Spatial variation in carbon isotope discrimination across the thalli of several lichen species

摘要

ABSTRACTVariations in stable carbon isotope discrimination (δ) were investigated across the thalli of several lichen species possessing different photobiont associations. Lichens containing (i) green algae (phycobiont), (ii) green algae in association with cyanobacteria confined in cephalodia, or (iii) cyanobacteria (cyanobiont) as the photobiont partner were studied. Carbon isotope discrimination was analysed in different thallus sections, which varied in distance from the margin and in age. The marginal thallus region is considered to be youngest, while the central region is thought to be oldest. This analysis showed a clear variation in δ across the thallus related to distance from the growing margin. In most of the species examined, the highest δ values were found in marginal regions (younger), while the central and basal regions (older) showed significantly lower δ. To investigate the effects of the historical increase in atmospheric CO2concentration and the concurrent decrease in the13C content of atmospheric CO2on the δ of lichens, experiments were carried out on herbarium samples ofLobaria pulmonariacollected from the mid 19th Century to 1953. The results obtained showed a pattern of variation of δ consistent with that of freshly collected samples; δ decreased substantially with increasing distance from the thallus margin, irrespective of the collection date. Moreover, no consistent variation of discrimination was found among different collection dates. These results demonstrate that the observed variation in δ is caused by age‐related changes in the physiological behaviour of different thallus sections, and that the past 150 years of increasing CO2concentration have not had significant effects on A inL. pulmonaria. Photosynthesis measurements, chlorophyll analysis and observations using optical microscopy, performed on freshly collected lichens, showed significant changes in morphological and physiological characteristics across the thallus. Particularly, remarkable variations in thickness were found across the thallus. These anatomical changes may be responsible for the variation in δ, through variations in CO2 transfer resistance and, consequently, in CO2availability across the thallus. The lack of age‐dependent variation in δ in cyanobiont lichens is possibly attributable to the operation of a CO2‐coneentrating mechanism and, therefore, to a more constant CO2environment across the thallus in th