UV-B Radiation Affects Photosynthesis-Related Processes of Two Italian Olea europaea (L.) Varieties Differently

Given the economical importance of the olive tree it is essential to study its responses tostress agents such as excessive UV-B radiation, to understand the defense mechanisms and toidentify the varieties that are able to cope with it. In the light of the analysis carried out in this study,we argue that UV-B radiation represents a dangerous source of stress for the olive tree, especiallyin the current increasingly changing environmental conditions. Both the varieties considered(Giarraffa and Olivastra Seggianese), although resistant to the strong treatment to which they wereexposed, showed, albeit in different ways and at different times, evident effects. The two varietieshave different response times and the Giarraffa variety seems better suited to prolonged UV-Bstress, possible due to a more efficient and quick activation of the antioxidant response (e.g.,flavonoids use to counteract reactive oxygen species) and because of its capacity to maintain thephotosynthetic efficiency as well as a relatively higher content of mannitol. Moreover, pigmentsreduction after a long period of UV-B exposure can also be an adaptation mechanism triggered byGiarraffa to reduce energy absorption under UV-B stress. Olivastra Seggianese seems less suited toovercome UV-B stress for a long period (e.g., higher reduction of Fv/Fm) and has a higherrequirement for sugars (e.g., glucose) possible to counteract stress and to restore energy.