Response of Photosynthesis and Water Relations of Rice (Oryza sativa) to Elevated Atmospheric Carbon Dioxide in the Subhumid Zone of Sri Lanka

The objective of the present paper is to determine the response of the physiological parameters related to biomass production and plant water relations in a standard Sri Lankan rice (Oryza sativa) variety (BG‐300) to elevated CO₂ (i.e. 570 µmol/mol). During two seasons, rice crops were grown under three different experimental treatments; namely, at 570 µmol/mol (i.e. ‘elevated’) and 370 µmol/mol (‘ambient’) CO₂ within open top chambers, and at ambient CO₂ under open field conditions. Leaf net photosynthetic rate in the elevated treatment increased by 22–75 % in comparison to the ambient. However, the ratio between intercellular and ambient CO₂ concentrations remained constant across different CO₂ treatments and seasons. CO₂ enrichment decreased individual leaf stomatal conductance and transpiration rate per unit leaf area, and increased both leaf and canopy temperatures. However, the overall canopy stomatal conductance and daily total canopy transpiration rate of the elevated treatment were approximately the same as those achieved under ambient conditions. This was because of the significantly greater leaf area index and greater leaf–air vapour pressure deficit under CO₂ enrichment. The leaf chlorophyll content increased significantly under elevated CO₂; however, the efficiency (i.e. photochemical yield) of light energy capture by Photosystem II (i.e. F_v/F_m) in chlorophyll a did not show a significant and consistent variation with CO₂ enrichment.