Introduction
Currently, salinity stress is one of the most important challenges in the agriculture and is the main growth limiting factor of many plant species. Saline stress adversely affects the plant's physiological and biochemical process which leads to a considerable reduction of plant growth and yield. Shallot (Allium hirtifolium Boiss.) is a perennial plant from the Alliaceae family, which is one of the native and valuable plants of Iran and wildly grows in the slopes of the Zagros Mountain range. The nutritional and medicinal value of shallots is due to the presence of sulfur compounds, especially allicin in the bulbs. The present study was conducted to investigating the effect of foliar application of salicylic acid on the changes of growth, physiological and biochemical traits of shallot under different levels of salinity stress in factorial design based on a completely randomized design with three replications. The investigated treatments were four levels of salinity (0, 30, 60 and 90 mM NaCl) and four levels of salicylic acid (0, 1, 1.5 and 2 mM). Distilled water (control) and salicylic acid (1, 1.5 and 2 mM) were foliar sprayed on the whole plants at 4, 6 and 8 weeks after sowing date. At the end of the growing season (beginning of yellowing of the leaves of the plants), growth, physiological and biochemical traits were evaluated. Evaluation of lipid peroxidation, osmolality compounds and activity of antioxidant enzymes was carried out in the leaves of treated plants and the amount of pyrovat and allicin was measured in harvested shallot bulbs.
Results and discussion
Various abiotic stresses restrict plant productivity, and many efforts have been done to reduce plant growth inhibition by alleviating the disorder’s effects of these stresses. Exogenous application of plant growth regulators has been reported as an economic procedure to improve plant resistance to environmental stresses. It has been previously reported that salicylic acid as a signaling molecule alleviated the adverse effect of different stress condition. In this experiment, shallot resistance to saline condition was enhanced by the foliar spray of salicylic acid. The results showed a significant link between salicylic acid treatment and improvement of bulb biomass under saline conditions. Lipid peroxidation regards to accumulation of malondialdehyde and hydrogen peroxide increased with increasing the salinity intensity. Also, saline stress significantly enhanced the proline and glycine betaine content in stressed plants of shallot. The plant antioxidant activity induced under stress condition by increased the total phenol content as well as the activity of catalase, peroxidase, ascorbate peroxidase and superoxide dismutase enzymes. The pyrovat and allicin content of shallot bulb was increased with increasing salinity stress. The decreased in the photosynthetic pigments (total chlorophyll and carotenoids) led to a decrease in plant growth with the intensifying of stress level. So that the lowest leaf area, fresh and dry weight of bulbs were obtained in severe salinity stressed plants (90 mM NaCl). Foliar application of salicylic acid through increasing the antioxidant compounds (total phenol, pyrovat and allicin) and the activity of antioxidant enzymes (CAT, POX, APX and SOD) limited the of hydrogen peroxide accumulation and lipid peroxidation. The induced accumulated osmolyte compounds proline and glycine betaine was decreased in treated plant with salicylic acid. The treatment of salicylic acid considerably improved the chlorophyll and carotenoids content especially in salinity stressed plants. So that the applied of salicylic acid, especially at concentration of 2 mM, reduced the harmful effects of salinity stress on plant growth and bulb yield by increasing the photosynthesis pigments and consequently photosynthesis efficiency. Also increased growth in the treated plants with salicylic acid has been attributed to changes in the concentration of plant hormones, especially auxins and cytokinins (the most important plant growth stimulating hormones). Nevertheless, the growth reactions of treated plants to salicylic acid are different depending on the used concentration, the plant species and the growth stage at the treatment time.
Conclusion
In the several literatures the salicylic acid effects on plant growth enhancement under unfavorable environmental condition attributed to salicylic acid-induced changes in plant biochemical and physiological processes. Based on the obtained results, the salinity resistance of the shallot plant in response to salicylic acid is related to the increased antioxidant capacity of the stressed plants, which leads to the improvement of the photosynthetic pigments, and consequently plant growth and bulb biomass under saline condition. Although the present study was performed in the glass greenhouse, the obtained finding showed that salicylic acid application could also be a promising treatment for improving salinity tolerance of A. hirtifolium under field conditions. |
