Exogenous Application of Ascorbic Acid Mitigates the Adverse Effects of Salt Stress in Two Contrasting Barley Cultivars through Modulation of Physio-Biochemical Attributes, K+/Na+ Homeostasis, Osmoregulation and Antioxidant Defense System

Among cereals, barley (Hordeum vulgare L.) is considered as tolerant to salinity stress; however, its performance and global production are still dramatically affected by salinity. Thus, the development of suitable management practices to minimize the deleterious effects of salt stress on this crop plant has become necessary. In this study, we explored the potential function of ascorbic acid (AsA) in mitigating the adverse effects of salt stress on barley at the seedling stage. To this end, greenhouse pot experiments were performed to study genotypic differences in response to 150 mM NaCl alone or combined with 1 mM AsA using two barley cultivars with different salt sensitivities, Rihane (salt-tolerant) and Testour (salt-sensitive). Obtained results showed that, although Rihane was well adapted and performed better compared to Testour genotype, the exposure to salt stress conditions resulted in reduced growth characteristics (dry weights and relative growth rates), associated with significant decreases in relative water contents (RWCs), total chlorophyll, soluble proteins, total carbohydrates as well as K+ contents. However, Na+ and MDA concentrations were markedly increased in the both salt-stressed cultivars, as compared to controls. Addition of AsA to the saline media boosted barley-seedling growth by decreases in Na+ contents, increases in total chlorophyll content, K+ contents as well as K+/Na+ ratios. Likewise, supplemental AsA modulated the antioxidant defense system in salt-treated barley seedlings by increasing both non-enzymatic (total phenolics and flavonoids) and enzymatic antioxidant components (mainly SOD, CAT and GPX) for the both cultivars, resulting thereby in a remarkably decrease in leaves’ and roots’ MDA contents. Additionally, proline and glycine betaine were noticeably accumulatedas a result of AsA addition, confirming the beneficial role of AsA in retaining additional osmolality to uphold the RWC and promote seedling growth, particularly in the salt-sensitive genotype. Overall, this comprehensive study provided evidence that the salt-induced damages were more effectively alleviated in Testour than Rihane genotype upon AsA supplementation to saline media. Therefore, exogenous AsA application may be suggested as a potential strategy to overcome the salinity-mediated impairment of barley, particularly salt-sensitive genotypes, at the early growth stage through improving osmotic and ionic homeostasis and upregulating the antioxidant defense system.