Phenolic Antioxidant Pool in Cross-Adaptation Induced by Salt Stress Contributing to Drought Tolerance in Olive

Drought stress severely affected olive cultivation by impairing physiological and biochemical functions. However cross-adaptation has emerged as a promising strategy to enhance plant drought tolerance. In this study, cross-adaptation was induced by exposing olive plants to 200 mM NaCl for three weeks, followed by two-months of recovery and a subsequent exposure to drought for one month. Our results indicated that this strategy enhanced drought tolerance in olive plants by promoting growth, maintaining photosynthesis, preserving water status and reducing oxidative damage. To elucidate the biochemical basis of this improved tolerance, we investigated the involvment of phenolic compounds as key contributors in the antioxidant defense system. Notably, salt pretreated olive plants exhibited significantly elevated phenolic content relative to non-pretreated ones. This enhancement was associated with the specific accumulation of key phenolic compounds, including secoiridoids (oleuropein), hydroxycinnamic acid derivatives (verbascoside) and glucoside flavonoids (luteolin 7-O-glucoside and apigenin 7-O-glucoside) together with disometin. The elevated level of these bioactive metabolites together with the alleviation of oxidative stress, as revealed by the decline in oxidative stress markers and the high antioxydant activity of salt pretreated olive plants suggested that phenolic compounds act as effective antioxidants under drought conditions. However, a slight but significant reduction in the content of the phenolic polymer, lignin, was observed in salt pretreated olive plants, possibly reflecting a metabolic shift in nitrogen allocation. To the best of our knowledge, this study is the first to highlight cross-adaptation’s role in selectively modulating phenolic molecules for adaptive drought responses.