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

Abstract

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.

Morphological and physiological adjustments of Brazilwood (Caesalpinia echinata Lam.) to direct solar radiation

“…Phenolic compounds and flavonoids tend to accumulate in the epidermis and mesophyll of tropical trees under intense irradiance (Melgar et al, 2009), as observed in plants of C. echinata in full sun. However, the presence of phenols was also observed in the epidermis of leaves of C. echinata under shade.…”

“…However, the presence of phenols was also observed in the epidermis of leaves of C. echinata under shade. The accumulation of phenols, especially quercetin glycosides luteolin, indicates an efficient antioxidative defense system (Guidi et al, 2008) acting in the scavengers of several reactive oxygen species (ROS) and singlet O 2 in the chloroplasts of plants under intense irradiance (Melgar et al, 2009). …”

“…In addition to the energy metabolism, the monosaccharides, disaccharides and RFOs were also related to the antioxidant, acting as scavengers (Mendes et al, 2001;Melgar et al, 2009) and inhibitors of the synthesis of ROS (Coueé et al, 2006) under different kinds of stress. Therefore, the investigation of antioxidative metabolism and its relations with the soluble carbohydrates could provide important information to understande the mechanisms of adaptation of shade-tolerant tropical trees when exposed to intense irradiance.…”

“…The physiological adjustments of shade plants when exposed to high radiation also involve the decreased concentration of total chlorophyll (Chl total ) or an increase in the ratio of xanthophylls and violaxanthin cycle in relation to Chl total (Wyka et al, 2007;Melgar et al, 2009). Under intense irradiance, some trees present inhibition of photosystem II (PSII) electron transport associated with the higher concentration of carbohydrates (Valladares et al, 2005;Melgar et al, 2009) and increased levels of flavonoids (Tattini et al, 2006).…”

“…Under intense irradiance, some trees present inhibition of photosystem II (PSII) electron transport associated with the higher concentration of carbohydrates (Valladares et al, 2005;Melgar et al, 2009) and increased levels of flavonoids (Tattini et al, 2006).…”