Radiant energy and light environment of crops

Agricultural Meteorology, 14(1974) 211--225 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands RADIANT ENERGY AND LIGHT ENVIRONMENT OF CROPS*

E. T. KANEMASU and G. F. ARKIN

EvapotranspirationLaboratory, Kansas State University, Manhattan, Kansas (U.S.A.) Texas Agricultural Experiment Station, Temple, Texas (U.S.A.) (Received October 1, 1973; accepted May 20, 1974)

ABSTRACT Kanemasu, E. T. and Arkin, G. F., 1974. Radiant energy and light environment of crops. Agric. Meteorol., 14: 211--225. Components of the radiation and energy balance are described and related to the growth, physiology, and structure of crop canopies. The net radiation below the canopy is used to evaluate inter-row advection in narrow-row and wide-row sorghum. The light environment in crops is discussed with respect to the reflective properties of plant leaves and the soil surface. A simplified model to predict the intercepted photosynthetically active radiation for a sorghum canopy is presented. When the light model is applied to narrow- and wide-row sorghum, the plants in the narrow-row spacing intercepted more light and predicted an 11.6% increase in net photosynthesis over the wide-row sorghum. RADIANT ENVIRONMENT Plants require e n e r g y t o live. E n e r g y is e x c h a n g e d b e t w e e n the p l a n t and its e n v i r o n m e n t b y the processes o f w a t e r e v a p o r a t i o n , c o n d u c t i o n , convect i o n and radiation. W h e n s t u d y i n g one o f the processes, one m u s t c o n s i d e r interrelationships with all o t h e r processes to i n t e r p r e t the results p r o p e r l y . The radiative p r o p e r t i e s o f a c r o p surface are a m o n g the m o s t i m p o r t a n t p a r a m e t e r s in s t u d y i n g p l a n t g r o w t h . Plants, f r o m e m e r g e n c e to harvest, are e x p o s e d to i n c o m i n g streams o f radiation. S h o w n in Fig.1 are t w o o f the p r i m a r y streams o f i n c o m i n g r a d i a t i o n : t o t a l i n c o m i n g ( T t ) and solar ( S t ) . Their relationship is given b y : Tt =Lt

+St

(1)

w h e r e L t is t h e i n c o m i n g longwave radiation and t h e arrows represent t h e d i r e c t i o n o f the radiation stream. T t and S t can be m e a s u r e d with an all-wavelength r a d i o m e t e r and a p y r a n o m e t e r , respectively. * Contribution no.1345 from Agronomy Department, Evapotranspiration Laboratory, Kansas Agricultural Experiment Station and Texas Agricultural Experiment Station, in cooperation with USDA-ARS, Blackland Research Center, Temple, Texas.

212

2.0

A TOTAL INCOMING RADIATION o SOLAR RADIATION • NET SOLAR RADIATION

15

~ T R,~tAT

1.0 T

~[

0.5 0.0

-0.5 0000

0400

0800

1200

LOCAL TIME

1600

2000

(hours)

Fig.1. Diurnal trends in radiation flux densities on a clear summer day at Manhattan, Kansas.

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Fig.2. Deviations of measured incoming longwave radiation from the Swinbank e q u a t i o n for two consecutive days.

213 The incoming longwave, L4, can be obtained by difference (T$ - $4) or computed from an empirical equation such as suggested by Swinbank (1963): L4 = 7 . 6 2 - 10-'6 T~ (ly min-' )

(2)

where Ta is the screen height temperature in o K. However, such empirical equations can produce systematic deviations from measured values. Fig.2 shows deviations of the measured L$ from the calculated values using eq.2. Systematic deviations, such as those shown in Fig.2, are thought to originate from elevated daytime temperatures near the ground. Thus one would expect eq.2 to overestimate during daytime and underestimate at nighttime because of usual inversion conditions (Idso, 1972). Paltridge (1970) suggests that temperature should be measured at 200 to 300 m instead of at screen height. RADIANT EXCHANGE Radiation exchange of a crop largely determines the water and COs balance of the plants. The longwave exchange associated with surface energy balance is given by: L n=L4-Lt

=Rn-Sn

(3)

where Ln is the net longwave radiation; R n is the net radiation, and Sn is the net solar radiation (Sn = $4 - ~S$ ; ~ = albedo). Outgoing longwave radiation, L t , is composed o f thermal radiation emitted and reflected by the surface: L t = eaT4s + (1 - e)L~

(4)

where e is the thermal emissivity; a is the Stefan-Boltzman constant; and Ts is the surface temperature. Thermal emissivities of crop canopies have been estimated at about 0.97 (Fuchs and Tanner, 1966). The net longwave is usually a small negative number obtained by subtracting one large term from another (Fig.l); therefore, large absolute errors in Ln can result (Stanhill et al., 1968). However, for some analyses, only changes in L n are required (Gay, 1971). Monteith and Szeicz (1961) suggest that net longwave radiation is related to the thermal characteristics of the surface, and, further, that the heating coefficient, ~, would be unique index to surface heating. Gay (1971) developed a model that includes the albedo and a new longwave exchange coefficient, ~. The nondimensional coefficient, 7~, is equal to -a~ where a is the constant from the regression equation, Rn = a Sn + b, and b is another constant. The heating coefficient, fl, is developed from an assumed dependence of Ln on Rn, while X is based on a dependence of Ln on Sn. Details on the regression models are given by Gay (1971) and Idso (1971). The regression of net longwave on net solar radiation is shown in Fig.3A for July 7 and 9. The longwave exchange coefficients (7~) of - 0 . 0 8 9 (~ = 0.091) and 0.015 (/3 = - 0 . 0 1 6 ) are obtained for dry and wet soil, respectively. The slightly positive X over the wet soil suggests that net longwave remains

214 BARE

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Fig.3. A. Regression o f net longwave against net solar radiation for a w e t and dry bare soil surface. B. Trends in albedo with solar elevation in the morning (azimuth < 1 8 0 ° ) and afternoon (azimuth > 1 8 0 °).

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(ho~s)

Fig.4. Diurnal trends in net radiation, evaporation and soil heat flux densities o f w e t and dry bare soil.

215 nearly constant with increasing net solar radiation; the large negative ~ over dry soil indicates a large degree of longwave exchanged for given amounts of absorbed solar energy. It follows that the dry surface is heating because the absorbed radiation was not being dissipated as latent heat flux. ENERGY EXCHANGE Absorbed radiant energy (Rn) is dissipated as evapotranspiration (ET), soil heat flux (G), and sensible heat to the air (H) and can be expressed as: (5)

Rn= ET + H + G

Shown in Fig.4 are the energy balance c o m p o n e n t s of wet and dry bare soil. Evapotranspiration rates are determined with the weighing lysimeters; net radiometers and heat flux plates are used to measure net radiation and soil heat flux, respectively. Sensible heating o f the air is obtained by difference using eq.5. The magnitudes of the energy balance c o m p o n e n t s are consistent with the ~ values. Dry soil surface partitions most o f its energy into H and G. Evapotranspiration is the primary energy c o m p o n e n t for the wet soil surface. Daytime ratio of net radiation for dry to wet soil is 0.74; net radiation is greater over wet than dry soil because wet soil has lower albedo (Fig.3B) and surface temperature. If indeed the nondimensionai coefficient, ~, is a sensitive radiative indicat or o f a surface drying and heating, it may be useful in determining heat or water stress of a crop canopy. In 1972 a sorghum canopy ( L A I = 3) was allowed to undergo water stress for 10 days, during which time energy and radiation balance were measured. Sorghum leaf-water potential decreased f r o m - 1 2 to - 1 8 bar and plants were visibly wilted. Table I shows the energy balance terms for the first two and last t w o days of the 10
Rn

ET

H

G

1 2 9 10

410 403 336 365

329 366 408 443

59 15 -98 -104

22 22 26 26

216 canopy temperature did not elevate above air temperature. Consistent with the radiation balance, ~ values did not change significantly from day to day, and each h had a correlation coefficient of at least 0.93. Such data indicate the need to determine the energy and radiation balance concurrently to properly interpret results (Idso, 1971). Radiant energy absorbed by the canopy -- the difference between the net radiation above (Rna) and below (Rnb) the canopy -- is related to the canopy energy balance by: R n a - Rnb = T + Hp

(6)

where T is the transpirational flux and Hp is the sensible heat exchange between the air and the plants. Energy balance at the soil surface can be described by: Rnb=Es+Hs+

G

(7)

where Es is the evaporation flux from the soil; Hs is the sensible heat exchange between the soil surface and the air; and G is the soil heat flux. Eq.5 can be derived by combining eqs.6 and 7 and using the relationships E T = Es + T a n d H = H s + Hp. Tanner et al. (1960) summarized the energy exchange of canopies having low and high plant populations. When population is high and water does not limit the plant or soil surface, Rnb is less than when populations are low; consequently, Es is lower. However, the high-population canopy, because it intercepts more energy than the low, will transpire more. The low population will transmit more energy to the soil and allow greater evaporation; thus, E T will be about the same for both populations. Under conditions of low soil moisture, what happens is not so clear. When the upper soil surface layer is dry and evaporation is limited by moisture (and not energy) supply, E T approximately equals transpiration. Depending upon climatic conditions, evapotranspiration may be greater from a high population than from a low population field (Tanner et al., 1960). Some researchers (Hanks et ai., 1971; Ritchie and Burnett, 1971), finding the reverse to be true, attributed the greater E T from low population to inter-row advection. Rnb, when partitioned primarily into Hs, becomes a significant source of energy for transpiration in the lower leaves of the canopy. Figs.5 and 6 show the energy balance terms associated with eqs.6 and 7 for sorghum spaced in wide (0.91 m) and narrow (0.46 m) rows. Measurements were during the early heading stage. Net radiation (Rna and Rnb), evapotranspiration ( E T ) , and soil heat flux (G) were measured. Evaporation from the soil, Es, was assumed to be zero because of extremely low soil moisture conditions (3-bar tension at the 90 cm depth); therefore, lysimeter estimates o f E T were taken to be equal to transpiration. Hp and Hs were determined from eqs.6 and 7. Hs, a major component in H, probably contributed greatly to the transpiration rate of lower leaves, especially in the wide-row

217

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WIDE ROW SORGHUM

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..d"

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0800

I000

1200

.1400

1600

1800

2000

LOCAL TIME (hours) Fig.5. Above canopy minus below canopy net radiation (Rna - Rnb), evapotranspiration

(lysimeter), sensible heat flux between the plants and the air (Hp) and sensible heat flux between the soil and air (Hs) for wide-row sorghum.

0.8

0.6

Z >.J

NARROW ROW SORGHUM 1972

•

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0.0 Hp ~'"~ . . . . . . ~ . . . . .

-O2 -04 I 0800

I000

1200

1400

1600

1800

2000

LOCAL TIME (hours)

Fig.6. Above-canopy minus below-canopy net radiation (Rna - Rnb), evapotranspiration (lysimeter), sensible heat flux between the plants and the air (Hp) and sensible heat flux between the soil and air (Hs) for narrow-row sorghum.

218 sorghum. ET (24-h total) of the wide-row sorghum was 10% greater than that of the narrow-row sorghum. On a seasonal basis, the ET of the wide-row sorghum also was 10% greater than that of the narrow-row sorghum. LIGHT

ENVIRONMENT

The reflectances of the crop canopy of individual leaves are of major importance in determining a canopy's radiation balance. Multiple reflection in the canopy space relates directly to the canopy profiles of photosynthesis and transpiration. Plant leaves are effective filters; they have a sharp absorption cut-off at about 700 nm. Leaves absorb strongly in the visible wavelengths (photosynthetic-active light) but weakly in the near-infrared. Optical characteristics of the closed canopy resemble those of individual leaves; however, canopy spectralreflectance characteristics depend on leaf area, leaf orientation, and plant species in addition to the sun angle and soil reflectances (Scott et al., 1968). The reflectance of solar radiation, a, is the weighted sum of the reflectances of visible and near infrared radiation. Because visible and near infrared energy compose approximately 48 and 52% of the solar radiation (Gates, 1965), respectively, the investigator should determine their refiectances separately, especially in that photosynthetic-active radiation (PAR) is confined to the visible wavelengths. Fig.7 shows the diurnal trends in albedo, visible, and near-infrared reflectance over a near-closed sorghum canopy (leaf erea index = 3). Stanhill et al. (1971)

i

SORGHUM of ~

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50 r

AUGUST !l, 1972

klA

z

ks_

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40i

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Z

o

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F VISIBLE

08

I0

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14

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LOCAL TIME (hours) Fig.7. Diurnal canopy.

trends

in n e a r i n f r a r e d ,

visible and solar radiation

reflectance

of a sorghum

219 suggested that marked diurnal variation (such as shown in Fig.7) may result from different reflectivities for diffuse and direct-beam radiation; their analyses indicated a greater reflectivity for direct-beam than for diffuse radiation. Near-infrared reflectance values shown in Fig.7 agree with those found over a ground-nut crop (Stanhill et al., 1971). Rosenberg (1969) concluded that increasing canopy reflectance could effectively decrease evapotranspiration. Applying reflective material to the leaves should increase the a m o u n t of reflected visible light, with the film itself absorbing very little. In the near-infrared region, the applied material should have lower absorptance and higher reflectance than the untreated leaf. Abou-Khaled et al. (1970) found that coating leaves with white clay (kaolinite) increased their reflectance. The radiation load being reduced by the coatings decreased transpiration of individual leaves by more than 20%; however, the treatment decreased photosynthesis when solar radiation was reduced below 0.6 ly m i n - ' . Sorghum leaves have a natural white bloom that increases visible reflectar,ce by about 10% with very slight increase in nearinfrared reflectance*. Because of the interactions among leaves and between leaves and the soil, laboratory results on single leaves cannot be generalized to the field canopy. Fuchs (1972) developed a set of equations that permit assessing interactions between the soil and the canopy foliage; he considered the plant c o m m u n i t y as a two-layer system -- canopy and soil surface. Though we cannot determine radiation distribution within the canopy by Fuchs's equations, we can make quantitative estimates for controlling the solar radiation balance by plant density, optical properties of foliage, and/or reflectance of soil surface. By Fuchs's theoretical analysis, a synergetic decrease in absorption of solar radiation can be expected when both leaves and soil of a dense canopy are coated with a reflective agent. In addition, increased soil reflectance in the visible wavelengths should increase the absorption of visible radiation by foliage. Response to soil surface reflectance should be greatest on light colored soils at high solar elevations. Photosynthesis may not be reduced by increasing the visible reflectance of leaves in light-saturated canopies but will be affected in canopies that are not light-saturated. Light saturation of a canopy depends on plant species, plant morphology, and stand geometry. Fig.8 shows net photosynthetic rates determined, using a field chamber, for grain sorghum of two row spacings (0.46 and 0.91 m) with the same linear row density. On a leaf area basis, the wider row spacing had greater photosynthetic flux density and appeared to become light-saturated by l l h 0 0 . Although photosynthesis followed solar radiation, photosynthetic flux at near maximum rates occurred over most of the daylight hours (09h30--15h30) on both canopies. In such cases, the canopy might be considered light-saturated, and an applied reflective agent may not reduce seasonal net photosynthesis significantly; however, during cloudy * Unpublished data from Evapotranspiration Laboratory, Manhattan, Kansas.

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Fig.8. N e t p h o t o s y n t h e s i s , e v a p o t r a n s p i r a t i o n a n d s o l a r r a d i a t i o n f o r t w o p l a n t d e n s i t i e s in s o r g h u m .

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Fig.9. M o d e l e d p l a n t l e a f a r e a i n t e r a c t i o n . A. I l l u s t r a t i o n o f t w o l a y e r s o f p l a n t l e a v e s o v e r l a p p i n g . B. T w o a n d t h r e e l a y e r s o v e r l a p p i n g l e a v e s d e v e l o p as p l a n t l e a f a r e a i n c r e a s e s

221 or overcast periods, the photosynthetic fluxes probably would be reduced. According to Hesketh and Baker (1967), the main factor determining a canopy's light requirement is its physical capacity to intercept. To calculate intercepted photosynthetically active radiation for a sorghum canopy, Arkin and Ritchie* developed a simplified model based on total canopy leaf area and plant spacing -- thus eliminating the necessity to define individual leaf orientations at several strata in the canopy. In this model, the effective leaf area of a plant (that portion of the foliage intercepting light and, hence, accounting for mutual shading of leaves) is represented as a horizontal disc with no openings. As plants in the c o m m u n i t y grow, the plant area intercepting direct light continues to increase until nearby plants begin to compete for the incoming light. At that point the effective area of each plant is diminished (Fig.9A). Mensuration formulae are then applied to determine interplant competition. The area of the two segments formed when the circular areas intersect is defined as:

A = t-nr~01 -rZsin0'l \ 360

2

×2 !

(8)

0 ~, the angle between the two radii, is calculated as: 01 = 180 ° - [2 sin-~ (X~/2r)]

(9)

X1 in eq.9 represents the spacing between plants within a row. X~/2, therefore, is the perpendicular distance of the chord from the eenter of the plant. The effect of plant interaction when r/> X~/2 is then expressed as: A'-A A'

= ~-(~01/180)+

sin0~

(10)

where A' is the area of a circle (Trr2 ). Eq.10 continues to apply as two or more plants within the row overlap and layering increases (Fig.9B). A form of eq.10 may be used to calculate intrarow light interactions. Transmitted light measured in grain sorghum communities having three different row spacings but equal plant populations resulted in an expression for the extinction coefficient that is a function of plant leaf interactions. This empirical expression was found for calculating the extinction coefficient, K, at solar noon: g = -0.18 - 0.34 (1 -

A/A')

(11)

The extinction coefficient varies with the solar altitude, especially at low sun angle (Clegg et al., 1969). This equation was found to express the changing

* Unpublished data from the Blackland Research Center, Temple, Texas.

222 extinction coefficient as a function of solar altitude, 7 : K --1.75 Kmax

sin 7 + 2.15

(12)

wherein Kmax is the extinction coefficient of maximum value normally determined for the day shortly after sunrise or just before sunset. With K a function of both solar altitude and leaf area interactions, the fractional transmitted photosynthetically active radiation is computed using the Bouguer-Lambert law in this form: I -- z exp ( - K LAI) Io

(13)

where I/I o is the fraction of P A R transmitted through the plant c o m m u n i t y canopy and L A I is the leaf area index. The magnitude of the effect of changing solar altitude on intercepted PAR is illustrated in Fig.10. Intercepted daily quanta flux computed using a single value for the extinction coefficient was 46.3 E m -2 ; daily quanta flux computed using an extinction coefficient varying with solar aitittide was 55.4 E m-2 . The predicted intercepted PAR values plotted agreed with field measurements reported by Clegg et al. (1969). Both the field data and the values predicted using an extinction coefficient varying with solar altitude showed little change in intercepted PAR over a

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0025

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Fig. 10. Measured and predicted values o f intercepted p h o t o s y n t h e t i c a l l y active radiation illustrating the relatively u n c h a n g i n g values for large portions o f the day.

223 significant p o r t i o n o f t h e m i d d a y period. H e n c e , w h a t m a y appear t o be light s a t u r a t i o n in a c a n o p y during field p h o t o s y n t h e s i s studies m a y be, in-

stead, a result of light interception remaining constant with increasing incident PAR.

Applying the light model to the data presented in Fig.8, we derived this equation ( F i g . l l ) : Po = 41.7 ( [ ) 1/2 - - 47.2

(14)

wherein, Po is the potential net photosynthesis on a ground area basis and i is the intercepted P A R . At light levels less than about 1.3 E m-2 h-' net photoSynthesis was zero, caused perhaps by closed stomata, not yet responsive to light or respiration rates exceeding gross photosynthesis rates. The effect of row spacing on net photosynthesis is illustrated by comparing the daily net photosynthesis of t w o grain sorghum communities each having an L A I of 4.2 and a plant population of 2.9 • l 0 s plant ha-1 and row spacings of 0.46 and 0.91 m, respectively. Computing intercepted P A R based

80

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(D 09 W I t,-Z >(/) 0 I--12) I O_ t-w Z

50

/

4O

@ @@

i 30~

20

I0

0

2

3

(INTERCEPTED PAR)~2- (E m-2 hr-I)'~2 Fig.11. Net photosynthesis as a function of intercepted photosynthetically active radiation.

224 on total canopy leaf area by the m e t h o d proposed by Arkin and Ritchie and using eq.14, we found t ha t the daily net photosynthesis of the plants having a 0.46-m row spacing (497.5 mg dm -2 day-~), exceeded by 11.6% the plants having a 0.91-m row spacing (445.6 mg dm -2 day). It is evident that the closer-row spacing results in more light being intercepted by the canopy. Because of improved plant orientation in the field, the canopy surface provided by close-row spacing intercepts light more efficiently than does one provided by wide-row spacing. A plant's leaf angle has been suggested as c o n t r i b u t o r to light interception, and, hence, to p h o t o s y n t h e t i c rates of the canopy (Duncan, 1971; Loomis et al., 1971; Wallace et al., 1972). Newton and Blackman (1970) suggest that a desirable geometry for leaf angle would be vertical (erect) upper leaves and horizontal lower leaves. In such canopies the extinction coefficient (K), as defined by the Bouguer-Lambert law, would increase with depth of canopy. Such a profile of K, however, has not been found in natural communities. Soybean, for example, has a high K at the top of the canopy, decreasing K in middle, and increasing K at the b o t t o m -- K values associated with horizontal upper leaves and near-vertical lower leaves ( L u x m o o r e et al., ] 971 ). Water stress on soybean, snapbeans, and sorghum, however, allows the upper leaves to orientate in a near-vertical position (Kanemasu and Tanner, 1969; L u x m o o r e et al., 1971). Shearman et al. (1972) found that sorghum leaves can undergo relatively large increases in water stress w i t h o u t affecting photosynthesis of individual leaves; hence, it may be desirable to allow such plants to undergo moderate water stress to p r o m o t e increased light penetration and increase canopy photosynthesis. CONCLUSIONS Apparently promising avenues of optimizing a field canopy's light and radiant energy include: (a) increasing the reflectance of the soil and of the plants' leaves artificially or genetically and, in addition, using the spectral reflectance of the coating to control the spectral composition of the absorbed radiation; (b) planting crops in narrower rows than currently r e c o m m e n d e d ; (c) breeding plants for short-season varieties adapted to high population; (d) breeding plants for short leaves that tend to remain erect; and (e) allowing certain species to undergo moderate water stress before irrigating. REFERENCES Abou-Khaled, A., Hagan, R. M. and Davenport, D. C., 1970. Effects of kaolinite as a reflective antitranspirant on leaf temperatures, transpiration, photosynthesis and water use efficiency. Water Resour. Res., 6:280--289. Clegg, M., Biggs, W., Eastin, J., Maranville, S. and Sullivan, C., 1969. Light transmission in field communities of sorghum. In: The Physiology of Yield and Management of Sorghum in Relation to Genetic Improvement. Ann. Rept. No.3. Univ. of Nebr., USDAARS, and Rockefeller Foundation, pp.35--51.

225 Duncan, W. G., 1971. Leaf angles, leaf area, and canopy photosynthesis. Crop Sci., 11: 482--485. Fuchs, M., 1972. The control of the radiation climate of plant communities. In: D. Hillel (Editor), Optimizing the Soil Physical Environment Toward Greater Crop Yields. Academic Press, New York, N.Y., pp.173--191. Fuchs, M. and Tanner, C. B., 1966. Infrared thermometry of vegetation. Agron. J., 58: 597--607. Gates, D. M., 1965. Radiant energy, its receipt and disposal. Meteorol. Monogr., 6:1--26. Gay, L. W., 1971. The regression of net upon solar radiation. Arch. Meteorol., Geophys. Bioklimatol., Ser. B, 19 : 1--14. Hanks, R. J., Allen, L. H. and Gardner, H. R., 1971. Advection and evapotranspiration of wide-row sorghum in the central Great Plains. Agron. J., 65:520--527. Hesketh, D. J. and Baker, D., 1967. Light and carbon assimilation by plant communities. Crop Sci., 7:285--293. Idso, S. B., 1971. Relations between net and solar radiation. J. Meteorol. Soc. Japan, 49: 1--12. Idso, S. B., 1972. Systematic deviations of clear sky atmospheric thermal radiation from predictions of empirical formulae. Q.J.R. Meteorol. Soc., 98:399--401. Kanemasu, E. T. and Tanner, C. B., 1969. Stomatal diffusion resistance of snap beans, I. Influence of leaf-water potential. Plant Physiol., 44:1547--1552. Loomis, R. S., Williams, W. A. and Hall, A. E., 1971. Agricultural productivity. Ann. Rev. Plant Physiol., 22:431--468. Luxmoore, R. J., Millington, R. J. and Marcellos, H., 1971. Soybean canopy structure and some radiant energy relations. Agron. J., 63:111--114. Monteith, J. L. and Szeicz, G., 1961. The radiation balance of bare soil and vegetation. Q.J.R. Meteorol. Soc., 87:159--170. Newton, J. E. and Blackman, G. E., 1970. The penetration of solar radiation through leaf canopies of different structure. Ann. Bot., 34:329--348. Paltridge, G. E., 1970. Day-time long-wave radiation from the sky. Q.J.R. Meteorol. Soc., 96:645--653. Ritchie, J. T. and Burnett, E., 1971. Dryland evaporative flux in a subhumid climate, II. Plant influence. Agron. J., 63:56--62. Rosenberg, N. J., 1969. Research in Evapotranspiration. Final Rept. to OWRR., University of Nebraska, Lincoln, Nebr., 153 pp. Scott, D., Menalda, P. H. and Brougham, R. W., 1968. Spectral analysis of radiation transmitted and reflected by different vegetation. N.Z.J. Bot., 6:427--449. Shearman, L. L., Eastin, J. D., Sullivan, C. Y. and Kinbacher, E. J., 1972. Carbon dioxide exchange in water stressed sorghum. Crop Sci., 12:406--409. Stanhill, G., Cox, J. T. and Moreshet, S., 1968. The effect of crop and climatic factors on the radiation balance of an irrigated maize crop. J. Appl. Ecol., 5:707--720. Stanhill, G., Fuchs, M. and Oguntoyinbo, J., 1971. The accuracy of field measurements of solar reflectivity. Arch. Meteorol. Geophys. Bioklimatol., Ser. B, 19:113--132. Swinbank, W. C., 1963. Long-wave radiation from clear skies. Q.J.R. Meteorol. Soc., 89:339--348. Tanner, C. B., Peterson, A. E. and Love, J. R., 1960. Radiant energy exchange in a corn field. Agron. J., 52:373--379. Wallace, D. H., Ozbun, J. L. and Munger, H. M., 1972. Physiological genetics of crop yield. Advan. Agron., 24:97--146.