CARBON UPTAKE BY LEAVES AND ROOTS OF LITTORELLA-UNIFLORA (L) ASCHERS

The characteristics of CO2 uptake by leaves and roots of Littorella uniflora (L.) Aschers. from oligotrophic Lake Kalgaard, Denmark, were compared. At identical external CO2 concentrations, the uptake on a dry weight basis was the same for both tissues but leaf uptake was 2-3 times higher per unit of surface area. At the natural, low CO2 concentrations around leaves (0.015 mM) and high concentrations around roots (1-5 mM), root uptake predominates. The CO2 concentrations in the sediments seem to limit photosynthesis under natural uptake conditions, i.e. with no stirring around the roots. Plants growing at low CO2 concentrations in sand are small but have an extensive root system relative to the leaf weight. These plants are photosynthetically efficient at low CO2 concentrations compared with the larger plants with small root systems that grow naturally in more organic sediments of higher CO2 concentrations. Reliable results of 14C production experiments with Littorella uniflora can only be obtained when incubation time is adjusted to minimize (1) the effect of the delayed equilibrium of 14C with 12C fixation at the start of an experiment and (2) the effect of CO2 limitation during long exposures, and when natural stirring forces are simulated. The secretion of dissolved organic carbon in short time incubations is low (0-3.3% of carbon fixation) and need not be corrected for. © 1979.