Applying nitrogen fertilizer improves the indica rice (Oryza sativa L.) quality by coordinating enzyme activity and grain‐filling rate

Abstract Background and Objectives The effects of nitrogen (N) fertilization on rice quality, grain‐filling rate, and starch synthase activity have been widely studied, but whether there is a relationship between grain‐filling rate and starch synthase activity has not been reported. We conducted a 2‐year field experiment using the indica rice cultivar Shenliangyou 5814 to investigate the effects of N fertilization (0, 90, 180, 270, and 360 kg·ha –1 ) on rice quality. Findings N fertilization notably increased grain yield, head rice rate, and protein content while reducing chalkiness degree and rate, thereby enhancing milling, appearance, and nutritional quality. Meanwhile, N fertilizer application (N = 270–360 kg·ha –1 ) delayed the time to reach the maximal grain‐filling rate, extending it to approximately 18.4–19.9 days after flowering, and enzyme activity also peaked around the same time. Grain‐filling rate was positively correlated with the activities of AGPase and SBE after N fertilization. They were also significantly correlated with milling, appearance, and nutritional quality after 22 days of flowering. Conclusions N fertilization achieved a harmonious balance between the grain‐filling rate and starch synthase activities, ultimately improving milling, appearance, and nutritional quality. Significance and Novelty N fertilization regulated the coordination between grain‐filling rate and starch synthase activity for rice quality.