Water‐Related Physiological Traits and Aquaporin Gene Expression in Flag Leaves and Panicles of Rice Cultivars Differing in Tolerance to High‐Temperature‐Induced Grain Quality Deterioration

ABSTRACT Deterioration in the appearance quality of brown rice due to heat‐stress‐induced ripening damage is a severe agricultural problem exacerbated by global warming. Five rice ( Oryza sativa L.) cultivars differing in their tolerance to heat‐stress‐induced ripening (‘Fukei 227,’ ‘Fusaotome,’ ‘Komanomai,’ ‘Sasanishiki,’ and ‘Hatsuboshi’) were used to measure the water potential and aquaporin‐related gene expression in flag leaves and panicles at heading to examine their correlations with brown rice appearance traits at harvest. Post‐heading mean air temperatures exceeded 26°C for all cultivars, confirming ripening under heat‐stress conditions. In heat‐stress‐tolerant cultivars, higher water potentials of flag leaves and elevated OsPIP2;2 and OsTIP1;2 expression levels were associated with lower milky‐white and immature grain percentages. In panicles, lower water potentials combined with the suppressed expression of OsPIP2;1 , OsPIP2;2 , OsPIP2;4 , and OsNIP2;1 genes and the upregulated expression of OsTIP2;1 , OsTIP2;2 , and OsTIP4;2 genes were associated with lower percentages of all grain categories with poor appearance quality. These results indicate that in heat‐stress‐tolerant cultivars, a coordinated organ‐specific water‐control strategy operates at heading. The aquaporin‐mediated increase in water transport maintains source function in flag leaves, whereas the suppression of the plasma‐membrane‐specific aquaporin genes and the upregulation of the tonoplast‐specific aquaporin genes stabilize sink function in panicles, collectively preserving brown rice appearance quality. ‘Hatsuboshi’ maintained a high appearance quality through a distinct mechanism independent of these water‐related physiological traits.