Anatomy and photochemical behaviour of Mediterranean Cistus incanus winter leaves under natural outdoor and warmer indoor conditions

Plants in Mediterranean environments can adopt photoprotective mechanisms to cope with winter temperatures, thus avoiding photoinhibition. An increase in global temperature is expected in the future as a consequence of climate change. The aims of this work were (i) to analyse anatomy and photochemical activity in winter leaves of Cistus incanus L., to identify characters having a potential role in photoprotection under natural winter conditions, and (ii) to evaluate the effect of higher temperature on such traits. Leaves from plants grown in a greenhouse (indoor) were compared with those from plants grown outdoors on the basis of anatomical and photochemical traits, including indexes evaluating the exposure of chloroplasts at the cell surface. Leaves developed outdoors were characterized by anatomical traits related to the optimization of light harvesting and to gas-exchange control. Such traits were accompanied by a higher pigment content and an increase in the thermal dissipation of excess absorbed light. Indoor leaves showed physiological and morphoanatomical adjustments addressed to invest more of absorbed light in photochemistry. Overall, our analysis showed that (i) photoprotection in C. incanus relies on adaptive traits at both the morphoanatomical and physiological level and (ii) C. incanus leaves seem to be able to acclimatize to warmer winters.