Photosynthetic carbon metabolism in Hydrilla verticillata Royle

Abstract

520-00Photosynthetic characteristics of the aquatic angiosperm Hydrilla verticillata Royle were studied under different experimental conditions. Photosynthetic rate closely paralleled the free CO2, concentration when only free CO2, was available to the leaves. When total inorganic carbon concentration was maintained at a constant level, photosynthetic rate varied inversely with pH changes. The highest rate was obtained at pH 5.0 and the lowest at pH 9.0. Thus free CO2, is the active species of carbon used in photosynthesis. Photosynthetic rate was linear with increasing CO2, concentration up to 2000 ??l/I. The CO2, compensation point for Hydrilla was found to be 0 ??l/I. Saturation rates of photosynthesis were obtained only at a very high CO2, concentration, suggesting high resistances to CO2, diffusion in the leaves. At pH 8.2, which was the pH of lake water at the time of collection, the photosynthetic rate was 17 ??moles ^14CO2 fixed per milligram chlorophyll per hour, a rate which was slightly higher than was expected from the free CO2, available in the medium. The possible reasons for this are discussed. RuBP carboxylase activity was comparatively much higher than PEP carboxylase activity in leaf extracts. 3-phosphoglyceric acid was the major initial stable photosynthetic product of CO2 fixation in light. There was a stimulation of the release of prefixed carbon from, leaves with increasing oxygen saturation both in the light and in the dark. Light/dark respiration ratios were less than one at all levels of O2 studied. Respiration was stimulated with increasing oxygen levels but apparent photorespiration was absent. Photosynthesis in Hydrilla leaves resembles that of the bundle sheath cells of C4 plants.