The distribution of leaf area, radiation, photosynthesis and transpiration in a Shamouti orange hedgerow orchard. Part II. Photosynthesis, transpiration, and the effect of row shape and direction

Agricultural and Forest Meteorology, 40 (1987) 145-162

145

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

THE D I S T R I B U T I O N OF L E A F A R E A , R A D I A T I O N , P H O T O S Y N T H E S I S A N D T R A N S P I R A T I O N IN A S H A M O U T I O R A N G E H E D G E R O W ORCHARD Part II. P h o t o s y n t h e s i s , t r a n s p i r a t i o n , and t h e effect of row s h a p e and direction*

S. COHEN, M. FUCHS, S. MORESHET and Y. COHEN

Department of Agricultural Meteorology, Agricultural Research Organization, The Volcani Centre, Bet Dagan 50-250 (Israel) (Received June 17, 1985; revision accepted December 17, 1986)

ABSTRACT Cohen, S., Fuchs, M., Moreshet, S. and Cohen, Y., 1987. The distribution of leaf area, radiation, photosynthesis and transpiration in a Shamouti orange hedgerow orchard. II. Photosynthesis, transpiration and the effect of row shape and direction. Agric. For. Meteorol., 40: 145-162. The influence of the distribution of radiation in an orange canopy on transpiration and photosynthesis was examined by developing a model of these processes. The leaf energy balance, microclimate relationships and climatic data are combined with radiation, leaf conductance, and leaf carbon uptake models to simulate orchard photosynthesis and transpiration over 2 days. Calculated hourly values of transpiration showed good agreement with measured values of sap flow in the orange orchard. Calculated carbon uptake during the six summer months was 22kg CO 2 per tree; however, experimental estimates of annual dry matter production yield 55 kg CO 2 per tree. The calculated figure is therefore considerably in error and indicates that present information used in carbon balance modeling of Citrus is inadequate. Even so, it is shown that radiation levels deep in the canopy, where a significant amount of leaf area and transpiration is located, are too low for significant carbon uptake to occur. As an example of the usefulness of the model, the distributions of photosynthesis, transpiration and photosynthetic radiation were simulated in hedgerow canopies of three different shapes following current pruning practices in Israel. The distribution of foliage inside the given hedgerow cross-section was calculated based on the relationship of average measured foliage density to calculated diffuse photosynthetic irradiance in the canopy. The simulation was run for rows oriented n o r t h - s o u t h and east-west and for climatic conditions of midsummer. The results of the simulation indicated that: (a) The highest photosynthesis in citrus orchards is obtained by covering the largest ground areas possible with a thick canopy, i.e., maximum leaf area index (LAI). Under such conditions most photosynthesis occurs in the upper 1 m of the canopy. (b) Although rows with slanted walls do not have the highest photosynthesis, they allow more light penetration into the canopy and have productive regions on the periphery of the canopy at all heights within the orchard. (c) Whereas row orientation has little influence on total photosynthesis of the orchard, a N-S orientation allows more light penetration into rows with slanted walls and/or wide * Contribution from the Agricultural Research Organization, The Volcani Centre, Bet Dagan, Israel. No. 1393-E, 1985 series.

0168-1923/87/$03.50

© 1987 Elsevier Science Publishers B.V.

146 inter-row alleys, thus reducing spatial variation in the computedphotosynthesis. (d) Water use of vertically pruned citrus orchards can be decreased significantly without seriously affecting photosynthesis by reducing canopy height to as low as 3m.

INTRODUCTION Prediction of the influence of orchard structure on photosynthesis and transpiration is important for orchard planning. This paper describes the submodels necessary for predicting the distribution of photosynthesis and transpiration in the orchard using the radiation data provided by a model developed in a previous paper (Cohen and Fuchs, 1987). One application of the model is t he prediction of the influences of pruning and row direction on canopy processes. For this purpose a model of the distribution of leaf area in the canopy is presented. The simulated distributions of radiation, leaf area, photosynthesis and transpiration in orchards are used to examine the consequences of different hedgerow shapes and directions. METHODS Sap flow in well-irrigated trees in the orchard was measured hourly by a calibrated heat pulse technique (Cohen et al., 1981) on July 25, 1978 (two trees) and J u n e 25, 1979 (four trees). Two thermopile pyranometers (Kipp and Zonen, Delft, Holland, type CM3), one using a shadow band as described by Drummond (1956), monitored global and diffuse irradiance. Air temperature and humidity, and windspeed were sampled at a height of 8 m with an aspirated psychrometer and a sensitive anemometer (Rimco, Melbourne, Australia). Environmental data were logged on a system equipped with counters and an integrating digital voltmeter. The instruments were scanned once a minute and data were averaged hourly. Cloud cover was assumed to be average for the season (see below). Atmospheric radiation was calculated from a semi-empirical relationship suggested by Campbell (1977) based on air temperature. Total radiant flux density in the photosynthetic wave band (400-700 nm) was assumed to be 50% of the global radiant flux density (Szeicz, 1974; Stanhill and Fuchs, 1977). Diffuse radiant flux density in the photosynthetic wave band was assumed to be 75% of the total diffuse radiant flux density (Szeicz, 1974). The photon flux in the photosynthetic band was calculated from the mean frequency of photosynthetic radiation and found to be 1 pE s 1 per 0.235 W. Long-term hourly averages for each month of temperature, relative humidity, windspeed and cloud cover were obtained from the Israel Meteorological Service for the Bet Dagan station (34°49'E, 32°00'N, 30 m M.S.L.). The data are average monthly means measured over 9-12 years between 1963 and 1975. Long-term hourly averages of global radiation (1962-1968) at Bet Dagan were taken from Manes et al. (1970). The ratio of diffuse to global radiation was determined from hourly means measured in 1967 and 1968 (Manes et al., 1970).

147 THEORY P h o t o s y n t h e s i s is k n o w n to be related to t e m p e r a t u r e , p h o t o s y n t h e t i c irradiance, PAR, and leaf c o n d u c t a n c e . A n empirical model s u m m a r i z i n g these r e l a t i o n s h i p s was developed (Cohen, 1983) and applied here. The model's parameters were c a l c u l a t e d from field m e a s u r e m e n t s of leaf gross p h o t o s y n t h e s i s and e n v i r o n m e n t a l factors in a S h a m o u t i o r a n g e orchard. R e s p i r a t i o n comp o n e n t s were modeled using relationships to t e m p e r a t u r e and gross photosynthesis (Cohen, 1983). L e a f c o n d u c t a n c e in S h a m o u t i o r a n g e has been s h o w n to be related to e n v i r o n m e n t a l factors as well as i n t e r n a l factors (Cohen and Cohen, 1983). The r e l a t i o n s h i p s to e n v i r o n m e n t can be s u m m a r i z e d in a multiple n o n l i n e a r model (Jarvis, 1976). In this work, c o n d u c t a n c e was related to p h o t o s y n t h e t i c irradiance (Q), leaf-to-air v a p o r pressure difference (Ae), and leaf t e m p e r a t u r e (7"1). The a c t u a l e q u a t i o n s used are presented t o g e t h e r with the p a r a m e t e r estimates in Table I. L e a f t r a n s p i r a t i o n was e v a l u a t e d using the leaf e n e r g y balance, after F u c h s (1979). B o u n d a r y l a y e r r e s i s t a n c e values m e a s u r e d by P e a r m a n et al. (1972) on artificial leaves were used. The l a t t e r showed good a g r e e m e n t with values m e a s u r e d by K a l m a (1969) in a S h a m o u t i o r a n g e orchard. The leaf e n e r g y b a l a n c e requires inputs of windspeed, air t e m p e r a t u r e , and w a t e r v a p o r c o n c e n t r a t i o n at the leaf. CO2 c o n c e n t r a t i o n is required for calc u l a t i n g CO2 exchange. K a l m a ' s (1969) m e a s u r e m e n t s of the d i s t r i b u t i o n of windspeed in the o r a n g e TABLE I Equations, parameter estimates, and significance levels for the parameters for the model of leaf conductance (g~) as related to environmental factors. Individual data points were used in the regression and convergence of the error sum of squares was obtained for these parameters. The data are described in detail by Cohen and Cohen (1983). N = 519, r2 = 0.11 Parameter

Estimate

Significance level

a~ b, a2 b2 a3 b:~

4.0 0.024 21.2 0.191 1.95 0.20

0.7 0.7

Equations: gl

=

f~(Q)[~(Ti)f3(Ae)

f~ ( Q ) = a 1 b I Q / ( a l + b I Q) f 2 ( T l ) = ( t a n h ( b 2 ( T ~ - a2) ) + f3(Ae) = (-tanh ( b 3 ( A e a3))

1)/2 + 1)/2

0.8 0.995 0.75

148

o r c h a r d w e r e a p p r o x i m a t e d by a l i n e a r f u n c t i o n of c u m u l a t i v e d o w n w a r d l e a f a r e a index, L (Fig. 1): U/Uo

0.83 - 0.092 L

=

(1)

w h e r e u a n d u0 are w i n d s p e e d at l e a f h e i g h t and a b o v e the c a n o p y , respectively. By d e s c r i b i n g t h e r e s u l t s in this m a n n e r , the e r r o r of p r e d i c t i o n a t t h e b o t t o m of t h e c a n o p y is small. This is i m p o r t a n t b e c a u s e low windspeeds h a v e a s t r o n g influence on the r e s i s t a n c e t e r m s in the e n e r g y balance. A l o w e r limit of 0.35 m s ~ w i n d s p e e d was set, b a s e d on K a l m a ' s m e a s u r e m e n t s . F o r c a l c u l a t i n g the d i s t r i b u t i o n of processes across the h e d g e r o w c a n o p y , h e d g e r o w cross-sections w e r e divided into cells, e a c h w i t h a s q u a r e crosssection of 0.25m 2 (Cohen a n d Fuchs, 1987). C o m p u t a t i o n of r a d i a n t fluxes at a n y p o s i t i o n in the cross-sections h a s b e e n discussed (Cohen a n d Fuchs, 1987). T h e a b s o r p t i o n of direct r a d i a t i o n in a cell of the h e d g e r o w cross-section was c o m p u t e d as the t r a n s m i s s i o n difference b e t w e e n two p a r a l l e l walls of the cell, one closer a n d one f a r t h e r from the sun. T h e s o l a r a n g l e w i t h r e s p e c t to the rows, 0, was e m p l o y e d to decide w h i c h walls of the cell to use for the calculation (for a d e s c r i p t i o n of 0 see C o h e n a n d Fuchs, 1987). T h e top a n d b o t t o m of the cell w e r e used w h e n - ~ / 4 < 0 < ~/4 a n d the two sides w e r e used w h e n 0 > ~/4 or 0 < - ~ / 4 . T r a n s m i s s i o n at e a c h 0.5 m-long cell wall was t a k e n as the a v e r a g e for five points at 0.1 m i n t e r v a l s on the cell wall. T h e points were t a k e n so as to fall on the walls of a p a r a l l e l o g r a m s u p e r i m p o s e d on the cell outline, w i t h its c e n t e r c o r r e s p o n d i n g to the cell's c e n t e r but w i t h two sides p a r a l l e l to the direct rays. T h e f r a c t i o n of diffuse r a d i a t i o n a r r i v i n g at the c e n t e r of the b o t t o m of e a c h cell was a s s u m e d to be r e p r e s e n t a t i v e for the e n t i r e b o t t o m of the cell. T h e difference in c o m p u t e d r a d i a t i o n a r r i v i n g at the top a n d b o t t o m c e n t e r of the cell was t a k e n as a n e s t i m a t e of t h e a b s o r p t i o n of diffuse r a d i a t i o n inside the 1.0

0.8

0.6

0.4 .-2"

0.2

0 0

I I

I 2

I 3

I 4

I 5

[ 6

Cumulative LAI Fig. 1. T h e r e l a t i o n b e t w e e n r e l a t i v e w i n d s p e e d (U/Uo) a n d c u m u l a t i v e d o w n w a r d leaf a r e a index (LAI) in a S h a m o u t i o r a n g e o r c h a r d ( t a k e n from K a l m a , 1969). R e l a t i v e w i n d s p e e d m e a s u r e d in October, 1968, is p l o t t e d a g a i n s t L A I of J u n e , 1968 (e O) a n d N o v e m b e r , 1967 ( o - - - o ) . T h e line is t h e l e a s t s q u a r e s f i t : u / u o = a + b L A I , a = 0.83, b = - 0 . 0 9 2 , r 2 = 0.88.

149

cell. Although the angle factor for diffuse interchange between the top and bottom of a square elongated prism is only 0.41 (Sparrow and Cess, 1966), more refined estimates gave negligibly different results. Absorption of scattered radiation was estimated from the absorption of direct and diffuse radiation (Cohen and Fuchs, 1987). T r eatmen t of the various radiant fluxes which comprise net radiation was as follows. Diffuse radiation, scattered direct radiation, and long-wave radiant interchange between leaves and sky were all assumed t o b e evenly distributed over the leaf area in the cell. The diffuse flux was calculated as described above. The long-wave exchange between leaves and sky was assumed to be diffuse and therefore the shape factor for this interchange was the same as for diffuse sky radiation (Fd). No scattering of long-wave radiation was considered. Net longwave fluxes between leaves and between leaves and ground were considered negligible. The leaf area in the cell was divided into sunlit and shaded leaf area. The amount of direct radiation absorbed by the sunlit leaves was computed. The difference between this and total direct radiation absorbed in the cell (see above) was divided evenly over the leaf area in the cell. In addition, sunlit leaf area was divided into two groups of equal area, one at high light and the other at low light, with mean direct irradiances of 0.25D/cos ~8 and 0.75 D/cos ~s, respectively, where D is direct irradiance on a horizontal surface above the canopy and ~/8 is the solar zenith angle. The distribution of leaf area in the hedgerow cross-section was measured in only one orchard. In the absence of measurements from real canopies with various shapes, a method was developed to predict the distribution of leaf area in canopy cross-sections of various shapes. No simple functions relating the density of leaf area to the position in the canopy were found for canopy cross-sectional distributions measured both before and after pruning. However, casual observations show t hat in well-lit parts of the canopy there is a dense growth of leaves, and deep in the canopy leaf density is reduced. This phenomenon is probably caused by the influence of light on leaf bud development (e.g. Erez et al., 1966). Therefore it was assumed th at leaf area density at a given point in the canopy depends on the relative average photosynthetic irradiance at the point. The computed average yearly relative photosynthetic irradiance is closely related to the calculated relative diffuse irradiance, Fd, as seen in Fig. 2. This relationship is due to the fact that the sun tracks a large portion of the sky during the course of the year and because the orchard's hedgerows were oriented close to nor t h- s out h. Since Fd is computed much more easily than yearly photosynthetic irradiance, F d was chosen as the expression for average relative light intensity. The relation between relative diffuse irradiance above, and leaf area density in, cells of the row cross-section is shown in Fig. 3. The values for leaf area density in the canopy are from the mean leaf area distribution in three row cross-sections measured in 1980. Densities in cells located symmetrically with

150 e•

rr

60

•

n _>, 40

•

20

•

• •0

o

0 20

40

Absorbed

60

diffuse

80

radiation,

%

Fig. 2. The relationship of calculated yearly photosynthetic radiation absorptance to calculated diffuse photosynthetic radiation absorptance in the canopy.

respect to the c e n t r a l vertical axis of the row (the axis of the tree t r u n k s ) were averaged. This c a n o p y m a t r i x is referred to later as the s y m m e t r i c average. The a v e r a g i n g was n e c e s s a r y because differences in leaf area density from one side of the h e d g e r o w to the o t h e r c a n n o t be related to c a l c u l a t e d incident diffuse r a d i a t i o n w h i c h is assumed to come from an isotropic sky. The c u r v e in Fig. 3 is the least squares fit, where L is leaf area density in m2m 3 in a 0.5 × 0.5 × 0.5m cubical space in the canopy, and Fd is the relative diffuse r a d i a t i o n a r r i v i n g at the c e n t e r of the upper surface of the cubical space. The r 2 for the c u r v e is 0.54. L =

-0.114/(Fd + 0.0278) + 3.46,

F~ >

0.0056

L

0,

Fd <

0.0056

=

(2)

C o n f i r m a t i o n t h a t this r e l a t i o n s h i p applies for a different c a n o p y shape is

~.

•

May

1978

E "T

6

~

4

E

o

~

Summer 1 9 8 0

•

•

•

>,

00

s

•

•

•

2

o .

_J

0

I

I 0.2

•

I

I 0.4

I

I 0.6

I

.

I 0.8

Rel. diffuse irradiance, Fd

.

J

I 1.0

0 ~ 0

.

,

i 0.2

0.4

I 0.6

0.8

1.0

Rel. diffuse irradiance, Fd

Fig. 3. The relationship of the density of leaf area to the diffuse radiation absorptance for black leaves. Data are from leaf area m e a s u r e m e n t s of summer, 1980. Density of leaf area is in m '2 leaves m 3. Fig. 4. As in Fig. 3. D a t a are from May, 1978, 2 m o n t h s after pruning.

151 taken from the data collected after pruning in 1978 (Fig. 4). Confidence intervals (95%) for parameters for a r e c t a ngul a r hyperbola based on the data in Fig. 4 include the parameters given in Eqn. (2). The procedure for calculating the distribution of leaf area is as follows: a desired canopy outline shape is given as a condition. Starting from the cells of the canopy matrix highest from the ground, relative diffuse downward radiation above each cell is calculated and the appropriate leaf area density according to Eqn. (2) is assigned to the part of the cell falling within the canopy outline. The procedure then proceeds to the next height, and so forth until it reaches ground level. The leaf area density distributions in the cross-sections computed by the above procedure are constant with respect to row orientation. If the canopy outline is symmetrical with respect to the central vertical axis of the row, then the distribution is symmetric as well. The procedure was applied to the following three basic canopy shapes: (A) Vertical hedgerow walls with l m alleys (i.e., bare soil) between rows. (B) Vertical hedgerow walls with 2 m alleys between rows. (C) T ri angul ar hedgerow cross-sections with walls slanting 25° from the vertical. These shapes will be referred to by letters A, B, and C. The maximum height for the canopies is 6 m and the row planting distance is 6 m (row width). These three shapes are presently used in Israeli Citrus orchards (Aloof, 1982). Foliage area density distributions derived from Eqn. (2), together with average radiation and climate conditions prevailing in May and June, were used to calculate PAR absorption, photosynthesis and transpiration within the canopy. The simulation was made for hedgerows oriented N-S and E W. May J u n e conditions were selected because results for May and June are representative for summer months (Table IV) and calculations for winter months are based on few measurements (Cohen, 1983). May and June are recognized as the critical season for fruit set and initial fruitlet development in Shamouti orange (Zucconi et al., 1978). RESULTS A comparison of calculated transpiration and the measured sap flux in the orchard is shown in Fig. 5. The sap flux was measured in two and four abundantly irrigated trees on J ul y 25, 1978 and June 25, 1979, respectively. The calculations are for a 4 m long canopy cross-section (the length of the average tree along the row) and utilize the leaf conductance model described above. There is large variation in sap flow measured in different trees growing in the same orchard and conditions, as seen in Fig. 5. This variation prevents more precise determination of the accuracy of the calculated transpiration from this comparison. However, it is clear that calculated transpiration is within the range of the tree to tree variations in measured sap flow. A map of the canopy cross-section showing the distribution of May October absorption of photosynthetic radiation is presented in Fig. 6A. Values range

152

July 25, 1978 _

6

E 0

-

-I,.-

. . o ~ - ~ - -o~o. ,o_.~ ,o,~O~'~o>~°"° -° -o."~,%,~ ,o,o

Q_ (/) ¢E~

~0

AT

l

IZ~ t-

tO

I

i

I

I

I

J

I

I

June 25, 1979

o_df-o-~' Q ~ // ~--O'

I~f 6

/%

0 Q_

'o',,,

(,9

O~#i

I

6

8

i

I

I

=

I0

t

I

12

I

=

14

I

=

16

18

Hour of day Fig. 5. Daily courses for two summer days of trunk sap fl0w of several trees (o - - o ) and calculated orchard transpiration ( o - - - e ) . Two and four trees were measured on July 25, 1978 and June 25, 1979, respectively. Calculations were made for times when complete measurements of orchard climate were made. A

r---/

/ - - - \

B

\__

/'-

--\

E ¢.°--

-I-

/ _/

/" --\

I 3 2 North -

i

.s~.. I ~__/ \_____ I

I

=

I

0

i

I

i

I

West

Distance

i

2

I

3 SouthEast from

I

t

1

i

2 North-

I

I

I

center,

I

I

I

t

r

t

2

I

3 SouthEast

West

row

I

0

m

Fig. 6. Contour maps of the computed distribution of leaf absorbed photosynthetic radiation (A) and gross photosynthesis (B) from May through October. Units are E m -2 (180 days) ~ (A), and g m 2 (180 days)-1 (B). The broken line in the middle of A is the compensation line (see text). The other two broken lines show the borders of the foliage.

153 TABLE II Summary of relative leaf area, gross photosynthesis (P), transpiration (T) and water use efficiency (%/%) in regions of the canopy determined by their leaf gross photosynthesis. The regions are seen as contour intervals in Fig. 6B Interval (gCO.2m "(180 days)- 1)

08o

80-200 200-400 400 800 > 800

% Leaf area

% Gross P

% Transpiration

29 53 17 20 11

12 16 37 32

18 17 32 24

9 }27

15

%P/%T

0.66 0.94 1.2 1.3

from 120 to a p p r o x . 4000/zE m ~ l e a f a r e a p e r 180 days. T h e b r o k e n line is t h e c o m p u t e d c o m p e n s a t i o n level for n e t p h o t o s y n t h e s i s , i.e., t h e level at w h i c h gross p h o t o s y n t h e s i s e q u a l s r e s p i r a t i o n . F i g u r e 6B gives t h e s a m e t y p e of p r e s e n t a t i o n for gross p h o t o s y n t h e s i s . T h e r a n g e of c a l c u l a t e d g r o s s p h o t o s y n t h e s i s is from < 0.1 k g m 2 to > 1 . 0 k g m 2 p e r 180 days. The c o m p e n s a t i o n level c o n t o u r for n e t p h o t o s y n t h e s i s is a b o u t 0.1 k g m - ~ gross p h o t o s y n t h e s i s . C a l c u l a t e d r e l a t i v e g a s e x c h a n g e a n d l e a f a r e a of t h e five c a n o p y r e g i o n s s h o w n in Fig. 6B a r e g i v e n in T a b l e II. In t h e r e g i o n of l o w e s t g r o s s p h o t o s y n t h e s i s p e r l e a f a r e a , 25% of t h e t r e e ' s t o t a l l e a f a r e a c o n t r i b u t e s o n l y 4% of t h e t r e e ' s g r o s s p h o t o s y n t h e s i s b u t a l m o s t 10% of t h e t r a n s p i r a t i o n . T h e s e r e s u l t s s u g g e s t t h a t t h e l e a f a r e a in t h e c e n t e r of t h e t r e e s m a y be e x c e s s i v e . W a t e r use efficiency is h i g h e s t for t h e o u t e r p a r t s of t h e c a n o p y a n d d e c r e a s e s w i t h decreasing photosynthesis per leaf area. M a p s of l e a f a r e a d e n s i t y in t h e s i m u l a t e d c r o s s - s e c t i o n s a r e g i v e n in Fig. 7. The t o t a l L A I v a r i e s from 6.7 for C a n o p y C to 7.9 for C a n o p y A, d e c r e a s i n g as t h e c a n o p y b e c o m e s n a r r o w e r a n d lower. H o w e v e r , t h e d e c r e a s e in L A I is less t h a n p r o p o r t i o n a l to t h e d e c r e a s e d c a n o p y c r o s s - s e c t i o n a l a r e a . The d i s t r i b u t i o n s of l i g h t i n t e n s i t y on t h e l e a v e s in c a n o p y c r o s s - s e c t i o n s for t w o o r i e n t a t i o n s a r e s h o w n in Fig. 8. T o t a l p h o t o s y n t h e t i c p h o t o n flux d e n s i t y for t h e a v e r a g e d a y in M a y a n d J u n e on a h o r i z o n t a l s u r f a c e is 27 E m ~ d a y 1 Canopy

Canopy

A

6FI

- ~.l

I

E

6

q

F-if

z

I

[

3

11~

2

Canopy

B

C

1

J= G) "1-

2

o/I, 5

~ ~ 2

I

[

,

\ K

I ,

0

I

2

5

0

i 2

Dishmce

I

from

0

I

row

2

5

I 2

I I

] 0

I I

I 2

I 5

center,m

Fig. 7. Contour maps of simulated leaf area density for the three canopy shapes used for simulating hedgerow shape and direction. Units are m2m ~3. The broken line shows the borders of the foliage.

154

Deep in the c a n o p i e s the lowest a v e r a g e v a l u e s v a r y f r o m 0.4 E m 2 d a y - 1 in C a n o p i e s A a n d B o r i e n t e d E - W to 0.8 E m 2 day-1 in c a n o p y C o r i e n t e d N - S . F i g u r e 8 shows t h a t well-lit leaves are c o n c e n t r a t e d on the top of the c a n o p y for v e r t i c a l l y p r u n e d canopies, while s l a n t e d walls allow well-lit leaves to be e v e n l y d i s t r i b u t e d o v e r the p e r i p h e r y of the canopy. The difference b e t w e e n the two o r i e n t a t i o n s is small in c a n o p i e s A a n d B b u t is l a r g e r in c a n o p y C, w h e r e for the N S o r i e n t a t i o n leaves inside the c a n o p y r e c e i v e twice as m u c h p h o t o s y n t h e t i c r a d i a t i o n as i n t e r i o r leaves in the E - W o r i e n t a t i o n . M a p s of gross p h o t o s y n t h e s i s per l e a f a r e a are v e r y s i m i l a r to the m a p s in Fig. 8. E q u i v a l e n t v a l u e s of gross p h o t o s y n t h e s i s for the c o n t o u r lines 1, 2, and 10 E m 2 day ' are 0.3, 0.6 and 2.5 g CO2m 2 d a y 1. This c o r r e s p o n d e n c e demonRow orientation 0 ° (N-S) -~ iO

A6

E

2

/

4

Row orientation 9 0 ° (E-W) 6

I

---~I

I0

4

_

]

2

Jl

t-

I I

-a~ r 2

0 3 W

I 2

I\ I

B 6

} 0

I' I

,o

I1, 2 3 E

0

3 N

I

l I

r_

I 0

I I

f, 2

I i 3 S

2

I 3 S

io

I

I E

l 2

I-

4

2 ~

.E 13n G) 2 -r

0 3 W

I

0

I

2

I 3 E

C 6! E

0

3 N

2

I l

s-

l 0

I I

A

4

t.--1"

2

2

0 3 W

I 2

I I

I 0

1 I

Distance

I 2

I 3 E

//// 3 N

\\

I

I

I

I

I

I

2

I

0

i

2

3 S

from row c e n t e r ,

m

Fig. 8. C o n t o u r m a p s of c o m p u t e d a b s o r p t i o n of p h o t o s y n t h e t i c r a d i a t i o n (E m - z day ~) on t h e a v e r a g e day in M a y a n d J u n e in t h e t h r e e c a n o p i e s for n o r t h s o u t h a n d e a s t w e s t o r i e n t a t i o n s .

155

strates that in a canopy the response of gross photosynthesis of groups of leaves to photosynthetic photon flux is close to linear even at high photon flux densities. Maps of the distribution of gross photosynthesis on a canopy volume basis are shown in Fig. 9, and demonstrate the importance to canopy productivity of the various parts of the canopy. In canopies with vertical hedgerow walls (Canopies A and B) most of the gross photosynthesis occurs within the upper meter of the canopy. Where the hedgerow wall is sloped (Canopy C) the highly productive region is thinner but is spread along the larger cross-sectional length of the hedgerow walls.

A 6 E

Row orientation (N-S)

0°

I-I0 I._----- 51---.~,

Row orientation 9 0 ° (E-W)

I

~ 2 o ~

4

-(

6

,0 " I ..__-:-~

"1

4- l ~ o . T s

-I

.lt--

a) "1-

2

I I

B 6 E

I I 2

I I

' 0

I I

I I, 2 3 E

0

l

3 N

I 2

I I

'-

I0 .1..----5

I 0

I

o

-

I 3 S

'3

5------'I

4

o.2'

el E~

0.25

I

-r-

I I 2

I

C 6-

0

I

2

I 3 E

~

2

I I

I 0

I I

f 2

I 3 S

I 2

I I

I 0

I I

I 2

J 5 S

row c e n t e r ,

m

0

N

6

4 /~\ °_

4

E

"1-

2

2

2

r

I 2

I I

I 0

I I

Distance

I 2

I 3 E

from

3 N

Fig. 9. C o n t o u r m a p s of c o m p u t e d d i s t r i b u t i o n of g r o s s p h o t o s y n t h e s i s on a v o l u m e basis (g m 3 day ~). O t h e r d a t a as in Fig. 8.

156 Total gross photosynthesis, average leaf gross photosynthesis, total transpiration, LAI, and water use efficiency of the simulated treatments along with results for the average measured canopy and the symmetric canopy derived from the latter are given in Table III. Differences in total gross photosynthesis demonstrate that gross photosynthesis increases with increasing ground coverage by leaves and not necessarily with increasing LAI. Differences in water use efficiency are small. Average leaf gross photosynthesis shows the greatest amount of variability, being lowest for Canopy B. Averaging (Table III) diminishes the differences between the two orientations seen in maps of the distribution of absorbed photosynthetic radiation and photosynthesis in the canopy (Figs. 8 and 9). DISCUSSION The good agreement between calculated tree transpiration and measured sap flow (Fig. 5) lends support to the calculations including the model of leaf conductance (Table I). In the early morning, the calculated tree transpiration is 11 h ~ more than the sap flow. If this is correct, then the tree's water storage is depleted by several liters during the morning. Although calculations for Shamouti orange show tree capacitance to be small (Cohen et al., 1983), morning hour differences of this magnitude between lysimeter-measured transpiration and sap flow have been observed in Valencia orange in South Africa (Y. Cohen and G.C. Green, 1981, unpublished data). Calculated transpiration during daylight hours for the actual canopy on average days in summer months (Table IV) is in agreement with Kalma's (1969) estimated orchard transpiration from intensive measurements of water balance made during 2 years in an orchard located a few kilometers from the orchard studied here. Kalma measured 493 mm evapotranspiration during May through October, but Shalhevet et al. (1976) and Moreshet et al. (1983) reported 735 and 746 mm water use, respectively, for the same season. The cause of this difference is not clear, since climatic variation does not seem to be a factor (Israel Meteorological Service, unpublished climatic norms; Stanhill, 1970). No measurements of mature orange orchard photosynthesis have been reported in the literature. Approximate yearly dry matter production in the orchard was estimated from the fresh fruit yield of approx. 100kg tree 1, or 20 kg tree 1 dry matter (Samish and Cohen, 1949). Leaf fall, or turnover, measured in our orchard is 9 kg dry weight per tree per year. Other components of the annual dry matter production of a 20-year-old Shamouti orange tree are estimated to total 8.5kg (Goldschmidt and Monselise, 1977). Converting to weight of CO2 based on molecular weights of carbohydrate (CH20) and CO2 gives 55kgCO2 per tree per year. The calculated half-yearly net photosynthesis, 22 kg CO2 per tree, which does not include night-time respiratory losses, is clearly too low. One source of error is probably in the calculation of leaf

Vertical p r u n i n g , i m alley Vertical p r u n i n g , 2 m alley 65 ° p r u n i n g (see text), 0.5m alley Symmetrical a v e r a g e a Symmetrical a v e r a g e (38°)a Avg. m e a s u r e d axis (38°) b

14.0 11.9 11.8 14.6 14.4 14.8

13.9 11.1 11.7 14.8

2.17 1.92 1.91 2.19 2.17 2.21

2.15 1.82 1.92 2.23

90 o

0o

0o

90 o

Transpiration (nun d a y - 1)

Gross photosynthesis (gm 2 ground day -1)

6.45 6.20 6.18 6.67

0o

6.64 6.70

6.47 6.10 6.09 6.64

90 o

W.U.E. (gCO 2/kg H 2O)

1.77 1.57 1.76 2.06

0o

2.03 2.08

1.76 1.47 1.75 2.08

90 o

Gross photosynthesis ( g m 2 leaves day 1)

a As above. Leaf densities in cells symmetrically located w i t h respect to the c e n t r a l vertical axis of the r o w s were averaged. b Mean leaf area d i s t r i b u t i o n in t h r e e r o w cross-sections m e a s u r e d in 1980. Row o r i e n t a t i o n 38 °.

A. B. C. D. E. F.

Canopy d e s c r i p t i o n / o r i e n t a t i o n

7.9 7.6 6.7 7.1 7.1 7.1

LAI

Calculated results for several c a n o p y cross-sections. E n v i r o n m e n t a l data used for the c a l c u l a t i o n s w a s for a v e r a g e days in May and J u n e . C a n o p i e s A, B, and C were 6 m h i g h

TABLE III

158 TABLE IV Results of the simulation of transpiration and photosynthesis for the average row cross-section. Orchard transpiration measured by Kalma (1969) is also given Month

Calculated transpiration (mm day- 1)

Transpiration (Kalma)

April May June July Aug. Sept. Oct.

1.44 2.00 2.45 2.45 2.41 2.12 1.59

1.7 2.0 2.3 2.2 1.8 1.5 1.6

Mean S.D.

2.07 0.42

1.87 0.30

Photosynthesis (g day- 1 t r e e 1) Net

Gross

81 147 153 135 103 115 70

219 323 379 382 358 323 237

respiration which is based on little experimental data. For this reason, further simulations omit this component and predict only gross photosynthesis. Calculations of radiant fluxes, sunlit leaf area, and distribution of leaf area with respect to light intensity all assumed that the angular distribution of leaf area was random. The analysis of measured distributions (Cohen and Fuchs, 1987) showed that the largest deviations from this assumption occur at midday in May. In order to evaluate the influence of this assumption, the model was run for midday in May using measured values of the distribution in the computation of absorption of direct radiation and sunlit leaf area, and the measured distribution of leaf area with respect to light intensity in the computation of photosynthesis and transpiration. Fluxes of diffuse and scattered radiation were calculated as before. Calculated total absorption of photosynthetic radiation increased by 7% due to the increased extinction of radiation. However, the increases in calculated transpiration and photosynthesis were < 2% and 1%, respectively. This result indicates that as far as photosynthesis and transpiration are concerned in this study, the deviation of the angular distribution of leaf area from random is not significant. Leaves growing under low light conditions inside the canopy have been shown to be on the borderline of productivity if not parasitic to the tree's carbon balance (see Results). It may be advantageous to rid the orchard of these leaves by reducing canopy height. Figure 10 shows the relationship of gross photosynthesis to height in the canopy for N-S oriented rows. The optimum canopy height for photosynthesis and dry matter production depends on the light intensity at which net photosynthesis becomes negative. At present this point is unknown. The cost of producing a leaf per day of leaf life can be estimated from the leaf

159 6

I~ E

5

• canopy A

\\

ocanopy

B

o~ 4 (-

~ 2

I0

L

0

I

I

,

I

2

i

I 5

,

T

4

T

T ~

5

6

Depth in canopy, m Fig. 10. Mean calculated gross photosynthesis of leaves plotted against depth in the canopy for the three simulated hedgerow shapes and n o r t h - s o u t h row orientation. U n i t s for the vertical axis are g m -2 d

dry weight divided by the average leaf life expectancy. Dry weight, per unit leaf area, increases with height in the canopy. The minimum, for Valencia leaves low in the canopy, is 120mgm -2 (G. Stanhill, M. Fuchs and G.C. Green, 1981, unpublished). Assuming that the average leaf life expectancy is 2 years, and converting carbohydrate to COs, the leaffs cost is 240 mg COs m- ~leaves day Leaves more than 3 or 4 m from the top of the canopy in vertically pruned canopies (A and B) are not productive (Fig. 10). This implies that photosynthetic productivity of vertically pruned canopies should hardly decline if their height is reduced to 3-4m. In the canopy with sloped walls (Canopy C), the average leaves are still productive 6 m deep in the canopy. This is because the leaves on the sides of the row are in high light conditions. A reduction in height of vertically pruned rows should also improve the water use efficiency (WUE) of the orchard. This is seen in Fig. 11, where the gross photosynthesis of canopies of increasing height is compared with the corresponding WUE (taken as gross photosynthesis per transpiration). It can be seen that for canopies A and B reductions in height from 6 m to 2 and 3 m, respectively, would increase the WUE greatly while reducing gross photosynthesis by about 10%. The corresponding decrease in transpiration would be about 20%. The findings of the simulation can be summarized as follows: (1) The highest photosynthesis in orchards can be obtained by covering all of the ground area with a thick canopy. Under these conditions maximum photosynthetic rates occur in the upper 1 m of the canopy. (2) Although rows with slanted walls do not have the highest photosyn-

160

9~C

B A

~c~

2

~6

0

I00

200

i

I

i

500

I

400

Gross pho?osynthesis g (tree doy) -I

Fig. 11. The influenceof canopyheight on the relationship between water use efficiencyand gross photosynthesis.Letters signifyhedgerowshape and numbers give orchard height in m. Hedgerow width is 6 m and direction is north-south. thesis, they allow more light penetration into the canopy and have productive regions at all heights on the periphery of the canopy. (3) Whereas row orientation has little influence on total photosynthesis of the orchard, a N S orientation allows more light penetration into rows with slanted walls and/or wide inter-row alleys. (4) Water use of vertically pruned orchards can be decreased significantly without seriously affecting photosynthesis by a reduction in height to as low as 3m. This study has developed a framework and model for predicting Shamouti orange hedgerow photosynthesis and transpiration. As predicted before for other crops (Liang and Chan, 1977; Jackson, 1980), this model predicts advantages for north-south row orientation (in Israel's latitude), and for dense orchards which maximize ground coverage. Previous models for woody plants have aimed for the prediction of bulk canopy processes (e.g. Liang and Chan, 1977; Stamper and Allen, 1979; Hsia, 1979). Prediction of the distribution of these processes in the canopy is a significant improvement upon previous models in that maps of the processes in the hedgerow allow speculation about other processes occurring in the canopy that at present cannot be modelled or even expressed quantitatively. An example could be in considering the location of fruit in the canopy, which is of considerable economic importance in harvest. Schertz and Brown (1966) found that 90% of the fruit is located within 1 m of the periphery of the canopy. This corresponds to the region of highest predicted photosynthesis. If fruit is located in regions of high photosynthesis, then, from the maps shown in Fig. 9, average fruit height will be lowest in the diagonally pruned canopy. This response of fruit height to diagonal pruning has been observed in field pruning trials with Shamouti orange (Aloof and Tal, 1980). The consideration of fruit height might

161 offset a d v a n t a g e s in predicted p h o t o s y n t h e s i s (see Table III) to o r c h a r d planners. The p r e s e n t model was u n a b l e to predict the o p t i m u m leaf a r e a index, or o p t i m u m size, for the canopy, with respect to p h o t o s y n t h e s i s , because the r e s p i r a t i o n c o m p o n e n t was u n c e r t a i n . Initial c a l c u l a t i o n s showed t h a t some regions of the c a n o p y are too d a r k for leaves to h a v e a positive c a r b o n balance. O p t i m u m c a n o p y size is an i m p o r t a n t p a r a m e t e r t h a t could be predicted if r e s p i r a t i o n and leaf a d a p t a t i o n to low light were b e t t e r understood. It is therefore i m p o r t a n t t h a t more r e s e a r c h be done on a d a p t a t i o n of photosynthesis in shade leaves to low light conditions. Yield is of p r i m a r y interest to a g r i c u l t u r a l planners. A l t h o u g h seasonal p h o t o s y n t h e s i s has been s h o w n to be directly related to yield in some crops (Zelitch, 1982), this has not yet been s h o w n for citrus. A complete fruiting model of citrus will be n e c e s s a r y in order to predict yields. S u c h a model could be used to extend the predictions of the p r e s e n t model. A l t h o u g h the problems and c h a l l e n g e s of such a model h a v e been discussed (Goldschmidt and Monselise, 1977), it is clear t h a t m u c h i n f o r m a t i o n is needed before t h a t goal can be achieved. Meanwhile, it would be well w o r t h w h i l e to a d a p t the p r e s e n t model to v a r i o u s o r c h a r d s and see if yield is c o r r e l a t e d with predicted p h o t o s y n t h e s i s . ACKNOWLEDGEMENTS We are grateful to Dr. G. Stanhill for his c o n s t r u c t i v e criticism of the m a n u s c r i p t . This r e s e a r c h was supported in part by a g r a n t from the S e a g r a m F u n d for Research, D e v e l o p m e n t and T r a i n i n g in Soil and Water.

REFERENCES Aloof, S., 1982. [Pruning and thinning the orchard.] Alon haNotea, 36:255~267 (in Hebrew). Aloof, S. and Tal, D., 1980. [Hedgerow pruning in Shamouti orange.] Hassadeh, 61: 409-413 (in Hebrew). Campbell, G.S., 1977. An Introduction to Environmental Biophysics, Springer-Verlag, New York, NY., 159 pp. Cohen, S., 1983. Light relations of an orange canopy. Ph.D. thesis, Hebrew Univ. of Jerusalem, Jerusalem, Israel (in Hebrew with English summary). Cohen, S. and Cohen, Y., 1983. Field studies of leaf conductance response to environmental variables in citrus. J. Appl. Ecol., 20: 561-570. Cohen, S. and Fuchs, M., 1987. The distribution of leaf area, radiation, photosynthesis and transpiration in a Shamouti orange hedgerow orchard. Part I. Leaf area and radiation. Agric. For. Meteorol., 40: 123-144. Cohen, Y., Fuchs, M. and Cohen, S., 1983. Resistance to water uptake in a mature Citrus tree. J. Exp. Bot., 34: 451460. Cohen, Y., Fuchs, M. and Green, G.C., 1981. Improvement of the heat pulse method for determining sap flow in trees. Plant Cell Environ., 4: 391-397. Drummond, A.J., 1956. On the measurement of sky radiation. Arch. Met., Wien B, 7: 414~436. Erez, A., Samish, R.M. and Lavee, S., 1966. The role of light in leaf and flower bud break of the peach (Prunus persica). Physiol. Plant., 19: 656659.

162 Fuchs, M., 1979. Atmospheric transport processes above arid-land vegetation. In: R.A. Perry and D.W. Goodall (Editors), Arid-land Ecosystems: Structure, Functioning and Management, Vol. I. I.B.P. 16, Cambridge Univ. Press, Cambridge, U.K., pp. 393-408. Goldschmidt, E.E. and Monselise, S.P., 1977. Physiological assumptions toward the development of a citrus fruiting model. Proc. Int. Soc. Citricult., 2: 668-672. Hsia, Y.J., 1979. Computation of radiation balance and transpiration of a Douglas-fir tree in a forest. Ph.D. thesis, Univ. of Washington, Pullman, WA. Jackson, J.E., 1980. Light interception and utilization by orchard systems. Horticult. Rev., 2: 208-267. Jarvis, P.G., 1976. The interpretation of the variations in leaf water potential and stomatal conductance found in canopies in the field. Phil. Trans. R. Soc. London, Ser. B, 273: 593-610. Kalma, J.D., 1969. Some aspects of the water balance of an irrigated orange plantation. Ph.D. thesis, Hebrew University of Jerusalem, Israel. Liang, T. and Chan, W.M., 1977. Estimating solar energy absorption potential for Macadamia nut orchard design: a theoretical approach. Trans. Am. Soc. Agric. Eng., 20: 1045-1049. Manes, A., Teitelman, A. and Fruehling, I., 1970. Solar radiation and radiation balance at Bet Dagan, Central Meteorological Institute. Meteorological Notes, Ser. A, No. 25. Israel Met. Service, Bet Dagan. Moreshet, S., Cohen, Y. and Fuchs, M., 1983. Response of mature "Shamouti" orange trees to irrigation of different soil volumes at similar levels of available water. Irrig. Sci., 3: 223-236. Pearman, G.I., Weaver, H.L. and Tanner, C.B., 1972. Boundary layer heat transfer coefficients under field conditions. Agric. Meteorol. 10: 83-92. Samish, Z. and Cohen, A., 1949. Composition of oranges in Israel. Bull. Agric. Res. Stat., Rehovot, pp. 51, 53. Schertz, C.E. and Brown, G.K., 1966. Determining fruit bearing zones in citrus. Trans Am. Soc. Agric. Eng., 9: 366-368. Shalhevet, J., Mantell, A., Bielorai, H. and Shimshi, D., (Editors) 1976. Irrigation of field and orchard crops under semi-arid conditions. Intl. Irrig. Information Center (Bet Dagan) Publ. No. 1, 110 pp. Sparrow, E.M. and Cess, R.D., 1966. Radiation Heat Transfer. Brooks/Cole, Belmont, CA. 322pp. Stamper, J.H. and Allen, J.C., 1979. A model of the daily photosynthetic rate in a tree. Agric. Meteorol., 20: 459-481. Stanhill, G., 1970. Measurements of global solar radiation in Israel. Israel J. Earth-Sci., 19: 91&6. Stanhill, G. and Fuchs, M., 1977. The relative flux density of photosynthetically active radiation. J. Appl. Ecol., 14: 317-322. Szeicz, G., 1974. Solar radiation for plant growth. J. Appl. Ecol., 11: 617~36. Zelitch, I., 1982. The close relationship between net photosynthesis and crop yield. BioScience, 32: 796-802. Zucconi, F., Monselise, S.P. and Goren, R., 1978. Growth abscission relationships in developing orange fruit. Sci. Horticult., 9: 137-146.