The radiation balance of a glasshouse rose crop

Agricultural Meteorology, 11 (1973) 385-404 © Elsevier Scientific Publishing Company, Amsterdam-Printed in The Netherlands

THE RADIATION BALANCE OF A GLASSHOUSE ROSE CROP* G. STANHILL, M. FUCHS, J. BAKKER** and S. MORESHET

Division of Agricultural Meteorology, Agricultural Research Organization, Volcani Center, Bet Dagan (lsrael) (Accepted for publication January 25, 1973)

ABSTRACT Stanhill, G., Fuchs, M., Bakker, J. and Moreshet, S., 1973. The radiation balance of a glasshouse rose crop. Agric. Meteorol., 11: 385-404. The radiation-balance components of a glasshouse rose crop were measured on five days representing successive stages of crop development from planting to a fully developed flowering canopy. The glasshouse, a heated, aluminum structure glazed with diffusing glass and orientated N - S , transmitted between one-half and two-thirds of the incident global radiation with an unchanged near-infrared fraction. There was considerable spatial and diurnal variation in the fraction transmitted, most of which could be explained by reference to the angle of incidence to the roof. 20% of the solar radiation reaching the mature canopy was reflected, compared with 28% for individual leaves. Reflection in the near infrared range was twice as great for the canopy. 30% of the incident global radiation was transmitted to the floor of the glasshouse below the mature canopy, largely via the pathway between the rose beds. The measured absorptivity agreed to within 10% with the value calculated from canopy gap frequency assessed by hemispherical photographs and the calculations showed the importance of decreasing the diffuse component and the path space in increasing canopy absorption. 4% of the photosynthetically active radiation absorbed by the canopy was fixed in dry matter production. The transpiration flux for the mature canopy, computed from measurements of water concentration difference and diffusive resistance of the leaves of the upper canopy, agreed with the values derived from soil water content measurements. The latent heat equivalent of the transpiration flux exceeded that of short wave absorption, the difference being satisfied by a convective heat flux independently demonstrated by measurements of a temperature inversion and downward flux of net terrestrial radiation during the day. Close agreement between the net radiation flux above the canopy and its transpiration flux suggests that the convective heat flux was derived from that fraction of the radiation flux not directly absorbed by the canopy.

* Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel. 1972 Series, No.2203-E. ** Present address: Agricultural University, Wageningen, The Netherlands.

386

G. STANHILLET AL.

I NTRODUCTION Successful crop production in heated greenhouses aims at ensuring that the flux density of photosynthetically active solar radiation is the limiting growth factor and the high winter insolation in Israel is presumably the main climatic reason for its rapidly expanding greenhouse industry. The main crop grown is roses for winter flower export to Europe and approximately half of the 85 ha area currently devoted to this crop is planted with the variety Baccara. Unpublished local data support the conclusion of Post and Howland (1946) that the rate of rose flower production in heated greenhouses is controlled by the rate of insolation and the data show that, within the six winter months, there is an approximately linear relationship between these two rates: the production inside the greenhouse of each rose flower requires 3.2 Mcall. solar energy outside the greenhouse. The economic significance of this relationship is enhanced by the inverse relationship between the price of rose flowers in Europe and the radiation flux density. The investigation reported was undertaken to elucidate the factors controlling the radiation climate of this crop and, in particular, the interaction between the radiative characteristics of the greenhouse and the rose canopy. SITE, RADIATIONBALANCEAND CROP MEASUREMENTS Site

The measurements were made at the Glasshouse Research Center at Bet Dagan in the central coastal plain of Israel in an aluminum structured greenhouse manufactured by Frampton-Ferguson and glazed with diffusing glass. The house was orientated N - S and was l 5 m in length and 12.5 m wide with a height of 3 in to the eaves and 6 m to the ridge. Heating was by hot water circulated through 4- and 6-cm diameter pipes at soil level. Ventilation was provided automatically when the air temperature within the greenhouse exceeded 28°C. Outside air was cooled and humidified at the point of entry with a water spray and then distributed throughout the length of the house by two perforated tubes of transparent plastic 60 cm in diameter. Irrigation was applied every three days through a permanent line of low-pressure sprinklers set at soil level in the center of each bed. A vertical cross section of the greenhouse is presented to scale in Fig.l, showing the position of beds, canopy and measurement instruments. Rose plants, var. Baccara on R. indica rootstock, were planted during the third week of February 1971 at a distance of 30 cm in the row, with three rows in each of the six, 1-m wide beds. The beds, covering 45% of the greenhouse floor area, were raised 30 cm above ash-covered paths and were filled to a depth of 50 cm with a mixture of light sandy loam and organic matter.

RADIATION BALANCE OF A GLASSHOUSE ROSE CROP

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Fig. 1. Vertical cr~ss section of experimental greenhouse, to scale (width 12.5 m, length 15 m, height to ridge, 6 m). Position of heating pipes indicated by open circles. Key to position of instruments (Jan. 5, 1972): 1 = Eppley precision spectral pyranometer, K$,~; 2 - 6 = Kipp pyranometers, K,~; 7 = Kipp pyranometer on reversing stand, KS; 8 = as above, Kt ; 9 = as above plus hemispherical RG8 filter, KIR$; 10 = as above, KIRt; 11 = Swissteco linear net pyranometer, K*; 12 = Eppley pyrgeomete: LS; 13 = Middleton net pyrradiometer on reversing stand, Q*; 14 = perforated, plastic ventilating sleeve; 15 = flower minus air temperature, T R - TA; 16 = Assman psychrometer, TA, TW; l 7 = Barnes IT-3 infrared thermometer, 3° field of view, Tc.

R a d i a t i o n balance m e a s u r e m e n t s

Radiation balance measurements were made at the different stages o f canopy d e v e l o p m e n t listed in Table 1. The location o f the radiation sensors is indicated in Fig.1. Solar radiation balance was measured with Kipp solarimeters calibrated against an Eppley precision spectral p y r a n o m e t e r maintained as a secondary standard. Kipp solarimeters whose outer glass domes were replaced by hemispherical RG8 glass filters, were used to measure the energy flux density of the solar spectrum b e t w e e n 690 and 2,800 nm. The incoming solar radiation K$S was measured above the greenhouse roof. Between the r o o f and the canopy, measurements included d o w n w a r d solar KS and d o w n w a r d solar infrared KirS, upward solar K t and upward solar radiation infrared Kirt. In addition, the net radiation Q* was determined with a F u n k net p y r r a d i o m e t e r . In all cases, the field o f view of the upward-facing radiometers was unobstructed by vegetation, while the d o w n w a r d facing radiometers were sited so that 90% o f the upward signal was obtained from a 7-m 2 area of foliage and p a t h w a y . Every 15 min the position of radiometers measuring KS, K t and Q* was inverted through 180 ° to minimize instrumental bias.

388

G. STANHILL ET AL.

TABLE I Radiation balance measurements in a greenhouse, Bet Dagan Date

1971 Feb. 15

Crop stage

prior to planting

Canopy height (cm)

Noon solar elevation (degrees)

Global radiation (cal. cm-2 day-~) above atmosphere

glasshouse

canopy

0

46

596

381

206

54

81

972

534

363

open canopy

109

80

966

704

415

Sept. 23-24 first flowers, almost closed canopy

141

59

747

382

195

232

35

455

254

135

June 7-8

15 weeks after planting, very open canopy

July 7 - 8

1972 Jan. 4 - 5

middle of flowering flush, fully grown canopy

Net short wave radiation below the canopy, K*, was measured with a recently calibrated, 3-m long linear strip net pyranometer. The output from the radiometers were scanned within 3 min at a frequency of 4 h -~ with a clock-controlled stepping switch which fed the signals to a single strip chart recorder. The measurements of K~4, and K~ were fed to integrating millivoltmeters read at 15-min intervals. The accuracy of both measuring systems was equivalent to 0.01 cal. cm '2 rain -1.

Crop measurements

Immediately after the final set of radiation balance measurements which were made above a fully developed canopy during a flowering flush, hemispherical photographs of the roof were taken from each of the six above-canopy pyranometer sites with a Spiratone 180 ° fisheye lens o f 7-ram focal length. Thirteen hemispherical photographs of the canopy were taken from the soil surface across approximately the same transect occupied by the below-canopy linear net pyranometer (instrument no.11, Fig. 1). Typical photographs of the canopy viewed from beneath the center of the bed and from the center of the pathway are presented in Fig.2A and B, respectively. The canopy distribution was analyzed from the thirteen hemispherical photographs by the method recommended by Anderson (1971) and the results for both under-canopy and pathway photographs are presented in Fig.3 as the mean probability that direct solar radiation at a given angle of solar elevation will be intercepted by canopy elements.

RADIATION BALANCE OF A GLASSHOUSE ROSE CROP

389

Fig.2. Hemispherical photograph of rose crop canopy. (Bet Dagan, Jan. 5, 1972). A. Beneath centre of bed; and B. Center of path.

390

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Fig.3. Probability of rose canopy intercepting direct radiation as a function of solar elevation. (Bet Dagan, Jan. 5, 1972.) * = from mean of 13 hemispherical photographs; • = from vertical projection of canopy outline; o = from mean of 7 hemispherical photographs under canopy; a = from mean of 6 hemispherical photographs across pathway.

2 m 2 of canopy immediately above this transect were then harvested in seven depth increments and separated into green and non-green leaves, stems and flowers. In addition to fresh and dry weight, the green leaf area was determined from the area-to-weight ratio of leaf disc samples. The green stem surface area was calculated as a cylinder from measurement of the total stem length and the diameter of sample stems. The chlorophyll a + b content of 2-m length samples of the outer tissue of green stem was determined by a spectrophotometric method (Bruinsma, 1963). The chlorophyll content of the green leaf tissue was measured in the same way on leaf disc samples. The total leaf area index expressed on a unit soil area basis was 6.14 and the stem area was 2.76. The total dry matter content of the harvested canopy was 1.56 kg m-2; 81% of the dry matter was in the stems, 17% in the leaves and 2% in the flowers. The dry matter content of the root system was 0.16 kg m -2, The canopy contained 3.11 kg m -2 of water, 74% of which was in the stems, 23% in the leaves and 3% in the flowers. The water content of the roots was 0.34 kg m -2. The total chlorophyll content of the leaves was 2.35 g m -2 with 0.51 g m -2 chlorophyll in the outer layers of the stem tissue. The vertical distribution of these parameters is presented in Fig.4. The spectral (diffuse and specular) reflectivity r, and the transmissivity t, of the upper surfaces of individual Baccara rose leaves were measured with a Beckman DK-2 recording spectrophotometer in March 1972. Samples of very young red leaves, young green leaves, and mature dark green leaves, were examined. The diffusive resistance to transpiration (R L, sec cm -1) of the lower surface of the leaves (the upper leaf surface in this species has no stomata and shows very high resistance values throughout the day) was measured from dawn till dusk on Jan. 5, 1972) with a condensation porometer (Moreshet and Yocum, 1972), Ten replicate measurements were

RADIATION BALANCE OF A GLASSHOUSE ROSE CROP

39

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Fig.4. Verical distribution of leaf and stem area, dry matter, water and chlorophyll a + b densities of rose canopy. (Bet Dagan, Jan. 6, 1972.) made at half-hourly intervals on leaves selected at random within the upper 70 cm o f the canopy. The surface temperature of the canopy was recorded continuously on a strip chart potentiometric recorder using a Barnes IT-3 infrared thermometer calibrated as described by Fuchs and Tanner (1966). The position of the sampling head (no. 17, see Fig.l) was such that its 3 ° field of view included approximately 80 cm 2 canopy surface. The temperature difference between a rose bloom and the surrounding air was recorded at a frequency of 4 h -~ with a differential thermocouple, the output of which was amplified by a chopper stabilized DC amplifier. One junction was within the center of a young flower, 1.35 cm in diameter and 2.0 cm high, which was freely exposed at the top of the canopy; the second junction, shielded from radiation, was freely exposed in the air at the same height. From dawn till dusk o f the same day, the relative water content (W %) of leaves within the upper 70 cm of the canopy was determined using the technique described by Barfs and Weatherley (1962). Three replicate sets of leaf discs were taken, each consisting of ten discs, 1 cm in diameter. The soil water content was measured at dawn and dusk from auger samples taken at 18 points to the full depth of the root zone (25 cm). Five core samples were taken to establish the density of this layer, enabling soil moisture content to be expressed volumetrically. Air temperature and humidity were measured at 15-rain intervals with an aspirated and radiation-shielded wet and dry bulb thermistor system, accurate to within 0.1°C. The sensors were situated just above the top of the canopy. RESULTS AND DISCUSSION

Transmission of the greenhouse The fraction of solar radiation transmitted into a greenhouse is a complex function of the sun's position in the sky, and the radiative characteristics of the greenhouse

392

G. STANHILL ET AL

structure and contents. A simplified method of calculating transmissivity has been presented by Smith and Kingham (1971), while Morris (1972)has discussed some of the difficulties in measuring this quantity caused by the formidable sampling problems. These were enhanced in the measurements described herein by the two large ventilating ducts between the pyranometers and the roof. The variability in time and space is illustrated in Fig.5, which presents diurnal transmission calculated from 15-min totals of K~$ and K~ measured on Jan. 5, 1972 at the six positions shown in Fig.1. The missing values during a midday hour were due to the malfunctioning of one of the integrators. L0

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The mean transmission coefficient throughout the day was 0.53. The spatial variation in transmission measured over 15-min periods, expressed as a standard deviation averaged 0.17. For periods of one hour, the average standard deviation was almost the same: 0.16. For totals of the five morning hours, the average transmission coefficient was 0.49 with a standard deviation of 0.12. Totals for the five afternoon hours gave a mean transmission coefficient of 0.56 with a standard deviation of 0.14. The spatial variation in transmission computed from daily totals of K$$ and K$ was very much reduced, the standard deviation being only 0.02. Thus under the conditions of measurement, six fixed points were insufficient to sample adequately the horizontal variation in the global radiation reaching the canopy across the greenhouse for periods shorter than one day, but were more than sufficient to obtain a reliable (+-2%) daily total of global radiation. The mean daily values of transmission listed in Table II are, with the exception of the last set of readings, based on two or three replicate measurements of global radiation within the greenhouse. The average transmission coefficient for all dates is 0.57, with

393

RADIATION BALANCE OF A GLASSHOUSE ROSE CROP TABLE II Transmission and spectral composition of global radiation, Bet Dagan Date

Transmission .1 of atmosphere

greenhouse

Number of pyranometers above canopy

Infrared fraction

0.64

0.54

3

0.54

-

0,55

0.68

2 *3

0.50

-

July 7 - 8

0.73

0.59

2

0.53

0.55

Sept. 2 3 - 2 4

0.51

0.51

3

0.52

0.52

0.56

0.53

6

0.51

0.53

in greenhouse

outside* 2

1971 Febr. 15 June7

8

1972 Jan. 4 - 5

,1 Ratios of daily totals; ,2 estimated by the equation of Yefimova (1971); ,3 including a 3-m long linear pyranometer. 90

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Fig.6. Transmissivity of greenhouse as a function of angle of incidence to roof. • = measured values at Bet Dagan (Jan. 5, 1972); each point is mean of 11 measurements, each based on 15-min totals measured under clear sun conditions and free from shade of ventilating sleeves, o = measured values from southeast England; each point is mean of two measurements for 60-min totals under clear sky conditions (in Morris, 1972, after Edwards and Lake). Solid line = calculated values for aluminum glazing bar array (from Morris, 1972) with horticultural glass (from Smith and Kingham, 1971).

some indication of the expected increased values under conditions of high sun and clear skies (Iwakari, 1969). The transmission coefficients observed are about 10% less than those measured by Edwards and Lake (1965) in England with a glasshouse of similar design.

394

G. STANtllLL El AL.

The rather low values of transmission in Israel cannot be completely explained by reference to the angle of incidence of direct solar radiation to the glasshouse roof at the time of measurement. Fig.6 shows that even at the same angle of incidence, transmission measured at Bet Dagan when the solar path was unobscured was generally less than that reported by Edwards and Lake (1965) for hourly periods measured in a similar glasshouse in southeast England. Both sets of measurements fall below the theoretical line calculated for an array of aluminum glazing bars (Morris, 1972) and horticultural glass (Smith and Kingham, 1971). Much of the spatial variation in KS and some of the low transmission can be attributed to the plastic ventilating sleeves. For a given angle of incidence, pyranometers shaded from the sun by these sleeves showed an average reduction of 40% in KS compared with readings from unshaded pyranometers. For the greenhouse as a whole, the reduction in KS caused by the ventilating sleeves was calculated to be 6%. Mean daily values of the infrared (>690 nm) fraction of KS measured inside the greenhouse are listed in Table II together with calculated fractions outside the greenhouse. The small reduction in the infrared fraction within the glasshouse is of the same magnitude as the relative decrease of the infrared transmissivity of horticultural glass. The conclusion that the spectral composition of solar radiation within the greenhouse is similar to that in the open may also be deduced from measurements of illumination and global radiation at Bet Dagan, reported by Zamir, Feigin and Dasberg (1972), which show values of luminous efficiency (= K lux/cal, cm -2 rain -1) to be similar to within +-5%, in the open and inside four different types of greenhouse at four different seasons.

Absorption of solar radiation by the crop canopy The absorption of the crop canopy, A -- [(KS - K t ) - K , ] K S -1 , w a s calculated from measurements of the net short wave flux density K . , over a 3-m long sample of path and soil surface beneath the rose canopy, which are available for the two later stages of canopy development listed in Table I. Mean daily values of A were 0.454 on Sept. 22-24, 1971, when the canopy had attained half its final height, and 0.51 s on Jan. 5, 1972, when canopy development was complete. On the last occasion the measured value of A can be compared with the value calculated from canopy distribution as assessed from hemispherical photographs (Fig.2). If the absorptivity of the individual canopy elements and of the soil is assumed to be 1.00, and KS is isotropically diffuse, then: 2~

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RADIATION BALANCEOF A GLASSHOUSE ROSE CROP

395

canopy element, i.e., the values plotted on the center line of Fig.3 andq~ is the azimuthal angle. The value of A computed from the thirteen hemispherical photographs for isotropically diffuse radiation was 0.423. If, by constrast, K4 consisted solely of direct, sunbeam radiation, then A I is the average value of p weighted by 'sin h along the diurnal solar track. The value of AI computed for purely direct radiation from the hemispherical canopy photographs was 0.63 sMeasurements made at the Central Meteorological Institute on Jan. 5, 1972 some 0.5 km west of the greenhouse gave the diffuse and direct components of K$$ as 0.32 and 0.68, respectively. Weighting the respective values ofA D and AI accordingly gives a calculated mean daily crop absorptivity of 0.567 compared with the measured value of 0.51 s. One reason for this 10% overestimate is the assumption of complete absorption of the intercepted radiation, i.e., the neglect of radiation transmitted through and reflected by the crop canopy. The fraction of K4 transmitted through the foliage was calculated from measured values of leaf transmissivity and leaf area index assuming leaf distribution to be isotropic and leaf reflection zero. The calculated canopy transmissivity was negligible (< 0.01 K4) for both global and infrared radiation. The fraction of K* reflected from the canopy was calculated with a simplified model from measured values of leaf reflectivity and hemispheric canopy cover assuming isotropic leaf distribution and zero canopy transmissivity and intra-canopy scattering. The calculated flux of reflected radiation agreed with the measured values but led to a value of crop absorptivity 9% lower than the measured value. Another reason for the discrepancy between the measured and calculated values of A is the assumption that the fraction of diffuse radiation within the greenhouse is the same as that outside. That this is most unlikely to be the case follows from the low transmission of the greenhouse previously discussed and has been demonstrated directly by Bonhomme (1969). The effect of seasonal changes in the sun's path on the canopy absorption of direct beam radiation was calculated assuming the canopy structure to be unchanged. Seasonal changes in mean daily values of Ai were less than 10%; the highest value, 0.691, was calculated for the summer solstice and the lowest, 0.616, at the equinoxes. As canopy absorption of diffuse radiation is much less than for direct radiation and is independent of the sun's path, it may be concluded that the effect of seasonal changes and greenhouse structure on the diffuse to direct components of K~ are the most important non-plant factors determining solar absorption by the crop canopy.

Reflection from the surface cover Mean daily values of reflection coefficient, Kf/K4, are listed in Table Ill. The low values of the unplanted house may be attributed to the black ash paths which then composed half of the ground area viewed by the pyranometers. At the later stages of

396

G. STANHILL ET AL.

TABLE 11I Short and long wave radiation balance components of greenhouse rose crop, Bet Dagan Date

Reflectivity coefficient ratio of daily totals

Net radiation balance (cal. cm -2)

Net long wave flux (cal, cm 2 min ~)

short wave

infrared

daytime

night tinre

daytime

nighttime

1971 Feb. 15

0.07

0.19

June 7 - 8

0.14

0.26

July 7 - 8

0.|7

0.23

339

-24

-0.01

-0.04

0.18

0.22

137

0

-0.01

0.00

0.20

0.41

109

29

0.02

0.05

Sept. 23

24

1972 Jan. 4 5

FLUX DENSITY OF GLOBAL RADIATION,% 25

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Fig.7. Spectral reflectivity and transmissivity of upper surfaces of rose leaves, var. Baccara. (Bet Dagan, March 24, 1972.) . . . . very young, red leaf (0.014 mg cm -2 chlorophyll a + b); = young, light green leaf (0.037 mg cm 2 chlorophyll a + b); - = mature, dark green leaf (0.039 mg cm ~ chlorophyll a + b). Note: Wavelength scale normalized for spectral distribution of K$ $ : thus, the area under any given wavelength interval is proportional to the flux density of the reflectance or transmittance. .

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RADIATION BALANCE OF A GLASSHOUSE ROSE CROP

397

canopy development, reflection is comparable to albedo values reported for other crops of similar height and leaf area, within the range of values listed in Table Ili. The mean solar reflectivity of individual rose leaves calculated for the spectral composition of global radiation under air mass conditions of Jan. 5, 1972 (List, 1966), and the measured spectral reflectivity of three leaves (Fig.7), averaged 0.28s for the entire solar spectrum and 0.443 for infrared radiation (>700 nm). Differences between the three leaf types were small; for the total solar spectrum reflectivity was 0.29 for both the very young and young leaves and 0.27 for the mature leaf: for the infrared spectrum reflectivities were 0.45 and 0.44, respectively. No marked diurnal trends were discernible in the reflection coefficient for either total or infrared radiation, most of the -+ 10% variation during the day being associated with sudden changes in K,k. Net and long wave radiation balance o f the canopy It has been known since 1909 that the flux density of net terrestrial radiation L* within a glasshouse is very small (Businger, 1963) and hence the net radiation balance Q* is essentially equal to the net short wave flux, K* = (K4- - Kt). The measurements at Bet Dagan listed in Table 1II support this conclusion. The negative values of L* measured during the night hours in midsummer were larger than those reported from The Netherlands by Scholte Ubing ( 1 9 6 1 ) , - 0 . 0 0 5 to 0.015 cal. cm -2 rain -I , but similar to those measured in Japan by Iwakari (1969), - 0 . 0 3 cal. cm -2 rain -~. Positive fluxes of L* of the same magnitude were measured in midwinter throughout the day (Fig.8); they do not appear to have been previously reported in the literature.

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398

G . S T A N H I L L ET A L

Canopy and flower temperatures The canopy temperature of 80 cm 2 surface area consisting of the upper foliage in the center of one of the middle beds is shown in Fig.9 as measured by infrared thermometry. The temperature of the upper canopy as sensed by the infrared thermometer was consistently 2.0°C less than that of the air within the greenhouse throughout the night. During the hours of daylight the average canopy temperature was also 2.0°C below that of the air, with some fluctuations in the difference, 'although it can be seen from Fig.9 45

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Fig.9. Temperature of air inside and outside greenhouseand of upper surface of canopy. (Bet Dagan, J a n . 5, 1972.) - . - = air t e m p e r a t u r e o u t s i d e g r e e n h o u s e m e a s u r e d in s t a n d a r d t h e r m o m e t e r s c r e e n at 2 m; = air t e m p e r a t u r e inside g r e e n h o u s e m e a s u r e d b y a s p i r a t e d p s y c h r o m e t e r ; ,~ = c a n o p y s u r f a c e t e m p e r a t u r e m e a s u r e d by i n f r a r e d t h e r m o m e t e r . H a t c h e d c o l u m n s i n d i c a t e operation of forced ventilation system with evaporative cooling.

that fluctuations in the air temperature within the greenhouse induced by operation of the forced ventilation system were closely paralleled by similar fluctuations in the surface temperature of the upper canopy. The temperature of the rose flower was within + 0.5°C of the air temperature throughout the night hours. During the morning and noon hours, the flower temperature averaged 2.0°C less than that of the surrounding air, i.e., the flower temperature was similar to that of the upper le~,ves of the canopy. During this period the temperature difference between the flower and the air varied between - 5.0 ° and 0.O°C, the minimum being noted during periods of forced ventilation. At 14h00, when the forced ventilation was stopped, the difference changed sign and by late afternoon the flower temperature exceeded that of the surrounding air by 2.0°C; by sunset this difference had become much smaller.

399

RADIATION BALANCE OF A GLASSHOUSE ROSE CROP

Water content, flux and resistance in air, canopy and soil The diurnal course of air water content on Jan. 5, 1972 is illustrated in Fig.10; values for the upper leaves of the canopy were calculated from the infrared thermometer readings, assuming the air within the leaves to be fully saturated. The water content of the air within the greenhouse was considerably more than that of the outside air, especially during the daylight hours. Operation of the forced ventilation system with its water spray humidifier ifi.troduced considerable short-term fluctuations in the water content of the inside air which reached 10 mbar vapour pressure at noon

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,

21

,

24

,i

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,

03 06 09 HOURS (IS.T.) '

,

12

,

15

18

Fig. l 0. Water concentration of air inside and outside greenhouse and in the upper surface of canopy. (Bet Dagan, Jan. 5, 1972.) • = concentration in air outside greenhouse; A = concentration in air inside greenhouse; o = concentration in air within upper leaves; hatched columns indicate operation of evaporative cooling with forced ventilation system.

I I I I I I I I I I I I It io

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Fig. 1 1. D i f f u s i v e resistance and relative w a t e r c o n t e n t o f leaves w i t h i n upper 70-cm layer o f rose c a n o p y . (Bet Dagan, Jan. 5, ] 9 ? 2 . ) o = mean l e a f resistance, v e r t i c a l lines represent t w o standard deviations; • = mean relative w a t e r c o n t e n t .

400

G. STANHILL ET AL.

The magnitude of the canopy to air vapour pressure difference, which averaged 5.2 mbar by day and 4.2 mbar by night, suggests a high potential rate of water loss from the canopy The diurnal curve of resistance to water vapour diffusion of the lower surfaces of leaves within the top quarter of the canopy js shown in Fig.11 together with the standard deviation of the mean values. The mean measured values o f leaf diffusive resistance (R L, sec cm -~) were fitted, by the method of least squares, to the flux density of solar radiation fK~, cal. cm -2 min-~), as measured by the pyranometer nearest to the site of resistance readings, the mean measured relative water content of the leaves (W %, also shown in Fig.l 1), and the canopy surface temperature measured by infrared thermometer (To, °C), resulting in the following linear, multiple regression equation I R E = 2.097K~ + 0 . 1 0 7 W - 0.782Tc + 11.304 The multiple regression coefficient was r = 0.936 and the standard error o f predicted values o f R L was 0.91 s e c c m -1 , nearly twice as large as the mean standard error of the measured values, which was 0.53 sec cm -~ . Analysis of variance of the regression showed that 85% o f the diurnal variation in R E could be attributed to variations in Tc, while variations in K+ and W each accounted for less than 2%. The correlation matrix showed an intercorrelation o f r = 0.76 between K~. and Tc, of r = - 0.29 for K~, and W, and of 0.41 for W and Tc. Values of R E were also related to K by the Michaelis-Menten equation in the following form: R E = (1 +

kKU~)RL rain

where k is the rate constant of the reactions (presumably the opening and closing light response o f stomatal aperture) and R E rain was taken as 3.6 sec cm -~ and 2.0 sec cm -a for the opening and closing reactions, respectively (see Fig. 11). Data during the period of morning opening fitted the above equation for values of K~. less than 0.3 cal. cm -2 min I , yielding a rate constant of k = 0.08. During the afternoon closing period, the relationship was only linear for values o f K+ less than 0.1 cal. cm -2 rain -~, yielding a rate constant of k = 0.02. The measured difference of water vapor concentration in the leaves and in the air, A×, was used with measurements of leaf resistance R E to calculate the transpiration flux, T = A× .RE -~ . The external air resistance was not known, but its neglect is unlikely to have caused an overestimate of T of more than 10%, as this additional resistance was probably less than 0.2 sec cm -~ (Monteith, 1965). The dawn to dusk total of computed values of T was 0.069 g cm -2, equivalent to 1.87 mm for the top 70 cm of leaf area (L.A.I. = 2.7) for which the values o f R L and leaf water concentration are representative. If these values are assumed to be representative for the total leaf area (L.A.I. = 6.1), then the daytime transpiration equals 4.27 ram. This figure agrees with the measured difference in the soil water content at dawn and dusk on the same day which, expressed on a soil area basis, was equivalent to the total water flux of 4.3 ram. The very low water content and hydraulic conductivity o f the

RADIATION BALANCE OF A GLASSHOUSE ROSE CROP

401

ash pathways suggests that evaporation from the path area can be neglected and that the daylight water loss expressed on a total ground surface basis was 1.93 mm. Expressed on the same area basis, the latent heat equivalent of short wave radiation absorbed by the canopy, i.e., K J, - K]" - (K,), during the same period, was 1.18 mm. The difference between the latent and radiative terms in the energy balance is the convective heat transfer from air to foliage in response to the 2.0°C inversion that was measured during the day. The origin of this convective heat flux to the canopy was evidently that fraction of the radiation flux which was not absorbed by the canopy, as can be seen from the close agreement between the measured dawn to dusk net radiation flux above the canopy Q, = 1.85 mm, and the computed and measured value of T = 1.93 mm. A close agreement between net solar radiation above a canopy and the water loss therefrom has also been reported for closed canopy crops of tomatoes growing under glasshouse conditions in England (Rothwell and Jones, 1961). The diurnal course of T also showed a good general agreement with K+ but detailed analysis for periods less than one day is hardly justified, in view of the spatial variation in the micrometeorological measurements.

Productivity and photosynthetic efficiency of the greenhouse rose crop A mean annual fresh weight production of 14.24 kg m -2 bed area has been reported for Baccara roses grown in a variety of greenhouses at Bet Dagan (Feigin et al., 1972). Assuming a relative water content similar to that measured in this study, the rate of dry matter production, expressed to a total greenhouse floor area basis, is 2.14 kg m -2 yr -1, which converted to its calorific equivalent, represents an energy flux of 0.85 Kcal. cm -2 yr -1 . This figure may be compared with the estimated flux of photosynthetically active radiation which, calculated on the basis of mean measured values of K~$, glasshouse transmissivity and infrared fraction at Bet Dagan, was 47 Kcal. cm -2 yr -~ . The photosynthetic efficiency is 1.8% when expressed on the basis of incident energy and 3.9% when expressed on the basis of absorbed energy. Both values are in the upper range of values reported in the literature for agricultural and natural plant communities on an annual basis (Phillipson, 1966; Botkin and Malone, 1968). The relatively high photosynthetic efficiency can be attributed to the high absorptivity of the canopy obtained by the continuous cropping system which maintains an almost uniformly high leaf area index throughout the year.

SUMMARY AND CONCLUSIONS A series of radiation measurements made within an aluminum-structured greenhouse glazed with diffusing glass and orientated N - S , showed the daily total flux density of

402

G. STANtIILLET AL.

solar radiation at crop canopy level to vary between one-half and two-thirds of that measured outside the greenhouse. The near infrared fraction of the solar flux within the greenhouse was within 2% of that in the open. At a given point of measurement the transmission varied widely during the day and, for any given period shorter than one day, there was also considerable spatial variation, confining the following conclusions to daily total values only. 20% of the solar radiation reaching a fully developed rose canopy was reflected, the fraction being twice as large for the nonphotosynthetic wavelengths. The soil and path surfaces of the unplanted house reflected 0.07 and 0.19 of the total and near infrared solar fluxes, respectively. Individual rose leaves reflected 0.29 of the total and 0.44 of the near infrared fluxes, showing only small effects of leaf development stage. 30% of the solar radiation reaching the top of a fully developed rose canopy in midwinter was transmitted through the canopy, ahnost entirely through gaps in the canopy or through the pathways between rose beds. Visual examination of hemispherical photographs taken under the canopy suggests that the pathway gap was the most important route for transmitted radiation. The measured transmissivity of individual rose leaves was 0.27 of the total radiation flux and 0.44 of the near infrared flux. Measurements showed that half of the incident solar flux was absorbed by the fully grown canopy of the rose crop. Canopy absorptivity was also calculated on the basis of hemispherical photographs and the results agreed with the measured values within 10%. The calculations indicate that canopy absorptivity increases with the size of the direct component of solar radiation and the horizontal homogeneity of foliage distribution. On an annual basis nearly 4% of the photosynthetically active solar radiation absorbed by the canopy was fixed in dry matter production and the high overall rate of productivit) 2.14 kg m -2 yr -1 , was attributed to the high absorption of radiation resulting from the consistently large leaf area. The radiant energy absorbed by the mature canopy provided less than two-thirds of the latent energy equivalent of transpiration, as derived from differences in soil water content measured on a typical midwinter's day. Transpiration measured in this way agreed with the flux calculated from measurements of the water concentration gradient between the air and the upper leaves of the canopy and the latter's diffusive resistance. This agreement implies a low external resistance to water diffusion and an equality in the transpiration rate throughout the canopy. The diurnal course of leaf resistance during midwinter was statistically found to be almost entirely dependent on the surface temperature of the leaves; the light response exhibited a marked diurnal asymmetry; stomatal opening in the morning being much slower than the evening closure. Most of the solar radiation which was not transmitted by the greenhouse was absorbed by the roof structure; a smaller amount, approximately 5%, was absorbed or reflected out of the greenhouse by the ventilating ducts, which also considerably

RADIATION BALANCE OF A GLASSHOUSE ROSE CROP

403

increased the spatial n o n h o m o g e n e i t y o f solar radiation at the crop surface. A significant fraction of the radiant energy absorbed by the greenhouse structure is dissipated in the f o r m o f sensible heat in the air within the greenhouse. This is evidenced by the slight net d o w n w a r d long wave radiant flux, and the t e m p e r a t u r e inversion above the canopy which was shown by direct measurements and deduced from the sign of the sensible heat flux density in the a p p r o x i m a t e energy budget.

ACKNOWLEDGMENTS We wish to thank L. G. Morris o f the F.A.O. High Value Crops Project for useful discussion and the loan o f e q u i p m e n t , and Y. Cohen for his assistance with the measurements. This research was supported by Grant FG-Is-275 o f the U.S. D e p a r t m e n t o f Agriculture. REFERENCES Anderson, M. C. 1971. Radiation and crop structure. In: Z. Sestak, J. Catsky and P. G. Jarvis (Editors), Plant Photosynthetic Production - Manual o f Methods. Junk Publ., The Hague, pp.412-466. Barrs, H. D. and Weatherley, P. E., 1962. A re-examination of the relative turgidity technique for estimating water deficits in leaves. A ust. J. Biol. Sci., 15:413-428. Bonhomme, R., 1969. Contribution fi l'~tude de la composition spectrale des rayonnements d'origine solaire fi l'air libre et sous une serre. Ann. Agron., 20:183-200. Botkin, D. B. and Malone, C. R., 1968. Efficiency of net primary production based on light intercepted during the growing season. Ecology, 49:438-444. Bruinsma, J., 1963. The quantitative analysis of chlorophylls a and b in plant extracts. Photochem. PhotobioL (Chlor. Metabol. Symp.), 2:241-250. Businger, J, A., 1963. The glasshouse (greenhouse) climate. In: W. R. Van Wijk (Editor), Physics o f Plant Environ men t. North-Holland, Amsterdam, pp. 277 - 318. Edwards, R. 1. and Lake, J. V., 1965. Transmission of solar radiation in a large span east-west glasshouse. J. Agric. Eng. Res., 10:125 - 131. Feigin, A., Dasberg, S. and Lachover, D., 1972. The suitability of natural soils for growing roses in greenhouses. In: Effects o f Environmental Conditions in Greenhouses on Rose Growth. Prelim. Rept. Agric. Res. Organ., Bet Dagan, Israel, pp.9-25. (Hebrew, with English summary) Fuchs, M. and Tanner, C. B., 1966. Infrared thermometry of vegetation. Agron J., 58:597-600. lwakari, S., 1969. Climate in a greenhouse, 2. Radiation and humidity conditions..L Agric. Meteorol. Tokyo, 24:17-24. List, R. J., 1966. Smithsonian Meteorological Tables. Smithsonian Inst., Washington, D.C., 6th revised ed., Table 148. Monteith, J. L., 1965. Evaporation and Environment. In: G. E. Fogg (Editor), The State and Movement o f Water in Living Organisms. Symp. Soc. Exp. Biol., 19, Cambridge Univ. Press, pp.205 234. Moreshet, S. and Yocum, C. S., 1972. A condensation type porometer for field use. Plant Physiol., 49:944-949. Morris, L. G., 1972. Solar radiation in greenhouses: A brief review. Isr. J. Agric. Res., 22(2):85-97. Phillipson, J., 1966. Ecological Energetics. Edward Arnold, London, 57 pp. Post, K. and Howland, J. E., 1946. The influence of nitrogen and light intensity on the growth and productivity of greenhouse roses. Proc. Am. Soc. Hortic. Sci., 47:446-450.

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Rothwell, J. B. and Jones, D. A. G., 1961. The water requirement of tomatoes in relation to solar radiation. Exp. Hortic., 5:25 30. Scholte Ubing, D. W., 1961. Short wave and net radiation under glass as compared with radiation in the open. Agron. J., 53:295-297. Smith, C. V. and Kingham, H. G., 1971. A contribution to glasshouse design. Agric. Meteorol., 8:447 468. Yefimova, N. A., 1971. Geographical distribution of the sums of photosynthetically active radiation. Soy. Geogr. Rev. Transl., 12:66-74. Zamir, N., Feigin, A. and Dasberg, S., 1972. The effect of ventilation system and covering material of the glasshouse on the microclimate and rose yields. In: Lffects of Environmental Conditions in Greenhouses on Rose Growth. Prelim. Rep. Agric. Res. Organ., Bet Dagan, Israel, p.. 27-38 (Hebrew, with English Summary)