Comparative analysis of water deficit and salt tolerance mechanisms in Silene

Comparative analyses of the responses to abiotic stress in related taxa with different degrees of tolerance can provide useful information to elucidate the mechanisms of stress tolerance in plants. This kind of study has been carried out in four Silene species, which were subjected to salt and water deficit treatments under controlled greenhouse conditions. Growth parameters and leaf levels of photosynthetic pigments, ions, osmolytes, malondialdehyde (MDA, an oxidative stress biomarker), total phenolic compounds and flavonoids, were determined in control and stressed plants. The degree of stress-induced growth inhibition allowed establishing the relative tolerance of the studied species, identifying S. vulgaris as the most tolerant to salinity and S. sclerocarpa to water deficit; these data correlated well with the characteristics of their natural habitats. All four species showed a high resistance to stress-induced leaf dehydration, and a good negative correlation was found between tolerance and the degradation of photosynthetic pigments. Salinity tolerance is mostly based on the inhibition of Na+ transport to the leaves and the maintenance of relatively high leaf K+ levels in the salt-treated plants. Proline is a reliable stress biomarker but does not appear to be involved in tolerance mechanisms in this genus, as it accumulates at higher levels in the more sensitive species. MDA contents did not increase significantly in response to the stress treatments - except in water-stressed S. latifolia, the species most affected by water deficit - suggesting that the plants were not affected by secondary oxidative stress under the experimental conditions used. Accordingly, the measured variations in the levels of total phenolic compounds and flavonoids were not statistically significant or did not correlate with the relative stress resistance of the studied species. Therefore, stress responses based on the activation of antioxidant systems do not seem to be relevant for abiotic stress tolerance in Silene.