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Isoetes howellii: A Submerged Aquatic Cam Plant?

Jon E. Keeley
American Journal of Botany
Vol. 68, No. 3 (Mar., 1981), pp. 420-424
Stable URL: http://www.jstor.org/stable/2442779
Page Count: 5
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Isoetes howellii: A Submerged Aquatic Cam Plant?
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Abstract

In the leaves (but not corms) of the submerged aquatic plant Isoetes howellii, malic acid concentration fluctuates from 1-3 mg g-1FW in the evening to 7-13 mg g-1FW 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-1FW change in acidity between morning and evening) of the plant extract. 14CO2-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 CO2 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 CO2 for photosynthesis during the day than during the night in its aquatic environment.

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