Cultivar differences in the grain protein accumulation ability in rice (Oryza sativa L.)

The demand for rice grain protein content (GPC) differs in different regions of the world. Despite large differences in GPC among cultivars, evaluation of the effects of genotype on GPC is difficult because GPC is influenced not only by cultivar traits (such as nitrogen uptake ability, sink size and heading date) but also by the environment. We hypothesized that grain protein accumulation ability (GPA) also affects GPC. The objective of this study was to clarify the differences in GPA among six lodging-tolerant, high-yielding Japanese cultivars: Bekoaoba, Habataki, Takanari, Hokuriku193, Momiroman, and Akenohoshi. To produce a wide variation in nitrogen availability per unit sink capacity (Nav), we used nitrogen topdressing at heading and spikelet-thinning treatment. In each cultivar, we found a logarithmic relation between GPC and Nav: GPC = A × Ln(Nav) + B, where A is the regression coefficient and B is a constant. A highly significant difference in regression coefficients among cultivars was found (P < 0.01). The regression coefficient was considered to be a measure of GPA; it varied from 0.969 in Bekoaoba to 1.820 in Takanari. This relation suggests that GPC is determined by Nav and GPA and that the environment affects GPC through Nav. GPA is a good criterion for evaluation of the effects of genotype on GPC. Nitrogen harvest index was highly significantly explained by multiple regression with GPA and the ratio of sink capacity to total dry matter production as independent variables, suggesting the influence of GPA on plant nitrogen dynamics during the grain-filling period. Therefore, it would be useful to determine the cultivars' GPA values for optimizing nitrogen management.