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

Photosynthesis and respiration of the submerged plant Elodea nuttallii (Pla nch.) St, John were investigated over a wide range of environmentally-relev ant conditions of pH (pH 5-9), O-2 (31-625 mu M) and dissolved inorganic ca rbon (0.15-24 mM) concentrations. Increases in O-2 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 wit h O-2 concentration (6.0 mu M at 62.5 mu M O-2, 16.3 mu M at 313 mu M O-2, and 54.6 mu M at 500 mu M O-2), pH caused a decrease in photosynthesis abov e pH 7 and below pH 6, The former was attributed to low CO2 availability un der these conditions, and the latter to a direct effect of acid conditions on the plant. The same response was found using buffered and unbuffered med ia. 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 b oundary layer thickness. Within the range of concentrations found in stands of E. nuttallii in the field, the effect of decreasing CO2 availability wa s capable of reducing photosynthesis. (C) 2000 Elsevier Science B.V. All ri ghts reserved.