The movement of labelled glucose from the phycobiont of the lichen Peltigera canina v. praetextata Hue., to the mycobiont, has been examined under contrasting environmental conditions of temperature and moisture. The ratio of labelled mannitol : labelled photosynthetic products is used as a relative measure of the rate of transport of glucose and its incorporation into mannitol.
The relative amount of label appearing in mannitol remains fairly constant over a wide range of experimental temperatures, but there is a marked interaction with the level of thallus hydration. At 100% thallus water content by weight, when high rates of photosynthesis are still maintained, only a small proportion of the total label appears in mannitol. It is suggested that this retention of photosynthate at low levels of thallus saturation is essential to supply the basic metabolic requirements of the phycobiont. Conversely the requirements of the fungal partner are satisfied at higher levels of thallus saturation. Thus alternate wetting and drying cycles are essential to maintain the metabolic requirements of each of the bionts and hence the physical integrity of the lichen itself. The results are briefly discussed in terms of the facilitated diffusion of glucose proposed by Smith (1974).