Post‐Heading High Nighttime Temperature Impairs Grain Protein‐Starch Balance and Rice Quality Through Altering Nitrogen Metabolism

High nighttime temperatures (HNT) tend to diminish rice quality by disrupting assimilate translocation and grain filling process in rice (Oryza sativa L.). However, there is controversy remains regarding whether source or sink limitation are the primary driver under HNT during grain filling period. Additionally, the physiological mechanisms underlying the genotypic variation in the response of grain protein content to HNT and its effect on rice quality have been less explored. To address whether nitrogen remobilization from leaves to grains during grain filling determines genotypic differences in grain quality under high night temperature, two cultivars - HHZ (Huanghuazhan, an indica inbred line) and YY4949 (Yongyou4949, an indica-japonica hybrid) - were treated with 30/22°C (day/night, CK) and 30/27°C (HNT) over two consecutive years. Significant genotypic variation in the response of grain storage substances to HNT was observed between the two cultivars. Under HNT, YY4949 exhibited a significant increase in grain protein content and glutelin/prolamin ratio, and this shift negatively impacted rice eating and cooking quality. Notably, the protein/amylose ratio exhibited a stronger correlation with chalkiness degree and pasting characteristics of rice flour. Under HNT, accelerated nitrogen remobilization from leaves to grains in YY4949 - driven by enhanced chloroplast degradation and upregulated expression of nitrogen metabolism-related enzymes and transporters exacerbated source limitation to rice quality and disrupted the balance between starch and protein in grains. Collectively, these findings suggest that genetic modulation of nitrogen remobilization could facilitate the breeding of climate-resilient rice cultivars with superior grain quality.