The contribution of crassulacean acid metabolism to the annual productivity of two aquatic vascular plants.

Net annual productivity and annual carbon budgets were determined for populations of Littorella uniflora var. americana and Isoetes macrospora in a mesotrophic and oligotrophic lake in northern Wisconsin, to assess the contribution of Crassulacean Acid Metabolism (CAM) to annual productivity of the species in their natural environment. Nocturnal carbon accumulation (CAM), daytime uptake of external CO2 via the C3 mechanism, and refixation of endogenously generated CO2 from daytime respiration were the sources of carbon income. CAM activity as diurnal acid rhythms reached maxima of 89 to 182 μeq·g-1 leaf fresh weight for the various populations.Maximum rates of daytime 14C uptake ranged from 0.56 to 1.46 mg C·g-1 leaf dry wt.·h-1 for the study populations. Refixation of daytime respired CO2 averaged 37% for the four populations. Carbon loss was due largely to "dark" respiration, during the day and night. Nocturnal carbon accumulation, daytime CO2 uptake and 24-h dark respiration were of similar magnitude, indicating dark respiration was equivalent to ∼50% of gross photosynthesis.Net annual production was measured for each population by following leaf turnover. Turnover rates for the Littorella populations were 1.56 and 1.72·yr-1, and for the Isoetes populations, 0.85 and 1.00·yr-1. Measured net annual productivity and calculated net annual productivity (based on carbon exchange) agreed within an average of 12% for the four populations. While CAM activity was greater for the more productive population of each species, the results suggest that the contribution of CAM to annual productivity is greater for the less productive population of each species. CAM contributed 45 to 55% of the annual carbon gain for the study populations.