Growth and yield response of field-grown tropical rice to increasing carbon dioxide and air temperature

Although the response of rice (Oryza sativa L.) to increasing atmospheric CO2 concentration and air temperature has been examined at the greenhouse or growth chamber level, no field studies have been conducted under the tropical, irrigated conditions where the bulk of the world's rice is grown. At the International Rice Research Institute, rice (cv. IR 72) was grown from germination until maturity for the 1994 wet and 1995 dry seasons at three different CO2 concentrations (ambient, ambient + 200, and ambient + 300 mu L L(-1) CO2) and two different air temperatures (ambient and ambient + 4 degrees C) using open-top field chambers. Averaged for both seasons, increases in CO2 concentration alone (+ 200, + 300 mu L L(-1)) resulted in a significant increase in total plant biomass (+ 31%, + 40%) and crop yield (+ 15%, + 27%) compared with the ambient control. The increase in crop yield was associated with an increase in the number of panicles per square meter and a greater percentage of filled spikelets. Simultaneous increases in CO2 and air temperature did not alter the biomass at maturity (relative to elevated CO2 alone), but plant development was accelerated at the higher growth temperature regardless of CO2 concentration. Grain yield, however, became insensitive to CO2 concentration at the higher growth temperature. Increasing both CO2 and air temperature also reduced grain quality (e.g., protein content). The combination of CO2 and temperature effects suggests that, in warmer regions (i.e., > 34 degrees C) where rice is grown, quantitative and qualitative changes in rice supply are possible if both CO2 and air temperature continue to increase.