Seasonal variation in the transpiration of glasshouse plants

Agricultural Meteorology - Elsevier Publishing Company, Amsterdam - Printed in The Netherlands

SEASONAL VARIATION IN T H E T R A N S P I R A T I O N OF PLANTS

GLASSHOUSE

J. V. LAKE,J. D. POSTLETHWAITE,G. SLACKAND R. I. EDWARDS National Institute of Agricultural Engineering, Wrest Park, Silsoe, Bedj'ord (Great Britain)

(Received December 21, 1964)

SUMMARY The transpiration of Helxine solierolii ("Mind your own business") was measured by growing it in soil kept close to field capacity on the pan of a weighing machine in a glasshouse in southern England. The glasshouse was not heated except to exclude frost. The plants provided a nearly horizontal mat of green foliage at all times of the year and the monthly mean daily transpiration, E', was related to the corresponding solar radiation R'c falling on the plant surface and to the vapour-pressure deficit ea-~a of the outside air by the following equation: E' - 0.38 R'c + 0.17 (ca-ca)-0.17 mm/day

where R'c is expressed as evaporating power in mm of water, e,, is the saturation vapour pressure (mm Hg) at the mean of the daily maximum and minimum air temperatures, and ed the actual vapour pressure (mm Hg) of the air, determined from the wet, and dry-bulb temperatures at 09h00 daily. R'c can be estimated from the solar radiation measured outside, if the light transmission of the glasshouse is known, so that the data required for estimating E' can all be obtained from agro-meteorological stations and the equation promises to be of practical value in calculating the water requirements of commercial glasshouse crops more accurately than has been hitherto possible.

INTRODUCTION In an earlier series of experiments (MORRIS et al., 1953, 1957) in which plants were grown in the pan of a sensitive weighing machine in a glasshouse, the hourly and daily evapotranspiration, E', from tomatoes, carnations and lettuces in soil near field capacity was correlated closely with the total solar radiation, R'c, received on a horizontal surface inside the glasshouse during the same period. The energy balance inside the glasshouse was considered on a daily basis. The Affr. Meteorol., 3 (1966) 187-196

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net heat exchange with the soil and plants was neglected and the daily potential evapotranspiration was written as:

E' = R'c (I-r)-R'~-K'

(1)

where all values are in units of evaporation, e.g., mm water/day. The reflection coefficient, r, of the crop surface was taken (MORRIS et al., 1953) as 0.2. R'B, the net longwave back radiation from the crop to the glass, and K', the sensible heat transferred to the air, were calculated as approximately 0.2 R'c and 0.1 R'c respectively. This gave: E' 0.5 R'c which is much lower than many of the values obtained experimentally. Apart from one case, when E' z 0.23 R'c, but in which the coefficient of correlation between 18 pairs of daily observations was only 0.41, E' in all the more recent experiments was within the range 0.44-0.98 R'c and the coefficients of correlation in sets of 12-27 daily observations all exceeded 0.8. The unexpectedly high values of E' were attributed to the lack of an adequate array of guard plants on all sides of the experimental area. Such guarding could have (/) prevented plants at the sides of this area receiving more solar radiation than those near the centre and (2) increased the ratio of evaporating area to dry path area in the glasshouse (MoRRlS and LAKE, 1962), although this was already near the practicable maximum. Effect 1 would be greater with tall plants than with short ones and the highest values of E'/R'c were, in fact, obtained with tall tomato plants, while some of the lowest were obtained with lettuces. However, the highest values were obtained in the summer, whereas the experiments with lettuces, the shortest plants used, were conducted in the winter, so that in addition to any effect of plant height, a seasonal effect could not be ruled out. The equation of PENMAN (1948) allows for the fact that outdoors the proportion of solar radiation, Rc, used to supply sensible heat to the air above the crop varies with the air temperature and water vapour pressure, so that evapotranspiration, E, is a smaller proportion of Rc in winter than in summer. MORRIS et al. (1957) give a graph showing E'/R'c increasing with the height of the experimental tomato plants, and ROTHWELL and JONES (1961) give a somewhat similar graph for a tomato crop in a 1/10-th acre glasshouse, but in neither case is the possible seasonal effect taken into account quantitatively; if this effect is large such curves could give misleading results when applied to crops sown at other times of the year. To find a more exact quantitative relation between E' and R'c and to separate seasonal effects from effects of changes in plant height, it is desirable to use similar plants at all times of the year and to completely surround them with guard plants. For this purpose, MORRIS (1959) has designed a sensitive weighing machine, in which all the mechanism is below the level of the pan. Evaporimeters have been successfully used to provide estimates of the water requirements of crops under glass (e.g., HUDSON, 1960; NIELSEN and CHRISTENSEN, 1960; JONES and ROTHWELL, 1964), but there is a need for quantitative information about the dissipation of solar radiation in a glasshouse not only in connection with crop water requirements but also with respect to the control of heating and ventilation. Agr. Meteorol., 3 (1966) 187-196

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METHOD

Choice of plant To find the effect of time of year on the proportion of radiation used for transpiration, a perennial plant was sought which would provide a green surface not changing appreciably in general physical properties for a period of twelve months or more. Grass was rejected because of the abrupt change, including the exposure of cut surfaces, which occurs at each mowing. Helxine solierolii was chosen because, once established. it completely covers the soil surface with a dense array of small, nearly horizontal leaves and the only appreciable change which occurs with time is in the thickness of this mat. The flowers, which when the plant grows outdoors appear from May to October (under glass the flowering season is sometimes longer), are very small and inconspicuous and are normally hidden by foliage if the plant is growing strongly. The stomata are mostly in the lower epidermis of the leaves and there are hydathodes in the upper epidermis. In a preliminary trial, the roots were found to penetrate rapidly to the bottom of a 50 cm deep box of soil so that there appeared little likelihood of inadequate root growth causing water stress. The choice of this plant rather than one of economic importance was further supported by the fact that it so completely covered the soil that the water loss could be considered as transpiration rather than evapotranspiration.

Management of the plants The foliage was pressed with a board from time to time, in order to keep the surface as nearly horizontal as possible. As the mat grew in thickness, the walls of the weighing machine pan and of the solarimeter tubes (see p.191) were extended upwards with strips of reinforced aluminium foil so that the horizontal area of the transpiring surface was maintained constant. Some preliminary experiments preceded the present work so that the mat of foliage was already thick and after the first three months, i.e., at the end of July 1959, it became necessary to mow it off. Measurements of the transpiration in August, 1959, are excluded from the present report because the plant cover was incomplete. The next mowing was in September 1960, after the end of the present experiments.

Description of glasshouses To provide an estimate of the reproducibility of the results, a recording weighing machine (MORRIS, 1959) was arranged centrally in each o f two similar metal glasshouses of the type used in earlier experiments (MORRIS et al., 1957). The ridge of each house was 3.1 m high and ran north-south, the area was 25 m z. The floors consisted of a deep ( ~ 30 cm) layer of dry pea grit, over concrete, so that evaporation was negligible. After some preliminary experiments, a canopy of

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polyethylene film was fitted inside each roof, about 50 cm from the glass, to prevent water from rain or condensation falling on the planted area. The canopy did not extend over the surrounding paths, so interference with air change through the roof was minimised. As direct solar radiation became appreciably diffused on passing through this canopy, other sheets of film were placed on the upper parts o f the eastern, southern and western walls o f the house so that any direct sunlight falling on the planted area passed through one layer of glass and one o f polyethylene. The side ventilators were below the level o f the experimental plants and were not obstructed by the polyethylene film. The position of the ventilators was adjusted manually, to keep the houses cool enough for good plant growth in summer and warm enough in winter. For protection against frost, electrical tubular heaters were arranged on the eastern and western walls, below the level o f the surface o f the experimental plants. The heating was controlled by an air thermostat so adjusted as to prevent the temperature of a spirit-in-glass thermometer resting on the surface of the foliage from falling below 1°C. Possible errors in weighing

The sensitivity of the mechanism of the weighing machines was 0.005 of the range of weight change available on a chart about 17 cm wide. Three ranges were available, 1, 5.5 and 10 kg, suitable for daily records of transpiration in mid-winter, spring or autumn, and mid-summer, respectively (1 kg ~ 0.3 mm water). Errors due to changes in buoyancy (MORRIS, 1959) could arise from changes in atmospheric pressure or temperature. Daily transpiration was taken as the change of weight from sunrise on one day to sunrise on the next, so that errors due to temperature differences (,~ 3 g/°C) were usually negligible but errors due to changes in barometric pressure could attain about :t: 30 g. The dry weight o f the plant material removed from the West weighing machine after mowing in September 1960 was 5.0 kg. The total water loss during its 12-month period of growth was about 350 mm (Table I), or 1,050 kg, so that the weight gain TABLE I COEFFICIENTS OF CORRELATION BETWEEN 19 MONTHLY MEAN DAILY VALUES OF TRANSPIRATION~

E ' , AND

VARIOUS ENVIRONMENTAL FACTORS

Factors correlated

Factors eliminated

Correlation coefficient

P

E', R" C E', Ca--Cd E', R' C E',Ta

-

+ 0.988

<

R'c ea---ed R'C, ea-ed R'c, ea-ed

+ 0.814 + 0.967 + 0.359 @-0.303

< 0.001 < 0.001 not significant not significant

E', Tmax.

0.001

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due to carbon assimilation by the plants was less than 0.5 ~ o f the loss due to transpiration and it was therefore ignored.

Arrangement of guard boxes Each weighing-machine pan, 3 rn z in plan area and 50 cm deepwas surrounded by l-m wide guard boxes of the same depth, two of them moveable to allow access to the weighing mechanism. The exposed sides of these boxes were covered with reinforced aluminium foil to reflect solar radiation and long-wave radiation emitted by the heating pipes. The weighed pan and guard boxes were all filled with John Innes No.1 potting compost (LAWRENCE and NEWELL, 1939).

Watering Water was added at night and dripped onto the soil through holes drilled 24 cm apart in small rigid p.v.c, pipes arranged 24 cm apart below the plant foliage. The experiments began when, after wetting the soil thoroughly, drainage from outlets in the floors of the pans had become negligible. Thereafter, the deficit was never allowed to exceed 3 mm. The guard boxes were normally watered with about 7 ~ more water per unit area than the pans, as they were expected to have a slightly higher transpiration rate. Nylon and stainless steel resistance blocks (POSTLETHWAITEand TRICKETT, 1956) provided a check on their soil-moisture content and additional water was occasionally applied.

Measurement of solar radiation R'c was measured with Moll-Gorczinski solarimeters (Kipp en Zonen) and the calibrations were checked every 6 months against a sub-standard maintained at the N.I.A.E. They were always set up with the same orientation o f the rectangular thermopile, so that successive readings were subject to similar azimuth errors. They were connected to magnetic amplifiers (Electro-Methods Ltd.), calibrated at intervals of a fortnight or less, and electrolytic meters (Siemens Schuckert), all housed in a pit to minimize temperature fluctuations. A solarimeter measuring Rc was on the roof of a building 6 m high and 50 m from the glasshouses. The solarimeter inside each glasshouse had its shade ring reduced to a diameter o f 14 cm and was supported at foliage level above an empty, 50 cm long, tube of the same diameter, sunk into the soil. Outdoor wet- and dry-bulb temperatures These were measured in a Stevenson screen about 300 m from the experimental glasshouses, as part of the routine observations at the Silsoe agrometeorological station.

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RESULTS

Apart from a break in August 1959, nearly complete records were obtained from May 1959 to August 1960 (Table I). From September to December 1959 both machines were used simultaneously, with the plant arrays maintained as nearly similar as possible, and the standard deviation (s) and coefficient of variation (V) of E' and R'c for these months have been calculated. It is possible that real differences in R'c due to mutual shading and reflections between the houses may have contributed to V. Real differences in E' may also have occurred because of this or because of wind affecting one house more than the other. The maximum monthly mean daily value of E' (July 1959) was 27 times as great as the minimum (December 1959) but the ratio of the corresponding values of R'c was only 14. Thus E'/R'c varied greatly with time of year, but within any month it remained relatively constant and the coefficient of correlation between E' and R'c exceeded 0.9 in all but three cases. Low values of this coefficient were associated with condensation occurring on the metal sides and base of the weighing machine pan. The estimated maximum weight of condensed water which remained on the sides of the pan without running off was comparable with the daily transpiration in December, so that the maximum error in the monthly mean value of E' was only 2 or 3 %. Thermal insulation of the pan would probably have greatly reduced this error. The seasonal variation was associated with changes both in the slope of the line of regression of E' on R'c and in the intercept on the E' axis, which ranged from large negative values in some summer months to near zero in winter. The amount o f energy supplied by the electrical heating system to combat frost has been divided by the total planted area and converted to evaporating power in mm o f water. It was sometimes

une

U°.o o o

/

~OMor'~h ,

1

2

,

3 R~- ( r a m / d a y )

,

,

4

5

Fig.1. Relation between monthly mean daily transpiration, E', and the corresponding quantity, R'c, of solar radiation falling on a horizontal surface at the top of the foliage.

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SEASONAL VARIATION IN TRANSPIRATION OF GLASSHOUSE PLANTS

193

large (greater than 2 R'c in January 1960) and might have been expected to affect the intercept on the E' axis, but the results show no evidence of this. When monthly mean daily values of E' were plotted against corresponding values of R'c (Fig. 1), the results obtained in the spring fell below the regression line while the autumn values were above it. E' thus appeared likely to depend not only on R'c, but also on the properties of the outside air and if a correlation with one of the routine meteorological observations could be established this would have practical value. The properties considered were T,, the mean of the daily maximum and minimum temperatures; Tm.... the daily maximum temperature and e,,-~d the mean vapour-pressure deficit, e,, was taken as the saturation vapour pressure at the mean temperature and ed, the actual vapour pressure of the air, was found from the wetand dry-bulb temperatures read at 09h00 G.M.T. The partial correlations between daily values of E' and T,, with R'c held constant, and those between E' and ea-e,~with R'c held constant, were calculated for several summer and winter months, but they did not attain significance at a probability P = 0.05. However, when monthly values were considered (Table II) R'c and e~,-e~t had significant and independent effects on E', greater than those of T~, or Tma×.did not. The equation of regression of monthly mean daily transpiration on solar radiation and vapour-pressure deficit was: E'

, 0.38 R'c + 0.17 (e,-ed)-0.17 ram/day

(2)

DISCUSSION

The seasonal trend of E'/R'c (Fig.l) is qualitatively similar to that of E/Rc found by PRUITT (1962) for a field of perennial ryegrass at Davis, California, but he did not appear to find an effect of vapour-pressure deficit. The plants were watered more frequently than is usually possible in experiments in the open air and the weighing machines provided a direct check that the soil was always near field capacity. Stomatal closure through water stress was therefore likely to be relatively unimportant. Closure at low values of R'c would make the relation between E' and R'c non-linear, but the very high value of the correlation coefficient (Table II) indicates that any such non-linearity was negligible. It therefore appears that the effect of stomatal resistance was either very small or constant. The temperatures of the plants and the air in a glasshouse are generally different from that of the outside air, so it is hardly surprising that little dependence of E' on To could be found. As ventilation was adjusted manually it is remarkable that, on a monthly mean basis, the effect of vapour-pressure deficit was adequately defined by ec,-ed measured in the outside air. With the ventilators closed and with condensation sealing the glass overlaps, the rate of air change was probably similar to that

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TABLE II MONTHLY MEAN DAILY VALUES OF TRANSPIRAT[ON~ Er~ AND THE MEASURED ENVIRONMENTAL FACTORS

Month

House

No. of R' C observations (mm )

78.5 /

RC (mm )

58.5

Light transmission of

E" (ram)

E'/R'c

R'c/Rc (74,)

1959 May June July

east east east

20 30 24

4.14 4.44 4.59

7.62 7.74 7.94

54 57 58

1.70 2.25 2.52

0.41 0.51 0.55

Sept. Sept. Oct. Oct. Nov. Nov. Dec. Dec.

east west east west east west east west

30 30 31 31 30 30 3l 31

2.66 2.68 1.49 1.51 0.71 0.74 0.34 0.32

4.69 4.69 2.76 2.76 1.21 1.21 0.65 0.65

57 57 54 55 59 61 52 49

1.28 1.27 0.70 0.73 0.19 0.20 0.09 0.10

0.48 0.47 0.47 0.48 0.27 0.28 0.26 0.31

1960 Jan. Feb. Mar. Apr. May June July Aug.

west west west west west west west west

31 29 31 30 31 29 30 22

0.50 1.05 1.41 2.91 3.75 4.76 3.63 3.48

0.86 1.96 2.56 5.12 6.62 8.44 6.25 5.70

58 54 55 57 57 56 58 61

0.13 0.31 0.49 1.14 1.70 2.24 1.68 1.68

0.26 0.29 0.35 0.39 0.45 0.47 0.46 0.48

S

v(%)

0.02 1.2

0.01 2.1

found in commercial glasshouses under the same conditions, i.e., less than one volume per hour. In the absence of condensation, the experimental houses would normally have been less airtight than many commercial ones, but during the present work the polyethylene film will have reduced the rates o f air change to more typical values. With the ventilators open, the rate o f air change varies very widely between different designs o f commercial glasshouse a n d depends greatly o n the weather a n d the exposure of the house, but it appears that in the present experiments variations in ventilation rate were either u n i m p o r t a n t or were strongly correlated with R ' c a n d it is therefore likely that eq.2 will hold g o o d in other glasshouses with sufficient accuracy for calculating crop p l a n t water requirements. F o r this purpose, an estimate o f the water loss may be required for a period o f a b o u t a week, a n d m e a s u r e m e n t s o f ca-ca, together with an estimate o f R ' c based on a m e a s u r e m e n t o f Rc, could be obtained by a glasshouse n u r s e r y m a n from his local meteorological station and used to estimate E' more accurately t h a n has been hitherto possible. Agr. Meteorol., 3 ('1966) 187-196

195

SEASONALVARIATIONIN TRANSPIRATIONOF GLASSHOUSEPLANTS TABLE II (continued)

'oejficient of 7rrelation between "and R'C

Slope of line of regression of E" on R' C

Intercept on E' axis (ram)

Mean of max. and min. temperatures of outside air T a

Vapour-pressure deficit in outside air ea-ed (mm Hg )

(°c)

Heat added electrically (ram) (cal./cn/'-~ ~ s8.5

•990 •967 •937

0.48 0.53 0.62

-0.27 -0.08 -0.34

11.8 15.1 17.5

2.5 3.9 4.6

.933 •918 ~.957 b.962 ~.948 ~.909 ~.851 ~.827

0.50 0.46 0.54 0.55 0.26 0.29 0.23 0.26

-0.04 +0.03 -0.11 -0.11 +0.01 -43.01 +0.01 +0.02

14.7 14.7 12.3 12.3 6.6 6.6 5.9 5.9

2.5 2.5 1.5 1.5 0.8 0.8 0.9 0.9

0.40 0.41 0.16 0.20

1.870 1.940 L984 ).986 ).964 ).905 ).972 ).968

0.28 0.32 0.39 0.47 0.47 0.53 0.53 0.58

-0.01 -0.03 -0.06 -0.22 -0.06 -0.26 -0.25 -0.33

3.9 4.2 6.0 8.8 12.7 15.8 15.2 14.8

0.5 0.8 0.2 0.7 3.1 3.8 2.9 2.4

1.05 1.27 0.23 -

/

It appears that little oi none of the energy supplied by the heating system was used for transpiration. This agrees with the result obtained with lettuces by MORRIS et al. (1953) and may be accounted for by the fact that the heaters were below the level o f the plants and were in use mainly at night when the s t o m a t a were likely to be closed. I f a glasshouse is heated during the day and the plants are so arranged that r ad i an t heat falls on them, there m a y be some increase in transpiration, but the quantity supplied to m o s t glasshouse crops during the day is rather small c o m p a r e d with the solar r a d i a t i o n and n o t m o r e than h a l f is likely to be in the f o r m o f r ad i an t heat. T h e use o f " d a y pipe h e a t " in a t o m a t o crop has been f o u n d to have only a small effect on the water r e q u i r e m e n t o f the plants (LAKE et al., 1963). T h e i m p r o v e m e n t in g u a r d i n g obtained with the new weighing machines can be assessed, as H e l x i n e solierolii was g r o w n for a short time in the layout used by MORRIS et al. (1953) and the value o f E ' / R ' c for N o v e m b e r 1956 was 0.40 c o m p a r e d with only 0.27 for N o v e m b e r 1959 in the present experiments. Th e r at i o o f planted

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j.v. LAKEet al.

area to total floor area was n e a r l y the s a m e in b o t h cases, b u t the p a n o f the first w e i g h i n g m a c h i n e was o n l y 48 c m wide a n d 127 c m l o n g a n d there were n o g u a r d p l a n t s a l o n g o n e l o n g edge. T h e p o l y e t h y l e n e l i n i n g s m i g h t have b e e n expected to r e d u c e s o m e w h a t the t r a n s m i s s i o n o f solar r a d i a t i o n by the glasshouses, so p r o d u c i n g values o f R ' c lower t h a n those f o u n d in typical c o m m e r c i a l houses. H o w e v e r , the m e a s u r e d values o f

R'c varied b e t w e e n 49 a n d 61 ~ o f Rc ( T a b l e !), a r a n g e similar to that (48 65 ~o) in 1/'lllth acre n o r t h - s o u t h v i n e r y houses used c o m m e r c i a l l y for t o m a t o g r o w i n g

(EDWARDS, 1963).

ACKNOWLEDGEMENTS

Thanks are due to Dr. A. G. Erith, formerly of the Agricultural Botany Department of Reading University, for suggesting the use of Helxine solierolii and to L. G. Morris for many helpful discussions.

REFERENCES EDWARDS,R. I., 1963. Transmission of solar radiation by glasshouses. Exptl. Hort., 1963 (9) : 1-8. HUDSON, J. P., 1960. A sensitive evapotranspiration gauge. Bull. Res. Council Israel, Sect. D, 8 : 263-272. JONES, D. A. G. and ROTHWELL,J. B., 1964. This evaporimeter is cheap to make and effective in use. Grower, 62 : 951-952. LAKE, J. V., BOWMAN,G. E. and MORRIS,L. G., 1963. The use of "day pipe heat" in tomato growing. Exptl. Hort., 1963 (8) : 1-11. LAWRENCE,W. J. C. and NEWELL, J., 1939. Seed and Pottin7 Composts. Allen and Unwin, London, 166 pp. MORRIS, L. G., 1959. A recording weighing machine for the measurement of evapotranspiration and dewfall. J. A~r. En(. Res., 4 (2) : 161-173. MORRIS, L. G. and LAKE, J. V., 1962. The water loss from plants and soil under glass. In: J.-C. GARNAUD(Editor), Advances in Horticultural Science and its Applications. Pergamon, Oxford, 3 : 323-328. MORRIS, L. G., NEALE, F. E. and POSTLETHWAITE,J. D., 1957. The transpiration of glasshouse crops and its relationship to the incoming solar radiation. J. A,~r. En~. Res., 2 (2) : I I 1-122. MORRIS, L. G., POSTLETHWAITE,J. n . , EDWARDS, R. I. and NEALE, F. E., 1953. The dependence of water requirements of glasshouse crops upon the total incoming solar radiation. Nat. Inst. A~r. En~., Tech. Mere., 86 : 16 pp. NIELSEN, B. F. and CHRISTENSEN,S. A., 1960. Evaporation in glasshouses for determining the water requirements of the plants. Horticultura, 14 (12) : 207-221. PENMAN, H. L., 1948. Natural evaporation from open water, bare soil and grass. Proc. Roy. Soc. (London), Set. A, 193 : 120-145. POSTLETHWA1TE,J. D. and TRICKETT,E. S., 1956. The measurement of soil moisture. J. A(r. En~. Res., I : 1-8. PRUITI, W. O., 1962. Diurnal and seasonal variations in the relationship between evapotranspiration and radiation. Ann. Rept. Invest. Energy and Mass Transfers Near the Ground, 2ml, Univ. Cahf. --U.S. Army Tech. Program, Contract No. DA-36-039-SC-80334 : 45 pp. ROTHWELL,J. B. and JONES, D. A. G., 1961. The water requirements of tomatoes in relation to solar radiation. Exptl. Hort., 1961 (5) : 25-30.

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