DIURNAL CHANGES IN PHOTOSYNTHETIC EFFICIENCY AND CAROTENOID COMPOSITION OF THE LICHEN ANAPTYCHIA-CILIARIS - EFFECTS OF HYDRATION AND LIGHT-INTENSITY

Diurnal changes in chlorophyll fluorescence and carotenoid composition were studied in artificially hydrated thalli of the lichen Anaptychia ciliaris to assess the concomitant variations in photosynthetic efficiency and xanthophyll cycle associated energy dissipation under contrasting light conditions. One group of thalli was kept continuously wet (over 50% of maximum water content) while another was kept dry and hydrated only 15 minutes before each measurement. Half of the thalli of each group were kept in the shade (natural environment) while the other half were exposed to direct sunlight. The photochemical efficiency of photosystem II (assessed by the fluorescence parameter Fv/Fm) decreased significantly towards the end of the day in thalli of A. ciliaris kept wet under direct sunlight, but not in wet, shaded thalli. The relative proportions of antheraxanthin (A) and zeaxanthin (Z) of wet thalli exposed to direct sunlight showed a significant increase in the afternoon and a sharp decrease during the evening. Dry thalli hydrated just 15 minutes before the measurements showed similar diurnal changes in Fv/Fm and in the relative proportions of A + Z to the continuously wet thalli under the same light environment. Predawn data from naturally dried thalli indicated a sustained high proportion of A + Z that accounted for a relatively low photochemical efficiency at the beginning of the experiments.