Moisture content and CO exchange of lichens. II. Depression of net photosynthesis in Ramalina maciformis at high water content is caused by increased thallus carbon dioxide diffusion resistance.

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 s light intensity (PAR) were obtained during drying of the thalli. At ambient CO concentrations from 200 to 1,000 ppm, CO uptake 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 CO concentration, and it was fully absent at 1,600 ppm ambient CO. Under these conditions of CO saturation, 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 CO uptake of the water saturated lichen at lower (e.g. natural) ambient CO must be due exclusively to increased resistance to CO diffusion from the external air to the sites of carboxylation.