Use of Photosynthetically Active Radiation by Sunflower and Sorghum

Eur. J. Agron., 1993, 2(2), 131-139

Use of Photosynthetically Active Radiation by Sunflower and Sorghum F. Rachidi, M. B. Kirkham*, L. R. Stone, and E. T. Kanemasu Evapotranspiration Laboratory, Department of Agronomy, Kansas State University, Manhattan, Kansas 66506, U.S.A.

Accepted 29 April 1993. (*) To whom correspondence should be addressed.

Abstract

Intercepted, reflected, and transmitted photosynthetically active radiation (PAR) of sunflower (Helianthus annuus L., cv. Hysun 354) with predominantly horizontal leaf orientation and sorghum (Sorghum bicolor (L.) Moench, cv. Funk's G522DR) with predominatly erect leaf orientation, two

important dryland crops, were measured to contrast their light use. In addition, the extinction coefficient (k), light-use efficiency (LUE), and the relationship between evapotranspiration (ET) and PAR were compared. Sunflower intercepted more PAR than sorghum. It also reflected less and transmitted less PAR than sorghum. Maximum interception of PAR was 98 per cent for sunflower and 95 per cent for sorghum. Maximum interception occurred at an LAI of 3.14 in sunflower and 3.55 in sorghum. The k value of sunflower (0.83) was higher than that of sorghum (0.62). The LUE value of sorghum (2.8 g m-2 per MJ m·2 ) was higher than that of sunflower (2.3 g m-2 per MJ per m-2). Sunflower had a higher ET than sorghum, for the same amount of PAR intercepted. It lost 0.80 mm of water per MJ m-2 of intercepted PAR compared to 0.69 mm water per MJ m-2 for sorghum. The results suggested that, because sunflower had a higher ET per unit intercepted PAR, the ET of sunflower had a higher proportion of transpiration than did the ET of sorghum. Key-words : Sunflower, sorghum, photosyntheticaly active radiation (PAR), intercepted PAR, reflected PAR, transmitted PAR, evapotranspiration.

INTRODUCTION

depends upon the leaf area index (LAI) of the canopy (Asrar et al., 1984).

Two important crops in dry areas are sorghum and sunflower. They are grown because they are drought resistant (Maertens and Bose, 1981 ; Hattendorf et al., 1988; Bremner and Preston, 1990). Little work has been done to compare their light-use characteristics, even though light is the source of energy that permits photosynthesis and evapotranspiration to take place (Bonhomme and Varlet-Grancher, 1977). Many studies have shown that the maximum amount of dry matter accumulated by a crop is strongly correlated with intercepted photosynthetically active radiation (PAR) (Monteith, 1977 ; Azam-Ali et al., 1984; Squire et al., 1984). The amount of PAR intercepted

An equation relating the amount of direct-beam short-wave radiation transmitted through a canopy to LAI is given by Campbell (1977; Eq. 10 5, p. 129). We adapt his equation to consider only the PAR part of short-wave radiation, as follows : I/I 0 = e-kL (1) where It is the transmitted PAR in MJ m- 2 , I0 is the PAR above the crop (MJ m·2), k is the extinction coefficient, and L is the leaf area index. In studies of light, k is an important parameter to determine, because differences in k show differences in leaf orientation and adaptation to the environment (Fitter and Hay, 1981, p. 37). Sunflower has predominantly horizontally oriented leaves and, therefore, the extinction coefficient of the crop is expected to be close to unity. However, when Saugier (1976) reviewed the literature on sunflower, he found that k varied between 0.6 and 0.9. The k value for sorghum, having predominantly vertically oriented leaves, has been reported to be smaller (0.19-0.49) (Clegg et al., 1974).

F. Rachidi is now with the Departement d'Ecologie Vegetale et Pastoralisme, Ecole Nationale d' Agriculture, Meknes. Morocco, and E.T. Kanemasu is now with the Department of Agronomy, University of Georgia, Griffin, Georgia, U.S.A. Contribution No. 91-294-J from tbe Kansas Agricultural Experiment Station, Manhattan, Kansas. The material in this paper is from a dissertation presented in partial fulfillment of the requirement for the degree of PhD in the Department of Agronomy at Kansas State University. lSSN ll6l-0301193/02!131 9 $ 4.00/ © Gauthier-Villars· ESAg

Light-use efficiency (LUE) can be estimated from the regression of above-ground dry weight on accu-

132

F. Rachidi, M. B. Kirkham, L. R. Stone and E. T. Kanemasu

mulated intercepted PAR. Light-use efficiency of C3 and C4 crops from different locations around the world was compared by Kiniry et al. (1989), who found that LUE of sunflower (C3) and sorghum (C4), based on shoot dry weight, averaged 2.2 g m-2 per MJ m- 2 and 2.8 g m-2 per MJ m-2 , respectively, if high values (4.8 and 5.0 g m-2 per MJ m-2) found for sunflower in southern France (Toulouse, France) were not included. The high values for sunflower were even higher than those found for sorghum (3.8 and 3.3 g m-2 per MJ m-2) in a different experiment at the same location in southern France (Kiniry et al., 1989). The relationship between evapotranspiration (ET) and intercepted PAR is of special importance in dry regions. In these regions, a crop that has less ET for the same amount of PAR intercepted would be better adapted to the environment, because it would conserve water. If the relationship between ET and intercepted PAR is known for two or more crops growing under the same environmental conditions, one can judge relatively the partitioning of soil evaporation and transpiration in ET. A canopy that loses more water (has a higher ET) for the same amount of PAR intercepted should transpire more. Models have been developed to separate soil evaporation and plant transpiration in ET (Jagtap and Jones, 1989). Sadras et al. (1991), who directly measured evaporation with evaporimeters, found that cultivars of sunflower had similar apportionment of ET. But no studies comparing partitioning of ET in sorghum and sunflower appear to have been published. The objective of this study was to compare intercepted, reflected, and transmitted PAR of these two major dryland crops, sorghum and sunflower. In addition, the three important parameters described above, i.e., k, LUE, and the relationship between ET and PAR, were compared. MATERIALS AND METHODS The experiment took place during two years (1988 and 1989) at a field site 5 km from the Kansas State University campus in Manhattan, Kansas (39° 12 N, 96° 35' W; 325m above sea level). To reduce space, only data for 1989 are presented here. Data for 1988 were similar to those of 1989. Data for 1988, as well as raw data for both years, are presented by Rachidi (1990). Grain sorghum (Sorghum bicolor (L.) Moench, cv. Funk's G522DR) and sunflower (Helianthus annuus L., cv. Hyson 354) were planted on the same day (16 May 1988 and 26 May 1989) in north-south rows in two, large plots to achieve uniform microclimatological conditions. A month before planting, the plots were fertilized with 56 kg N ha- 1 and 62 kg P ha- 1 • The sunflower plot was 48 m x 79 m, and the sorghum plot, adjacent to and west of

the sunflower plot, was 42 m x 79 m. There was no replication of the plots. We used large plots because small plots are not suitable for obtaining reliable evapotranspiration data (Tanner, 1957). The plant density at harvest was 5.9 plants m- 2 for sunflower and 11.8 plants m- 2 for sorghum. The crops were grown under rainfed conditions, except in early May, 1989, when a preplant irrigation (0.10 m) was applied to the plots to ensure good crop establishment. The soil type was a Muir silt loam (fine-silty, mixed, mesic Cumulic Haplustoll) (Jantz et al., 1975). It is developed from alluvial deposits. The texture is silt loam to 0.14 m depth; silty clay loam from 0.4 m through 0.50 m ; silty clay from 0.50 m through 0.81 m ; and silt loam below 0.81 m. It is free from root-restricting features (Kaigama et al., 1977). Twenty years of experiments at the field site (e.g., see Darusman et al., 1991) have shown that the physical properties are uniform across the entire area on which the sunflower and sorghum grew. The soil water content at different soil water potentials in 0.15 m increments, between 0 and 1.50 m, as well as the soil bulk density, are given by Mayaki (1975, p. 113) (Table 1). Table 2 gives the average temperature and total rain that fell in May, June, July, and August of 1989 and the departures from the 30-year (1951 through 1980) mean (NOAA, 1989).

Table 1. soil. Depth em 0-15 15-30 30-45 45-60 60-75 75-90 90-105 105-120 120-135 135-150

Water content and bulk density of the silt loam

Soil water content - 0.01 MPa - 1.50 MPa m-3 m' 0.391 0.377 0.405 0.404 0.379 0.377 0.377 0.386 0.411 0.410

Soil bulk density Mg m- 3

0.173 0.193 0.196 0.223 0.076 0.057 0.073 0.049 0.078 0.051

1.37 1.29 1.38 1.30 1.28 1.28 1.30 1.37 1.32 1.38

Table 2. - Average temperature and rainfall, and departures from normal, during the 1989 growing season in Manhattan, Kansas. Temp. Month

oc

May June July August

18.1 22.7 26.8 25.6

Dep.

oc

Rain mm

Dep. mm

- 0.4 - 1.1 0.2 - 0.1

56 85 37 135

- 56 - 49 - 63 55 Eur. J. Agron.

133

Radiation use by sunflower and sorghum

Nine neutron probe access tubes were installed in the central area of each plot. Soil water content was measured with a neutron probe, approximately once a week during the growing season. Measurements began on 16 June and ended on 31 August. The measurements were made to a depth of 3.1 m in 0.15 m increments. Water content of the surface 0.08 m deep layer of soil was determined gravimetrically. Mean daily evapotranspiration was calculated by adding the change in total profile water between two dates to rainfall and dividing the sum by the number of days in the time period under consideration. Runoff and deep drainage were assumed to be negligible from the plots, which had berms (ledges) around them. Plant samples were taken on 23, 30 June; 5, 10, 14, 19, 24, 28 July; 2, 7, 11, 16, 28 August; I, 12, and 20 September to monitor LAI and above-ground dry weight. These dates were 28, 35, 40, 45, 49, 54, 59, 63, 68, 73, 77, 82, 94, 98, 109, and 117 days after planting (DAP), respectively. Each day, ten plants per crop were chosen randomly from each plot for these measurements. The plants were brought back to the laboratory for dry weight determination and leaf-area analysis. Leaf area was measured by using a laboratory model leaf area meter (Model LI-3100, Li-Cor, Inc., Lincoln, Nebraska, U.S.A.). Only green leaves were used to calculate LAI. For sunflower, bud formation and physiological maturity occurred on 6 July and 31 August, respectively. For sorghum, boot stage, 50 per cent bloom, soft dough, and physiological maturity occurred on 27 July, 2 August, 11 August, and 21 September, respectively. A t-test was used to determine any significant differences in dry weight or LAI between the crops (Snedecor and Cochran, 1980, p. 83-106). Measurements of PAR began on 29 June (34 days after planting) and were taken every day until 28 August (94 days after planting), except for 16 June and 30 July. Incident PAR (1 0 ) and reflected PAR (lr) were measured 1.0 m above the canopy with quantum sensors (Model LI-190SB Quantum Sensor, Li-Cor, Inc., Lincoln, Nebraska, U.S.A.) with 1.0 cm2 of sensor surface area. One sensor was used to measure incident PAR for both crops. Two sensors were used for each crop to measure the reflected PAR, one located above the row and the other above the area between rows. The average of the two measurements was used in calculations. Before canopy closure, these values varied by about 0.5 per cent. Transmitted PAR (11) at the soil surface was measured with a light bar (Model LI-191SB Line Quantum Sensor, Li-Cor, Inc.). The surface of the sensor measured 1.0 m in length and 0.12 m in width. Two light bars were used, one for each crop, and their locations were changed daily in the field. They were placed on the soil between the rows and perpendicularly to the rows. The other sensors were fixed at the same place throughout the study. The sensors were connected to a micrologger Vol. 2, no 2- 1993

data-acquisition system (Model CR21X, Campbell Scientific, Inc., Logan, Utah, U.S.A.), and the output was averaged every 10 minutes, 24 hours a day (i.e., output from the sensors was integrated within the 10minute period)!. The readings, averaged every 10 minutes, were used to calculate daily incident, reflected, transmitted, and intercepted PAR. Gallo and Daughtry (1986) describe the relationships among incident, reflected, transmitted, and intercepted PAR. They give an equation to determine intercepted PAR, assuming the reflected PAR to be insignificant. Following their procedure, we calculated intercepted PAR by subtracting transmitted PAR from the incident PAR. Daily percent interception of PAR was calculated as the ratio of intercepted PAR [~ (1 0 - 11)] to incident PAR during each day multiplied by 100. Extinction coefficients were determined by plotting the daily averages of transmitted PAR as a function of LAI, determined on the same dates, and then using Eq. (1) to calculate the values. Regression analyses were done to determine if there were significant relationships between variables (SAS Institute, 1982). Comparison of the slopes of linear regressions was done by using the method described by Zar (1984). RESULTS Interception of photosynthetically active radiation by both sorghum and sunflower increased rapidly from a minimum 34 DAP to more than 90 per cent around 60 DAP (Figure 1). During this time, LAI also quickly increased (Figure 2). No significant difference

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Time, Days after planting Figure 1. -

Intercepted PAR versus days after planting for sorghum and sunflower.

134

F. Rachidi, M. B. Kirkham, L. R. Stone and E. T. Kanemasu

was found between the LAI of the crops until they attained their maximum LAI (Figure 2). Sorghum had a higher maximum LAI (4.17) than sunflower (3.57). The maximum LAI occurred 63 days after planting for each crop. In both years, due to senescence, sunflower LAI dropped to a low value at the end of the season, while sorghum maintained a high value. Redelfs et al. ( 1987) found similar results for LAI development of sunflower and sorghum. Maximum LAI and maximum interception of PAR occurred at about the same time (ca. 60 DAP). Percent interception then remained relatively constant until about 75 DAP, when it started to decrease gradually for both crops (Figure 1). Maximum PAR interception by the sunflower canopy was 98 per cent of the daily incoming PAR (74 DAP); for sorghum, the value was 95 per cent (71 and 75 DAP). The interception of PAR (Figure 3) was, in general, high early in the day, declined to a minimum value around solar noon, and then started to increase at the end of the day. For all dates shown (9 July ; 13, 18, and 24 August), sunflower intercepted more radiation than sorghum. The amount of PAR intercepted changed during the growing season. When LAI was small, the percent of PAR intercepted during each hour of the day depended upon the angle of the sun. On 9 July 1989 (44 DAP; LAI of sorghum = 2.5; LAI of sunflower 2.2), the percentage interception was least at midday (Figure 3) with the sun at its highest elevation. Hipps et al. ( 1983) also reported that interception values of wheat (Triticum aestivum) early in its growing season were lowest near midday when solar altitude angles were large. This was because the horizontal component of the direct beam was nearly parallel to the rows at this time, due to their northsouth orientation. Our rows also were oriented in the north-south direction. The extinction coefficient, which can be considered to be the ratio of shadowed area to the actual leaf area (Jones, 1992, p. 35), changes during the day, due to changing solar and leaf angles. As LAI increased, interception of PAR increased. By 13 August 1989 (79 DAP), when the leaf area index of both crops had reached its maximum, sorghum and sunflower intercepted large proportions of PAR (greater than 80 per cent) at all sun angles (Figure 3). Late in the season (24 August 1989, 90 DAP), the percentage interception remained relatively high, because much of the PAR was intercepted by senescent leaves. On 13, 18, and 24 August (Figure 3), there was a tendency for the amount of PAR intercepted to vary diurnally, especially for sunflower on 18 and 24 August, as the sun angle changed. At the end of the season (24 August), it is interesting to note that sunflower intercepted 100 per cent of the PAR before mid-morning and after midafternoon. This date was near the time when the

0 0

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Time, Days after planting Figure 2. - Leaf area index of sorghum and sunflower. The dates of measurement were 23, 30 June; 5, 10, 14, 19, 24, 28 July; 2, 7, 11, 16, 28 August; 1, 12, and 20 September. The means for each value shown were compared by a t-test, and a star indicates that the probability of a larger value of t was > 0.95.

plants reached physiological maturity (31 August), and the senescing leaves were intercepting the maximum percentage of PAR early and late in the day. Before LAI was at maximum, reflected PAR (Figure 4, 10 July, 45 DAP) tended to parallel intercepted PAR (Figure 3, 9 July). That is, values were high around 08.00 and 18.00 h and decreased at midday. Sorghum intercepted less PAR than sunflower (Figure 3, 9 July), and it reflected more PAR (Figure 4, 10 July). Later in the season, on 13 August (79 DAP) (Figure 4), the midday decrease was not as obvious, and values increased during the afternoon to peaks for both crops at 18.00 h. On this day, sorghum reflected more PAR than sunflower until 16.00 h, and then sunflower's reflectance surpassed that of sorghum. Peak values of reflected PAR were 7.1 per cent for sorghum (18.58 h) and 7.5 per cent (18.43 h) for sunflower. But, over the day, sunflower reflected less PAR than sorghum. As LAI increased, the percentage of PAR interception increased (Figure 1), and, therefore, the percentage transmitted decreased (Figure 5). Sunflower transmitted less PAR at any value of LAI than sorghum. Conversely, sunflower intercepted more PAR than sorghum, as LAI increased (Figure 1). Using the data in Figure 5 and Eq. (1), we calculated that sunflower had an extinction coefficient of 0.83 and sorghum had one of 0.62. Eur. J. Ag ron.

Radiation use by sunflower and sorghum

135

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Figure 3.

Diurnal variation of intercepted PAR for sorghum and sunflower. LA! of sorghum on IO July, II August, 16 A~
Light-use efficiency values in the literature are expressed in radiometric units, such as W m-2 or MJ m-2 d- 1, rather than the units of photosynthetically active radiation, which are in photon units, such as )lE m- 2 s- 1 or )lmol m- 2 s- 1• Thus, to compare our LUE values with those in the literature, we followed the procedure of Kiniry et al. ( 1989). The total incoming solar radiation in MJ m-2 d- 1 (Table 3) was converted to PAR assuming that 45 per cent of incoming radiation energy was PAR. These values were then multiplied by the percentage interception by sorghum and sunflower (Figure 1). The resulting Vol. 2,

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2

1993

values, along with the dry weight data, are shown in Table 3. Again, to compare our values with those in the literature, we plotted cumulative dry matter versus cumulative intercepted PAR, even though, for accurate work, one should plot the rate of increase in dry matter versus the rate of interception of PAR (Russell et al., 1989, p. 27). We summed dry matter and intercepted PAR for the periods between plant samplings. This resulted in eleven periods (eleven data points) each for sorghum and sunflower (i.e., for days after planting of 35-40, 40-45, 45-49, 49-54, 5459, 59-63, 63-68, 68-73, 73-77, 77-82, and 82-94).

F. Rachidi, M. B.. Kirkham, L. R. Stone and E. T. Kanemasu

136

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For sorghum, a plot of cumulative dry matter versus cumulative intercepted PAR (with eleven data points) gave a line with the following equation : y = 2.8x 67.1, and the correlation coefficient was 0.98. For sunflower, the equation of the line (for eleven data points) was y = 2.3x + 123, and the correlation coefficient was 0.98. Therefore, the LUE values of sorghum and sunflower were 2.8 and 2.3 g m-2 per MJ m- 2 , respectively.

- Cumulative evapotranspiration of sorghum and as a function of cumulative intercepted PAR. of the two lines are : sorghum, y = 0.69x - 22.3 ; y = 0.80x + 17.7.

Sunflower had a higher evapotranspiration for a given level of intercepted PAR than sorghum (Figure 6). Sunflower lost 0.80 mm of water per MJ m-2 of intercepted PAR compared to 0.69 mm of water per MJ m- 2 for sorghum. These two values differed statistically at the 5 per cent level. The correlation coefEur. J. Ag ron.

137

Radiation use by sunflower and sorghum

Table 3. - Solar radiation, dry weight, and intercepted PAR of sorghum and sunflower on dates of measurement, also given as days after planting (DAP). Date

DAP

29 June 30 June I July 2 July 3 July 4 July 5 July 6 July 7 July 8 July 9 July 10 July 11 July 12 July 13 July 14 July 15 July 16 July 17 July 18 July 19 July 20 July 21 July 22 July 23 July 24 July 25 July 26 July 27 July 28 July 29 July 30 July 31 July I Aug. 2 Aug. 3 Aug. 4 Aug. 5 Aug. 6 Aug. 7 Aug. 8 Aug. 9 Aug. 10 Aug. II Aug. 12 Aug. 13 Aug. 14 Aug. 15 Aug. 16 Aug. 17 Aug. 18 Aug. 19 Aug. 20 Aug. 21 Aug. 22 Aug. 23 Aug.

34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89

Vol. 2,

2- 1993

0°

Dry weight Daily solar Intercepted PAR Sunf. Sorg. Sorg. Sunf. radiation t MJ m- 2 g m- 2 63

117

130

194

224

284

298

484

239

478

455

631

498

743

605

940

778

1000

973

1110

1060

1090

25.8 21.1 19.5 19.5 28.3 27.6 29.4 29.2 27.7 28.1 28.2 23.6 27.1 25.3 21.0 19.3 15.4 6.6 19.7 23.4 27.2 28.0 25.5 16.1 14.6 22.4 15.8 18.8 26.2 24.3 20.0 19.9 21.5 20.9 19.6 18.0 25.8 20.5 25.7 27.5 23.9 25.9 20.9 22.7 21.2 15.9 19.9 11.8 23.7 20.7 20.7 18.3 18.0 15.6 12.1 21.8

4.5 4.0 5.1 5.5 8.6 8.7 9.9 9.3 9.1 9.3 9.5 8.3 9.8 9.1 7.7 7.0 5.7

7.0 6.5 6.8 6.8 10.4 10.7 11.4 11.3 11.1 11.0 11.0 8.8 10.7 10.3 8.6 8.1 6.5

7.4 8.8 10.4 11.0 10.4 6.6 6.1 9.4 6.6 7.9 11.0 10.1 7.7

8.5 10.1 11.8 12.1 11.0 6.9 6.3 9.6 6.7 8.2 11.3 10.5 8.1

8.5 8.2 7.8 6.9 10.9 8.7 10.4 11.2 9.7 11.1 8.8 9.4 8.7 6.5 8.1 4.9 9.6 8.3 8.1 7.5 7.5 6.0 4.6 8.5

9.1 8.7 8.4 7.5 10.6 8.4 11.0 11.7 10.6 10.5 8.4 9.3 8.6 6.6 8.3 4.7 9.4 8.3 8.2 7.2 7.5 6.7 5.0 8.4

Table 3. Date 24 25 26 27 28

Aug. Aug. Aug. Aug. Aug.

(Continued).

DAP 90 91 92 93 94

Dry weight Daily solar Intercepted PAR Sunf. Sorg. Sorg. Sunf. radiation t MJ m- 2 g m- 2

1410

1220

17.6 18.6 23.3 15.6 21.0

6.4 6.9 8.1 5.3 6.8

7.5 7.7 8.8 6.0 7.8

t Data from the Weather Library, Kansas State University + Missing values

ficients between evapotranspirat ion and intercepted PAR were 0.97 for sunflower and 0.99 for sorghum. A high correlation coefficient (0.96) between ET of sorghum and intercepted PAR also was found by Steiner (1986).

DISCUSSION We found that maximum PAR interception for sorghum and sunflower were 95 per cent and 98 per cent, respectively. Many factors affect maximum radiation interception by plant canopies (Jones, 1992 ; p. 30-42; Fitter and Hay, 1981, p. 31-38). One important factor is leaf orientation. Kiniry et al. (1989) attributed the difference in light interception between the two crops to the fact that leaves of sunflower are more horizontal than those of sorghum. Also, heliotropic movements can have large effects. For example, a cowpea (Vigna sp.) leaf that is continually oriented perpendicularly to the solar beam receives nearly 50 per cent more radiation than a horizontal leaf (Jones, 1992, p. 38-40). Because sunflower leaves are phototropic (Saugier, 1976, p. 92), they are expected to intercept more light per unit leaf area. Rate of development of leaf area also affects the timing of maximum PAR interception. Zaffaroni and Schneiter (1989) studied interception of PAR by sunflower hybrids in North Dakota in the U.S.A. They found that 95 per cent of the maximum intercepted PAR was obtained with an LAI of 3.7 at 69 DAP. In our study, maximum PAR interception by the sunflower canopy (74 DAP) occurred with an LAI of 3.14 (measured 73 DAP). For sorghum, maximum PAR interception (71 and 75 DAP) occurred with an LAI of 3.55 (measured 73 DAP). Thus, sunflower obtained maximum interception of PAR at a lower LAI than sorghum. The extinction coefficients that we determined in this experiment (0.62 and 0 83 for sorghum and sunflower, respectively) should not be taken as absolute

138

F. Rachidi, M. B. Kirkham, L. R. Stone and E. T. Kanemasu

values, because they vary with LAI, changes in solar altitude angle during the day, and season. Zaffaroni and Schneiter (1989) and Rawson and Hindmarsh (1983) found that, as LAI of sunflower increased, the extinction coefficient decreased until a steady phase was reached. Their conclusion was that canopies with lower LAI were more efficient in light interception per LAI unit than canopies with higher LAI. Similar results were reported by Clegg et al. (1974) for sorghum and by Hipps et al. (1983) for wheat.

Bonhomme R. and Varlet-Grancher C. (1977). Application aux converts vegetaux des lois de rayonnement en milieu diffusant. I. Etablissement des lois et verifications experimentales. Ann. agron., 28, 567582.

The LUE values found in this study (2.3 and 2.8 g m- 2 per MJ m-2 for sunflower and sorghum, respectively) are almost identical to those reported by Kiniry et al. (1989) for sunflower (2.2 g m-2 per MJ m-2 ) and sorghum (2.8 g m-2 per MJ m-2 ). The LUE of sorghum falls within the range of values (2.6 to 4.3 g m-2 per MJ m- 2 ) given by Hammer and Vanderlip (1989) and approximately agrees with the number reported by Sivakumar and Virmani (1984) (3.09 g m- 2 per MJ m-2 ). Differences in LUE can result not only from different photosynthetic pathways (C3 versus C4 ), but also by differential partitioning between shoots and roots and chemical composition of the grain. Sorghum yields less grain energy than sunflower (Bremner and Preston, 1990).

Campbell G. S. (1977). An introduction to environmental biophysics. New York : Springer-Verlag. 159 pp.

Sunflower had a higher ET per unit intercepted PAR (Figure 6), intercepted more PAR (transmitted less PAR) (Figures 1 and 5), and reflected less PAR than sorghum (Figure 4). Therefore, the results suggest that the ET of sunflower had a higher proportion of transpiration than did the ET of sorghum. One might use plots of ET versus intercepted PAR to compare, indirectly and qualitatively, the partitioning of evaporation and transpiration of two or more different crops. This proceedure is valid, however, only for comparative studies with crops growing under similar environmental conditions. For example, for the same intercepted PAR, if Crop A has a higher ET than Crop B, then the ET of Crop A should have a higher proportion of transpiration than the ET of Crop B. The results would have to be quantified by direct measurement of transpiration and evaporation, using methods such as those developed by Sadras et al. (1991).

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