Development of a leaf-level canopy assimilation model for CERES-Malize

Calculating crop growth rate as the product of intercepted light and radiation use efficiency may yield inaccurate predictions under stress conditions. A more mechanistic simulation of photosynthesis and respiration may be required to improve model accuracy under stresses. We developed a leaf-level photosynthesis and respiration model for maize (Zea mays L.) and linked it to CERES-Maize. The new model has three components that simulate light absorption, instantaneous leaf gross assimilation, and canopy respiration. Daily solar radiation was fractioned into hourly direct and diffuse components, and transformed into hourly photosynthetically active radiation (PAR). Extinction coefficients for direct and diffuse PAR were calculated, and a hedgerow approach followed to restrict the calculation of leaf area index (LAI), light interception, and photosynthesis to the fraction of ground shaded by the canopy. Light absorption was calculated for sunlit and shaded fractions of LAI. Instantaneous gross assimilation per leaf considered the effects of light intensity, leaf age, and air temperature, and was integrated for daylight hours and green leaf area. Maintenance and growth components of respiration are computed separately. The new model was linked to CERES-Maize and provided reasonable estimates of instantaneous leaf gross assimilation as well as daily trends of canopy gross assimilation and respiration. It also conforms to the standards of CERES-Maize, thus requiring only a minimum set of daily weather inputs (solar radiation, maximum and minimum temperature, and rainfall). The model supports the simulation of leaf-level processes such as hail damage, mechanical damage, herbivory, leaf pathologies, detasseling, and others.