Photosynthetic HCO3− Utilization and OH− Excretion in Aquatic Angiosperms

The utilization of HCO(3) (-) as carbon source for photosynthesis by aquatic angiosperms results in the production of 1 mole OH(-) for each mole CO(2) assimilated. The OH(-) ions are subsequently released to the medium. In several Potamogeton and Elodea species, the site of the HCO(3) (-) influx and OH(-) efflux are spatially separated. Described here are light- and dark-induced pH changes at the lower and upper epidermis of the leaves of Potamogeton lucens, Elodea densa, and Elodea canadensis.In the light, two phases could be discerned. During the first phase, the pH increased at both sides of the leaves. This pH increase apparently resulted from CO(2) fixation. During the second, so-called polar phase, the pH at the upper side increased further, but the pH at the lower side dropped below the pH of the ambient solution. The pH drop at the lower epidermis indicates that the K(+) influx exceeds the net CO(2) (HCO(3) (-) + CO(2)) influx slightly. This may result either from a proton pump driving an extra K(+) influx or from CO(2) diffusion from the cells into the outer medium previously taken up as HCO(3) (-). In the dark, a CO(2) gush was observed at both sides. During the polar phase, the upper side becomes electrically negative with respect to the lower side. Subsequent depolarization in the dark revealed that this potential difference consisted of a fast and a slow component.