Genotypic Responses of Rice to Ammonium and Nitrate under Contrasting Water Regimes: Agronomic, Biochemical, and Gene-Expression Analyses

Considering the increasing water and labor constraints in rice production, direct-seeded rice is becoming increasingly important. However, direct-seeded rice is vulnerable to water stress and inefficient nutrient use. Nitrogen use efficiency in rice is generally low, and improving nitrogen acquisition and assimilation is important for sustainable productivity. In this study, 48 recombinant inbred lines were evaluated under ammonium and nitrate nutrition in irrigated and rainfed environments across two cropping seasons. Agronomic, physiological, biochemical and molecular responses were assessed. Significant effects of genotype, nitrogen form, environments and their interactions were observed for several traits. Rainfed conditions reduced grain and biological yield compared with irrigated conditions. Ammonium produced a higher mean grain yield than nitrate by approximately 26.2 g m-2 under irrigated conditions and 154.2 g m-2 under rainfed conditions, although the responses varied among genotypes. Biological yield and SPAD values were generally higher with ammonium, whereas total chlorophyll showed season and environment-dependent responses. Glutamine synthetase activity was highest under ammonium treatment, while glutamate synthase activity was higher under nitrate treatment. Grain yield showed strong positive correlations with biological yield, but correlations between grain yield and GS or GOGAT activity were not significant. Expression of OsGln1.2, OsGln2 and OsGlt1 varied according to genotype, tissue and nitrogen treatment. Overall, the results demonstrate that responses to nitrogen forms are genotype- and environment-dependent, and that ammonium may provide a yield advantage under rainfed conditions. Integrating field performance with biochemical and molecular traits may help identify rice genotypes suited to water-limited systems.