Effect of water regime on carbon isotope composition of lichens

δ 13C values of the lichens Ramalina duriaei and Teloschistes villosus collected in their natural habitat were repeatedly measured during 2 years. Results show variations in the stable carbon isotope ratios (13C/12C). Such variations are correlated to the seasonal rainfall, i.e. low values of δ 13C of the lichens during the winter and high values of δ 13C during the dry summer. Relatively low δ 13C values were obtained also in laboratory experiments with lichens grown under controlled humid conditions and in lichens collected from humid habitats. The variations in carbon isotopes were associated with quantitative metabolic changes. Under humid conditions an increase was obtained in the total amount of the extracted water-soluble fraction of the plant tissues as well as in the relative content of soluble carbohydrates. Analysis of the possible factors which may cause such variations indicates that the quantity of precipitation and the exposure time to high humidity were the main environmental factors causing seasonal variations in the δ 13C values of the lichens. Such variations are dependent mainly on enzymic reactions and are probably less influenced by purely physico-chemical processes. In view of the data presented here the balance between carboxylation and decarboxylation reactions seems to be the major factor for the observed seasonal differences. Winter accumulation of 12C enriched components causes an over-all decrease in δ 13C. During the summer, those storage materials are respired with concomitant increase in the δ 13C of the residual plant material.