Salinity-induced changes in phenolic compounds in leaves and roots of four olive cultivars (Olea europaea L.) and their relationship to antioxidant activity

Phenolic compounds are constituents of all higher plants. However, their biosynthesis is often induced when plants are exposed to environmental stresses, such as salinity. The aim of the present research was to determine the phenolic status (total phenol content, oleuropein and hydroxytyrosol) and antioxidant activity of four olive cultivars under saline conditions in two different plant parts (leaves and root). We also tested the possible relationship between oleuropein and glucose, since the latter compound is a part of the former molecule and accumulates under saline conditions. The data indicate that salinity stimulated the biosynthesis of phenols and oleuropein, especially in leaves, whereas the hydroxytyrosol concentration was either negatively or not affected by the salt stress. Oleuropein was the main phenolic compound in both tissues regardless of NaCl treatments. In leaves, glucose showed a totally inverse response to salinity than that of oleuropein, while a highly negative correlation existed between these two substances (R = −0.90, −0.80, −0.88 and −0.84 for ‘Zard’, ‘Ascolana’, ‘Koroneiki’ and ‘Arbequina’, respectively). A possible explanation for this relationship is that oleuropein acts as a glucose-reservoir for osmoregulation or high energy-consuming processes required for plant adaptation to salinity. A highly significant correlation was recorded between total phenol content and antioxidant activity in both leaves and roots. Finally, there is no indication pointing to translocation of phenolic compounds between leaves and root owing to exposure to salt stress.

► Salinity induced phenylpropanoid metabolism in the olive tree. ► Oleuropein was the main phenolic compound in olive tree involved in protection against salinity stress. ► Oleuropein in the leaves could act as a glucose reservoir. ► There is no indication of a transport mechanism between leaves and root. ► Oleuropein may act as an additional constituent of the antioxidant defense system of olive tree.