Spatiotemporal Heterogeneity Characteristics of Rice Grain Quality and Its Response to Nitrogen Management

Optimizing nitrogen (N) management is crucial for high-quality rice (Oryza sativa L.) production. However, how N affects grain quality at different positions within a panicle remains unclear. This study evaluated the effects of different N application regimes on the milling, appearance, eating, and nutritional quality of grains at varying panicle positions. We used a japonica cultivar Wuyunjing 31 in a controlled pot experiment with three N treatments: N32:0 (early heavy N), N16:16 (split application with late N topdressing), and N16:0 (low-N control). Results showed that late N topdressing (N16:16) significantly improved head rice yield across all grain positions, which was linked to higher storage protein accumulation (especially glutelin) and larger length-to-width ratio. Conversely, late N application deteriorated appearance quality by increasing the chalky grain rate and chalkiness. This negative effect was most pronounced in superior grains on upper and middle branches. Furthermore, the N16:16 treatment consistently decreased amylose content while increasing albumin, prolamin, and glutelin levels, demonstrating a clear trade-off between carbon (C) and N sinks. We speculated that these intra-panicle differences result from increased competition for carbon resources between starch and protein synthesis pathways. Overall, precision N management should account for spatial differences in grain development to effectively balance rice yield and quality.