Light might regulate divergently depside and depsidone accumulation in the lichen Parmotrema hypotropum by affecting thallus temperature and water potential

Depsides and depsidones are the most common secondary products uniquely produced in lichens by the fungal symbiont, and they accumulate on the outer surface of its hyphae. Their biological roles are subject to debate. Quantitatively the compounds typical of a given lichen can vary dramatically from thallus to thallus. Several studies have addressed whether this variability is correlated with the light reaching different thalli, but the conclusions are contradictory. We addressed the question with the lichen Parmotrema hypotropum growing on unshaded, vertical tree trunks, a controlled natural environment where the light absorbed by each thallus over its lifetime is the only major position-dependent variable. The exact north-east-south-west orientation of each thallus was used to calculate its yearly light exposure based on astronomical and meteorological considerations. The calculated irradiation around the trunk, distributed over a continuous 40-fold intensity range, then was compared with the amount of compound per unit thallus weight, determined by quantitative thin layer chromatography. P. hypotropum accumulates the depside atranorin in the cortex and the depsidone norstictic acid in the medulla and around the algae. A direct correlation was observed between the yearly amount of light reaching the lichen and the amount of atranorin. In contrast, the amount of norstictic acid decreased with increasing light. Although we did not measure thallus temperature and water potential, a unifying interpretation of these and other published data is that depside/depsidone accumulation in lichens is mediated by localized changes in temperature and water potential produced by light absorption within each thallus. This suggests water relations-based functions for depsides and depsidones.