**1. Introduction**

244 Solar Radiation

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This chapter focuses upon the effects of solar radiation on the flower opening time and panicle temperature, both of which significantly affect heat-stress-induced sterility in rice, although solar radiation affects crop growth rates and production through photosynthesis, the fundamental physiological function of green plants. Three possible traits that may mitigate the damage caused by high temperature during the flowering period in rice have been proposed (Zhao et al., 2010). The first trait is the formation of a long dehiscence at the base of the anther, but this trait is under genetic control so that it is not affected by solar radiation. The second trait is early morning flower opening to avoid high temperatures during anthesis. The flower opening time can be affected by solar radiation (Kobayasi et al., 2010). The third trait is panicle temperature, which can also be affected by solar radiation as well as microclimates and cultivar-related factors such as transpirational conductance and panicle shape (Yoshimoto et al., 2005). The latter two traits are discussed in this chapter.

Two experiments were conducted in Shimane Prefecture, Japan to reveal the roles of solar radiation in determining the flower opening time and panicle temperature. In section 2, the effects of solar radiation on the flower opening time have been examined. Early morning opening of rice flowers is a beneficial response to avoid heat-stress-induced sterility because the sensitivity of rice flowers to high temperatures decreases during the 1-hr period after flower opening (Satake & Yoshida, 1978). The effects of solar radiation on the flower opening time were evaluated using generalized linear models. Cultivar differences in the contribution of solar radiation to the flower opening time were estimated. Furthermore, the roles of the diurnal change in solar radiation on determining the flower opening time were examined. In section 3, the effects of solar radiation on panicle temperature have been examined. Atmospheric temperature is typically used to estimate temperature-dependent stress. However, solar radiation also affects plant tissue temperature. An empirical model to estimate the contribution of solar radiation to panicle temperature was developed using generalized linear models.
