Rice responses to drought under carbon dioxide enrichment. 2. Photosynthesis and evapotranspiration

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Abstract
Future climate change is projected to include a strong likelihood of continued increases in atmospheric carbon dioxide concentration ([CO2]) and possible shifts in precipitation patterns. Due mainly to uncertainties in the timing and amounts of monsoonal rainfall, drought is common in rainfed rice production systems. The objectives of this study were to quantify the effects and possible interactions of [CO2] and drought stress on rice (Oryza sativa, L.) photosynthesis, evapotranspiration and water‐use efficiency. Rice (cv. IR‐72) was grown to maturity in eight naturally sunlit, plant growth chambers in atmospheric carbon dioxide concentrations [CO2] of 350 and 700 μmol CO2 mol–1 air. In both [CO2], water management treatments included continuously flooded controls, flood water removed and drought stress imposed at panicle initiation, anthesis, and both panicle initiation and anthesis. Potential acclimation of rice photosynthesis to long‐term [CO2] growth treatments of 350 and 700 μmol mol–1 was tested by comparing canopy photosynthesis rates across short‐term [CO2] ranging from 160 to 1000 μmol mol–1. These tests showed essentially no acclimation response with photosynthetic rate being a function of current short‐term [CO2] rather than long‐term [CO2] growth treatment. In both long‐term [CO2] treatments, photosynthetic rate saturated with respect to [CO2] near 510 μmol 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 [CO2] enrichment allowed photosynthesis to continue for about one to two days longer during drought in the enriched compared with the ambient [CO2] control treatments.

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