**2.2.1 Variations in the flower opening time during the observation periods**

Although flower opening time is under genetic control, wide ranges in flower opening time were recorded in all seven cultivars. Cultivar 'Xiaomazhan' flower opening time recorded 1–2 hr earlier than other cultivars. The flowers of the indica cultivars ('Xiaomazhan', 'IR72', and 'Takanari') opened earlier than those of the japonica cultivars ('Fujihikari', 'Koshihikari', 'Milky Queen' and 'Asahi'). The flowers of 'Milky Queen' did not open earlier than those of the other japonica cultivars although its flowers opened early in Japan (Kobayasi et al., 2010) and China (Zhao et al., 2010). The range in the flower opening time was higher than 2 hr in 'Fujihikari', 'Xiaomazhan', and 'IR72'.

## **2.2.2 Correlations between solar radiation and the flower opening time**

The six cultivars other than 'Takanari' showed negative correlations between the flower opening time and mean hourly solar radiation for every hour between 0500 and 1000 (Fig. 4). The cultivars can be classified into two groups. One group showed negative, high correlations and comprised the indica cultivar 'IR72' and japonica cultivars 'Fujihikari', 'Asahi', and 'Milky Queen'. The other group showed relatively weak correlations and comprised the indica cultivars 'Takanari' and 'Xiaomazhan' and the japonica cultivar 'Koshihikari'.


Table 2. Variations in the flower opening time, mean flower opening time, and range in flower opening time expressed as Japan Standard Time for seven *O. sativa* cultivars in 2010.

Fig. 4. Correlation coefficients between the flower opening time and hourly solar radiation for each hour from 0500 to 1000 in seven *O. sativa* cultivars.

Fig. 5. Correlation coefficients between the flower opening time and hourly solar radiation for each hour over seven 1-hr periods based on the mean flower opening time in each cultivar (successive 1-hr periods from 7 hr before mean flower opening time until 1 hr before flower opening time)

To examine the difference in response hours to solar radiation before flower opening among cultivars, correlation coefficients between the flower opening time and hourly solar radiation were calculated for each hour over seven 1-hr periods based on the mean flower opening time in each cultivar (successive 1-hr periods from 7 hr before mean flower opening time until 1 hr before flower opening time; Fig. 5). The correlation coefficients of the cultivars classified as the group that showed negative, high correlations (Fig. 4) dropped rapidly at five hours before flower opening. At five and four hours before flower opening, the correlation coefficients of the group with negative, high correlations remained at approximately −0.7. Although the obtained negative correlation was weak, the correlation coefficients of 'Takanari' and 'Koshihikari' dropped at 5 hr before flower opening. We did not calculate the correlation coefficients between flower opening time and solar radiation in 'Xiaomazhan' because the flowers of this cultivar opened before 1000.

Fig. 4. Correlation coefficients between the flower opening time and hourly solar radiation

Fig. 5. Correlation coefficients between the flower opening time and hourly solar radiation for each hour over seven 1-hr periods based on the mean flower opening time in each cultivar (successive 1-hr periods from 7 hr before mean flower opening time until 1 hr before

To examine the difference in response hours to solar radiation before flower opening among cultivars, correlation coefficients between the flower opening time and hourly solar radiation were calculated for each hour over seven 1-hr periods based on the mean flower opening time in each cultivar (successive 1-hr periods from 7 hr before mean flower opening time until 1 hr before flower opening time; Fig. 5). The correlation coefficients of the cultivars classified as the group that showed negative, high correlations (Fig. 4) dropped rapidly at five hours before flower opening. At five and four hours before flower opening, the correlation coefficients of the group with negative, high correlations remained at approximately −0.7. Although the obtained negative correlation was weak, the correlation coefficients of 'Takanari' and 'Koshihikari' dropped at 5 hr before flower opening. We did not calculate the correlation coefficients between flower opening time and solar radiation in

'Xiaomazhan' because the flowers of this cultivar opened before 1000.

for each hour from 0500 to 1000 in seven *O. sativa* cultivars.

flower opening time)

#### **2.2.3 Evaluation of the effects of solar radiation, on cultivar's, air temperature, vaporpressure deficit, and wind speed on the flower opening time**

The multiple correlation coefficients determined by generalized linear model increased after 0400 and peaked between 0700 and 0800 (Fig.6) and then slightly decreased after 0800. The highest multiple correlation coefficient (adjusted R2 = 0.849, *p* < 0.001) was obtained for the 0800–0900 period (Table 3). During this period, three factors (cultivar type, solar radiation, and air temperature) were significant (*p* < 0.05). The standardized partial regression coefficient for air temperature was −0.131, indicating that higher air temperature during this period resulted in an earlier flower opening time. Similarly, the standardized partial regression coefficient of solar radiation was −0.002, indicating that higher solar radiation during this period also resulted in an earlier flower opening time. The standardized partial regression coefficients for the other two weather factors (vapor-pressure deficit and wind speed) were not significant except for wind speed during the 0500–0600 period. The estimated contributions (hr) of each cultivar were −2.16, −0.89, −0.37, −0.01, 0, 0.41, and 0.437 for 'Xiaomazhan', 'IR72', 'Takanari', 'Fujihikari', 'Asahi', 'Koshihikari', and 'Milky Queen' respectively.

Fig. 6. The multiple correlation coefficients (adjusted R2) obtained from multiple regression analysis using general linear models. The correlations for four weather factors (air temperature, solar radiation, vapor-pressure deficit, and wind speed) and the flower opening time for the 1-hr periods based on Japan Standard Time were analyzed.



ns (not significant). \*, \*\*, and \*\*\* (significant at *p* < 0.05, *p* < 0.01, and *p* < 0.001) respectively.

Table 3. Results of multiple-regression analysis using general linear models for the correlations between five hourly-averaged weather factors (air temperature, solar radiation, vapor-pressure deficit, and wind speed) and the flower opening time for the 1-hr periods based on Japan Standard Time and the significance of the rice cultivar.

The contributions of the four weather factors (air temperature, solar radiation, vaporpressure deficit, and wind speed) were estimated for each cultivar (Table 4.). The contributions of solar radiation and air temperature were higher than those of vaporpressure deficit and wind speed. Among cultivars, the variation in solar radiation was larger than that in air temperature. The contribution of solar radiation in 'Fujihikari' was 45.2 min, whereas its contribution in 'Xiaomazhan' was 3.9 min. The contributions of solar radiation in the cultivars classified as the group that showed negative, high correlations (Fig. 4.) were relatively high.


Table 4. Estimated contributions (expressed as min) of the four weather factors (air temperature, solar radiation, vapor-pressure deficit, and wind speed) using the results of general linear models (Table 3.).

#### **2.2.4 Observation of a diurnal pattern in multiple correlation coefficients in each cultivar**

Two distinctive peaks were observed in the diurnal change of multiple correlation coefficients in each cultivar (Fig. 7). One peak was observed immediately after sunset (2000– 2300) and the other was observed immediately after sunrise (0500–0700). In 'Fujihikari', 'IR72', and 'Milky Queen', highest multiple correlation coefficients of 0.816, 0.559, and 0.571, respectively were obtained for the 0600–0700 period. In 'Xiaomazhan', the highest multiple correlation coefficient (0.720) was obtained for the 0500–0600 period. In these four cultivars, the peak immediately after sunrise was higher than that immediately after sunset. In 'Fujihikari' and 'Koshihikari', highest multiple correlation coefficients of 0.570 and 0.484, respectively were obtained for the 2100–2200 period. The highest multiple correlation coefficient (0.716) in 'Asahi' was obtained for the 2200–2300 period. In these three cultivars, the peak immediately after sunrise was lower than that immediately after sunset.

Fig. 7. Multiple correlation coefficients (Adjusted R2) for each hour between 1800 on the day before flowering and 0900 on the flowering day for each cultivar.
