Dark CO2-fixation and diurnal malic acid fluctuations in the submerged-aquatic Isoetes storkii.

In the leaves (but not corms) of the submerged aquatic Isoetes storkii malic acid concentration fluctuated from 22 μeg g FW-1 in the evening to 171 μeg g FW-1 in the morning. Associated with this was a change in titratable acidity of 152 μeg g FW-1 between morning and evening. 14C carbon was fixed in both the light and the dark, though the amount of carbon fixed in the light was more than that fixed in the dark. Autoradiographs show 88% of 14CO2 fixed in the dark is recovered after 1 h, in malic acid and the remainder in one other unidentified product, whereas these two products contain less than 15% of the 14C fixed after 1 h exposure to 14CO2 in the light. It is suggested that CAM metabolism in this aquatic species may be related to the low availability of CO2 for photosynthesis during the day in its aquatic environment and that this metabolic pathway may prove common in the genus Isoetes.