RICE RESPONSES TO DROUGHT UNDER CARBON-DIOXIDE ENRICHMENT .2. PHOTOSYNTHESIS AND EVAPOTRANSPIRATION

Future climate change is projected to include a strong likelihood of c ontinued increases in atmospheric carbon dioxide concentration ([CO2]) and possible shifts in precipitation patterns. Due mainly to uncertai nties in the timing and amounts of monsoonal rainfall, drought is comm on in rainfed rice production systems. The objectives of this study we re to quantify the effects and possible interactions of [CO2] and drou ght stress on rice (Oryza sativa, L.) photosynthesis, evapotranspirati on and water-use efficiency. Rice (cv. IR-72) was grown to maturity in eight naturally sunlit, plant growth chambers in atmospheric carbon d ioxide concentrations [CO2] of 350 and 700 mu mol CO2 mol(-1) air. In both [CO2], water management treatments included continuously flooded controls, flood water removed and drought stress imposed at panicle in itiation, anthesis, and both panicle initiation and anthesis. Potentia l acclimation of rice photosynthesis to long-term [CO2] growth treatme nts of 350 and 700 mu mol mol(-1) was tested by comparing canopy photo synthesis rates across short-term [CO2] ranging from 160 to 1000 mu mo l mol(-1). These tests showed essentially no acclimation response with photosynthetic rate being a function of current short-term [CO2] rath er than long-term [CO2] growth treatment. In both long-term [CO2] trea tments, photosynthetic rate saturated with respect to [CO2] near 510 m u mol mol(-1). Carbon dioxide enrichment significantly increased both canopy net photosynthetic rate (21-27%) and water-use efficiency while reducing evapotranspiration by about 10%. This water saving under [CO 2] enrichment allowed photosynthesis to continue for about one to two days longer during drought in the enriched compared with the ambient [ CO2] control treatments.