Annual dynamics of photosynthetic activity and photoprotective strategies in seedlings of Phillyrea angustifolia L.

Plants of Mediterranean Basin are adapted to cope with wide thermal excursion over the year in order to maintain an optimal photosynthetic performance. In this study the annual dynamics of photosynthetic activity and photoprotective strategies were investigated in P. angustifolia, one of the most common species of Mediterranean maquis. To evaluate the contribute of photochemical and thermal dissipation in PSII photoprotection, gas exchange and chlorophyll fluorescence measurements were performed at saturating irradiance on two-years-old seedlings grown outdoor. The lowest values of net photosynthesis, stomatal conductance and apparent carboxylation efficiency were found in July and February; in these months the Ci/Ca ratio was the lowest and the highest, respectively. No difference was observed in quantum yield of PSII electron transport and maximal PSII photochemical efficiency throughout the year, except for the strong decrease in February. Non-photochemical quenching peaked in July and February, when the highest and the lowest ETR/AN ratio, respectively, occurred. Data suggest that in July, photochemical sinks other than CO2 fixation and thermal dissipation are able to dissipate safely the excess of energy to reaction centres. In February thermal dissipation becomes the most important safety valve for photosynthetic apparatus, but is unable to prevent efficiently photoinhibition as indicated by the maximal PSII photochemical efficiency decrease.