Flower opening in the early morning helps to avoid sterility of rice (
Oryza sativa 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 (
Ta). However, the effects of
Ta, 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
Ta,
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,
Ta,
R s, and VPD on FOT, we constructed general linear models (GLMs) and calculated mean
Ta,
R s, and VPD every 3 hr from 0000 to 1200. The GLMs revealed that the average
Ta,
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
Ta and
R s were negative and those of VPD were positive, indicating that higher
Ta, 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
Ta,
R s, and VPD to FOT differ with the cultivar.
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