Effects of Temperature, Solar Radiation, and Vapor-Pressure Deficit on Flower Opening Time in Rice

Flower opening in the early morning helps to avoid sterility of rice (Oryza saliva L.) caused by heat stress at anthesis. Although flower opening time (FOT) is under genetic control, it is also affected by weather, particularly by air temperature (T(a)). However, the effects of T(a), solar radiation (R(s)), and vapor-pressure deficit (VPD) on rice FOT are unclear, making it difficult to predict FOT. Therefore, we investigated the correlation of FOT with T(a), R(s), and VPD during various periods before anthesis under field conditions. By photographing spikelets at 10-min intervals, we determined the FOT of five cultivars. To evaluate the individual effects of cultivar, T(a), R(s), and VPD on FOT, we constructed general linear models (GLMs) and calculated mean T(a), R(s), and VPD every 3 hr from 0000 to 1200. The GLMs revealed that the average T(a), R(s), and VPD between 0600 and 0900 significantly affected FOT (adjusted R(2)=0.399; P < 0.001). The standardized partial regression coefficients of T(a) and R(s) were negative and those of VPD were positive, indicating that higher T(a), higher R(s), and lower VPD in the early morning result in earlier FOT. Moreover, multiple-regression analysis showed that the period affecting FOT the most, and the relative contributions of T(a), R(s), and VPD to FOT differ with the cultivar.