Phenols in reproductive and somatic structures of lichens: a case of optimal defence?

Optimal defence theory (ODT) attempts to explain variation in plant seconda ry compounds between different species, different growth conditions and dif ferent parts of individual plants. The theory is widely applied to vascular plants and more recently also to seaweeds. Surprisingly, ODT has gained li ttle attention as potential explanation on the distribution of lichen secon dary metabolites. In the present study, we analysed intrathalline variation in total phenol content and phenol spectra between reproductive and somati c structures of three foliose lichens. Xanthoria parietina, Vulpicida pinas tri and Hypogymnia physodes. The results showed that the concentration of p henolic compounds is higher in sorediate than in non-sorediate lobe ends of V. pinastri and H. physodes as well as in apothecia of X. parietina compar ed to other parts of the thallus. These results were in accordance with ODT predicting higher allocation of phenols in structures that are most import ant for the fitness of an individual genet or ramet. This pattern was paral lel in all species regardless whether the compounds originate from either a cetate-mevalonate or shikimic acid pathways. Moreover, both sexual (X. pari etina apothecia) and asexual (soralia of V. pinastri and H. physodes) repro ductive structures were higher in phenols compared to somatic tissue.