Characterizing the long-term variation in grain quality of super hybrid rice under the interactive effects of nitrogen and climate.

Super hybrid rice is widely cultivated for its high yield potential. However, systematic knowledge of how its grain quality has evolved alongside genetic yield improvement, and how it responds to variety factors, nitrogen (N) management, and climatic conditions, remains limited. Two-year field experiments were conducted at two subtropical sites using three representative super hybrid rice varieties representing different breeding phases and grown under four N application rates. In addition, historical grain quality data from all 136 super hybrid rice varieties released between 2005 and 2023 were analyzed to assess long-term trends associated with breeding progress. The historical analysis revealed that head rice rate increased by 0.25% per year, whereas chalky grain rate, chalkiness degree, and amylose content declined significantly over the same period. Consistent with these trends, field experiments showed that cultivars from more recent breeding phases exhibited higher head rice rates, lower chalkiness, and lower protein and amylose contents than earlier varieties. Random forest and structural equation modeling indicated that variety was an important contributor of variation in head rice rate, appearance traits, and amylose content under the present field conditions, with RF relative importance values ranging from 12.25% to 26.87% for these traits. Pre-anthesis temperature and solar radiation primarily influenced milling quality and chalkiness, whereas post-anthesis high temperature exerted stronger effects on grain morphology and starch-related traits. Nitrogen application showed trait-specific effects, with a strong association with protein content in the RF analysis (relative importance = 10.09%) and additional effects on milling and appearance traits under some N treatments. This study provides evidence that released super hybrid rice varieties showed clear changes in grain quality over release years, and that variety, nitrogen management and climatic conditions jointly affected grain quality traits under field conditions.