Photochemical activity and anatomical modifications in plants of Cistus incanus L. subsp. incanus subjected to cold.

Winter cold and high irradiance represent natural constraints in Mediterranean-type ecosystems that may limit plant growth and productivity, resulting often in photoinhibition of photosynthetic apparatus. However, evergreen species of Mediterranean maquis are able to adapt successfully to low temperatures adopting both physiological strategies and anatomical modifications. The aim of this study was to investigate in Cistus incanus L. subsp. incanus, a shrub widely diffused in Mediterranean maquis of Southern Italy, the role of photochemical and non-photochemical strategies in reducing the excess of light energy to reaction centres, as well as the leaf structural modifications related to cold acclimation. To this purpose, chlorophyll a fluorescence measurements were conducted on fully expanded leaves at midday, under saturating light intensity, on plants grown indoor at 20-25°C and outdoor during the winter. Anatomical and cytological features of leaves were also determined on both plant groups through light and fluorescence microscopy. Data obtained showed that leaf photochemical behaviour and structure change significantly in response to different growth temperatures: in outdoor plants we observed a significant decrease of electron transport rate (ETR), photochemical quenching (qP), photochemical efficiency of open PSII reaction centres (Fv'/Fm') and an increase of non-photochemical quenching (qN) and total chlorophyll and carotenoids content, accompanied by changes in tissue density. Conversely indoor plants exhibited a better capacity in photochemistry. However, the decrease of photochemical processes under cold did not lead to photodamage risks in outdoor plants, because no difference in maximal photochemical efficiency (Fv/Fm) was detected as compared to indoor plants. The overall analysis of data suggests that in outdoor plants the photoprotection at low winter temperatures is obtained by means of a combination of thermal dissipation processes and alteration of leaf structural characteristics.