LICHEN METABOLISM. 1. THE USE OF TRITIUM LABELLED WATER IN STUDIES OF ANHYDROBIOTIC METABOLISM IN RAMALINA CELASTRI AND PELTIGERA POLYDACTYLA

Abstract

SummaryA new radiochemical technique has been applied to a study of anhydrobiotic metabolism in the lichen species, Ramalina celastri (Sprengel) Krog & Swinsc. and Peltigera polydactyla (Neck.) Hoffm. Specimens were incubated in known atmospheric humidities of THO vapour which were found to equilibrate with cellular H2O within 5 days. This method has proved effective in a study of a wide range of in vivo metabolic processes occurring at both high and low cellular water contents.Qualitative and quantitative estimations of the 3H labelling of water soluble cell constituents (ammo acids, organic acids, sugars, sugar phosphates, and sugar alcohols), neutral and polar lipids, water soluble macromolecules, and insoluble cell residues have shown that diverse metabolic systems have significantly different levels of resistance to desiccation. Tricarboxylic acid cycle reactions and related amino acid interconversions were shown to be relatively desiccation resistant, whereas Calvin cycle metabolism appeared to be highly sensitive, a conclusion supported by 14CO8 tracer experiments. Conversely, the synthesis of sugar alcohols was found to be active beyond the limit of photosynthetic metabolism. Neither lipid nor macromolecular synthesis was shown to occur to a significant degree in either lichen in the desiccated state.It is proposed that the syntheses of amino acids and sugar alcohols at low cellular saturation levels are important physiological processes. Amino acid synthesis results in the accumulation of an available store of stable intermediates, ready to feed into the T.C.A. cycle by trans‐amination or deamination on rehydration of the lichen. Sugar alcohol metabolism assists in the maintenance of a high sugar alcohol concentration to act as a physiological buffer during extreme dehydration.