Responses of olive trees (Olea europaea L.) to nickel contamination during fruit development: physiological traits and fruit quality

Nickel (Ni) is an essential element for plants, but its excess in soil and water can negatively affect growth and physiology. Olea europaea L., a woody crop often grown in marginal areas and in water scarcity conditions, could be exposed to Ni contamination. Little is known about Ni effects on olive physiology and fruit quality. Olive trees (‘Cipressino’) were irrigated with increasing Ni concentrations (20, 200, and 2000 µg L⁻¹) from fruit pit hardening to turning stage to assess physiological responses, Ni accumulation, ionomic changes, fruit quality, and the expression of genes involved in unsaturated fatty acid and tocopherol biosynthesis. Under 2000 µg L − 1 a significant Ni increment in roots but a reduced mobilization to aerial parts was observed, hypothetically suggesting that roots can act as a selective barrier. Ni exposure has induced a complex response, mostly in an organ-dependent manner. Although most changes occurred in cations (i.e. Zn, Mn, Mg) mainly at root and stem level, boron decrement in most organs could have an important role in olive Ni response. Ionomic changes had potentially an impact on leaf greenness trends and pigment-related traits. In fruits, irrigation with 200 and 2000 µg L⁻¹ Ni altered the expression of genes involved in tocopherol ( OeMPBQ MT ) and unsaturated fatty acid ( OeSAD1 , OeSAD2 , OeSAD3 , OeFAD2-5 , OeFAD6 , and OeFAD7-1 ) biosynthesis, while corresponding changes in tocopherol and fatty acid levels were mainly observed as trends. Long-term studies are needed to determine whether these molecular responses translate into relevant changes in fruit composition and olive oil quality.