The present work analyses the morphology, anatomy, water relations and chlorophyll content of thalli of the lichen Ramalina capitata var. protecta from two different populations exposed to contrasting microclimatic conditions due to differences in the orientation of the rock surface. The population on the north-facing rock surface (NFS) was exposed to lower photosynthetic photon flux densities (PPFD), remained at high relative humidities for longer periods of time and was exposed to lower temperatures than the population on the south-facing surface (SFS). We proposed the hypothesis that the shadier the habitat the greater the ecological advantage for enhanced light harvesting. Thalli from the SFS had shorter and wider lacinia, thicker thalli, mostly due to increased medulla thickness, a higher water-retention capacity, a higher percentage of thallus volume occupied by the algal cells and a higher chlorophyll content than thalli from NFS. The phenotypic plastic response of the traits studied in R. capitata var. protecta was not directly related to differences in the light availability, at least for the range of PPFD experienced by the two populations studied, since the population exposed to higher PPFD exhibited larger amounts of light harvesting pigments. Both populations exhibited the same intrathalline distribution of algal cells and chlorophylls, which were more abundant in the apical than in the basal zones of air thalli studied. Periods of water-induced metabolic activity were shorter in the SFS than in the NFS, and structural and chlorophyll data indicated that thalli from the SFS were better prepared for the photosynthetic exploitation of these briefer periods and for maintaining thallus hydration into dry periods. These results suggest that differences in selective pressure between the two populations of R. capitata var. protecta studied involved maximization of the photosynthetic exploitation of the periods of metabolic activity when they are brief, as has been described for certain vascular plants from xeric environments. (C) 1997 Annals of Botany Company.
