The influence of environmental factors on apparent photosynthesis and respiration of the submersed macrophyte Elodea canadensis

Oxygen effects on apparent photosynthetic and dark respiratory O 2 exchange rates of detached leaves of Elodea canadensis Michx. (Hydrocharitaceae) were determined over a range of conditions which the submersed plant is likely to experience in shallow water. Apparent photosynthesis is inhibited by O 2 under all the experimental regimes of light, temperature, CO 2 concentration and pH. This inhibition is not caused solely by an accelerated rate of dark respiration, and the observed variations in O 2 inhibition are comparable to O 2 effects on photosynthesis and photorespiration of terrestrial C 3 plants. Percentage inhibition of apparent photosynthesis is enhanced by high O 2 and also by low CO 2 . These results indicate that high O 2 , high pH and low CO 2 conditions could cause major losses in photosynthetic activity under field conditions. This may account for some of the losses in biomass that are observed under still water conditions.