Energy budget and transpiration characteristics of rice grown under elevated co₂ and high temperature conditions as determined by remotely sensed canopy temperatures

The effects of elevated CO₂ concentration and high temperatures on transpiration and gaseous diffusive resistances of rice canopy were investigated. Akihikari and IR36 cultivars were grown under two CO₂ concentrations ([CO₂], 365 and 700 μL L^-1) × three temperatures (29.8, 30.4 and 32.5°C on average over the experimental period), created by two Temperature Gradient Chambers. From 2 August (panicle initiation) to 22 August (booting), measurements were made of dry and wet bulb temperatures, canopy surface temperatures (T_c) and net radiation along with evapotranspiration (E) measurements by microlysimeters. Aerodynamic resistance (r_a), obtained from the measured E and microclimate data, showed a fairly constant value (11.7 s m^-1)- Then, r_a, T_c and microclimates data were substituted into energy budget equations to obtain E and canopy resistance (rc). In all plots, calculated E was in good agreement with measurement by lysimeters, and r_c reached minimum values (rcjnm) at solar radiation above 500 W m^-2. Elevated [CO₂] at the lowest temperature plot increased rm by 40-49% and T_c by 1.4-1.6°C and it reduced E by 14-16% of those under ambient CO2 conditions. With the rising growth temperature, these effects of elevated [CO₂] drastically decreased. The observed r, responses to temperature and [CO₂] seemed to have reflected a long-term acclimation of rice to these environments. These results indicate that anticipated global warming significantly reduces the advantageous effects of elevated [CO₂] on plant water use.