EXPLORING PHENOTYPIC PLASTICITY IN THE LICHEN RAMALINA-CAPITATA - MORPHOLOGY, WATER RELATIONS AND CHLOROPHYLL CONTENT IN NORTH-FACING AND SOUTH-FACING POPULATIONS

The present work analyses the morphology, anatomy, water relations and chlorophyll content of thalli of the lichen Ramalina capitata var. pr otecta from two different populations exposed to contrasting microclim atic conditions due to differences in the orientation of the rock surf ace. The population on the north-facing rock surface (NFS) was exposed to lower photosynthetic photon flux densities (PPFD), remained at hig h relative humidities for longer periods of time and was exposed to lo wer 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 th e SFS had shorter and wider lacinia, thicker thalli, mostly due to inc reased 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 respo nse of the traits studied in R. capitata var. protecta was not directl y related to differences in the light availability, at least for the r ange of PPFD experienced by the two populations studied, since the pop ulation exposed to higher PPFD exhibited larger amounts of light harve sting pigments. Both populations exhibited the same intrathalline dist ribution of algal cells and chlorophylls, which were more abundant in the apical than in the basal zones of air thalli studied. Periods of w ater-induced metabolic activity were shorter in the SFS than in the NF S, 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 max imization of the photosynthetic exploitation of the periods of metabol ic activity when they are brief, as has been described for certain vas cular plants from xeric environments. (C) 1997 Annals of Botany Compan y.