OCCURRENCE AND HISTOLOGICAL DISTRIBUTION OF USNIC ACID IN THE RAMALINA-SILIQUOSA SPECIES COMPLEX

Usnic acid appears to be a constant secondary product of all chemotypes of the Ramalina siliquosa species complex. Its highly variable concentrations and apparent absence from some individuals are due to a preferential accumulation in apothecia and spermagonia, structures not necessarily included equally in all samples in routine chemical screenings. In a specimen of one chemotype analyzed in detail, the proportion of usnic acid to the beta-orcinol depsidone protocetraric acid was, surprisingly, highest in the pruina covering the outer surface of the apothecial disk and lowest, as expected, in the medulla. Elsewhere in the thallus, usnic acid accumulates in the spermagonia and contributes insignificantly to the color of the thallus. If usnic acid functions as a sun screen or allelopathic agent here, it is protecting reproductive structures rather than the algal symbiont as often proposed for this substance in lichens in which it occurs in tissues over the algae.