The significance of CAM as a carbon-conserving mechanism in two submerged aquatics, Littorella uniflora (L.) Aschers and hoetes lacustris L., was evaluated by determining (1) the loss of previously fixed CO2, released through decarboxylation of malic acid and (2) the quantitative importance of CAM relative to external CO2 uptake in photosynthesis.
Using a 14C-labelling technique it was found that the loss of CO2 derived from decarboxylation of malic acid constituted less than 2 % of nocturnal carbon uptake, confirming that the diurnal rhythm of acidity provides a good measure of the incorporation of carbon via CAM.
The exchange pattern of inorganic carbon and oxygen was measured for plants incubated in open flow-through systems. The contribution of internal and external CO2 to photosynthesis was determined as the difference in CO, uptake and oxygen release, where excess oxygen release reflected the assimilation of CO2 released from deacidification of malic acid. Despite a rapid deacidification, uptake of external CO2 was stimulated by 15 to 30% at intermediate external CO2 concentrations. It is suggested that this effect was due to a reduced photorespiratory activity caused by an enhanced internal CO2 concentration generated from malic acid. The simultaneous uptake of inorganic carbon from high internal and low external CO2 concentrations can only be explained by assuming a non-linear CO, gradient from the lacunal air to the bulk medium, with the CO2 concentration in the outermost cell layers being lower than both the bulk medium and the lacunal air.
The relative contribution of CAM to the total uptake of CO2 in daytime declined from 95% (both species) at an external CO2 concentration of 30μ CO2 to 38% (Littorella) and 34% (Isoetes) at 200μ CO2. This resulted from increased uptake of external CO2 at high external CO2 concentrations and a parallel suppression of internal decarboxylation of malic acid.
The observed suppression of decarboxylation was confirmed by following the time course in the content of titratable acidity of the leaves. A reversible inhibition of daytime deacidification was seen for external CO2 concentrations higher than 3.0 to 5.4 mM CO2 in both Littorella and Isoetes.
The functional significance of CAM for aquatics rests in the enhanced capacity for obtaining inorganic carbon resulting from the extension of the diel period in which inorganic carbon can be accumulated and in the high reassimilation efficiency of nocturnal respiratory CO2.