Disrupted Source–Sink Balance Under Whole‐Season Warming (1°C–5°C) Alters Photosynthesis, Biomass Allocation, and Yield in Rice

ABSTRACT Global warming poses a significant threat to rice ( Oryza sativa L .) production by altering physiological processes and yield formation. Global air temperature is projected to increase by 1.0°C–5.7°C by 2100. However, studies investigating the effects of sustained warming above 2°C over the whole‐season of rice remain limited. In this study, four warming treatments of ambient, +1°C, +2°C, and +5°C were implemented in the natural light chambers to assess the responses of net photosynthetic rate, dry matter accumulation and partitioning, and yield components in medium‐season rice. Prior to heading, elevated air temperature reduced net photosynthetic rate, while prolonged exposure to supra‐optimal air temperature suppressed dry matter accumulation. Especially under the +5°C treatment, dry matter in leaves and stems decreased by 8.35% and 21.39%, respectively. After heading, elevated air temperature disrupted the source‐sink relationship by diminishing grain sink capacity, reallocating assimilates to vegetative organs. Consequently, both the chlorophyll content index and leaf area index increased, indirectly enhancing net photosynthetic rate during the later growth stages. Nevertheless, yield formation was ultimately constrained, and grain yields decreased by 3.89%, 14.30%, and 71.92% under warming treatments of +1°C, +2°C, and +5°C, respectively. This study provides quantitative evidence that whole‐season warming within the IPCC AR6 projected range affects rice growth and yield through physiological mechanisms, particularly when air temperature increases up to 5°C. These findings offer insights for developing climate‐resilient rice cultivars and informing adaptive agricultural strategies under future warming scenarios.