Organic-inorganic fertilizer combination coordinates starch and protein accumulation in bottom panicle grains to reduce chalkiness and enhance head rice yield.

Non-uniform grain filling within a rice panicle, particularly in bottom panicle grains, remains a major constraint to overall quality improvement. Organic-inorganic fertilizer combination can improve grain yield and soil fertility, while its effects on starch-protein balance and subsequent chalkiness and head rice yield in bottom panicle grains remain poorly understood. This research intended to explore the influences of organic-inorganic fertilizer combination on head rice yield, chalkiness, and starch and protein contents in rice grains, with a focus on understanding their relationship with grain development in the bottom panicle. A two-year field experiment was conducted with two organic fertilizer levels from 0 to 3000 kg ha-1 and five nitrogen (N) rates from 0 to 360 kg N ha-1 during 2021 and 2022. Compared with single N fertilization, organic-inorganic fertilizer combination remarkably raised head rice yield by 6.45% and decreased chalkiness degree by 12.32%, with the most pronounced improvements observed in bottom panicle grains. Physiological analysis demonstrated that this combined fertilization strategy regulated key enzymes involved in starch synthesis (AGP, GBSS, SSS, SBE) and protein synthesis (GS, GOGAT, GPT, GOT) during grain filling, leading to a more balanced starch/protein ratio in bottom panicle grains. Importantly, the quality improvement from organic-inorganic fertilizer combination is evident only under moderate N rates (≤180 kg N ha-1). At high N levels (≥270 kg N ha-1), organic fertilizer provides no additional benefit. Organic-inorganic fertilizer combination optimizes rice quality primarily by balancing protein and starch synthesis in bottom panicle grains, and the combination of 180 kg N ha-1 and 3000 kg ha-1 organic fertilizer is proposed as a reliable technical approach for rice production. The present research establishes a theoretical foundation to support integrated nutrient management strategies aimed at simultaneously improving rice milling performance and yield stability.