PHYSIOLOGICAL‐ENVIRONMENTAL INTERACTIONS IN LICHENS

SUMMARY The potential control by temperature and moisture of the movement of glucose from the phycobiont in Collema furfuraceum, Peltigera polydactyla and Peltigera rufescens is examined. The ratio of labelled mannitol to labelled glucose after exposure to 14 CO 2 is used as a measure of the amount of glucose transported and converted to mannitol in the lichen thallus over a range of experimental temperatures and levels of thallus hydration. The results show that temperature has little effect on the proportion of the total label appearing in mannitol except at 5°C. This is interpreted as a minimum temperature requirement for the conversion of glucose to mannitol within the mycobiont. The effect of thallus hydration, however, shows that in Peltigera polydactyla the major movement of fixed carbon from the phycobiont to the fungal partner occurs at, or close to, full thallus saturation. Below 300% thallus moisture by weight the amount of label appearing in mannitol declines sharply. Conversely, in Collema furfuraceum and Peltigera rufescens , transport of glucose continues at a constant rate down to very low levels of thallus hydration. These results are discussed in relation to our previous findings using Peltigera praetextata , and in relation to the balance of total photosynthate between the lichen symbionts during a drying cycle. It is evident that the rate of transport of glucose is closely controlled by the level of thallus hydration which in turn correlates with the xeric or mesic preference of each species. Alternate wetting and drying cycles effectively ensure that each biont obtains an adequate supply of the fixed carbon resources, thus maintaining the integrity of the lichen.