Physiological, biochemical and metabolomic changes in two Italian cultivars of Olea europaea in response to UV-B radiations

The stratospheric ozone layer is undergoing a deterioration due to factors of natural and anthropogenic origin that decrease the amount of UV-B radiation filtered and consequently increase the damage to living organisms. Although many countries have signed the Montreal Protocol with the goal of reducing the release of ozone-depleting substances, many researchers are uncertain about the recovery of the ozone layer, whose status is therefore constantly monitored. This scenario includes studies on the effects that excessive UV-B exposure can have on organisms. Being sessile, plants are particularly at risk because they are more exposed to this stress factor. In this PhD thesis the effects of prolonged UV-B stress (14 hours per day for 8 weeks) have been analyzed and compared on two cultivars of olive tree (Olea europaea L.), Olivastra Seggianese (Tuscan cultivar) and Giarraffa (Sicilian cultivar). The choice of the olive tree is based on its economic importance and its large-scale cultivation in almost all the Italian territory. The analyses carried out in this study show that UV-B radiation is a dangerous source of stress for the olive tree. Both cultivars, while proving resistant to the treatment, showed prominent effects although in different ways and times. It was possible to determine the critical points at which the most evident effects occur, i.e., after the first two weeks of stress (T2) and during the last two weeks (T6-T8). The data obtained suggest possible mechanisms by which Giarraffa responds and resists to UV-B stress more effectively than the Olivastra Seggianese, including the ability to maintain photosynthetic efficiency, a more effective and rapid activation of the antioxidant response, higher availability of flavonoids, increased hydroxycinnamic acids derivatives, optimization of GPox use as well as a relatively high content of mannitol and Hsp70. Moreover, Giarraffa opted for a targeted choice of RubisCO isoforms and managed SuSy content, saving energy during the critical stress point. In addition, the Giarraffa cultivar is better adapted to prolonged UV-B stress due to a higher presence of palmitic acid, α-linolenic acid, and stearic acid, as well as some terpenes and long-chain alkanes. In conclusion, emerging data indicate that the Giarraffa cultivar has developed better adaptive capacities than the Seggianese cultivar, probably due to the area of origin where the incident solar radiation is higher than in the native area of the Tuscan cultivar.