Nitrogen management regulates grain quality of rice ratoon crop by altering starch structure and protein composition

Ratoon rice system, which permits two harvests from a single planting, is expanding rapidly. This study investigated how nitrogen (N) management (N₀: no N input; N_bud: N for bud initiation; N_tiller: N for regenerated tillers; N_bud + N_tiller: combined application) modulates starch fine structure, protein accumulation, and their relationships with grain quality of rice ratoon crop. Compared to N₀, N application significantly altered grain quality through modifications in apparent amylose content, starch structure and physicochemical properties, and protein composition. The N_bud + N_tiller reduced the honeycomb-like structure, number (ND) and volume (VD) weighted mean diameters, median (D (50 %)) and 90th percentile (D (90 %)) of volume, and relative crystallinity, which contributed to superior milling and appearance qualities. However, it concurrently impaired Cooking and eating quality (CEQ), as evidenced by increased setback viscosity, pasting temperature, ΔH, and hardness, alongside decreased breakdown viscosity, balance, taste, exterior and overall score. These detrimental effects were attributed to the significant increases in grain protein (4.8 %-14.8 %) and glutelin (6.1 %-17.7 %) content induced by N_bud + N_tiller, an effect that outweighed the modifications in starch structure. These findings underscore the critical need to optimize N_bud + N_tiller to balance starch structure and protein composition, thereby achieving synergistic improvements in milling, appearance, and CEQ of rice ratoon crop.