The effect of changing environmental variables in the surrounding water on the physiology of Elodea nuttallii

Photosynthesis and respiration of the submerged plant Elodea nuttallii (Planch.) St. John were investigated over a wide range of environmentally-relevant conditions of pH (pH 5–9), O2 (31–625 μM) and dissolved inorganic carbon (0.15–24 mM) concentrations. Increases in O2 concentration produced a linear decline in photosynthesis and a linear increase in respiration. The effect of reduced CO2 availability was substantial, and best described by the Whittingham-Hill equation. Carbon dioxide compensation point varied with O2 concentration (6.0 μM at 62.5 μM O2, 16.3 μM at 313 μM O2, and 54.6 μM at 500 μM O2). pH caused a decrease in photosynthesis above pH 7 and below pH 6. The former was attributed to low CO2 availability under these conditions, and the latter to a direct effect of acid conditions on the plant. The same response was found using buffered and unbuffered media. Elodea canadensis responded similarly to pH except below pH 6, where no decrease was found. The influence of the boundary layer on photosynthesis was demonstrated qualitatively, photosynthesis was reduced with increased boundary layer thickness. Within the range of concentrations found in stands of E. nuttallii in the field, the effect of decreasing CO2 availability was capable of reducing photosynthesis.