Effect of water deficit on leaf phenolic composition, gas exchange, oxidative damage and antioxidant activity of four Greek olive (Olea europaea L.) cultivars

The olive tree (Olea europaea L.) is often exposed to severe water stress during the summer season. In this study, we determined the changes in total phenol content, oleuropein and hydroxytyrosol in the leaves of four olive cultivars (‘Gaidourelia’, ‘Kalamon’, ‘Koroneiki’ and ‘Megaritiki’) grown under water deficit conditions for two months. Furthermore, we investigated the photosynthetic performance in terms of gas exchange and chlorophyll a fluorescence, as well as malondialdehyde content and antioxidant activity. One-year-old self-rooted plants were subjected to three irrigation treatments that received a water amount equivalent to 100% (Control, C), 66% (Field Capacity 66%, FC66) and 33% (Field Capacity 33%, FC33) of field capacity. Measurements were conducted 30 and 60 days after the initiation of the experiment. Net CO2 assimilation rate, stomatal conductance and Fv/Fm ratio decreased only in FC33 plants. Photosynthetic rate was reduced mainly due to stomatal closure, but damage to PSII also contributed to this decrease. Water stress induced the accumulation of phenolic compounds, especially oleuropein, suggesting their role as antioxidants. Total phenol content increased in FC33 treatment and oleuropein presented a slight increase in FC66 and a sharper one in FC33 treatment. Hydroxytyrosol showed a gradual decrease as water stress progressed. Malondialdehyde (MDA) content increased due to water stress, mostly after 60 days, while antioxidant activity increased for all cultivars in the FC33 treatment. ‘Gaidourelia’ could be considered as the most tolerant among the tested cultivars, showing higher phenolic concentration and antioxidant activity and lower lipid peroxidation and photochemical damage after two months of water stress. The results indicated that water stress affected olive tree physiological and biochemical parameters and magnitude of this effect depended on genotype, the degree of water limitation and duration of treatment. However, the severity as well as the duration of water stress might exceed antioxidant capacity, since MDA levels and subsequent oxidative damage increased after two months of water deficit.

► Olive tree response depended on genotype and the degree and duration of water stress.
► Optimal photochemical efficiency of PSII was constant, revealing an adaptation response.
► Water stress induced phenolic compounds accumulation.
► Hydroxytyrosol may act in favour of oleuropein synthesis as water stress progresses.
► Moderate water stress conditions could be used to enhance oleuropein concentration.