Photosynthesis and photorespiration in submerged aquatic vascular plants

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.