Paraheliotropism in Robinia pseudoacacia plants: an efficient mean to cope with photoinhibition.

The exposure to high irradiance and temperature may result in photoinhibition. Paraheliotropism may represent an important strategy to decrease the potential for photodamages. The aim of this study was to assess the contribution of leaf movements to PSII photoprotection at high light and temperature in Robinia pseudoacacia. For this purpose, gas exchange and chlorophyll fluorescence measurements were performed at 10:00 and 12:00 am, and at 15:00 and 18:00 pm on attached blocked leaves (BL) and on control unblocked leaves (UL). At midday as well as at 15:00 and 18:00 pm, BL showed a decrease of net photosynthesis (An), stomatal conductance (gsH2O), quantum yield of electron transport (FPSII), percentage of inhibited photosynthesis by O2 (IPO), and PSII efficiency factor (F’q/F’v) as compared to values measured in the morning, whereas Ci/Ca ratio and NPQ increased significantly. Differently from BL, UL maintained the photosynthetic performance elevated throughout the day and higher compared to constrained leaves. At 18:00 pm, An, gsH2O, FPSII and F’q/F’v of BL showed a tendency to recovery compared to 15:00 pm even if the values remained lower than those measured at 10:00 am and in UL. Conversely to UL, no recovery was found in maximal PSII photochemical efficiency at the end of the study period. Data suggest that in R. pseudoacacia leaf movements represent an efficient and reversible strategy to overcome environmental stresses such as high light and temperature. Moreover paraheliotropism is able to protect photosystems avoiding photoinhibitory damage risks at no cost for plant in terms of An reduction.