 12 pm the soil temperatures were
the same for both sets of plants (Fig. 2).
Change in Growth
Vegetative growth of the split-night tomato plants was
slower than the controls. Final height or weight, however,
was similar because the split-night plants continued to
grow longer. The time course of height of both popula-
tions are shown in Fig. 3. The slower growth of split-night
plants was most obvious in the analyses of the Mid-
growth Date parameter of the logarithmic growth curve
where the delay of 5 days was significant at the 10% level
of probability (Table 1A). The rate of growth of split-
night plants given by the growth curve was about 15%
slower than the controls. Since the duration of their
growth was extended 3 days, they grew to the same final
height when vegetative growth ended during fruit
formation.
Table 1A Parameters describing the vegetative growth of
Patio hybrid tomatoes.
Growth
Parameter
Control Split Night Average
Final Weight (gm)
25.0 ±4.9
24.2 ±4.2
24.6 ±4.4
Mid-growth Date (da)
55.4 ±6.6
60.4 ±4.9
57.9 ±6.2
Rate* (gm/da)
0.96±0.16
0.83±0.14
0.89±0.16
Duration (da)
26.4 ±6.6
29.4 ±4.4
27.9 ±5.6
'Significantly different at the P>0.10 level of probability.
Table 1B Parameters describing the fruit growth of Patio
hybrid tomatoes.
Growth
Parameter
Control
Split Night
Average
Final Volume* (ml)
666 ±102
582 ±66
627 ±94
Mid-growth Date (da)
117 ± 6
115 ± 4
116 ± 5
Rate (ml/da)
32. 3± 5.3
30.3± 3.6
31.3± 4.6
Duration (da)
20.9± 2.9
19.3± 1.6
20.1± 2.4
"Significantly different at the P>0.05 level of probability.
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Fig. 4 The average number of open flowers of control (*) and
split-night (o) tomato plants. The I bar Indicates the standard
deviation from the sample mean.
The average number of leaves per tomato plant did not
differ between control and split-night populations, and
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increased linearly at one leaf every 5 days from day 1 to
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Figures 4 and 5 show that control and split-night plants
flowered and set fruit at the same rate but the split-night
plants lagged several days. This lag became smaller as
plant development and fruit-set progressed.
The only statistically significant difference between the
control and split-night populations during fruit growth
was in the final yield of fruit. Control tomato fruit grew
slightly faster and longer than the split-night fruit. These
small differences led to a 13% decrease in yield (significant
at the 5% probability level). The difference in yield for a
large population of plants grown under the conditions of
this experiment could vary from 5 to 22% within the
standard error of the mean of the sample studied.
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Fig. 5 The average number of fruit of control (*) and split-
night (o) tomato plants and the standard deviation, I.
Fig. 6 The average number of open flowers of control (•) and
split-night (o) Easter lilies.
The control tobacco plants grew considerably faster
than the split-night plants. Although plant-to-plant varia-
bility obscured the difference in the elongation rate of
individual leaves, visual observation of the size and
number of leaves after a month of growth suggested that
the controls grew about 50% faster than the split-night
plants.
The growth and flowering of Easter lilies were affected
little by the temperature at night. The cooling near the
lilies however, was not as great as in other parts of the
split-night greenhouse because they were closest to the
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Fig. 7 Net photosynthesis ot a control (•) and a split-night (o)
tomato plant. The control and split-night data are for different
days.
steam heating pipes. The average height of the lilies
increased linearly at about 0.5 cm • day"1 and at any time
was about the same for both the control and the split-
night plants. The mean final height of 42 cm was attained
on day 80. The timing of flowering and the average
number of flowers open per day was also the same for the
two growth conditions (Fig. 6). However, the control
plants seemed to flower a few days longer than the split-
night plants.
Diurnal Course of Net Photosynthesis
and Stomatal Opening
To learn if different levels of respiration and transpira-
tion during the cool part of the night may persist to the
following morning, we measured the net photosynthesis
and transpiration of individual plants for a day. Figures 7
and 8 show net photosynthesis, normalized by the dry
weight of the plant, for tomato and tobacco plants and
show the sunlight and temperature. Since control and
split-night plants were measured on different days, their
photosynthesis cannot be directly compared. When the
sun rose, however, photosynthesis in split-night plants
rose as fast as in the control plants. Photosynthetic effi-
ciency in the early morning is especially important in
winter since days often become cloudy by mid-morning
and remain cloudy for the rest of the day. Thus, a signifi-
Flg. 8 Net photosynthesis of a control (*) and a split-night (o)
tobacco plant. The control and split-night data are for different
days.
cant reduction of photosynthesis from 7 am to 10 am
could include up to 30% of the total photosynthesis for
the day.
Figures 7 and 8 show that photosynthesis depends on
the sunlight. To compare the behavior of split-night and
control tomato plants, the photosynthesis data from six
experiments are plotted against sunlight intensity in
Fig. 9. On the average, plants from both treatments
respired at the rate of 0.06mgCO2g ' min'1 in the dark.
The rate of photosynthesis rises linearly to 0.42mgCC>2
g~'min~' at a sunlight intensity of 0.25 langley • min-1.
During cloudy winter weather, photosynthesis is sat-
urated at only 25% of full sunlight. In the brightest sun,
control plants fixed CO2 at a slightly higher rate than the
split-night plants. The data corresponding to light inten-
sities of 0.0 to 0.20 langley min-1, which include the early
morning hours, show that cool nights do not reduce
photosynthesis during the early morning. In the dim sun-
light during winter there is little difference in the photo-
synthesis of control and split-night plants.
Respiration during the night depends on temperature;
thus, there was a noticeable decrease in respiration of the
split-night plants during the cool part of the night. The
split-night tobacco clearly showed this effect (Fig. 8).
While not quite so obvious for tomato (Fig. 7), respira-
tion decreased when the temperature of the split-night
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Fig. 9 Net photosynthesis of Individual tomato plants from six
separate experiments as a function of sunlight intensity. Filled
symbols (•, ■) represent control and open symbols (o, O) rep-
resent split-night plants. The circles are for measurements in
the morning and the squares are for the afternoon.
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Fig. 10 The stomatal resistance to diffusion of water from the
leaves of control (*) and split-night (o) tobacco plants. Filled
symbols represent control and open symbols represent split-
night. The I represents the standard deviation.
plant fell; there was no corresponding decrease in respira-
tion of the control plant during the night. Although this
behavior might seem beneficial for conserving assimilated
CO2, the growth analysis suggests that this decreased
respiration was accompanied by decreased metabolism
and development of the split-night plants.
Measurement of net photosynthesis under controlled
laboratory conditions confirmed the independence of
photosynthetic efficiency from previous night tempera-
tures. Leaf discs of tobacco sampled at 9 am under control
and split-night environments had the same photosyn-
thetic rates. There was no significant difference between
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Fig. 11 The percentage of the total radioactivity found in the
leaves (y,V) and stem (■. D) of tomato plants during vegeta-
tive growth. Filled symbols represent controls and open sym-
bols represent split-night plants. The I represents the standard
deviation.
the 0.73 ±0.04 mg CO2 ■ gm~r • min-1 of split-night plants
and the 0.77 ±0.07 mg C02 gm_1 ■ min-1 of the controls.
Stomatal opening can be inhibited or delayed by cold
(Drake and Salisbury, 1972), and although greater stoma-
tal resistance could reduce CO2 assimilation in split-night
plants, the diurnal records in Figs. 7 and 8 showed no
reduced photosynthesis. Moreover, transpiration
depended only on sunlight and concurrent temperature
rather than on the previous temperature of the plant.
Figure 10 shows leaf resistance measured at four times
during a day. No significant differences in the average leaf
resistance between control and split-night plants occurred
at any time, even at 6:00-6:30 am, just after the green-
house began heating. However, the variability of stomatal
resistance among individual leaves did differ signifi-
cantly. Thus, at 6 am the standard deviation for 1 5 meas-
urements of split-night plants was ± 1 1 sec ■ cm"1 which is
more than twice the standard deviation (± 4 cm • sec-1) for
the same number of measurements on control plants. This
scatter gradually disappeared as the day progressed.
Connecticut Agricultural Experiment Station Bulletin 781
100
D
O
3
m
or
h-
Q
>
r-
o
<
o
<
0 6 12
TRANSLOCATION TIME, hrs
Fig. 12 The percentage of the total radioactivity found in the
leaves (y.V), stem (■, D) and fruit (*, o) of tomato plants dur-
ing reproductive growth. Filled symbols represent control and
open symbols represent split-night plants. The I represents the
standard deviation.
Diurnal Course of Translocation
The rate of movement and partitioning of recently
assimilated photosynthate was determined both at flow-
ering and early fruit filling stages. In Figs. 1 1 and 12 the
radioactivity in the roots, stems, leaves and fruit of the
plants 6 and 1 2 hours after 14CC»2 labelling is compared to
the amount initially in the leaves. There were much
greater differences between the plants at the two dates
than between the control and split-night plants for a given
time. During vegetative growth, little assimilated ' C was
exported from the leaf, and 70-80% of the radioactivity
remained in the leaves, even after 12 hours (Fig. 11).
During reproductive growth, however, 14C-labelled sug-
ars were swiftly transported, primarily to the fruit, and
after 6 hours about 40% of the radioactivity was reco-
vered from the fruit. This amount increased to more than
50% by 1 2 hours (Fig. 1 2). Thus, the movement of sugars
out of the leaf is much greater during fruit filling than
during vegetative growth.
In either developmental stage, both the control and
split-night plants translocated a slightly higher percen-
tage of radioactivity in the evening than during the day, as
can be seen by comparing plants labelled at 5 pm to plants
labelled at 1 1 am. The control plants continued to translo-
cate 14C-labelled sugars rapidly during the latter part of
the constant temperature night (11pm to 5 am). The
reduction of translocation in split-night plants during the
cool part of the night was the most obvious difference
between the control and split-night plants in both trans-
location experiments.
During fruit filling more radioactivity was exported in
6 hours from the leaves of split-night plants than from
controls. This occurred for both the 1 1 am and 5 pm
labelling times. The rapid translocation between 5 and
1 1 pm by the split-night plants largely counteracted the
slow translocation during the next 6-hour period so that
after 12 hours there was little difference between the total
amount of radioactivity translocated in the two treat-
ments. The split-night plants had only 2-3% less l4C in the
fruit, even after 6 hours of cool night temperatures (see
the 5 pm label in Fig. 12).
The radioactivity in the root tissues was only about 1%
of the total radioactivity in split-night and control plants,
except for the plants harvested at 5 am after 12 hours of
translocation. At 5 am during vegetative growth, the
roots of the control plants contained substantially more
radioactivity than the roots of the split-night plants (4.8%
vs. 2.9%). During reproductive growth, however, there
was less radioactivity in the roots and no difference
between split-night and control plants.
Diurnal Course of Carbohydrate
The first carbohydrate analyses were made when the
plants were about 28 cm tall, had 12 leaves, and were
beginning to flower. On February 28, 1979, following a
sunny day, sugars had accumulated to similar levels in the
leaves of control and split-night plants (see Fig. 13A,B).
Starch was the major storage product, and had accumu-
lated to approximately 12% of leaf dry weight by 5 pm.
Starch levels were depleted in the leaves of control plants
at a nearly linear rate to 4.9% at 5 am, after which no
further metabolism was observed (Fig. 13 A). Starch
depletion in leaves of split-night plants was rapid only
between 5 and 1 1 pm when the plants were at 60 F (15 C)
or above, and more starch remained in these leaves than
in the warmer control leaves. There were no significant
differences in amount of sucrose, the major translocated
sugar, between leaves of the two treatments.
Both sucrose and starch accumulated in tomato stems
(Fig. 13). In stems of control plants, starch remained high
until 1 1 pm but was lower by 5 am. In stems of split-night
plants, on the other hand, starch was depleted during the
early evening but during the cool part of the night, accum-
ulated to the previous level (Fig. 13B) and then was
depleted again after the greenhouse temperature rose.
Initially, levels of sucrose in the stems were high for both
treatment groups (about 5.5%). Sucrose levels in the con-
trols then declined during the night to 2.8% (Fig. 13C) as
observed in the leaf, but sucrose levels in stems of split-
night plants remained between 4 and 5% throughout the
day and night.
Direct effects of temperature on the carbohydrate
Split-night Temperatures in a Greenhouse
I
>-
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rr
0.
5pm llpm 5am Ham 5pm llpm 5am Ham
TIME OF DAY
Fig. 13 Carbohydrate levels during the vegetative growth of
tomato. Starch levels in percent of dry weight (panels A and B)
and sucrose levels In percent of dry weight (panels C and D)
are shown for the leaves (t.v) , stems (■, D) and the roots
(A, A). Filled symbols represent control and open symbols
represent split-night plants. The I represents the standard
deviation.
metabolism of the roots were not readily apparent
(Fig. 13). Roots of split-night plants, however, had lower
carbohydrate levels at nearly all times, indicating a
decreased availability of carbohydrate. In both treat-
ments starch decreased sharply after 1 1 pm while sucrose
remained constant, suggesting that starch, rather than
imported sucrose, is the carbon for root respiration at
night.
The second carbohydrate analyses were when the
plants were about 60 cm tall, had 14 to 16 leaves, and
seven immature fruit weighing 13 g. Harvests were again
made on a clear, sunny day following two cloudy, over-
cast days. Accumulation and depletion of carbohydrates
was more sensitive than translocation to the cool night.
Figures 14A and B illustrate the diurnal trends in starch
in the leaf. Apparently because of the cool night, the
normal periods of accumulation and depletion were offset
in time. By 5 pm, leaves of both populations had accumu-
lated substantial amounts of starch, 14.9% in control and
13.7% in split-night leaves. Degradation of starch in the
leaves of control plants was rapid only after 1 1 pm.
Leaves of split-night plants, however, had degraded their
starch levels to 8.1% prior to the onset of the cool period
at 1 1 pm. During the cooler part of the night, a slight but
not significant accumulation was detected. At 5 am,
leaves of split-night and control plants contained almost
the same amounts of photosynthate. Sucrose levels
remained constant at 1.5% during these large fluctuations
in starch accumulation.
The differences between control and split-night plants
in the time of accumulation and degradation of starch
were especially apparent in the stem. Stems of control
plants were only beginning to accumulate starch by 5 pm
of a clear, sunny day, but starch continued to accumulate
from 1 1% to a maximum of 19.9% by 1 1 pm. In stems of
split-night plants, however, starch accumulation had
apparently peaked several hours before 5 pm, and the
level of starch continued to decline to a minimum of
11.2% at 1 1 pm. During the cool part of the night, starch
rapidly accumulated in the stems of split-night plants
(Fig. 14B). Sucrose dropped from 7.8% to 4.6% during
this period of starch synthesis in the stem (Fig. 14D).
The decline in starch in stems of control plants between
5 am and 1 1 am came approximately 6 hours after the
decline in leaf starch. By 1 1 am, leaves and stems of
split-night plants had declined to approximately the same
level of starch.
25
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5pm II pm 5am Ham 5pm llpm 5am Ham
TIME OF DAY
Fig. 1 4 Carbohydrate levels during the reproductive growth of
tomato. Starch levels In percent of dry weight (panels A and B)
and sucrose levels for the leaves and stems, and reducing sugar
levels for the fruit, as percent of dry weight (panels C and D) are
shown for the leaves (y , v) > stems (■, □) and fruit (*, o). Filled
symbols represent control and open symbols represent split-
night plants. The I represents the standard deviation.
10
Connecticut Agricultural Experiment Station Bulletin 781
As expected, fruits were the major sink for photosyn-
thate. Sucrose is the form in which carbon is translocated
from the leaves, but no diurnal fluctuations in sucrose in
the fruit were observed in plants of either treatment.
Instead, soon after arrival in the fruit, sucrose was hydro-
lyzed to the reducing sugars fructose and glucose before
starch synthesis. The control fruit accumulated sugars
linearly throughout the night to 27.4%, while starch
accumulated after a 6 hour lag to a nearly identical level
(Figures 14A, C). In the fruit of split-night plants, no
significant accumulation of reducing sugars or starch
occurred during the night. After dawn, starch in the fruit
rose from 13% to 21.4% by 1 1 am. Final starch and sugar
at 1 1 am were generally, but not significantly, less in fruit
of split-night plants.
Relatively low levels of carbohydrates were observed in
roots of all plants. Roots of control plants contained 2.5%
sucrose and starch, while roots of split-night plants con-
tained about 2% sucrose and 1.5% starch. Variability in
measurement rendered most comparisons nonsignificant.
DISCUSSION
Change in Growth
A comparison of plants grown in constant (15 C) and
split-night (15C/7C) temperatures shows only a small
retardation by the cool night on the growth, development,
and yield. This generally agrees with the findings of others
(Carow and Zimmer, 1977; Parupis, 1978; Shanks, 1978;
Shanks and Link, 1979; Thome and Jaynes, 1977).
The inhibition of growth by cool nights may be small
only because of the slow growth of plants in winter. Dim
sunlight intensities and short days limit plant growth
during winter. Permanent biochemical and structural
adaptations to the dim light preclude rapid photosynthe-
sis on the infrequent sunny days. For example, the tomato
plants had maximum photosynthesis at only 25% of full
sun (Fig. 9). Prolonged sunlight provides accumulations
of starch that supply the plant on subsequent cloudy days.
The relatively slow winter-time growth observed for both
split-night and control plants is probably due to this
fluctuating, and often limiting, supply of carbohydrate.
The effects of split-nights, half warm (15 C) and half
cool (7 C), were much less than would be expected from
the behavior of tomato plants grown at constant tempera-
tures. At a constant 7C, little weight accumulates and
stems elongate little (Hussey, 1965; Went, 1944), while at
a constant 15 C, the rate of growth would be fully half the
maximum reached at 26 C. A naive calculation from the
rates at 7 and 15 C would predict that 8 hours at 7C
should decrease the vegetative growth of split-night
tomato plants by 33%. The difference in the rate of
growth, however, was only 15%. Even this small differ-
ence was insignificant for development as plants in both
treatments reached the same final size and produced the
same numbers of flowers and fruit.
The split-night temperature of 7 C did not inhibit flow-
ering of tomato nor promote fruit abortion, in contradic-
tion to the widely held belief that temperatures below 10 C
prevent fruit formation. The belief could have been
initiated by Went (1944) or by the prevention of fruit
formation by cool nights in the field. Our success may be
due to the choice of short-season varieties, to the fertiliza-
tion of flowers by vibration of the stem, or because the
plants were repeatedly exposed to cold.
Although the growth rate of fruit was not significantly
different between control and split-night plants, the final
yield was decreased by 1 3% in the split-night plants due to
slightly slower growth rate combined with a shorter devel-
opment time. At constant temperature, the growth of
tomato fruit at 7.5 C is one-third that at 15 C (Walker and
Thornley, 1977). Thus, fruit growth rate should decrease
by 25% for split-night plants if the timing of the cool
period is unimportant. In fact, we found that the rate of
fruit growth was only 5% less. Cool nights retard fruit
growth much less than vegetative growth. This effect is
also seen in sweet pepper (Rylski, 1973).
The lilies did not show any discernable effects of cool
nights on growth or flowering. The development and
flowering of several other plants have been shown to be
insensitive to a split-night regime as long as the cool
period is less tban 12 hours (Carow and Zimmer 1977,
Parups 1978, Shanks 1978, Shanks and Link, 1979). Of
the species tested here, tobacco growth was reduced the
most by cool nights.
Change in Physiology
Most processes were severely inhibited in the split-
night tomato plants during the cool period. Therefore,
their uninhibited growth requires two conditions: 1)
Assimilation processes, such as photosynthesis and nu-
trient uptake, are not inhibited by previous cool tempera-
tures; 2) Temperature-dependent growth processes are
completed before the onset of the cool period. The second
condition also implies that the metabolism of split-night
plants may be especially active during the warm period.
We tested one aspect of the first condition: the rate of
assimilation of CO2 or net photosynthesis. No substantial
inhibition due to split-night temperature was observed. In
particular, early in the morning and soon after being
warmed to 15 C, the photosynthesis of the leaves of split-
night plants responded to light the same as controls. This
response explains much of the success of the split-night
scheme. Others found that plants subjected to regular
cold nights showed the same photosynthesis as warm-
night plants when tested under identical warm, controlled
conditions (Hurd and Enoch, 1976; Kohl and Thigpen,
1979). However, plants grown at a steady warm tempera-
ture and then suddenly cooled to 5 or 10 C do not recover
photosynthetic capacity when rewarmed (Taylor and
Rowley, 1971; Crookston et al., 1974). This irreversible
behavior is caused by loss of stomatal control and general
tissue disruption (Drake and Salisbury, 1972; Breiden-
bach and Waring, 1977; Lyons, 1973; Chatterton et al.,
1972; Ivory and Whiteman, 1978). Apparently, long
adaptation to cool nights plays an important part in
eliminating harmful effects on stomatal function and
photosynthesis. The high photosynthetic capability soon
Split-night Temperatures in a Greenhouse
II
after a cool night satisfies the first requirement necessary
for split-night plants to grow as fast as the controls.
During the entire winter, the growth of both split-night
and control tomato plants was slow because carbohy-
drate was lacking. Following cloudy days and little pho-
tosynthesis, fewer hours of warm night temperatures
should be needed for distribution and metabolism of the
photosynthate than following sunny days. Thus,
temperature-dependent growth processes might not be
affected simply because they could be completed before
the onset of the cool period, without any other adaptation
or change in metabolism of the plants. This hypothesis
was not directly tested here, since the diurnal harvests for
the determination of translocation rates and carbohy-
drate levels were always made during and after sunny
days. After growing for 52 and 86 days under split-night
conditions, however, the plants continued in a long estab-
lished diurnal rhythm of physiological processes.
Our experiments showed that the split-night regime
distinctly accelerated the translocation and metabolism
of carbohydrate relative to the controls before the cool
part of the night. In this way the plants adapted to the
split-night regime and completed temperature-dependent
processes during the warm early evening.
Both translocation and metabolism of carbohydrate
are necessary for growth. These two processes are not
independent because starch reserves must be converted to
sucrose to be translocated from the leaves. Likewise, the
sucrose must be metabolized in the fruit and stems for
continued influx of sucrose. Both translocation and
metabolism were substantially inhibited during the cool
portion of the split-night regime. Translocation from the
leaves was inhibited by half or more (Figs. 1 IB, 12B) in
agreement with results of translocation in tomato under
constant temperature conditions (Walker and Ho, 1977).
Carbohydrate metabolism as measured by CO2 respira-
tion, was also reduced about half when the temperature
fell from 15 C to 7C (Figs. 7 and 8). Specifically, the
conversion of starch to sucrose was inhibited by the cool
temperature. Starch levels in the split-night plants tended
to rise from 1 1 pm to 5 am, even in the leaves, although the
net carbohydrate reserves must be depleted during these
hours of net respiration (Figs. 13B and 14B).
Split-night tomato plants did not adapt their metabo-
lism to the cool temperatures per se. Instead, the trans-
location and metabolism of carbohydrate was faster
during the afternoon and early evening in the split-night
plants. This behavior was most pronounced during fruit
filling, probably because the efficient translocation of
carbohydrate is necessary for fast fruit growth. The
amount of radioactive carbohydrate moved from the
leaves to fruit was greater in split-night plants during the
afternoon and evening (Fig. 12). Starch was degraded
rapidly during the warm period from 5 pm to 1 1 pm in
both the stems and leaves of split-night plants, causing
levels below those found in the control plants 6 hours later
(Fig. 14). In contrast, the starch levels in control plants
did not peak until after 5 pm and starch degradation
continued throughout the night. Thus, split-night tomato
plants showed a definite, although indirect, adaptation to
the repeated cool temperatures during reproductive
growth that allowed a growth rate comparable to the
controls.
The vegetative and reproductive stages of tomato
growth must be considered separately in an analysis of the
economic benefits of split-night greenhouse management.
The split-night regime did not change the diurnal cycle of
translocation and carbohydrate metabolism in vegetative
plants and their rate of growth was reduced. However, the
plants did reach the same final size and fruit bearing
capacity by extending growth for several days. Thus, the
split-night regime seems to be suitable for producing
bedding plants. The economic benefit of split-night man-
agement can be found simply by subtracting the cost of
extending the growing season by a few days from the
savings due to the split-night temperature.
Reproductive plants acclimated to the split-night
regime by speeding translocation and carbohydrate
metabolism during the day, and the rate of growth of their
fruit was not significantly reduced. However, fruit pro-
duction in split-night plants declined faster than in the
controls; the duration of fruit growth was shorter; and the
final yield was significantly reduced. It may be possible to
alleviate the faster decline in split-night plants by adjust-
ing soil temperature, fertility or watering. Nevertheless,
the time required to produce tomato fruit from seedlings
under split-night temperatures was about the same as
under constant temperature; thus, a significant amount of
fuel was saved. To calculate the economic benefit, this
savings must be compared to the reduction in economic
yield of the fruit.
ACKNOWLEDGEMENTS
Mr. William Loefstedt introduced to us the idea of
growing plants in split-night temperatures and has pro-
vided many stimulating discussions.
Mr. James Perito provided most of the assistance dur-
ing the course of the experiment. He took care of the
plants, measured their growth and development, and
accomplished the initial data reduction. Dr. George R.
Stephens provided valuable advice on the watering and
fertilization of the plants and arranged for the supply of
Easter lilies. Cindy Sudarsky did many of the carbohy-
drate analyses. Finally, Dr. David Oliver measured the
tobacco photosynthesis under controlled conditions.
12
Connecticut Agricultural Experiment Station Bulletin 781
APPENDIX
Calculation of Heating Degree Days
During Split-Night
Instead of measuring the fuel used in the greenhouse at
different temperatures, we calculated the savings from
reduced temperature by a modification of the familiar
method of degree-days. First, we define a heating degree-
day (HDD) that accounts for different inside tempera-
tures at different times of the day, then we examine how
the sun warms the greenhouse and affects HDD, and
finally we calculate the energy needed to warm a cold
greenhouse in the morning.
Heating degree-days are simply the differences between
a desired minimum temperature inside a greenhouse and
the daily mean temperature (TMN) outdoors, with nega-
tive values omitted. For a residence, we simply subtract
TMN from 65 F (18.5 C) where TMN is the mean of the
maximum and minimum outside temperatures, TMAX
and TMIN. This simple method serves because the same
inside temperature is assumed for all hours. To calculate
degree-days for heating the inside to different tempera-
tures at different hours, however, requires that we specify
the outside temperature, hour-by-hour.
The daily course of temperature outside is generally a
steady rise from TMIN at 6 am to TMAX at 2 pm and a
fall to TMIN at 6 am that can be approximated by the two
straight lines AB and BC shown in Fig. Al. Thus, the
temperature Tat time / hours after 6 am is approximately
T= TMIN + tj 8 (TMAX -TMIN)
(Al)
from 6 am to 2 pm, and
r= TMAX -(r- 8)/ 16 (TMAX -TMIN) (A2)
from 2 pm to 6 am. In general, the minimum temperature
differs from one day to the next and TMIN at point A is
not equal to TMIN at point C in Fig. Al. It can be shown,
o
UJ
rr
r>
r-
<
rr
UJ
0_
UJ
TMAX-
TMIN
6am 10 am 2pm 6 pm 10 pm 2 am 6 am
TIME, hr
Fig. A1 The variation of outdoor temperature during a winter
day Is shown by the solid line. The dashed lines AB and BC
approximate the actual temperature variation.
however, that calculations using equations ( A 1 ) and (A2)
give essentially the same results as do more complicated
equations that account for this difference in TMIN.
Therefore, we use the simpler equations (Al) and (A2).
From equations (Al) and (A2) for outside temperature
at every hour we can calculate a mean outside tempera-
ture for three periods of steady inside temperature. One
period is at night between 10 pm and 6 am when the
greenhouse is allowed to cool. We call the mean tempera-
ture for this period TN for "temperature-night." A second
period is when the sun heats the inside. Generally this is
between 9 am and 3 pm and we call the mean temperature
for this period TS for "temperature-sun." During the 10
remaining hours, 6 am to 9 am and 3 pm to 10 pm the
greenhouse would generally be held at a warm tempera-
ture by burning fuel. The mean temperature for this 10-
hour period is called TD for "temperature-day." The
mean temperatures TN, TS, and TD are simply found by
integrating the outside temperatures given by equations
(Al) and (A2) between appropriate limits. The results of
these integrations are:
for hours 10 pm to 6 am
TN = 0.25 TMAX + 0.75 TMIN (A3)
for hours 9 am to 3 pm
TS = 0.735 -TMAX + 0.265 TMIN (A4)
for hours 6 am to 9 am, and 3 pm to 10 pm
TD = 0.56 -TMAX + 0.44 TMIN (A5)
The HDD for a greenhouse maintained at temperature
TB for 24 hours of a day is simply the sum of the three
periods given by:
HDD24 = 8/24 (TB-TN) + 6/24 (TB -TS)
+ 10/24(TB-TD) (A6)
and is equivalent to the standard calculation of heating
degree days for a residence at a steady temperature. To
determine the relative fuel savings afforded by reducing
o
o
UJ
(T
3
I-
<
rr
Ui
CL
UJ
r-
TB
TSB-
TO
, ntTD
»_
^
1
\
\
1
\
1
1
\
\
\
1
*
"fc 1" *SB '
\
-
0
-C
off
heat
C
O
TIME, hr
Fig. A2 The temperature response to a reduced thermostat
setting for a greenhouse with time constant RC 0 (solid line)
and for one with RC greater than zero (dashed line).
Split-night Temperatures in a Greenhouse
13
the greenhouse temperature for 8 hours we must calculate
how much fuel would normally be used without the
reduction and how much would be used with the reduc-
tion and then compare the two numbers. To facilitate this
comparison we calculate separately the amount of these
HDD24 that is accumulated other than at night since this
will be the same with or without temperature reduction:
HDD' = 6/24(TB-TS)+10/24(TB-TD). (A7)
To obtain an estimate of the fuel savings due to tempera-
ture reduction at night we calculate the HDD at night
when the temperature is set back to TSB, add this result to
HDD' and subtract the sum from HDD24. That is, the
savings 5 are given by:
S = HDD24 (TB) -[HDD' (TB) + HDDN (TSB)]. (A8)
We now examine the effect of solar heating on our
calculation. The bright sun sometimes heats the green-
house above TB for a few hours so that no fuel is required
during this time. From pyrheliometer recordings, we have
derived a daily solar factor SR, which is either 1 or 0: SR
is set equal to 1 if the day had full or nearly full sun,
otherwise SR is set as 0. We obtained SR during the
months of December, January, February, March and
April for the last 5 years. Likewise, the HDD calculations
described below use TMAX and TMIN data for these
same periods.
To determine the influence of sun, we calculate a green-
house heating degree-day GHHDD24 defined by:
GHHDD24 = 8/24(TB-TN) + 6/24(l-SR)-
(TB-TS)+ 10/24 (TB-TD) (A9)
Clearly, for days with little or no sun:
GHHDD24 = HDD24
since SR = 0. For sunny days, no heating is required
during the solar period. Thus, SR=1 and
GHHDD24<HDD24.
As above, we calculate a
GHHDD'= 6/ 24 (1 - SR) • (TB - TS) + 10/24 (TB -TD)
for the part of the day that does not include 10 pm to 6 am.
Finally, since at night there is no sun and GHHDDN =
HDDN, we obtain the savings:
S = GHHDD24(TB)-
[GHHDD'(TB)+ HDDN(TSB)]
(A10)
Energy Savings Due to Split-Night Regime
The absolute savings and the percent savings of HDD
according to equation (A8), which ignores solar heating,
are shown in Table A 1 for December, January, February,
March and April during the last 5 years. The average
savings due to a reduction to 7.2 C compared with a
15.5 C setting is 19% during the entire winter. Finally, the
calculated savings according to equation (A 10), which
includes the effects of solar heating, are shown in Table
A2. The average savings for a 7.2 C reduction compared
with the standard 15.5 C is 21.5%.
Heating Inefficiency Due to Changing the Temperature
In the above calculation of fuel savings we have
assumed that the greenhouse temperature instantane-
ously becomes equal to the thermostat setting; that is, we
have ignored the thermal inertia of the greenhouse. We
examine the effects of finite cooling and reheating times
for the simple case of a constant outside temperature and
no heat supplied by the sun, i.e., an overcast day. The
temperature history of the greenhouse in this case is
shown in Fig. A2. The three temperatures TB, TSB and
TO refer to the upper thermostat setting, the lower ther-
mostat setting and the outside temperature, respectively.
Four times must be considered: The time tc is the time
required for the greenhouse to cool from TB to TSB, u is
the time to reheat from TSB to TB, ?sb is the time the
greenhouse remains at TSB and PER is the entire heating
period, in this case, 1 day.
For a hot-air heating system, no heat is required during
cooling. The heat, QSB, required during 1 day using the
reduced temperature regime is:
QSB = qB • tB + <7sb • /sb + qr h
(All)
where q%, qSn and q, are the rates of heat (cal/ sec) supplied
by the furnace during the steady upper temperature, dur-
ing the steady lower temperature and during reheating,
respectively. The percent savings due to temperature
reduction is obtained by dividing equation (All) by the
heating required if the temperature is not reduced, i.e.,
Q = qB • PER. Denoting the fractional times by /, e.g.,
?sb/PER=/sb, the fractional savings S is (Q - QSB)/Q or
S = [l -/B-(9SB/9B)/"sB)-(gr/?B).A] (A12)
If the greenhouse has an overall heat capacity C (cal/°C)
and an overall resistance to heat transfer R (sec °C/cal),
cooling and reheating will be characterized by a time
constant R ■ C. During cooling to TSB the temperature
obeys
TSB-TO = (TB-TO)exp(-*c/RC) (A13)
while during reheating
TB-TO-4rR = (TSB-TO ^R)exp(-/r/RC) (A14)
In addition, during steady state at TB and at TSB we have
9b = (1/R)(TB-TO) (A15)
<7sb = (1/R)(TSB-TO) (A16)
Solving equation (A 14) for t, and using this result
together with equations (A 15) and (A 16) in equation
(A 12), we obtain
5=l-/B-C(TSB-TO)/(TB-TO)ySB-(9r/9B)/r(A17)
where
/ = (RC/PER)ln[(l-?sB/qr)/(l-9B/9r)] (A18)
14
Connecticut Agricultural Experiment Station Bulletin 781
If the greenhouse time constant is very small, and heating
takes little time, the savings are approximately
S s 1 -/„ -[(TSB -TO)/(TB -TO)]/sb
(A 19)
Equation (A 19) for the case of constant outside tempera-
ture is equivalent to our method of calculating the savings
presented earlier in Tables Al and A2.
To calculate the effect of reheating on the savings given
by equation (A17), we must know the ratios of heating
rates qsv/q, and qe/qi. As an example, we assume that
<7B/<7r = 0.35. Then, if TB= 15.6°C, TSB = 7.2°C and TO
= -l.l°C, we have gsB/<?B = 0.5 and <7sB/<7r = 0.175. For
our greenhouse, RC/ PER = 0.25 so that f,= 0.06. We
must also have/B +/c +/sb +fr = 1 . For an 8-hour temper-
ature reduction, the savings calculated by equation (Al 7)
are about 2.5% less than the savings calculated by equa-
tion (A 1 9). Of course, if reheating is done inefficiently, the
savings will be reduced somewhat more (Zabinsky and
Parlange, 1977).
In our experiments, we added a certain amount of
water at temperature TB to the plants in the experimental
greenhouse each morning to speed the warming of the
soil. However, if no heat is expended for warming the
water during the cooling period, i.e., during time tc, then
this added heat is entirely accounted for by our calcula-
tions and will not affect the results.
In conclusion, an 8-hour temperature reduction during
the winter months in Connecticut should afford a relative
fuel savings of about 1 8%.
Table A1 Split-night fuel savings calculated from heating degree days assuming that green-
house temperature is reduced from 15.5 to 7.2C from 10 pm to 6 am each day.
HDD24
HDD'
HDDN
HDD24
Savings
%
Month
base 15.5°C
base 15.5°
C
base 7.2C
C
(set-back)
Savings
December
870
536
179
715
155
17.8
January
1025
637
233
870
155
15.1
February
853
523
190
713
140
16.4
March
664
389
123
511
152
22.9
April
351
185
41
226
125
35.6
Total for
3764
—
—
—
727
19.3%
Heating
Season
Table A2
Split-night fuel
savings calculated as In
Table
A1 except
that an allowance for
solar hearting is Included.
GHHDD24
GHHDD
HDDN
GHHDD24
Savings
%
Month
base 15.5C0
base 15.5°
C
base 7.2C
C
(set-back)
Savings
December
810
477
179
656
154
19.0
January
908
520
233
753
155
17.1
February
743
413
190
603
140
18.8
March
590
315
123
438
152
25.8
April
322
156
41
197
125
38.8
Total for
3373
—
—
—
726
21 .5%
Heating
Season
Split-night Temperatures in a Greenhouse
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