The Effects of FACE (Free-Air CO₂ Enrichment) on Temperatures and Transpiration of Rice Panicles at Flowering Stage

Elevated CO2 concentration raises canopy temperature through stomatal closure, which has the potential to exacerbate the heat-induced spikelet sterility (HISS) of rice. However, it is difficult to predict the panicle temperature change under increasing CO2. We observed the effects of FACE (free-air CO2 enrichment) on panicle temperature, and developed a heat balance model to simulate panicle temperature and its transpiration. Elevated CO2 reduced leaf stomatal conductance by about 30 % at the heading stage, which caused the leaf temperature to rise by about 1-2 ℃. The air temperature was higher by 0.5-1 ℃ and the relative humidity was lower by 5-8 % in elevated CO2 concentration (E-CO2) plot than in ambient CO2 concentration (A-CO2) plot. Panicle transpirational conductance changed diurnally in close association with atmospheric humidity. The mid-day panicle conductance was around 1 cm s–1 on the flowering day, but it decreased to 0.3 cm s–1 as the panicles aged and dried after flowering. A model simulation showed that the increase in panicle temperature purely due to E-CO2 was 0.5-1 ℃ on the flowering day, which is large enough to exacerbate HISS. It is also suggested that E-CO2 could accelerate panicle drying as well as warming with its transpirative stimulation.