Antioxidant defences and oxidative damage in salt-treated olive plants under contrasting sunlight irradiance

The interactive effects of root-zone salinity and sunlight on leaf biochemistry, with special emphasis to antioxidant defences, were analysed in Olea europaea, during the summer period. Plants were grown outside under 15% (shade plants) or 100% sunlight (sun plants) and supplied with 0 or 125 mM NaCl. Measurements were conducted of 1) the contribution of ions and soluble carbohydrates to osmotic potentials 2) the PSII photochemistry and the photosynthetic pigment concentration 3) the concentration and the tissue-specific distribution of leaf flavonoids 4) the activity of antioxidant enzymes 5) the leaf oxidative damage. The concentrations of Na+ and Cl- were significantly greater in sun than in shade leaves, as also observed for the concentration of the “antioxidant” sugar-alcohol mannitol. The de-epoxidation state of violaxanthin-cycle pigments increased in response to salinity stress in sun leaves: This finding agrees with a greater maximal PSII photochemistry (Fv/Fm) at midday detected in salt-treated than in control plants. By contrast, salt-treated plants in the shade suffered from midday depression in Fv/Fm to a greater degree than observed in control plants. The high concentration of violaxanthin-cycle pigments in sun leaves, suggests that zeaxanthin may protect the chloroplast from photo-oxidative damage, rather than dissipating excess excitation energy via non photochemical quenching. Dihydroxy B-ring substituted flavonoid glycosides accumulate greatly in the mesophyll, not only in the epidermal cells, in response to high sunlight. The activity of antioxidant enzymes varied little because of sunlight irradiance, but declined sharply in response to high salinity in shade leaves. Interestingly, control and particularly salt-treated plants in the shade underwent greater oxidative damages than their sunny counterparts. These findings, which conform to the evolution of O. europaea in sunny environments, suggest that under partial shading, the antioxidant defence system may be ineffective to counter salt-induced oxidative damage.