3.1 Differences in air-temperature and daylength between the two rice-cultivation seasons
Different patterns of air-temperature variation were found between the early (ES) and late (LS) rice-cultivation seasons (Figure 1a, grey lines), based on the analysis of daily air-temperature data (10 years) of Leizhou in Guangdong Province, China. The average air-temperature raised gradually in ES, and decreased in LS. In addition, the average air-temperature was significant 1.7 ℃ lower in ES (25.9 vs . 27.6 ℃, p <0.001) than that in LS (Figure 1b). Noticeably, much lower air-temperature was detected in the ES during the initial growth stages (Figure 1b). For example, during the seedling stage (S), the average air-temperature of ES was significant 8.2 ℃ lower (20.9vs . 29.1 ℃, p <0.001) than that of LS, and it was supported by the measured air- and soil-temperature in the common garden experiments (Figure S2). Similarly, average air-temperature was 4.6 ℃ lower (24.2 vs . 28.8 ℃,p <0.001) in ES during the tillering stage (T). These results indicated the significant lower air-temperature in ES than that in LS, particularly during the initial growth stages.
Results also showed different patterns of daylength between ES and LS (Figure 1a, black lines). The average daylength increased gradually in ES, and decreased in LS. The average daylength was significant 0.45 h longer (12.76 vs . 12.31 h, p <0.001) in ES than that in LS (Figure 1c). In addition, significant differences in daylength were detected at different rice growth stages, particularly during the flowering (F) and ripening stage (R) (Figure 1c). For example, the average daylength was 0.98 h longer (13.11 vs . 12.13 h, p <0.001) in ES during the flowering stage, and was 1.81 h longer (13.37 vs . 11.56 h, p <0.001) in ES during the ripening stage. These results suggested the significantly longer daylengths in ES than that in LS, particularly during the reproductive growth stages.