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

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Abstract
The presence and magnitude of photorespiration in the submersed freshwater angiosperm S. 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 (equal to O2 saturation at 20.degree. C), the rate was 0.4 .mu.g C (mg organic dry wt) per h, which was about 10% of net photosynthesis under the experimental conditions. Increasing the O2 concentration to 30 mg O2/l, enhanced photorespiration to 30% of net photosynthesis. The concentration of O2 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.