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 mumol/mol). During two seasons, rice crops were grown under three different experimental treatments; namely, at 570 mumol/mol (i.e. 'elevated') and 370 mumol/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.