A sensitivity analysis of the radiation use efficiency for gross photosynthesis and net carbon accumulation by wheat

Radiation use efficiency (RUE) provides a useful diagnostic approach for estimating carbon accumulation by terrestrial plant communities. A model for instantaneous gross photosynthesis by a canopy, considering sunlit and shaded leaves, variation of maximum rate of leaf photosynthesis within the canopy and a solution of the radiative transfer equation for propagation of direct and diffuse photosynthetically active radiation within the canopy, is numerically integrated in space (angular variation of radiances and depth within the canopy) and time (diurnal variation) to obtain daily total gross photosynthesis. Then, to obtain net carbon accumulation, growth and maintenance respiration at a prescribed temperature are calculated from nitrogen content of foliage, stem and root using field measurements for 27 canopies representing two cultivars of winter wheat and average data for five cultivars of spring wheat. The leaf area index (L0) of these canopies varies between 0.5 and 5.2. The results show that for any given irradiance, the coefficient of variation of RUE for gross photosynthesis and net carbon accumulation due to changes in L0 is generally less than 10%. Strongly linear relationships are found between the RUE and diffuse fraction of the incident radiation, with slope varying with L0. Temperature appears as an important factor determining RUE under predominantly cloudy conditions. The calculated RUE values are compared with observations.