Varietal Range in Transpiration Conductance of Flowering Rice Panicle and Its Impact on Panicle Temperature

Transpiration from rice (Oryza sativa L.) panicles can help lower the temperature of the panicle ( T p ), which is the susceptive organ for high temperature-induced spikelet sterility (HISS). By increasing the transpiration, the heat damage to the panicle predicted to occur due to global warming may be avoided. To examine the possibility of genetic improvement in transpiration conductance of intact rice panicles (gpI ), we measured gpI at the time of flowering in the open field in 21 rice varieties of widely different origins. We observed a difference in gpI among the varieties and three series of experiments, ranging from 0.15 to 0.67 cm s- 1 . We also estimated its impact on the difference between Tp and air temperature ( T a ) (Δtp, Tp - Ta) using a micrometeorology model, where T a was given as 28°C or 35°C. The varietal range in gpI was estimated to correspond to the range of 2.1°C in ΔTp under a humid atmospheric condition and the range of 3.5°C in Δ t p under a dry atmospheric condition. The estimated ΔTp ranges due to varieties may be useful for improving heat avoidance capacity under excessive heat at the critical stage. The sensitivity analysis of ΔTp to ranging gpI suggested that g pI higher than the highest gpI observed in this study may not be effective for additional cooling of T p . Thus, the target of improvement in gpI against HISS should be set at the level of the existing varieties with the highest gpI

Heat-Mitigation Effects of Irrigated Rice-Paddy Fields Under Changing Atmospheric Carbon Dioxide Based on a Coupled Atmosphere and Crop Energy-Balance Model

Known as the heat-mitigation effect, irrigated rice-paddy fields distribute a large fraction of their received energy to the latent heat during the growing season. The present hypothesis is that increased atmospheric CO2 concentration decreases the stomatal conductance of rice plants and increases the air temperature by means of an increased sensible heat flux. To test this hypothesis, a coupled regional atmospheric and crop energy-balance model is developed and applied to a 300 x 300 km(2) region in Japan. Downscaling meteorological variables from grid-mean values of mixed land use (3 x 3 km(2)) generates realistic typical diurnal cycles of air temperature in rice paddies and adjacent residential areas. The model simulation shows that, on a typical sunny day in summer, doubling the CO2 concentration increases the daily maximum grid-mean air temperature, particularly where rice paddies are present, by up to 0.7 degrees C. This CO2 effect on the grid-mean air temperature is approximately half the effect of the reduction in rice-paddy area that is postulated to occur on a time scale similar to that of the atmospheric CO2 change. However, within the internal atmospheric boundary layer of the rice paddies, the CO2 effect on the air temperature (+ 0.44 degrees C) still exceeds the effects of the land-use change (+ 0.11 degrees C). These results show a potentially important interplay of plant physiological responses regarding atmospheric CO2 in the heat-mitigation effect of rice-paddy fields under a changing climate