ISOETES HOWELLII: A SUBMERGED AQUATIC CAM PLANT?

Abstract

In the leaves (but not corms) of the submerged aquatic plant Isoetes howellii , malic acid concentration fluctuates from 1–3 mg g –1 FW in the evening to 7–13 mg g –1 FW in the morning. Associated with this is a change in pH (a.m. pH 3–4 vs. p.m. pH 5–6) and titratable acidity (75–200 μ eq g –1 FW change in acidity between morning and evening) of the plant extract. 14 CO 2 ‐fixation experiments indicate that carbon is fixed in both the light and the dark, though the amount of carbon fixed in the light is more than double that fixed in the dark. Autoradiographs show 89% of dark‐fixed CO 2 ends up in malic acid and the remainder in citric acid, whereas these two acids constitute less than 5% of the light‐fixation products. It is suggested that CAM metabolism in this aquatic species may be related to the lower availability of CO 2 for photosynthesis during the day than during the night in its aquatic environment.