Peer 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, CO2 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 CO2 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 CO2 assimilation rate and leaf N content. Radiation use efficiency within each species was nearly constant at high leaf CO2 assimilation rates, but decreased appreciably at low leaf CO2 assimilation rates. At the leaf CO2 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.

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