**2.3 Discussion**

254 Solar Radiation

Solar radiation

0500–0600 −0.108 −0.035 0.042 1.124 0600–0700 −0.129 −0.007 0.026 0.601 0700–0800 −0.102 −0.003 0.029 0.045 0800–0900 −0.131 −0.002 0.027 0.059 0900–1000 −0.147 −0.001 0.024 0.008

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

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

> Air temperature

'Fujihikari' 45.2 48.6 13.7 3.6 'Koshihikari' 28.9 39.4 15.3 4.0 'Xiaomazhan' 3.9 50.1 12.6 0.8 'Milky Queen' 28.1 26.0 15.3 3.4 'IR72' 37.1 48.0 14.4 1.4 'Takanari' 5.0 20.3 10.9 1.6 'Asahi' 33.1 45.4 11.5 1.2

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

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

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,

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

based on Japan Standard Time and the significance of the rice cultivar.

Standardized partial regression coefficient

Vapor-pressure deficit

Vapor-pressure deficit

Wind speed

> Wind speed

Period

relatively high.

**cultivar** 

Cultivar Solar

general linear models (Table 3.).

radiation

Air temperature

> Higher solar radiation after sunrise resulted in an earlier flower opening time because the six cultivars excluding 'Takanari' showed negative correlations between the flower opening time and mean hourly solar radiation for every hour between 0500 and 1000 (Fig. 4). High solar radiation in the morning is related to an early flower opening time in several rice cultivars (Kobayasi et al., 2010). Similarly, solar radiation from 0400 to 0800 influenced the flower opening time in 'Koshihikari', but not in EG0 (Nakagawa & Nagata, 2007). In this experiment, cultivar differences in the sensitivity of the flower opening time to solar radiation were observed. The cultivars examined in this experiment were classified into two groups: cultivars with negative, high correlations between solar radiation and the flower opening time and those with relatively weak correlations. However, some of our results were inconsistent in that 'Koshihikari' showed a high response to solar radiation in previous studies (Kobayasi et al., 2010; Nakagawa & Nagata, 2007), but not in this experiment. In 2010, record heat and sunshine occurred, and maximum air temperature was higher than 34°C for 23 days, while, in normal years, the maximum air temperature is rarely higher than 34°C. The mechanism of solar radiation and air temperature in determining the flower opening time is not unclear. Air temperature and solar radiation may synergistically affect the flower opening time by altering panicle tissue temperature.

> The correlation coefficients between the flower opening time and hourly solar radiation dropped rapidly at five hours before flower opening for each hour over seven 1-hr periods based on the mean flower opening time in the six cultivars except 'Xiaomazhan' whose flowers opened before 1000 (Fig. 5). The sensitivity of rice flowers to solar radiation during flower opening may increase before flower opening. Pollen grains continue to develop until

flowering occurs, and they accumulate with starch 1 day before anthesis (Koike & Satake, 1987). After the end of starch engorgement in the pollen grains, the starch is rapidly digested at the end of the grain opposite to the germ poles 3–4 hr before flower opening; and more than 70% of pollen grains become sugar-type grains by the time of anther dehiscence (Koike & Satake, 1987). These findings suggest that development in sugar-type grains and the digestion of starch in the pollen grains are related to the beginning of flower opening. The digestion of starch in the pollen grains starts immediately after sunrise except in 'Xiaomazhan'. These results suggest that an increase in solar radiation triggers flower opening processes. However, the mechanism(s) by which solar radiation triggers starch digestion remains unclear. Pollen grains in anthers receive little solar radiation because they are covered by the palea and lemma of a rice flower. Solar radiation may increase anther temperature and promote starch digestion.

We found the highest multiple correlation coefficient (adjusted R2 = 0.849, *p* < 0.001) for the 0800–0900 period (Table 3). During this period, three factors (cultivar, 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 contributions of vapor-pressure deficit and wind speed to the flower opening time were relatively small. These results indicate that it is necessary to consider two weather factors (air temperature and solar radiation) when analyzing the effects of weather on the flower opening time and, the contributions of vapor-pressure deficit and wind speed to the flower opening time were small. We estimated the contributions (hr) to the flower opening time. The contributions of 'Xiaomazhan', 'IR72', 'Takanari', 'Fujihikari', 'Asahi', 'Koshihikari', and 'Milky Queen' were −2.16, −0.89, −0.37, −0.01, 0, 0.41, and 0.437, respectively. Three indica cultivars ('Xiaomazhan', 'IR72', 'Takanari') showed early morning flowering, and the contribution to the flower opening time advancement was the highest in 'Xiaomazhan'. This result agreed with the previous results that 'Xiaomazhan' showed the earliest flower opening time in Japan (Kobayasi et al., 2010) and Jiangsu Province, China (Zhao et al., 2010). On the other hand, 'Milky Queen' did not show early morning flowering in this experiment. This result suggests that weather factors are also important in determining the flower opening time, and the consideration of all weather effects is required in breeding rice cultivars with early morning flowering.

Two distinctive peaks were observed in the diurnal change of multiple correlation coefficients in each cultivar (Fig. 7). One peak was immediately after sunset (2000–2300), and the other was immediately after sunrise (0500–0700). This suggests that other aspects of light conditions such as light cycle may influence the flower opening time in addition to the amount of solar radiation. The flowers of rice plants grown in a chamber tend to open 1–2 hr later than those grown outdoors (Imaki et al., 1982). Artificial dark or light treatments have been reported to affect the flower opening time (Nishiyama & Blanco, 1981). The effects of the diurnal cycle of light and temperature on the flower opening time in *P. nil* flowers have been studied experimentally (Kaihara & Takimoto, 1979, 1980, 1981a, 1981b, 1983). *P. nil* flowers subjected to various photoperiods bloomed approximately 10 hr after light-off when the light period was 10 hr or longer and approximately 20 hr after light-on when the light period was shorter (Kaihara & Takimoto, 1979). The higher air temperature during the dark

flowering occurs, and they accumulate with starch 1 day before anthesis (Koike & Satake, 1987). After the end of starch engorgement in the pollen grains, the starch is rapidly digested at the end of the grain opposite to the germ poles 3–4 hr before flower opening; and more than 70% of pollen grains become sugar-type grains by the time of anther dehiscence (Koike & Satake, 1987). These findings suggest that development in sugar-type grains and the digestion of starch in the pollen grains are related to the beginning of flower opening. The digestion of starch in the pollen grains starts immediately after sunrise except in 'Xiaomazhan'. These results suggest that an increase in solar radiation triggers flower opening processes. However, the mechanism(s) by which solar radiation triggers starch digestion remains unclear. Pollen grains in anthers receive little solar radiation because they are covered by the palea and lemma of a rice flower. Solar radiation may increase anther

We found the highest multiple correlation coefficient (adjusted R2 = 0.849, *p* < 0.001) for the 0800–0900 period (Table 3). During this period, three factors (cultivar, 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 contributions of vapor-pressure deficit and wind speed to the flower opening time were relatively small. These results indicate that it is necessary to consider two weather factors (air temperature and solar radiation) when analyzing the effects of weather on the flower opening time and, the contributions of vapor-pressure deficit and wind speed to the flower opening time were small. We estimated the contributions (hr) to the flower opening time. The contributions of 'Xiaomazhan', 'IR72', 'Takanari', 'Fujihikari', 'Asahi', 'Koshihikari', and 'Milky Queen' were −2.16, −0.89, −0.37, −0.01, 0, 0.41, and 0.437, respectively. Three indica cultivars ('Xiaomazhan', 'IR72', 'Takanari') showed early morning flowering, and the contribution to the flower opening time advancement was the highest in 'Xiaomazhan'. This result agreed with the previous results that 'Xiaomazhan' showed the earliest flower opening time in Japan (Kobayasi et al., 2010) and Jiangsu Province, China (Zhao et al., 2010). On the other hand, 'Milky Queen' did not show early morning flowering in this experiment. This result suggests that weather factors are also important in determining the flower opening time, and the consideration of all weather effects is required

Two distinctive peaks were observed in the diurnal change of multiple correlation coefficients in each cultivar (Fig. 7). One peak was immediately after sunset (2000–2300), and the other was immediately after sunrise (0500–0700). This suggests that other aspects of light conditions such as light cycle may influence the flower opening time in addition to the amount of solar radiation. The flowers of rice plants grown in a chamber tend to open 1–2 hr later than those grown outdoors (Imaki et al., 1982). Artificial dark or light treatments have been reported to affect the flower opening time (Nishiyama & Blanco, 1981). The effects of the diurnal cycle of light and temperature on the flower opening time in *P. nil* flowers have been studied experimentally (Kaihara & Takimoto, 1979, 1980, 1981a, 1981b, 1983). *P. nil* flowers subjected to various photoperiods bloomed approximately 10 hr after light-off when the light period was 10 hr or longer and approximately 20 hr after light-on when the light period was shorter (Kaihara & Takimoto, 1979). The higher air temperature during the dark

temperature and promote starch digestion.

in breeding rice cultivars with early morning flowering.

period resulted in a later flower opening time with the temperature during the last half of the dark period having a stronger effect than that during the first half (Kaihara & Takimoto, 1980). At the lower temperature, the flower opening time is probably determined by the time of the latest preceding light-off (or light-on) signal (Kaihara & Takimoto, 1981a). Rice plants grown in a glasshouse or growth chamber have been reported to open flowers 1–2 hr later than those grown outdoors (Imaki et al., 1982).

In *P. nil* flowers, Kaihara & Takimoto (1981b) found that petals of the buds are the sites of photo- and thermo-perception; flower opening is caused mainly by the epinasty of petal midribs. On the other hand, we do not know the sites of photo- and thermo-perception in rice. Rice flowers lack petals and sepals. Rice lodicules, which are considered to be organs homologous to petals, expand when a rice flower opens. However, lodicules are covered with a palea and lemma and receive a low level solar radiation. Furthermore, plant growth regulators affect the flower opening time. Among plant growth regulators, abscisic acid promotes the flower opening time in *P. nil* (Kaihara & Takimoto, 1983). In rice, methyl jasmonate affects the flower opening time (Kobayasi & Atsuta, 2010; Zeng et al., 1999). Methyl jasmonate is important for tapetum, stamen, and pollen development in rice (Hirano et al., 2008). Some of plant growth regulators in anthers or lodicules may be triggers for flower opening by increasing solar radiation and air temperature. Advanced study of the sites of photo- and thermo-perception and of the mechanism for flower opening time determination, with consideration of plant growth regulators is needed. Under field conditions, where light intensity varies, it may be difficult to separate the effects of light intensity and the cycle of light and dark. Future research is necessary to study the effect of the light cycle on the flower opening time using growth chambers or supplementary light under field conditions to separate this factor from the effects of light intensity.
