Ecological Physiology of the Lichen Hypogymnia Physodes

SUMMARY When air‐dry thalli of Hypogymnia physodes are immersed in water, four main features characterize the rewetting phase. (1) There is an immediate and substantial release of gas, the‘wetting burst’. Although the released gas appears to be mostly CO 2 , it is non‐metabolic in origin and probably the result of a previous adsorption of gas onto polysaccharide sheaths surrounding fungal hyphae. (2) There is a period of 1‐2 h increased respiration,‘resaturation respiration’, which, unlike basal respiration, is cyanide‐sensitive. The cause of resaturation respiration remains obscure, but could result from exposure of respiratory substrate to enzymes from which they are normally separated. (3) There is a brief period (1–2 min) in which there is a rapid loss of organic and inorganic solutes from the thallus. Since water is a structural component of membranes, removal of some of this water during drying probably caused increased permeability so accounting for the losses observed. (4) There is a very rapid recovery of processes including photosynthesis and nutrient uptake. Even 10 s after rewetting, incorporation of 32 P was 60% inhibited by DNP. The ecological significance of these features is discussed, particularly in relation to the apparent need of many lichens for periodic drying.