Photosynthesis and photorespiration in submerged aquatic vascular plants

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 CO 2 , however, could not be achieved, even at 3500 μl ℓ −1 CO 2 . 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 CO 2 and in aerial leaves of Myriophyllum saturation was being approached at 2100 μl ℓ −1 CO 2 .The high resistance of submerged leaves to CO 2 transfer makes accurate measurements of photosynthesis and photorespiration difficult. Apparent photosynthesis, though, was stimulated in 2% O 2 and inhibited in 50% O 2 compared with the rates in air. Oxygen sensitive CO 2 evolution into CO 2 -free air in the light was measured and compensation points of 31 to 75 μl ℓ −1 CO 2 were determined. These observations indicate that photorespiration exists in submerged aquatic vascular plants, but the rate of the process cannot be accurately measured.