Journal Article

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

Juan Carlos Melgar, Lucia Guidi, Damiano Remorini, Giovanni Agati, Elena Degl’innocenti, Silvana Castelli, Maria Camilla Baratto, Cecilia Faraloni and Massimiliano Tattini

in Tree Physiology

Volume 29, issue 9, pages 1187-1198
Published in print September 2009 | ISSN: 0829-318X
Published online September 2009 | e-ISSN: 1758-4469 | DOI: http://dx.doi.org/10.1093/treephys/tpp047
Antioxidant defences and oxidative damage in salt-treated olive plants under contrasting sunlight irradiance

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The interactive effects of root-zone salinity and sunlight on leaf biochemistry, with special emphasis on antioxidant defences, were analysed in Olea europaea L. cv. Allora, 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. The following measurements were performed: (1) the contribution of ions and soluble carbohydrates to osmotic potentials; (2) the photosystem II (PSII) photochemistry and the photosynthetic pigment concentration; (3) the concentration and the tissue-specific distribution of leaf flavonoids; (4) the activity of antioxidant enzymes; and (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 (F v/F m) at midday, detected in salt-treated than in control plants, growing in full sunshine. By contrast, salt-treated plants in the shade suffered from midday depression in F v/F m to a greater degree than that 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 mechanisms. 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 damage 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.

Keywords: antioxidant enzymes; chlorophyll fluorescence; electron paramagnetic resonance; flavonoid glycosides; fluorescence microspectroscopy; mannitol; violaxanthin-cycle pigments

Journal Article.  8396 words.  Illustrated.

Subjects: Plant Sciences and Forestry

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