Highlighting the differential role of leaf paraheliotropism in two Mediterranean Cistus species under drought stress and well-watered conditions

The differential degree by which paraheliotropism may counterbalance the deleterious impact of high irradiance between congeneric species in relation to different water availabilities has been poorly investigated. We followed the evolution of gas exchange, quenching analysis and OJIP parameters in restrained (R) and free (F) to move leaves of Cistus monspeliensis (CM) and Cistus salvifolius (CS) under drought stress (WS) and well-watered conditions (WW). Concerning gas exchange parameters, leaf restriction effect was overall not significant in CM except in apparent carboxylation efficiency (C-e) under WS, while CS showed a significant sensitivity of maximum net photosynthetic rate (A(max)), stomatal conductance (gs) and C-e even under WW. The recovery analysis highlighted also a faster gs recovery in F leaves. Furthermore, in both the species, restriction affected photon allocation pathways especially in terms of light-regulated and light-independent constitutive non-photochemical energy dissipation under WW, ultimately affecting electron transport rate (ETR). Nevertheless, the OJIP analysis provided us evidences that CM was characterized by a down-regulation of ETR while an impairment occurs in CS. In CM this was due to its ability to modify a certain fraction of reaction centers thus resulting in a higher capability for dissipation of excess light energy under well-watered conditions, not affecting electron transport efficiency. This response was not observed in CS. Overall, we demonstrated that congeneric species, even mostly sharing the same physiological targets, differ in the degree by which leaf movements help to counterbalance the negative effect of the high irradiance in relation with the amount of water available.