Lichen Metabolism

S ummary Metabolic studies were carried out on two lichen species ( Ramalina celastri and Peltigera polydactyla) using 3 HHO (g) and 14 CO 2 . Incubations in the presence and absence of light at 100 % r.h. resulted in 3 H and 14 C labelling of a variety of compounds. Tritium fixation during darkness was determined quantitatively as 4.1 and 8.2 % of ‘light’ labelling (for P. polydactyla and R. celastri , respectively), and was localized predominantly in polyols, alanine, glutamic acid, glutamine, aspartic acid and γ‐amino butyric acid. Under similar experimental conditions, only trace amounts of 14 C were found in sugar alcohols, whereas amino acids were substantially labelled. The dissimilarity between 3 H and 14 C labelling patterns has led to the conclusion that polyols synthesized in the absence of photosynthesis must be derived from other stored substrates within the thallus. The role of tricarboxylic acid (TCA) cycle‐related amino acids in lichen metabolism, and some potential synthetic modes of the unusual amino acid, γ‐amino butyric acid, are discussed.