Effects of nitrogen fertilization on nitrogen metabolism and grain protein in premium-tasty japonica rice

Premium-tasty japonica rice, distinguished by its unique flavor and enriched nutritional value, has attracted significant attention across Asia. Nitrogen metabolism plays a crucial role in determining the growth, development, and quality of rice grains. This study investigates the effects of nitrogen application rates on the enzymatic activities related to nitrogen metabolism, nitrogen metabolites, and grain protein content in the premium-tasty japonica rice variety, Suxiangjing 100. Using a range of nitrogen application rates (0, 80, 160, 240, 320, and 400 kg per hectare), comprehensive analyses were conducted across various developmental stages. Results indicate that as nitrogen application increased, there were distinct patterns in the enzyme activities and metabolite concentrations related to nitrogen metabolism in the leaves of premium-tasty japonica rice. The activities of nitrate reductase (NR), glutamine synthetase (GS), and glutamate synthase (GOGAT) peaked at moderate nitrogen applications, while excessive nitrogen led to a decrease in their activities. The activity of Glutamate Dehydrogenase (GDH) generally increased, but showed a decline after the filling stage. With increasing nitrogen application, the concentrations of nitrate nitrogen (NO3–N), ammonium nitrogen (NH4–N), free amino acids (FAA), and soluble proteins (SP) initially rose but declined at excessive application rates. The grain protein content increased, with levels of prolamin, glutelin, and globulin peaking at nitrogen applications of 240 kg/ha and 320 kg/ha, while changes in albumin levels were minimal. This study not only provides novel insights into the role of nitrogen in shaping the growth and quality of premium-tasty japonica rice, but also suggests refined nitrogen management strategies for sustainable agricultural practices.