THE ABILITY OF PORPHYRA-LINEARIS (RHODOPHYTA) TO TOLERATE PROLONGED PERIODS OF DESICCATION

Polphyra lineraris, is the uppermost intertidal macroalga on the eastern Mediterranean coast. It is able to regain photosynthetic activity upon reimmersion, even after drying in air to a stable water content of ca. 5% of fully hydrated plants for a period of three weeks. During desiccation experiments the plants continued net emersed photosynthesis at a maximal rate until their relative water content (RWC) dropped to 60%. Net photosynthetic gas exchange of plants with only 10% RWC was still positive. Photosynthetic rates, however, decreased during the drying process more rapidly than did the respiration rates. During rehydration, plants previously kept dry (5% RWC) for short periods (up to 24 h) regained net photosynthetic capability only when their RWC reached about 35%. The longer the thalli were exposed, the longer they had to be reimmersed in order to regain positive net photosynthesis, and the lower were their photosynthetic rates after full rehydration. After three weeks of exposure, the gas exchange systems of Porphyra plants could still recover, but after one month exposure the plants died. This more or less corresponds with the longest exposure encountered in nature, which is the period of two weeks between successive spring tides. It is suggested that the capability of the photosynthetic systems of P. linearis to recover after such long exposures to dry air is most important for its survival at the high intertidal level it occupies on the rocky shore.