Adaptation of Elodea and Potamogeton to different inorganic carbon levels and the mechanism for photosynthetic bicarbonate utilisation

The different abilities of three submerged aquatic macrophytes, Elodea canadensis, E. nutallii and Potamogeton lucens, cultured under high and low CO2 concentrations, to utilise bicarbonate was found to correlate with the ability to exhibit a polar leaf pH reaction (i. e. acidification of the medium on one side of the leaf). The utilisation of bicarbonate did not depend on the induction of extracellular carbonic anhydrase activity as found with unicellular algae. Although the bicarbonate utilisation was inhibited by acetazolamide, an inhibitor of carbonic anhydrase activity, there was no difference in the concentration of extracellular carbonic anhydrase between leaves with high and with low bicarbonate utilisation. In experiments using the isotopic disequilibrium method we found only a small contribution of bicarbonate to the carbon fixation of protoplast. The percentage of bicarbonate contribution to the fixation did not differ between protoplasts isolated from Potamogeton leaves with high bicarbonate utilisation (from a low CO2 culture) and from leaves with low bicarbonate utilisation (from a high CO2 culture). We conclude that bicarbonate utilisation depends on the polar leaf pH reaction and that CO2 is the carbon species that is taken up by the leaf