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 atmosphe ric CO2 concentration and air temperature has been examined at the gre enhouse 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 an d 1995 dry seasons at three different CO2 concentrations (ambient, amb ient + 200, and ambient + 300 mu L L(-1) CO2) and two different air te mperatures (ambient and ambient + 4 degrees C) using open-top field ch ambers. Averaged for both seasons, increases in CO2 concentration alon e (+ 200, + 300 mu L L(-1)) resulted in a significant increase in tota l plant biomass (+ 31%, + 40%) and crop yield (+ 15%, + 27%) compared with the ambient control. The increase in crop yield was associated wi th an increase in the number of panicles per square meter and a greate r percentage of filled spikelets. Simultaneous increases in CO2 and ai r temperature did not alter the biomass at maturity (relative to eleva ted CO2 alone), but plant development was accelerated at the higher gr owth temperature regardless of CO2 concentration. Grain yield, however , became insensitive to CO2 concentration at the higher growth tempera ture. Increasing both CO2 and air temperature also reduced grain quali ty (e.g., protein content). The combination of CO2 and temperature eff ects suggests that, in warmer regions (i.e., > 34 degrees C) where ric e is grown, quantitative and qualitative changes in rice supply are po ssible if both CO2 and air temperature continue to increase.