Results and discussion Directory UMM :Data Elmu:jurnal:A:Agricultural & Forest Meterology:Vol106.Issue4.2001:

B.J. Choudhury Agricultural and Forest Meteorology 106 2001 317–330 321 roots. Estimating L o or foliage nitrogen content using data from different sources introduce uncertainty in determining A g , and such uncertainty appears in de- termining R m T when shoot and root nitrogen con- tent have not been determined concurrently. However, such desirable sets of data were very limited, and thus measurements from different sources have been used to determine the needed stand characteristics. The impact of uncertainties in the input data on RUE and CUE has been addressed by sensitivity analysis Section 4.3.3. The L o and nitrogen content per unit ground area mmol m − 2 of foliage, stem and roots used in the calculations are summarized in Table 1. These data have been extracted from published tables and figures after enlargement to minimize error, or estimated using allometric relations. 3.2. Incident irradiance The direct and diffuse incident irradiance have been calculated from a model Choudhury, 2000 by pre- scribing the atmospheric conditions and times during the day being cloudy at Elora 43.7 ◦ N, 80.4 ◦ W during July, Los Banos 14.2 ◦ N, 121.3 ◦ E during April, and Kununurra 15.7 ◦ S, 128.7 ◦ E during June Table 2. These locations and time have been chosen for hav- ing RUE data and to capture the diversity of radia- tion regime which can prevail during growth of these crops.

4. Results and discussion

4.1. Illustrative results The variations of average RUE and CUE for the three crops with incident irradiance are given in Table 2 for Elora, Los Banos, and Kununurra. Calcu- lations have been done for representative mean daily air temperature of 20 ◦ C at Elora, and 26 ◦ C at Los Banos and Kununurra. Since the irradiance is similar at Elora and Los Banos, the effect of temperature on RUE and CUE can be assessed by comparing the results in this table. Thus, RUE and CUE values are seen to be higher at Elora as compared to those at Los Banos because of lower respiration at Elora Eq. 2d. The effect of temperature on RUE and CUE is seen to be more noticeable for rice as compared to maize and sorghum because the quantum efficiency of photo- synthesis by leaves of C 4 crops maize and sorghum is independent of temperature, while it decreases with increasing temperature for C 3 crops rice. As a consequence, canopy gross photosynthesis of rice de- creases with increasing temperature. Moreover, since quantum efficiency becomes progressively important determinant of gross photosynthesis as irradiance decreases, so also the effect of temperature. Thus, the RUE for rice decreases by 11 for clear skies 18 vs. 16 mmol mol − 1 ; the first line in this table at each location, and by 30 for overcast skies 27 vs. 19 mmol mol − 1 ; the last line in this table at each loca- tion in going from Elora to Los Banos. This sensitiv- ity to temperature has been elaborated in Choudhury 2000. One can see in this table that CUE for all crops de- creases with decreasing total irradiance, for the reason discussed in Section 2.3. However, RUE does not have any systematic pattern of variation with total irradi- ance. At Elora, the RUE of all three crops increases with decreasing irradiance. At Los Banos, the RUE of maize is relatively constant, while it is decreasing and then increasing for rice with decreasing irradiance. At Kununurra, the RUE of maize and sorghum remains steady and then decreases with decreasing irradiance, while it remains steady and then increases for rice. This lack of a systematic pattern appears because RUE is the product of RUE for gross photosynthesis RUE g and CUE Eq. 9, and RUE g also changes with inci- dent total irradiance Fig. 1. The RUE and CUE are found to be relatively in- sensitive to changes in leaf area index L o for maize Fig. 2 and sorghum Fig. 3, but they tend to de- crease with increasing L o when greater than three for rice Fig. 4. With plant growth, respiration remains closely in balance with gross photosynthesis by maize and sorghum. In Section 2.4 a theoretical maximum RUE RUE max for maize was found to be 39 mmol mol − 1 . The results at Elora for maize Table 2 show that the calculated average RUE 29 mmol mol − 1 is about 26 lower than the RUE max . In this case the cal- culated average CUE ca. 0.64 is about 14 lower than the maximum possible value, Y G 0.74. Simi- larly, the RUE max for rice in a tropical environment was found to be 34 mmol mol − 1 Section 2.4, while 322 B.J. Choudhury Agricultural and Forest Meteorology 106 2001 317–330 Fig. 1. The variation of average radiation-use efficiency for gross photosynthesis mmol CO 2 per mol intercepted photon; mmol mol − 1 by maize n = 24 and rice n = 14 canopies with incident total irradiance at Kununurra Australia. the calculated average RUE at Los Banos Table 2 is about 50 lower. In this case, the calculated average CUE ca. 0.57 is about 23 lower than Y G 0.74. The RUE max for sorghum was found to be 39 mmol mol − 1 Section 2.4. For clear sky con- dition at Kununurra Table 2, the calculated RUE 22 mmol mol − 1 is 44 lower, while the correspond- ing CUE is 23 lower than the respective maximum values. Thus, CUE is contributing to about half the departure for RUE. However, for overcast condition, the calculated RUE 20 mmol mol − 1 ; Table 2 is 49 lower than RUE max , but in this case CUE is 46 lower than the theoretical maximum values =0.74. Thus, the major fraction of departure of RUE from RUE max for overcast condition is occurring due to the departure of CUE from its theoretical maximum value. Fig. 2. The variation of radiation-use efficiency mmol CO 2 per mol intercepted photon; mmol mol − 1 and carbon-use efficiency mol CO 2 per mol CO 2 ; mol mol − 1 with leaf area index for maize at Elora Canada. 4.2. Comparison with observations 4.2.1. Radiation-use efficiency The reported values of RUE are based on field measurements of biomass accumulation generally above ground dry matter, although a few studies have included below ground dry matter and IPAR either measured in different ways or calculated from measured L o and incident irradiance. Some of the methodology related uncertainties in the reported data have been discussed by Sinclair and Muchow 1999, and the data selected for comparison are primarily those used by Sinclair and Muchow 1999 for pro- viding a realistic perspective on the upper limits of RUE that might reasonably be expected for individ- ual species under ideal conditions. The RUE values based on intercepted solar radiation have not been analyzed here because of substantial differences in B.J. Choudhury Agricultural and Forest Meteorology 106 2001 317–330 323 Fig. 3. The variation of radiation-use efficiency mmol mol − 1 and carbon-use efficiency mol mol − 1 with leaf area index for sorghum at Los Banos Philippines, and Kununurra Australia. Fig. 4. The variation of radiation-use efficiency mmol mol − 1 and carbon-use efficiency mol mol − 1 with leaf area index for rice at Los Banos Philippines. Also shown are the maximum rate of leaf photosynthesis mmol m − 2 s − 1 obtained from Eq. 2b. the radiation extinction within the canopy as com- pared to PAR. From the reported RUE, the RUE for carbon accumulation has been determined by taking carbon content of dry matter as 42 for maize and sorghum, and 40 for rice. The root biomass has been observed to be about 10–30 of shoot biomass for maize and sorghum towards the end of their veg- etative period, while this fraction is about 10–20 for rice. The RUE values have been increased by 20 for maize and sorghum, and by 15 for rice to account for root biomass, when it was not included. A difference of up to 10 for maize and sorghum, and 5 for rice, between the calculated and observed RUE cannot be resolved due to residual uncertainty in specifying the root biomass. The PAR and solar irradiance in MJ have been converted to molar units using a conversion factor of, respectively, 4.6 and 2.05. The estimated and calculated RUE values are given in Table 3, together with relevant meteoro- logical data. Although RUE is found to depend on both total irradiance and its diffuse fraction Section 4.1, only observed total irradiance were given with the reported RUE values. The diffuse fractions esti- mated from observations are included in this table. The calculated values are average for all canopies of each crop in Table 1, except for rice for which the 324 B.J. Choudhury Agricultural and Forest Meteorology 106 2001 317–330 Table 3 Comparison of estimated based on observations and calculated radiation-use efficiency for net carbon accumulation respectively, RUE e and RUE c ; mmol mol − 1 . Also given are the sources for the observations, mean daily irradiance S o , mol m − 2 per day, estimated range of diffuse fraction for the irradiance f d,e and mean daily air temperature T a , ◦ C corresponding to the RUE e , the irradiance S L: location; E for Elora, LB for Los Banos, and K for Kununurra corresponding to the calculated RUE highlighted figures in Table 2 and percent difference of the RUE values . The calculated RUE values for rice with asterisk are averages for the transplanted canopies n = 9, and subscript ‘o’ when observed temperature is used Crop Source a RUE e S o f d,e T a S L RUE c Percentage Maize 1 30 46 0.20–0.55 20 47 E 28 − 7 2 28 46 0.25–0.60 18 47 E 28 3 37 47 0.25–0.60 21 47 E 28 − 24 4 31 37 0.30–0.60 22 40 E 28 − 10 5 28 46 0.25–0.60 19 47 E 28 Sorghum 6 25 37 0.30–0.60 27 37 LB 24 − 4 7 22 39 0.10–0.30 26 38 K 22 Rice 8 16 33 0.35–0.70 26 30 LB 17 6 8 16 33 0.35–0.70 26 30 LB 19 ∗ 19 8 18 37 0.35–0.70 26 37 LB 16 − 11 8 18 37 0.35–0.70 26 37 LB 18 ∗ 9 25 32 0.45–0.80 22 30 LB o 20 − 20 9 25 32 0.45–0.80 22 30 LB o 22 ∗ − 12 a Sources: 1 Tollenaar and Bruulsema, 1988; 2 Andrade et al., 1992; 3 Andrade et al., 1993; 4 Kiniry, 1994; 5 Westgate et al., 1997; 6 Sivakumar and Huda, 1985; 7 Muchow and Coates, 1986; 8 Kiniry et al., 1989; 9 Horie and Sakuratani, 1985. averages for only transplanted canopies are also given. A brief elaboration of some of the data in Table 3 is given below. From the measured growth rate of above ground dry matter and the rate of PAR absorbed by maize grown at Elora 43.7 ◦ N, 80.4 ◦ W, Tollenaar and Bruulsema 1988 found the maximum RUE to be 0.753 g mol − 1 during the vegetative period. This value has been multiplied by 0.96 Tollenaar and Bruulsema, 1988 to convert RUE based on absorbed PAR to IPAR. Thus, the estimated RUE is 30 mmol mol − 1 =0.753×0.42×1.2×0.96×100012. The calculated value is 7 lower. From periodic measurements of above ground dry matter and interception of PAR by maize at Morris 45.6 ◦ N, 95.9 ◦ W, Westgate et al. 1997 found the maximum RUE to be 3.02 g MJ − 1 . The observed irra- diance at St. Cloud was considered in the calculation. From periodic measurements of total dry matter and L o for transplanted rice cv. IR54 grown at Los Banos 14.2 ◦ N, 121.3 ◦ E and incident irradiance, Kiniry et al. 1989 calculated IPAR and determined RUE to be 2.2 g MJ − 1 . The estimated RUE is 16 mmol mol − 1 . The calculated average RUE for all canopies is 6 higher than the estimated value, while it is 19 higher if only transplanted canopies are considered. A dis- crepancy of 19 would be more appropriate because the crop was transplanted. Kiniry et al. 1989 quote another value of RUE for above ground dry matter production for rice as 2.1 g MJ − 1 the citation for these data suggest that these measurements were at Los Banos. The IPAR was estimated from measured L o and incident irradi- ance. It is not stated whether these data are for trans- planted or direct seeded rice. The estimated RUE is 18 mmol mol − 1 =2.1×1.15×0.4×10004.6×12. The average RUE calculated for all canopies and those for transplanted canopies are, respectively, 16 and 18 mmol mol − 1 . Considering that transplanting is the most common practice for growing rice, the estimated and calculated RUE values are in good agreement. From measurements of above ground biomass and absorbed PAR at Tsukuba 36.2 ◦ N, 140.1 ◦ E, Horie and Sakuratani 1985 found the max- imum RUE to be 3.28 g MJ − 1 . The data pre- sented by the authors suggest PAR reflectance to be 7. The estimated RUE would be 25 = 3.28 × 1.15 × 0.4× 0.93× 10004.6× 12. The authors do not provide concurrent mean daily irradi- ance or temperature. From available observations, the B.J. Choudhury Agricultural and Forest Meteorology 106 2001 317–330 325 mean daily irradiance May–September at this loca- tion is about 32 mol m − 2 per day, while mean daily temperature at Shimodata 36.2 ◦ N, 140.0 ◦ E is found to 22 ◦ C. The calculated average RUE for transplanted canopies is 12 lower than the estimated value. For the above comparisons, the estimated and calculated RUE values differ by −24 to +19. The ensemble average n = 10 calculated RUE 25 mmol mol − 1 differs from the estimated value 26 mmol mol − 1 by 4. As noted above, discrep- ancies of up to 10 between the estimated and calculated RUE for maize and sorghum, and 5 for rice cannot be resolved due to residual uncertainty in specifying root biomass, when this was not included in the measurements. Although the diffuse fraction of the calculated incident irradiance Table 2 is within the range of observations Table 3, uncertainty in specifying this fraction for the observed incident ir- radiance was found to introduce additional ca. 2–7 variability in the calculated RUE Fig. 5. Thus, an attempt has been made in Section 4.3 to understand Fig. 5. The variation of average radiation-use efficiency mmol mol − 1 with diffuse fraction for a constant total incident irradiance of 33 mol m − 2 per day at Elora for maize n = 24, sorghum n = 13, and rice n = 14. rather large discrepancies for maize −24 and rice +19. 4.2.2. Carbon-use efficiency For potted maize plants, Yamaguchi 1978 found CUE to be in the range 0.49–0.73 average 0.60, n = 14. Field studies at Moscow 55.8 ◦ N, 37.6 ◦ E by Koshkin et al. 1987 gave CUE in the range 0.60–0.73 average 0.67, n = 10 during vegetative stage. Louwerse et al. 1990 provide measurements of gross photosynthesis, above ground biomass and respiration for a field study with maize at Wageningen 51.9 ◦ N, 5.7 ◦ E. These measurements suggest CUE to be in the range 0.60–0.65 during the vegetative stage, if root biomass is assumed to be 20 of the above ground biomass. The calculated values at Elora are within this range, but somewhat lower values are ob- tained at Los Banos and Kununurra because of higher temperature and thus higher maintenance respiration at these two locations Table 2. However, since the reported values are from temperate locations, com- parison with the results at Elora is more appropriate. Measurements reported by Wilson et al. 1980 for a growth chamber study of sorghum suggest CUE to be 0.56–0.64 average 0.60, n = 15 in the absence of severe stress daytime leaf water potential greater than −1 MPa. The CUE values reported by McCree 1988 based on growth chamber study are in the range 0.54–0.66 average 0.60, n = 29. The calculated values are fairly consistent with these data Table 2, Fig. 2. It is desirable to have field measurements for a better evaluation of the calculated values. A field study using rice cv. Peta grown by trans- planting at Los Banos, Tanaka and Yamaguchi 1968 found CUE to be in the range 0.40–0.70 average 0.59, n = 8 during the period from early growth stage to shortly after panicle initiation. Field obser- vations by Cock and Yoshida 1973 at the same lo- cation and method of growing gave average CUE of 0.60 for periods before and after flowering for seven varieties of rice. For potted plants grown in a cul- ture solution, Yamaguchi 1978 found CUE to be 0.54–0.67 average 0.62, n = 10 for period up to flowering. The data presented by Hirota and Takeda 1978 based on growth chamber study show that CUE decreases from 0.74 to 0.61 with growth during the vegetative phase average 0.65, n = 3. These ob- servations suggest CUE during vegetative period to 326 B.J. Choudhury Agricultural and Forest Meteorology 106 2001 317–330 be 0.59–0.65. The calculated range of average CUE for all canopies at Los Banos is 0.50–0.61 Table 2, but it is 0.54–0.64 when only transplanted canopies n = 9 are considered. The calculated average CUE for the transplanted rice at Los Banos for represen- tative mean daily irradiance during the vegetative pe- riod 0.59 at the irradiance of 30 mol m − 2 per day, 0.61 at the irradiance of 37 mol m − 2 per day, and 0.62 at the irradiance of 44 mol m − 2 per day, n = 9 compare well with field measurements at this loca- tion 0.59, Tanaka and Yamaguchi, 1968; 0.6, Cock and Yoshida, 1973, as quoted above, although the average values for all canopies is somewhat lower 0.56–0.59; Table 2. The calculated decreasing trend of CUE with increasing L o Fig. 3 is consistent with measurements reported by Hirota and Takeda 1978, but Cock and Yoshida 1973 did not observe such variation. The calculated CUE values for the three crops are consistent with the reported data. The model param- eters were not calibrated or adjusted for the above comparisons. Field measurements are desirable for a better evaluation of the calculated results for sorghum. A comparison of CUE values for the three crops in Table 2 and the results shown in Fig. 3 also suggest that CUE is less sensitive to changes in A max and dif- fuse fraction for a given total irradiance. 4.3. Sensitivity analysis 4.3.1. Maximum leaf photosynthesis Calculations for maize and sorghum reported above were done by taking the maximum rate of leaf pho- tosynthesis A max ; Eq. 2a, respectively, as 57 and 47 mmol m − 2 s − 1 , which are the mean maximum val- ues, while it was noted in Section 2.1 that the range for maize is 52–62 mmol m − 2 s − 1 , and for sorghum is 32–57 mmol m − 2 s − 1 . At Elora, calculations using A max of 57 for maize gave average RUE and CUE as, respectively, 28 mmol mol − 1 and 0.65 mol mol − 1 for incident irra- diance of 40 or 47 mol m − 2 per day Table 2. When calculations were re-done using the range of A max , changes in RUE and CUE were, respectively, about 3 and 1 from the base values. At Los Banos, calculations using A max of 47 for sorghum gave average RUE and CUE as, re- spectively, 24 mmol mol − 1 and 0.58 mol mol − 1 for incident irradiance of 37 mol m − 2 per day Table 2. When calculations were re-done, the RUE and CUE decreased by, respectively, 17 and 4 when A max was 32 mmol m − 2 s − 1 , and they increased by, respec- tively, 8 and 1 when A max was 57 mmol m − 2 s − 1 , as compared to the base values. At Kununurra, calculations using A max of 47 for sorghum gave average RUE and CUE as, respectively, 22 mmol mol − 1 and 0.59 mol mol − 1 for incident ir- radiance of 38 mol m − 2 per day Table 2. When calculations were re-done, the RUE and CUE de- creased by, respectively, 22 and 5 when A max was 32 mmol m − 2 s − 1 , and they increased by, respectively, 10 and 2 when A max was 57 mmol m − 2 s − 1 , as compared to the base values. For a given uncertainty in A max , the relative changes in RUE and CUE depend upon incident irradiance, and the changes in CUE are found to be lower than those in RUE. These differing sensitivity for RUE and CUE appears because changes in A max directly affects gross photosynthesis and RUE g , while maintenance respi- ration does not get affected. These sensitivity results suggest that, when grown in similar environmental conditions, cultivar related variability of RUE would be expected to be more noticeable in sorghum than in maize. The calculated RUE of maize, in one case, was found to be 24 lower than the estimated value Table 3. Consideration of the uncertainties in spec- ifying the maximum rate of leaf photosynthesis and the diffuse fraction associated with the observed in- cident irradiance Tables 2 and 3, suggests that the potential maximum RUE might be 30 mmol mol − 1 in- stead of 28 mmol mol − 1 given in Table 3 see further discussion below. Now, to reconcile the remaining difference 37 vs. 30 mmol mol − 1 , it is pertinent to analyze possible reasons for overestimation. If, instead of 20, root biomass is taken to be 10 of shoot biomass, the estimated RUE would be 34 =37×1.11.2 mmol mol − 1 . Moreover, Uhart and Andrade 1995 have noted that, because of measur- ing light interception around noon on sunny days, the IPAR used to determine RUE could have been underestimated by about 15 during the vegetative period. Thus, the estimated RUE could be about 30 =341.15 mmol mol − 1 . Thus, the difference of 24 between the calculated and estimated values of RUE might be reconciled considering the limits of B.J. Choudhury Agricultural and Forest Meteorology 106 2001 317–330 327 uncertainties in the measurements and in specifying input parameters of the model. By ignoring the above data due to varied uncertain- ties, the average n = 4 estimated and calculated RUE for maize are, respectively, 29 and 28 mmol mol − 1 , while those for sorghum n = 2 are, respectively, 24 and 23 mmol mol − 1 Table 2. Based on these aver- ages, the estimated RUE for sorghum is 17 lower than that for maize, while it is 18 lower for the calculated values of RUE. Thus, present calculations are providing a reasonable understanding about lower RUE observed for sorghum as compared to that for maize, which was not understood previously Sinclair and Muchow, 1999. The results in Table 2 show that when maize and sorghum are “grown” side-by-side, the RUE of sorghum would be, on the average, ca. 7–13 lower than that for maize. Higher difference found above is primarily because the RUE values for maize are from temperate locations, while those for sorghum are from tropical locations lower respiration at temperate loca- tions. The average potential observable maximum RUE, calculated by taking A max = 62 mmol m − 2 s − 1 for maize and A max = 57 mmol m − 2 s − 1 for sorghum, and the likely maximum fraction of diffuse radi- ation for a given total incident irradiance appears to be about 30 and 28 mmol mol − 1 , respectively, for maize and sorghum at Elora, and these values are, respectively, about 28 and 26 mmol mol − 1 at Los Banos Fig. 6. These potential maximum val- ues are not the same at both locations because of the difference in mean daily temperature; 20 ◦ C at Elora vs. 26 ◦ C at Los Banos. The potential maxi- mum value for maize at Elora is 23 lower than a theoretical maximum value of 39 mmol mol − 1 Sec- tion 2.4, and this shortfall of 23 is contributed about equally by the shortfall of CUE =0.65 from a theoretical maximum value Y G = 0.74 and RUE g =46 mmol mol − 1 from its potential maximum value = αε = 53 mmol mol − 1 . Loomis and Amthor 1999 estimated the potential maximum RUE of maize to be 4.6 g carbohydrate MJ − 1 IPAR which is equivalent to 33 mmol mol − 1 in the temperature range 20–30 ◦ C for a canopy which has a dry matter of 14 t ha − 1 and is intercepting 62 mol m − 2 per day. The present calculations are giving somewhat lower aver- age values ca. 10 at Elora, and 15 at Los Banos for the potential maximum RUE of maize for reason- Fig. 6. The variation of the average potential maximum radiation-use efficiency mmol mol − 1 of maize n = 24 and sorghum n = 13 with incident total irradiance at a Elora, and b Los Banos. able values of mean daily incident irradiance during the vegetative period Fig. 6. Nevertheless, consider- able room appears to exist for improving the RUE of sorghum over the presently available observations. 4.3.2. Leaf photosynthesis — nitrogen relationship Field measurements by Peng et al. 1995 gave the following relationship for rice: A m = 0.20n l + 30 11 While the average RUE for gross photosynthesis RUE g calculated using Eq. 1b is 31.0 mmol mol − 1 , it is 31.5 mmol mol − 1 obtained using Eq. 11 at Los Banos for the incident irradiance of 30 mol m − 2 per day. These values for only transplanted canopies are, respectively, 32.6 and 33.4 mmol mol − 1 . Thus, two in- dependently derived equations for leaf photosynthesis are found to give very similar RUE for gross photo- synthesis. The mean coefficient of variation in per- cent RUE for net photosynthesis and CUE calculated for transplanted canopies n = 9 are, respectively, 328 B.J. Choudhury Agricultural and Forest Meteorology 106 2001 317–330 20 9 mmol mol − 1 and 0.59 8 mol mol − 1 , which are nearly equal to those quoted above Section 4.1 ob- tained using Eq. 1b. Thus, application of Eq. 11 is not providing an explanation for the discrepancy of 19 between the estimated and calculated RUE Table 3. If one accepts the estimated RUE of 16 mmol mol − 1 Table 3 and CUE of 0.60 mol mol − 1 Tanaka and Yamaguchi, 1968; Cock and Yoshida, 1973 are representative for transplanted rice at Los Banos, it follows from Eq. 9 that a representative RUE g would be 27 mmol mol − 1 . The calculated mean RUE g of 33 mmol mol − 1 for transplanted canopies noted above in this section is 22 higher than the esti- mated RUE g . Such an error in RUE g can occur when the calculated canopy gross photosynthesis is higher or IPAR is lower than observations. Comparison of the calculated and observed canopy gross photosyn- thesis at Los Banos did not show the calculated gross photosynthesis to be noticeably higher than the ob- servations results not shown. As noted in Section 4.2.1, Kiniry et al. 1989 had estimated IPAR from measured L o and incident irradiance, assuming PAR extinction coefficient to be 0.65. The results from radiative transfer model Choudhury, 2000, used to determine IPAR and irradiance on leaves, gave an average extinction coefficient of 0.62. This difference in the extinction coefficient can cause ca. 3 differ- ence in IPAR when L o = 1, and the difference would be much less for higher L o . Thus, the above consid- erations are not providing an understanding of 19 discrepancy in Table 3. While Eqs. 1b and 11 describe variation of the maximum rate of leaf photosynthesis within a canopy due to variation in leaf nitrogen content, observations show that different cultivars of rice achieve varied maximum leaf net photosynthesis during its vegeta- tive phase: 19–32 mmol m − 2 s − 1 n = 50; Kawamitsu and Agata, 1987, and 38–40 mmol m − 2 s − 1 n = 2; Peng et al., 1995. When calculations were done replacing A max of 34 mmol m − 2 s − 1 in Eq. 1b by 25 mmol m − 2 s − 1 , the average RUE and CUE for transplanted canopies n = 9 were found to be, re- spectively, 17 mmol mol − 1 and 0.58 mol mol − 1 for incident irradiance of 30 mol m − 2 per day at Los Banos. This calculated RUE is 6 higher than the estimated value rather than 19 Table 3. However, these average RUE and CUE for incident irradiance of 37 mol m − 2 per day at Los Banos were found to be, respectively, 15 mmol mol − 1 and 0.59 mol mol − 1 , and this RUE is 17 lower than the estimated value, rather than 0 Table 3. These comparisons strongly suggest that a knowledge of A max or the maximum leaf nitrogen content for a specific variety, if Eqs. 1a and 11 are assumed to describe also the variability of A max among cultivars is highly desirable for a better quantitative understanding of RUE of rice. 4.3.3. Stand characteristics There are uncertainties in the input stand character- istics Table 1 because of inaccuracies in the reported data and synthesis of data from different sources. For example, Daughtry and Hollinger 1984 found the co- efficient of variation of the foliage biomass and spe- cific leaf area to be, respectively, about 18 and 5 in the vegetative phase based on measurements on 20 plants selected randomly from uniform plots. They also noted that the natural variability of leaf area per plant may exceed 10 of the mean in an uniform field. Determining L o from separate sources for fo- liage biomass and specific leaf weight introduces un- certainty in L o . The effect of uncertainties in L o and nitrogen con- tent of foliage, stem and roots on RUE and CUE of Maize at Elora for the incident irradiance of 40 mol m − 2 per day is given in Table 4. It is seen that increases decreases in the nitrogen content de- creases increases the CUE due to increase decrease in maintenance respiration. Increasing L o by 25 is seen to decrease RUE by 3, and such an effect Table 4 Percent change in RUE and CUE of maize n = 24 at Elora when the incident irradiance is 40 mol m − 2 per day due to uncertainties in the input stand parameters. The results given are the effect of either increasing RUE+ and CUE+ or decreasing RUE− and CUE− leaf area index by 25, nitrogen content of foliage and stem by 15, and of roots by 35. A positive value for percent change appears when the effect of changing a specific parameter resulted in an increase of RUE or CUE with respect to the base values given in Table 2 Parameter RUE+ RUE− CUE+ CUE− Maize Leaf area index − 2.9 1.9 0.3 − 0.8 Foliage 1.0 − 1.9 − 0.5 0.5 Stem − 0.4 0.4 − 0.5 0.4 Roots − 0.6 0.6 − 0.6 0.6 B.J. Choudhury Agricultural and Forest Meteorology 106 2001 317–330 329 appeared because, while canopy gross photosynthesis increased by 4, RUE g decreased by 3 i.e., IPAR increased more than gross photosynthesis. And, because of increase in gross photosynthesis, CUE increased by 0.3 Eq. 9. Increasing the foliage nitrogen content by 15 is seen to decrease CUE by 0.5, but RUE increases by 1. Such an effect appeared because, while canopy gross photosynthesis and RUE g increased by 1.6 as a result of increases in the leaf photosynthesis Eq. 2a, this increase was insufficient to negate the increase in maintenance respiration. And, as a result, the CUE decreased. An increase decrease in the nitrogen content will de- crease increase CUE, which get directly reflected in the RUE Table 4. The effect of changing L o or foliage nitrogen content on RUE or CUE is somewhat complex, as noted above. Nevertheless, the results in Table 4 suggest that limited uncertainties in the stand characteristics affect RUE or CUE generally by a few percent.

5. Summary and conclusions