Photosynthesis and photorespiration in submerged aquatic vascular plants

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Abstract
In experiments carried out in a water-vapour-saturated atmosphere, the photosynthesis of submerged leaves of Potamogeton and Myriophyllum was light saturated at a quantum flux of 200 μE m−2 s−1. Saturation of photosynthesis with CO2, however, could not be achieved, even at 3500 μl ℓ−1 CO2. In contrast, floating or aerial leaves of these plants showed saturation with light only at or above a quantum flux of 1200 μE m−2 s−1. In floating leaves of Potamogeton photosynthesis was saturated at 1000 μl ℓ−1 CO2 and in aerial leaves of Myriophyllum saturation was being approached at 2100 μl ℓ−1 CO2.The high resistance of submerged leaves to CO2 transfer makes accurate measurements of photosynthesis and photorespiration difficult. Apparent photosynthesis, though, was stimulated in 2% O2 and inhibited in 50% O2 compared with the rates in air. Oxygen sensitive CO2 evolution into CO2-free air in the light was measured and compensation points of 31 to 75 μl ℓ−1 CO2 were determined. These observations indicate that photorespiration exists in submerged aquatic vascular plants, but the rate of the process cannot be accurately measured.