Impact of weather conditions and drought stress on primary and secondary metabolites of olives from Slovenian Istra

Sugars, the primary metabolites of plants, play an important role in the process of olive ripening, as they provide energy for metabolic changes and serve as a source for the biosynthesis of fatty acids. Phenolic compounds, which are secondary metabolites, can be synthesized in plants as a reflection of stressful situations such as tissue damage and UV irradiation. The objective of this study was to assess the impact of different irrigation treatments (0, 15, 33, 40, and 100% crop evapotranspiration (ETc)) on primary metabolites (sugars, sugar alcohols) and secondary metabolites (phenolic compounds). In different phenological periods, the levels of oleuropein, sugars and sugar alcohols were determined in olives and leaf samples in the crop years 2014, 2015, and 2016. The levels of oleuropein in olive fruits and leaves were highest under the irrigation treatment of 40% ETc and decreased noticeably under the irrigation treatment of 100% ETc. The median levels of glucose were higher compared with the mannitol and fructose levels in olive fruits. Total determined sugars in olive fruits during maturation decreased. While significant correlations were found in olive fruits between determined sugars and sugar alcohols and different irrigation treatments, we did not observe any correlation between determined sugars and sugar alcohols in olive leaves and irrigation treatments. Our research confirmed the complexity of the content of oleuropein and sugars or sugar alcohols in the leaves and fruits of olive trees from Slovenian Istra, which depend on various factors, not just drought stress. Weaker and less obvious correlations between the studied compounds and drought stress might be a consequence of unusual rainfall in the studied crop years.