Adaptive response of cultivated rice (Oryza sativa ssp. indica) flowers to low sunlight intensity.

Records of sunlight intensity at anthesis of 388 rice landraces (with 32 replicated populations), flowering in the short-day season of 2022, reveal that under cloudy condition, the rice florets tend to open later, or the sunrise-to-anthesis duration (SAD) is longer when rice florets open than at sunny period. This difference in the length of SAD was statistically highly significant (p < 0.0001), confirmed by a two-sample permutation test with 10,000 iterations. This finding corroborates our general observation previously reported from a larger set of 1114 landraces (including the 388 landraces examined in this study). However, the intensity of sunlight at the flower opening time (FOT) may not remain uniformly sunny (high illuminance) or cloudy (low illuminance, < 40,000 lx) until the flower closing time (FCT). To understand the effect of uniformly low sunlight intensity, we subsequently recorded solar illuminance at FOT of 33 landrace populations (including 8 repeats). Half of each population was kept under artificial shade, compared to the other half exposed to sunlight. This experiment revealed that low illuminance, mimicking overcast days, significantly (p < 0.02) delays FOT and lengthens SAD, corroborating the pattern detected in our earlier findings. Permutation test with 10,000 iterations decisively confirms (p < 0.0001) the prolongation of SAD under shade and cloudy conditions. Experimental shading has an indeterminate effect on flower exposure duration (FED) of the same landraces. We surmise that the delayed FOT during natural cloudy period is an adaptation in rice plants in anticipation of rain, for protection of the pollen from rainwash.