Changes in Internal Atmosphere of Submersed Vascular Hydrophytes in Relation to Photosynthesis

Abstract

Gases were extracted from the internal atmosphere of Elodea canadensis and Ceratophyllum demersum in a closed system under reduced pressure. The volume of extractable gases varied throughout the daily period. Gases separated and quantitatively measured by gas chromatography included CO2, O2, N2, and CH4. Results from diurnal studies showed a lag between peak values for dissolved oxygen in the surrounding water and oxygen in the internal atmosphere. Lags were also observed in the CO2 values. As a result photosynthetic activity by these plants is not immediately reflected by changes in dissolved oxygen content of the surrounding water. The lacunal system apparently functions as a reservoir for metabolic gases. Experiments in whcih light intensity was controlled by floating screens indicated that at low intensities evidence of oxygen production in E. canadensis may be obtained only by observing the changes which occur within the internal atmosphere. At these intensities no increase in dissolved oxygen can be detected in the surrounding water although internal oxygen concentrations may increase slightly. Confirmation that carbon was being fixed by the plants under these conditions has been obtained by carbon—14 uptake studies using gas phase counting procedures. The observations cast furthur doubt on the reliability of using changes in dissolved oxygen as a sole measure of photosynthetic activity in aquatic ecosystems where submersed vascular plants are dominant.