LICHEN PHYSIOLOGY XII. THE ‘INHIBITION TECHNIQUE’

Abstract

SummaryDuring photosynthetic fixation of 14C by lichens, 14C‐carbohydrate moves from alga to fungus. If the 12C‐form of this mobile carbohydrate is added to the external medium, then the 14C‐form is released from the lichen. This observation forms the basis of the, inhibition technique', used to study movement of carbohydrates between symbionts within intact lichen thalli. The effect is specific since 14C is released from the lichen only if the 12C‐carbohydrate in the medium is identical or structurally very similar to that moving between the symbionts.This paper examines the usefulness of the technique in the study of carbohydrate movement. Lichens were exposed to a short pulse of NaH14CO3 before transfer to carbohydrate media. Relatively high concentrations of carbohydrate (20 mM–80 mM) were needed for maximum ‘inhibition’. More than half the lichens examined released over 50%, of a pulse of fixed 14C during maximum ‘inhibition’ Using 2‐desoxy‐glucose as an ‘inhibitor’ of glucose movement in Peltigera polydactyla, it was shown that the supply of glucose from the alga was more than sufficient for the metabolic requirements of the fungus. The rate release of 14C during ‘inhibition’ varied widely between different lichens, but it was not clear if this was due to differences in pool sizes of mobile caibohydrates, or to real differences in rates of carbohydrate supply. Much less carbohydrate movement between the symbionts occurred in the dark, than in the light. The effect of pH on 14C release during ‘inhibition’ contrasted sharply with effects of pH on release from freshly isolated algae. This raised further doubts whether the behavior of freshly isolated algae is similar to those in the thallus. The explanation of ‘inhibition’ on the basis of a competition between the high concentration of external carbohydrate and the much lower concentration of 14C‐carbohydrate released from the alga at fungal uptake sites may require modification in at least some lichens. Competition or exchange between 12C and 14C carbohydrates may begin at stages before the fungal uptake sites.