Photorespiration and internal recycling of CO2 in the submersed angiosperm Scirpus subterminalis

Abstract

The presence and magnitude of photorespiration in the submersed freshwater angiosperm Scirpus subterminalis Torr. was investigated by gas-exchange characteristics in an open water-flow system. The minimal rates of photorespiration during active photosynthesis were measured by following the time course of differential 14CO2 and 12CO2 uptake. At 8 mg O2 L−1 (equal to oxygen saturation at 20 °C), the rate was 0.4 μg C (mg organic dry weight)−1 h−1, which was about 10% of net photosynthesis under the experimental conditions. Increasing the oxygen concentration to 30 mg O2 L−1, enhanced photorespiration to 30% of net photosynthesis. It was shown that the concentration of oxygen affected net photosynthesis, CO2 evolution into CO2-free water in the light, the post-illumination CO2 burst, and the CO2 compensation point.The effect of the internal gas space on recycling of CO2 was investigated by comparing gas-exchange by intact and sectioned leaves. About 30% of the CO2 of photorespiratory origin was recycled internally within the lacunal system. The gas-exchange characteristics of Scirpus were similar to those observed in terrestrial C3 species although of a minor magnitude and a different time course. The ecological implications of these finds are discussed.