UV triggers the synthesis of the widely distributed secondary lichen compound usnic acid

Synthesis of the cortical dibenzofuran derivative usnic acid and the medullary depsidone salazinic acid was studied in Xanthoparmelia stenophylla thalli from which the compounds had been removed by acetone rinsing prior to a 21-day field experiment with UV absorbing and transmitting screens. Natural levels of ultraviolet radiation clearly induced the re-synthesis of usnic acid. The re-synthesis was boosted by the addition of ribitol, the carbohydrate delivered from the Trebouxia photobiont to the mycobiont. Salazinic acid was also weakly induced by UV. Re-synthesis was relatively low, up to 2.5 and 3.1% of start values for usnic and salazinic acid, respectively. However, given that the natural content of both compounds was high, constituting 12% of thallus dry weight, the absolute amounts of lichen compounds re-synthesised were not so small. We also studied the extractability of nine extracellular lichen compounds in three species X. stenophylla, Hypogymnia tubulosa, and Vulpicida pinastri, and found two distinct fractions of cortical compounds, one major that was completely extractable from living lichens and one minor that was only extractable with grinding. Medullary compounds were completely extracted without grinding. These findings did not influence the relative differences between treatments in our experiment, but may be of importance for future assessments of, e.g., quantitative studies of extracellular lichen compounds.