Mechanisms of productivity improvement through source–sink relationships in rice (Oryza sativa L.) breeding for cold, high–latitude environments

Context Stable, high-yielding rice ( Oryza sativa L.) is essential for ensuring global food security, yet rice cultivation in cold, high-latitude regions poses significant challenges because of narrow cultivation periods and low-temperature stress. While most high-latitude regions rely on panicle-weight-type varieties, panicle-number-type cultivars with extremely early flowering have been developed in Hokkaido. However, the high-yielding mechanisms underlying these early-flowering panicle-number-type cultivars remain unclear. Objective This study aimed to elucidate the traits of rice cultivars that achieve stable, high yields in cold, high-latitude regions through source–sink interactions. Methods Ten rice cultivars developed in Hokkaido were evaluated over two years (2022 and 2023) to identify the mechanisms underlying their success in cold environments. The sink traits included yield components and spikelet distribution on primary and secondary branches. The source traits included the leaf area index (LAI), canopy surface temperature difference (CTd), and dry matter remobilization (DRM). Results The grain yield ranged from 530.4 to 929.7 g m −2 in 2022 and from 460.0 to 773.2 g m −2 in 2023. By simultaneously enhancing the sink capacity through an increased grain number and improving source function via a higher LAI and a lower CTd, modern cultivars significantly increased yield. Principal component analysis clearly distinguished high-yielding from low-yielding cultivars, with the first two components explaining 74.7% and 66.5% of the total variance in 2022 and 2023, respectively. Partial least squares structural equation modeling revealed that preanthesis source ability strongly influenced both sink strength (path coefficient: 0.788) and grain-filling source ability (path coefficient: 0.834), which contributed equally to yield (path coefficient: 0.468 and 0.442, respectively). The model explained 71% of the yield variation. Notably, the “Sorayutaka” cultivar, which exhibited high yield potential in both years, achieved stable, high productivity through a favorable source–sink balance. This cultivar presented a moderate number of panicles with many grains per panicle, a consistently low CTd throughout the growing season, and a superior DRM. Conclusion These findings provide a comprehensive framework for understanding yield formation mechanisms in panicle-number-type cultivars with extremely early flowering. The results highlight the importance of increasing preanthesis source ability while optimizing source–sink relationships for rice breeding in cold, high-latitude regions.