Seasonal diurnal acid rhythms in two aquatic crassulacean acid metabolism plants.

The diurnal change in titrable acidity in two aquatic CAM plants, Littorella uniflora var. americana (Fern.) Gl. and Isoetes macrospora Duriev., growing at two sites, was monitored at biweekly intervals for two years during the ice-free period. Both plants exhibited the classic pattern of CAM acitivity, with deacidification 60 to 90% complete by noon. The maximum diurnal acid rhythms observed were 169±7, and 154±20 μeq·g-1 fresh weight for the two Littorella populations, and 182±9 and 133±16 μeq·g-1 fresh weight for the two Isoetes populations. The seasonal pattern of the diurnal acid rhythm was correlated with temperature and light. The maximum activity occurred in midsummer, with negligible activity under ice cover. This pattern was similar to that of terrestrial CAM plants from non-arid environments. Comparison of CAM activity for populations of the same species indicates that the magnitude of CAM activity is closely related to plant productivity, and appears to be related to light and perhaps CO2 availability. In these plants, CAM extends the diel period of carbon accumulation and contributes 40 to 50% of the annual carbon gain. The prolonged period of carbon acquisition and effective conservation of respired CO2 via CAM is of paramount importance in the growth and productivity of these plants in oligotrophic environments.