Leaf nitrogen, plant age and crop dry matter production in rice

Photosynthetic rates and radiation use efficiency (RUE) in rice (Oryza sativa L.) increase with higher concentrations of en, but this relationship changes with crop ontogeny. We propose a model of the rice crop that takes into account the effects of both leaf N status and ontogenetic stage on photosynthesis. The model is based on the photosynthetic rate of single leaves with the maximum rate expressed as a function of leaf N and plant age represented by a developmental index (DVI). The maintenance coefficient at 25 degrees C, M(25), is also expressed as a function of DVI and is derived from the fraction of metabolically active organs. The simulated crop performance gave a good presentation of actual crop performance under an independent range of different N and climatic conditions. The RUE, defined as the quotient of dry weight over intercepted radiation, was estimated to decrease gradually by about 0.5 g MJ(-1) between the mid reproductive stage and maturity even when leaf N concentration remained constant. Without the age functions that take development stage into account, estimates of biomass production may be out by ca. 15%. We conclude that this model can be combined with other submodels that describe LAI dynamics and N uptake to analyze the effect of N on rice growth over a wide range of circumstances.