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.
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