THE COMBINED EFFECTS OF DESICCATION AND IRRADIANCE ON MOSSES FROM XERIC AND HYDRIC HABITATS

The combined effects of desiccation and irradiance on the physiology of the sand dune moss Tortula ruraliformis (Besch.) Grout and the minerotrophic flush moss Dicranella palustris (Dicks.) Crundw. ex. E. F. Warb (D. squarrosa (Starke) Schp.) were studied. Damage as a result of desiccation in the dark, measured by loss of protein and the relative accumulation of thiobarbituric acid (TBA) reactive products (which gives an estimation of lipid peroxidation), was greater in D. palustris. Desiccation alone had no effect on the total concentrations of chlorophyll and carotenoids in either species. Water loss resulted in the cessation of measurable photosynthetic oxygen evolution in both species. Respiration was less sensitive to desiccation than was photosynthesis. A combination of irradiance and water stress prevented any recovery of photosynthesis during subsequent rehydration in D. palustris, but suppressed recovery only marginally (at the highest irradiance) in T. ruraliformis. The loss of protein, chlorophyll, and carotenoids, and lipid peroxidation were all substantially increased in D. palustris desiccated in the light, but these same conditions resulted in only minimal damage of T. ruraliformis. Continuous exposure to high irradiance was less deleterious to desiccated than hydrated T. ruraliformis. The data are discussed in relation to the habitat preferences of the two species, and also in relation to possible causal factors in the initiation of damage.