Physiological‐environmental Interactions in Lichens

Abstract

SUMMARYThe potential control by temperature and moisture of the movement of glucose from the phycobiont in Collema furfuraceum, Peltigera polydactyla and Peltigera rtifescens is examined. The ratio of labelled mannitol to labelled glucose after exposure to "CO.^ is used as a measure ofthe 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 lahel 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 "o thallus moisture by weight the amount of label appearing in mannitol declines sharply. Conversely, in Collema furfuraeeum 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.

The carbon economy of lichens

“…Relations between environmental conditions and carbohydrate allocation patterns have been studied even less (Armstrong & Smith, 1994), even though thallus hydration status seems to be important. For instance, in the Nostoc lichen Peltigera polydactyla, mannitol formation was significantly enhanced when WCs were increased (MacFarlane & Kershaw, 1982), whereas alternate drying and wetting was required for polyol metabolism and translocation in the Trebouxia lichen Hypogymnia physodes (Farrar, 1976). It has therefore been assumed that alternate wetting and drying cycles might be required to provide an adequate carbon supply to each of the symbionts.…”

Tansley Review No. 117

“…For instance, in the Nostoc lichen Peltigera polydactyla, mannitol formation was significantly enhanced when WCs were increased (MacFarlane & Kershaw, 1982), whereas alternate drying and wetting was required for polyol metabolism and translocation in the Trebouxia lichen Hypogymnia physodes (Farrar, 1976). It has therefore been assumed that alternate wetting and drying cycles might be required to provide an adequate carbon supply to each of the symbionts.…”

Effects of Light and Dark on the Ultrastructure of Lichen Algae

“…Starch synthesis has been correlated with the incubation of lichens at intermediate thallus water contents. It has been suggested that, compared to fully hydrated material, a smaller proportion of photosynthetic producís is released from the algal to the fungal symbiont so that more is retained and converted to starch (Harris and Kershaw, 1971;Macfarlane and Kershaw, 1982;Ascaso et al, 1986). While starch increases in senescent algal cells, probably also due to reduced translocation to fungal cells (Scott and Larson, 1984), the loss of starch has not previously been investigated in either senescent or undamaged material.…”