Oxygen Exchange With the Lacunae and Across Leaves and Roots of the Submerged Vascular Macrophyte, Lobelia Dortmanna L.*

SUMMARY The oxygen dynamics of Lobelia dortmanna were examined by continuous oxygen measurements in the lacunae and in the water around leaves and roots during light/dark cycles. Lobelia exchanges about six times more oxygen over the root than the leaf surface. The exchange rates per unit of surface area are two or three times higher across roots than leaves in accordance with a similar difference in permeability constants. The delay in reaching constant rates of exchange across roots is about 45 min after switching to dark and about 25 min after switching to light. This delay corresponds to the time necessary to change the oxygen concentration in the lacunae to a new steady state gradient with the water around roots. The oxygen pools in lacunae and root‐water change at the same rate at equilibrium. During incubation in a closed system it is necessary to correct for changes in the lacunal oxygen pool when it is significant relative to the oxygen pool in the external water. The lacunae of Lobelia are an effective pathway for gas movements between leaves and sediment. The extensive lacunae, and the diffusional barrier for gases in Lobelia , should facilitate recycling of oxygen, and probably carbon dioxide, within the plant‐sediment system.