**2. Experiment 1: Effects of solar radiation on the flower opening time in rice**

Global climate change poses a serious challenge to crop production around the world. In rice (*Oryza sativa* L.), temperatures higher than 34 °C at the time of flowering may induce flower sterility and decrease yields, even in temperate regions such as southern Japan, if the cropping season is not changed to avoid such temperatures (Horie et al., 1996; Kim et al., 1996). Crop simulation models (Horie et al., 1996) have suggested that yields of currently grown rice varieties in southern Japan would be reduced by up to 40% under future climate scenarios. Serious yield losses in rice due to flower sterility occurred in the Yangtze Valley of China in 2003, when the temperatures during the hottest summer in the region's history affected the reproductive stage of rice cultivated in this region (Wang et al., 2004).

Early morning opening of rice flowers is a beneficial response to avoid sterility caused by heat stress during anthesis because the sensitivity of rice flowers to high temperatures decreases during the 1-hr period after flower opening (Satake & Yoshida, 1978). Thus, a flower opening time, 1 hr earlier than normal may reduce the risk of sterility because it may lead to anthesis before the air temperature reaches the critical level; air temperature can rise at a rate of higher than 3 °C hr−1 starting at approximately 1000 (Nishiyama & Blanco, 1980). A controlled environment experiment revealed that flowers of 'Milyang 23' that opened earlier in the morning and at a lower temperature had higher seed sets than those that opened later (near midday) and at higher temperatures (Imaki et al., 1987).

Thus, the selection of cultivars with early flower opening times is an important method for reducing heat-induced sterility (Ishimaru et al., 2010; Jagadish et al., 2008; Nishiyama & Blanco, 1980). For example, the flowers of *O. glaberrima* Steud. open earlier than those of *O. sativa* L. (Jagadish et al., 2008; Nishiyama & Blanco, 1980), and the flowers of interspecific hybrids between *O. glaberrima* and *O. sativa* open earlier than those of *O. sativa* (Nishiyama & Satake, 1981). Reduced sterility in the early morning flowering line subjected to rising temperatures during anthesis in the greenhouse was attributed to an earlier flowering time compared with 'Koshihikari' (Ishimaru et al., 2010).

Although the flower opening time is under genetic control, it is affected by aspects of the weather such as solar radiation and air temperature (Hoshikawa, 1989; Imaki et al., 1983; Jagadish et al., 2007, 2008; Kobayasi et al., 2010; Nakagawa & Nagata, 2007; Nishiyama & Satake, 1981). The relationship between the flower opening time and solar radiation has been researched under field conditions (Kobayasi et al., 2010). Using correlation analysis, most japonica cultivars showed a negative correlation between solar radiation and flower opening times; this correlation showed that higher solar radiation resulted in earlier flower opening times. However, the indica cultivar 'IR72' did not show a negative correlation between the solar radiation and flower opening time. This result suggests that the contribution of solar radiation to the flower opening time is different among ecotypes, and compared with japonica cultivars, indica cultivars have lower sensitivity in determining the flower opening time by solar radiation. Moreover, the response of flower opening to high temperature differs among rice cultivars. The flower opening time occurs earlier at high temperatures in 'Milyang 23', whereas it occurs later in 'Nipponbare' (Imaki et al., 1983). In a study of indica cultivars, the flower opening time occurred approximately 45 min earlier at higher temperatures (Jagadish et al., 2007). Furthermore, the relationship between solar radiation and air temperature should be incorporated into the analysis of the flower opening time because the amount of solar radiation is one of the most important factors in determining air temperature. It has been suggested that synergistic effects on the flower opening time may exist between temperature and light (Kobayasi et al., 2010). In addition to solar radiation and air temperature, other weather factors such as vapor-pressure deficit and wind speed (Tsuboi, 1961) affect the flower opening time. However, the combined effects of

flower sterility and decrease yields, even in temperate regions such as southern Japan, if the cropping season is not changed to avoid such temperatures (Horie et al., 1996; Kim et al., 1996). Crop simulation models (Horie et al., 1996) have suggested that yields of currently grown rice varieties in southern Japan would be reduced by up to 40% under future climate scenarios. Serious yield losses in rice due to flower sterility occurred in the Yangtze Valley of China in 2003, when the temperatures during the hottest summer in the region's history

Early morning opening of rice flowers is a beneficial response to avoid sterility caused by heat stress during anthesis because the sensitivity of rice flowers to high temperatures decreases during the 1-hr period after flower opening (Satake & Yoshida, 1978). Thus, a flower opening time, 1 hr earlier than normal may reduce the risk of sterility because it may lead to anthesis before the air temperature reaches the critical level; air temperature can rise at a rate of higher than 3 °C hr−1 starting at approximately 1000 (Nishiyama & Blanco, 1980). A controlled environment experiment revealed that flowers of 'Milyang 23' that opened earlier in the morning and at a lower temperature had higher seed sets than those that

Thus, the selection of cultivars with early flower opening times is an important method for reducing heat-induced sterility (Ishimaru et al., 2010; Jagadish et al., 2008; Nishiyama & Blanco, 1980). For example, the flowers of *O. glaberrima* Steud. open earlier than those of *O. sativa* L. (Jagadish et al., 2008; Nishiyama & Blanco, 1980), and the flowers of interspecific hybrids between *O. glaberrima* and *O. sativa* open earlier than those of *O. sativa* (Nishiyama & Satake, 1981). Reduced sterility in the early morning flowering line subjected to rising temperatures during anthesis in the greenhouse was attributed to an earlier flowering time

Although the flower opening time is under genetic control, it is affected by aspects of the weather such as solar radiation and air temperature (Hoshikawa, 1989; Imaki et al., 1983; Jagadish et al., 2007, 2008; Kobayasi et al., 2010; Nakagawa & Nagata, 2007; Nishiyama & Satake, 1981). The relationship between the flower opening time and solar radiation has been researched under field conditions (Kobayasi et al., 2010). Using correlation analysis, most japonica cultivars showed a negative correlation between solar radiation and flower opening times; this correlation showed that higher solar radiation resulted in earlier flower opening times. However, the indica cultivar 'IR72' did not show a negative correlation between the solar radiation and flower opening time. This result suggests that the contribution of solar radiation to the flower opening time is different among ecotypes, and compared with japonica cultivars, indica cultivars have lower sensitivity in determining the flower opening time by solar radiation. Moreover, the response of flower opening to high temperature differs among rice cultivars. The flower opening time occurs earlier at high temperatures in 'Milyang 23', whereas it occurs later in 'Nipponbare' (Imaki et al., 1983). In a study of indica cultivars, the flower opening time occurred approximately 45 min earlier at higher temperatures (Jagadish et al., 2007). Furthermore, the relationship between solar radiation and air temperature should be incorporated into the analysis of the flower opening time because the amount of solar radiation is one of the most important factors in determining air temperature. It has been suggested that synergistic effects on the flower opening time may exist between temperature and light (Kobayasi et al., 2010). In addition to solar radiation and air temperature, other weather factors such as vapor-pressure deficit and wind speed (Tsuboi, 1961) affect the flower opening time. However, the combined effects of

affected the reproductive stage of rice cultivated in this region (Wang et al., 2004).

opened later (near midday) and at higher temperatures (Imaki et al., 1987).

compared with 'Koshihikari' (Ishimaru et al., 2010).

air temperature, solar radiation, vapor-pressure deficit, and wind speed on the flower opening time of various rice genotypes remain unclear, particularly under field conditions; this limits our ability to predict the flower opening time.

The cycle of solar radiation may also affect the flower opening time as well as the amount of solar radiation. Most studies on the effects of weather factors on the flower opening time have been conducted under controlled environments with artificial light conditions (Imaki et al., 1983; Jagadish et al., 2007, 2008; Nishiyama & Blanco, 1981) and the flowers of rice plants grown in a glasshouse or a growth chamber have been reported to open 1–2 hr later than those grown outdoors (Imaki et al., 1982). This suggests that solar radiation substantially affects the flower opening time; not only the strength of solar radiation but also light conditions can influence the flower opening time because the duration of anthesis increases under continuous light or dark conditions (Hoshikawa, 1989). The light intensity and cycle of light and dark may affect the flower opening time. The effects of a diurnal cycle of light on the flower opening time in dicotyledonous *Pharbitis nil* flowers have been experimentally studied under artificially controlled conditions (Kaihara & Takimoto, 1979).

In this section, first, the role of solar radiation in determining the flower opening time was evaluated using correlation analysis between solar radiation and the flower opening time. The correlations between the flower opening time and solar radiation averaged hourly, from 0500 to 1000 were analyzed. Second, we used general linear models to separately evaluate the effects of the type of cultivars, air temperature, solar radiation, vapor-pressure deficit, and wind speed on the flower opening time. In the second analysis, we used two types of 1 hr time spans: a 1-hr time span based on Japan Standard Time, and a 1-hr time span based on the time of mean flower opening times in each cultivar; this eliminated the effects of the circadian rhythm. Finally, we attempted to detect the roles of the diurnal change in solar radiation in determining the flower opening time.
