Sources of Inorganic Carbon Acquired through CAM inLittorella uniflora(L.) Aschers

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Abstract
Madsen, T. V. 1987. Sources of inorganic carbon acquired through CAM in Littorella uniflora (L.) Aschers.—J. exp. Bot. 38: 367–377. The CO2 dynamics of the lacunal air and the relative contribution of external and internal CO2 sources to dark CO2 assimilation was examined in the submerged aquatic CAM species Littorella uniflora (L.) Aschers. Refixation of internal CO2, released by dark respiration, constituted about 30–35% of the total dark CO2 assimilation. At a CO2 concentration of 0·2 mol m–3 around the leaves the external CO2 uptake through the roots increased from 45% of the total CO2 uptake at 0·7 mol m–3 CO2 to 100% at 1·6 mol m–3 and 3·1 mol m–3 CO2 around the roots. The negligible importance of leaf CO2 uptake at high CO2 concentrations around the roots was the result of a causative high CO2 concentration in the leaf lacunae. The CO2 permeability of Littorella leaves was high relative to root permeability. This has at least two ecological implications: (1) it enhances the potential diffusive release of CO2 from the sediment C02-pool via the lacunal system of the plants. This loss of CO2, however, was found to be greatly reduced by CAM activity of the plants. (2) The high permeability of the leaf surface to CO2 exchange allows the plants to assimilate CO2 from the water surrounding the leaves when the concentration is high, i.e. during extensive epiphyte dark respiration. Thus, CAM tends to facilitate retension of a high CO2 pool in the sediment-plant system and at the same time allows the plants to exploit the water column CO2 source when it is abundant. This result is in accordance with the general idea that CAM in aquatics constitute a carbon conserving mechanism.