Winter and summer leaves of Cistus incanus: differences in leaf morpho-functional traits, photosynthetic energy partitioning and poly-ADP-ribose polymerase (PARP) activity.

Leaf anatomy, photosynthetic performance, and the role of poly(ADP-ribosyl)ation in maintaining genomic stability in winter and summer leaves of Cistus incanus L. were investigated to evaluate the strategies allowing this species to grow in a Mediterranean climate. Measurements on summer leaves (SL) were conducted at midday and on the following morning to assess the reversibility of changes induced by high temperatures and irradiances reached at midday. Winter leaves (WL) were thicker with a lower leaf density and higher intercellular spaces compared with SL, which showed reduced net photosynthesis (Pn) and a decrease in Rubisco activity. Compared with WL, SL showed a higher thermal dissipation (ΦNPQ) in compensating for the reduced Pn. This allowed minimization of ROS production (low ΦNO) and hence prevented photo-oxidative damage. In SL, poly(ADP-ribose) polymerase (PARP) activity was down-regulated, probably allowing leaves to become tolerant to high temperatures and irradiances. SL are also characterised by chloroplasts occupying larger cell volume and well exposed to intercellular spaces. This may well favour better liquid-phase CO2 diffusion towards carboxylation sites, contributing to reducing the negative influence of high leaf density and the low percentage of intercellular spaces on photosynthesis.