Upper-leaf architecture was associated with grain yield variation in an indica-japonica rice recombinant inbred line...

Rice yield improvement depends on the coordinated optimization of plant architecture and physiological performance. In this study, an F8 indica-japonica recombinant inbred line (RIL) population derived from 'Zhongyouzao8' × 'Toyonishiki' was evaluated in a single field environment to examine how heading progression, flag-leaf SPAD dynamics around heading, and upper-leaf architecture were associated with grain yield. A total of 159 RILs and the two parents were phenotyped for grain yield, initial heading date, full heading date, pre-heading SPAD (SPAD1), post-heading SPAD at the onset of grain filling (SPAD2), flag leaf angle, plant height, and flag leaf traits. Substantial phenotypic variation and evident transgressive segregation were observed for most traits. Grain yield ranged from 2.62 to 9.88 t ha^-¹, and several traits showed moderate to high broad-sense heritability. Correlation, regression, principal component analysis, and high- versus low-yielding group comparisons suggested that, under the conditions evaluated, grain yield variation was more closely associated with upper-leaf architecture than with heading progression itself, whereas SPAD dynamics provided complementary but weaker information. However, the regression model explained only a limited proportion of yield variability, indicating that additional unmeasured yield components may also have contributed substantially to grain yield. Several preliminary candidate lines carrying relatively high grain yield and favorable combinations of flag leaf morphology and SPAD-related phenotypic profiles were identified from the single-environment screening. These findings are exploratory and require multi-environment validation before these trait combinations can be used as stable selection criteria in rice breeding.