Leaf Nitrogen, Photosynthesis, and Crop Radiation Use Efficiency: A Review

Effects of N in crops are profound, but much understanding of crop growth responses to N is empirical. This review attempts to develop a mechanistic understanding of the effects of N on crop biomass accumulation by elucidating quantitative relationships among leaf N content, CO 2 assimilation rate, and crop radiation use efficiency. Three crop species were considered: soybean ( Glycine max [L.] Men.), rice ( Oryza sativa L.), and maize ( Zea mays L.). The correlation between leaf N content and leaf CO 2 assimilation rates was high within each species, although the response functions were markedly different among species. A relationship was developed predicting crop radiation use efficiency (biomass accumulated per unit solar radiation intercepted) for each of the crops as a function of both leaf CO 2 assimilation rate and leaf N content. Radiation use efficiency within each species was nearly constant at high leaf CO 2 assimilation rates, but decreased appreciably at low leaf CO 2 assimilation rates. At the leaf CO 2 assimilation rates typical of a species, the radiation use efficiency was predicted to be about 1.2 g MJ −1 for soybean, 1.4 g MJ −1 for rice, and 1.7 g MJ −1 for maize. Simple calculations during early crop growth examined the competitive use of N for the construction of either large leaf area or high leaf N content. Maize had the greatest biomass accumulation because it had low leaf N contents that allowed the most crop leaf area growth, and it had high radiation use efficiencies. For each rate of N supply to leaves, an optimum leaf N content existed to maximize crop biomass accumulation.