Nitrogen topdressing at panicle initiation modulated nitrogen allocation between storage proteins and free nitrogenous compounds in grains of japonica rice

Nitrogen (N) topdressing often leads to a decline in cooking and eating quality due to alterations in grain chemical composition. Investigation into the response of N compounds in grains, such as protein-bound amino acids (PAA), free amino acids (FAA), and other residue N compounds (N_residue), would be beneficial for understanding the impact of N on rice quality. The present study selected five japonica rice cultivars and conducted a two-year field experiment employing two N fertilization treatments: the CK treatment, where all N fertilizer was applied as basal fertilizer, and the topdressing treatment, where 50% of the total N was applied as topdressing at the panicle initiation stage. Averaging across years and cultivars, N topdressing resulted in a 16.3% increase in PAA and an 8.65% decrease in FAA, with no significant impact on N_residue. N topdressing resulted in a decrease in the proportion of essential amino acids found in prolamins, as well as a reduction in the proportion of amino acids belonging to the aspartate family and arginine. Significant genotypic variations were observed in the response of nitrogen components to topdressing. Furthermore, elite cultivars with premium quality exhibited greater stability in amylose content compared to protein content.