ECOLOGICAL PHYSIOLOGY OF THE LICHEN HYPOGYMNIA PHYSODES

SUMMARY The effects of ‘severe’ and ‘mild’ cycles of wetting and drying on Hypogymnia physodes were investigated. Each 24‐h cycle of the ‘severe’regime consisted of 3 h soaking in buffer followed by 21 h in air (becoming air‐dry in 1‐2 h), all at 20°C in light or dark. After seven ‘severe’cycles in the light, polyol levels remained high; mannitol, arabitol and ribitol were continually turned over, since there was a heavy loss of previously fixed 14 C from each. In the dark, there was some decline in polyol levels after 7 days, presumably because the polyol pool could not be replenished by photosynthesis. During ‘severe’regimes, the carbon loss from the polyol pool was much greater than could be explained by respiration. Nevertheless, if maintenance of high rates of photosynthesis and large pools of polyols are taken as criteria of healthy thalli, then ‘severe’ cycles of wetting and drying are less deleterious than constant saturation. ‘Mild’cycles were carried out at 5°C, with a 12 h light/12 h dark photoperiod, and involved much more gradual drying and wetting than the ‘severe’ regime. Thalli showed a rise in ability to photosynthesize during the first 16 days of this treatment. It is concluded that in general, a fluctuating water content is more beneficial to lichens than constant saturation. The results described in this and other papers are used to formulate the concept of‘physiological buffering’. This postulates that the response of lichens to severe environmental stress is characterized by depletion and marked turnover of the polyol pool, so buffering insoluble material such as proteins against such stress. A substantial proportion of photosynthesis in lichens is devoted to maintaining the polyol pool at a high level, leaving only a small amount available for growth processes.