Enhanced Photosynthetic Capacity and Assimilate Transport Are Associated with Higher Yield in Super Hybrid Rice

Enhancing rice yield under high-input systems increasingly relies on optimizing physiological processes rather than further increasing external inputs. This study aimed to clarify the physiological basis underlying the yield advantage of super hybrid rice, focusing on photosynthetic capacity and assimilate transport. We compared super hybrid rice (Yliangyou 3218 and Yliangyou 5867) with super conventional rice (Zhendao 11 and Nanjing 9108) under field conditions in 2023–2024. Super hybrid rice consistently outperformed super conventional rice, with grain yield 19.7% higher in 2023 and 23.7% higher in 2024, primarily due to an increased number of spikelets per panicle, and grain yield was also positively correlated with photosynthetic capacity (net photosynthetic rate, stomatal conductance, maximum carboxylation rate, maximum electron transport rate and triose phosphate utilization rate). In 2024, spikelets per panicle and grain yield were also positively associated with phloem soluble sugar and vascular bundle number, indicating that enhanced assimilate transport contributed to higher spikelet formation. These results demonstrate that, compared to super conventional rice, the yield advantage of super hybrid rice is underpinned by coordinated enhancement of photosynthesis and assimilate transport, highlighting the importance of source–sink optimization for further yield improvement.