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

Summary

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.