Effects of salinity and drought on growth, ionic relations, compatible solutes and activation of antioxidant systems in oleander (Nerium oleander L.)

Nerium oleander is an ornamental species of high aesthetic value, grown in arid and semi-arid regions because of its drought tolerance, which is also considered as relatively resistant to salt; yet the biochemical and molecular mechanisms underlying oleander's stress tolerance remain largely unknown. To investigate these mechanisms, one-year-old oleander seedlings were exposed to 15 and 30 days of treatment with increasing salt concentrations, up to 800 mM NaCl, and to complete withholding of irrigation; growth parameters and biochemical markers characteristic of conserved stress-response pathways were then determined in stressed and control plants. Strong water deficit and salt stress both caused inhibition of growth, degradation of photosynthetic pigments, a slight (but statistically significant) increase in the leaf levels of specific osmolytes, and induction of oxidative stress-as indicated by the accumulation of malondialdehyde (MDA), a reliable oxidative stress marker-accompanied by increases in the levels of total phenolic compounds and antioxidant flavonoids and in the specific activities of ascorbate peroxidase (APX) and glutathione reductase (GR). High salinity, in addition, induced accumulation of Na+ and Cl- in roots and leaves and the activation of superoxide dismutase (SOD) and catalase (CAT) activities. Apart from anatomical adaptations that protect oleander from leaf dehydration at moderate levels of stress, our results indicate that tolerance of this species to salinity and water deficit is based on the constitutive accumulation in leaves of high concentrations of soluble carbohydrates and, to a lesser extent, of glycine betaine, and in the activation of the aforementioned antioxidant systems. Moreover, regarding specifically salt stress, mechanisms efficiently blocking transport of toxic ions from the roots to the aerial parts of the plant appear to contribute to a large extent to tolerance in Nerium oleander.