**3. Experiment 2: Effects of solar radiation on rice panicle temperature**

Global warming could increase the probability of heat-induced sterility in rice, which is most sensitive during anthesis (Matsui et al., 1997a). In addition, an elevated CO2 concentration raises canopy temperature through stomatal closure, thereby exacerbating heat-stress-induced sterility in rice (Matsui et al., 1997b). The mechanism of heat-stressinduced sterility is explained by the inhibition of anther dehiscence when flowering under high temperature (Satake & Yoshida, 1978). Atmospheric temperature is usually employed to estimate temperature-dependent stress, but panicle temperature is obviously the major determinant of heat-induced sterility (Zhao et al. 2010). The results in section 2 suggest that panicle temperature also affects the flower opening time, which is strongly related with heat-stress-induced sterility in rice. Together with other microclimates and cultivar-related factors such as transpirational conductance and panicle shape, solar radiation can affect panicle temperature and exacerbate or mitigate temperature-dependent stresses. However, it is difficult to predict panicle temperature change under field conditions because panicles have shapes and characteristics that differ from those leaves on transpiration (Ishihara et al., 1990), which generally leads to a sizeable difference between panicle temperature and leaf/air temperatures (Nishiyama 1981).

Yoshimoto et al. (2005) developed a heat balance model to simulate panicle temperature and its transpiration. However, the model needs many parameters of microclimates. We developed a simple and convenient empirical model to estimate the contribution of solar radiation to panicle temperature using generalized linear models in Experiment 2. The contribution of solar radiation for panicle temperature was estimated using this model.
