Excessive Leaf Rolling Reduces Grain Yield by Disrupting Source–Sink Balance in Rice (Oryza sativa L.)

Optimizing rice (Oryza sativa L.) plant architectype is an important approach to coordinating the source–sink relationship and unlocking yield potential. In this study, using the large-panicle rolled-leaf variety ST-12 and the small-panicle flat-leaf variety Nipponbare, we systematically compared plant architectype traits, photosynthetic characteristics, biomass accumulation, carbohydrate accumulation and remobilization, source–sink characteristics, and yield under two nitrogen levels in field conditions to test the hypothesis that excessive leaf rolling influences the accumulation and translocation of photosynthetic products and disrupts the source–sink balance. The results showed that Nipponbare exhibited significantly higher yield than ST-12 under both nitrogen levels, attributable to its higher number of productive panicles, grain-filling percentage, and thousand-grain weight. Although ST-12 had a higher single-leaf photosynthetic rate and leaf area index, its top three leaves were excessively rolled, reducing its canopy light interception and canopy photosynthetic rate, thereby leading to significantly lower stem NSC content at heading and biomass accumulation during grain filling compared with Nipponbare. Notably, ST-12 had higher contents of cellulose, hemicellulose, and lignin in the stem at heading, directing more pre-anthesis photosynthetic products into structural carbon, while the translocation of non-structural carbohydrates to grains was not affected. Further analysis revealed that ST-12 had a lower source capacity, sugar–spikelet ratio, source–spikelet ratio, and source–sink ratio than Nipponbare, whereas its total spikelet number and sink capacity were significantly higher. Correlation analysis showed that source characteristic indices and the source–sink ratio were positively correlated with yield, grain-filling percentage, and thousand-grain weight, while sink characteristic indices were negatively correlated with these traits. In conclusion, excessive leaf rolling impairs canopy photosynthesis, leading to a large sink but weak source imbalance. For large-panicle varieties, a higher source–sink ratio, not simply larger sink size or total biomass, is the key to high yield.