Digitonin abolishes the ability of the fungal component of Peltigera polydactyla to absorb glucose and increases the permeability of the fungal cell membrane so that substantial amounts of mannitol are released. Digitonin has no detectable effect on the algal component (Nostoc) so that it can be used as a selective inhibitor against the fungus in the intact lichen. During photosynthetic 14C fixation by the intact lichen in the presence of digitonin, there is a large release of 14C-glucose to the medium. This shows that in symbiosis, transport of glucose out of the alga is not primarily caused by or dependent upon fungal uptake acting as a sink.

Pretreatment of the lichen with glucose results in uptake of the sugar into both the algal and the fungal cells, increasing the pool size of algal glucose and stimulating efflux of previously absorbed glucose, and also depressing the rate of subsequent glucose uptake by the fungus. Pretreatment also causes a marked release of 14C-glucose to the medium during 14C-fixation, but the amount of 14C-glucose released when fungal uptake is completely prevented, as in an‘inhibition’experiment, is severely reduced.

Pretreatment of the lichen with 2-deoxy-glucose disrupts the fungal metabolism of glucose but has little effect on the algal cells, causing a large release of 14C-glucose whether or not 2-deoxy-glucose is present during the subsequent 14C-fixation.

Pretreatment with sorbose causes a marked reduction of 14C-glucose release from the alga in the lichen, as if absorbed sorbose successfully competes with glucose for the efflux mechanism. The affinity of the efflux mechanism for sorbose, together with its previously reported insensitivity to a wide range of inhibitors, suggests by analogy to studies in other organisms that it is a facilitated diffusion system.

Digitonin, 2-deoxy-glucose and sorbose represent a group of inhibitors of glucose movement between the symbionts which act at different sites in the process. Although external 12C-glucose can be used to interfere with 14C-glucose movement between the symbionts in ‘inhibition techniques’, it affects glucose transport mechanisms in both fungus and alga, so making interpretation difficult of quantitative data produced by the technique on rates and amounts of transport. The qualitative value of the‘inhibition technique’in identifying mobile compounds remains undoubted.

Prolonged illumination of the lichens (which increases the size of the glucose pool within the alga) or feeding with mannitol (the fungal product formed from glucose) has little effect on transport between the symbionts.