title: Moisture content and CO2exchange of lichens. II. Depression of net photosynthesis in Ramalina maciformis at high water content is caused by increased thallus carbon dioxide diffusion resistance.
abstract: Thalli of Ramalina maciformis were moistened to their maximal water holding capacity, thus, simulating actual conditions following a heavy rainfall. Time courses of net photosynthesis at 17° C and 750 μE m-2s-1light intensity (PAR) were obtained during drying of the thalli. At ambient CO2concentrations from 200 to 1,000 ppm, CO2uptake of the moist lichens was depressed at high water content. After a certain water loss, net photosynthesis increased to a maximal value and decreased again with further drying of the thalli. The degree of initial depression of photosynthesis decreased with increasing ambient CO2concentration, and it was fully absent at 1,600 ppm ambient CO2. Under these conditions of CO2saturation, net photosynthesis remained constant at maximum for many hours and decreased only when substantial amounts of water had been lost. We conclude that the carboxylation capacity of the lichen is not affected by high contents of liquid water. Therefore, the depression of CO2uptake of the water saturated lichen at lower (e.g. natural) ambient CO2must be due exclusively to increased resistance to CO2diffusion from the external air to the sites of carboxylation.
authors.affiliation: Lehrstuhl für Botanik II, Universität Würzburg, Mittlerer Dallenbergweg 64, D-8700, Würzburg, Germany.
authors.forename: O L
authors.initials: OL
authors.lastname: Lange
authors.affiliation: Lehrstuhl für Botanik II, Universität Würzburg, Mittlerer Dallenbergweg 64, D-8700, Würzburg, Germany.
authors.forename: J D
authors.initials: JD
authors.lastname: Tenhunen