Effects of free-air CO2 enrichment on leaf and panicle temperatures of rice at heading and flowering stage

The elevated CO2 concentration in the atmosphere could induce global warming and physiological changes in plants, e.g. stomatal closure. Through FACE (Free Air CO2 Enrichment) experiments in a paddy field, it has been confirmed that CO2-induced stomatal closure decreases transpiration, which causes a rise of leaf temperature (T-l). Thus, with elevated CO2, the probability of heat-induced spikelet sterility (HISS) of rice could increase. The objectives of this study were as follows: (1) Measure differences in stomatal conductance (g(s)), particle transpiration conductance (g(p)), T-l, and particle temperature (T-p) of rice plants between FACE plots (E-CO2) and ambient plots (A-CO2) at Wuxi, China (31 degrees 37' N, 120 degrees 28' E) at heading and flowering stages. (2) Model the heat balance on a panicle surface. (3) Simulate T-p and clarify the impacts of FACE on T-p using the model. Panicles emerged and flowered about 4 days earlier in E-CO2 than in A-CO2. The measured T-p was 1 - 2 degrees C higher in E-CO2 than in A-CO2, which was equivalent to or even higher than the difference in T-l between in the two plots. The g(p) decreased with panicle age after heading, while g(s) increased asymptotically with the increase of photosynthetically active radiation. But, under both CO2 conditions, g(p), was not significantly different for the same days after heading. By modelling heat balance on a panicle surface and by simulating T-p, influences of elevated CO2 on T-p were quantified. First, lower g(p) due to older age after flowering. Second, higher air temperature (T-a) followed by higher T-l. Third, higher long wave radiation emitted from leaves (L-l). The effects of T-a and g(p) on T-p were significant but the effect of g(p) on T-p was not significant. On the same number of days after heading, the net effect of g(p) on T-p would be almost zero. Therefore, raised T-a would be a critical factor for the increase in T-p.