Rice Growth, Yield and Photosynthetic Responses to Elevated Atmospheric Carbon Dioxide Concentration and Drought

J.T. Baker and L.H. Allen, Jr.

Rice is a major food crop that should respond favorably to expected future increases in atmospheric carbon dioxide concentration. Due to uncertainties in the timing and amounts of monsoonal rainfall, drought is common in some rainfed rice production systems. In this paper, we summarize results of experiments conducted by the University of Florida and USDA-ARS at Gainesville, FL, USA where the effects and interactions of elevated atmospheric carbon dioxide concentration and periodic drought were examined in relation to grain yield and canopy-scale gas exchanges, specifically photosynthesis, respiration, and evapotranspiration. Elevated CO2 increased rice growth, grain yield and canopy photosynthesis while reducing evapotranspiration by about 10%. During drought stress cycles, this water savings under elevated CO2 allowed photosynthesis to continue for one to two days longer compared with the ambient CO2 treatment. Rice canopy photosynthesis saturated with respect to CO2 near 500 µmol mol-1 and we found little evidence of photosynthetic acclimation or down-regulation in response to long-term CO2 enrichment treatments of 350 and 700 µmol mol-1. Under a much broader range of long-term CO2 treatments (160 to 900 µmol mol-1), a significant degree of photosynthetic down regulation was detected. Daytime CO2 enrichment resulted in higher canopy dark respiration compared with the ambient grown controls when compared at a common, near ambient nighttime CO2. We also detected a rapid and reversible, direct inhibition of canopy dark respiration rate with rising chamber CO2 at an air temperature of 28°C.