Morphophysiological responses of Nigella sativa L. to salicylic acid under salinity stress

Nigella sativa is an herbaceous and annual plant in the family of Ranunculaceae, which cultivated in different parts of Europe and Asia, including in Iran (various regions, especially Arak and Isfahan). The plant has small leaves and white flowers with blue margin. Salinity stress is one of the major environmental stresses due to low osmotic potential of soil solution (osmotic stress), ionic toxicity (ionic stress) and nutritional disorder, or a combination of these factors in the plant as one of the most important factors limiting growth and development of plants. In order to study the morpho-physiological and biochemical responses of Nigella sativa plant to salicylic acid under salinity stress conditions a split plot experiment with two factors carried out based on completey randomized design with 3 replicates in the greenhouse of the Medicinal plants research center of Shahed University in 1396. The salinity factor with 4 levels (0, 3, 6, 9 dS/m) in main plots and salicylic acid levels in 4 levels (0, 25, 5 0 and 0.75 mM) were investigated in sub plots. Salicylic acid levels were sprayed on plants one week before applying salinity stress (about 4 to 6 leaves) and three weeks after salinity stress conditions. After 21 days of treatment, the effect of salinity stress on growth traits (shoot length, number of branches, number of leaves, root length, fresh and dry weight of shoot, fresh and dry weight of root), physiological traits (chlorophyll a, chlorophyll b and carotenoid, proline, malondialdehyde and catalase and superoxide dismutase were investigated. The results showed that different salinity concentrations had significant effects on morphological and physiological traits. By increasing salinity levels, the growth indices such as number of branches, number of leaves and chlorophyll b and the amount of superoxide dismutase and malondialdehyde decreased, while by increasing the salinity levels the content of proline and catalase in leaf increased. The results showed that applying salicylic acid under salinity stress reduced the growth indices and photosynthetic pigments. The highest number of branches, number of leaves and chlorophyll b was obtained at 3 dS/m salinity and 0.5 mM salicylic acid. Also, by increasing salicylic acid level, the activity of the proline content and leaf cattalase increased, but with increasing the salicylic acid levels, the amount of malondialdehyde and superoxide dismutase enzymes decreased. Based on the results, it can be concluded that the Nigella sativa is a semi-sensitive saline plant. The overall results showed that salicylic acid increased the plant tolerance to salinity stress. However, for a better conclusion on the effect of salicylic acid, the use of this compound is needed in a wider range. For example, in most of the studied physiological and biochemical traits, no significant difference was found between the effects of various salicylic acid concentrations. While positive effect of salicylic acid cannot be ignored in improving the damage caused by stress. In general, the use of salicylic acid resulted improving in biochemical parameters and increased plant tolerance to salinity stress. Therefore, it can be concluded that applying the salicylic acid can be a suitable procedure to reduce the harmful effects of salt stress in Nigella sativa.