Modeling radiation- and carbon-use efficiencies of maize, sorghum, and rice

A previously developed model for radiation-use efficiency (RUE) for gross photosynthesis and net carbon accumulation by wheat before anthesis [Agric. Forest Meteorol. 101 (2000) 217], with some improvement, has been applied to maize, sorghum, and rice during their vegetative period under unstressed conditions. The objective of the present study is to assess the extent to which the model can provide RUE for net carbon accumulation and carbon-use efficiency (CUE; the ratio of daily net carbon accumulation and gross photosynthesis) for a range of incident irradiance and leaf area indices of maize, sorghum and rice, recognizing that (a) these crops are grown in substantially different environmental conditions than those for wheat, and (b) while rice is a C3 crop like wheat, maize and sorghum are C4 crops. The calculated RUE values for net carbon accumulation differed from observations (five for maize, two for sorghum and three for rice) by −24% to +19%, with an average (n=10) of −4% (underestimation), while the calculated CUE values are found to be within the range of observations. The model parameters have not been calibrated or adjusted for these comparisons. Calculations suggest that there is much room to increase the RUE of sorghum over the currently available measurements, although it might not attain the potential maximum value for maize. The effects of variations in the maximum rate of leaf photosynthesis among cultivars, changes in the relationship between maximum rate of photosynthesis by leaves and its nitrogen content, and uncertainties in the input canopy parameters on RUE and CUE are assessed by sensitivity analysis.