Calcium Improves Germination and Growth of Sorghum bicolor Seedlings under Salt Stress

Salinity is a major constraint limiting plant growth and productivity worldwide. Thus, understanding the mechanism underlying plant stress response is of importance to developing new approaches that will increase salt tolerance in crops. This study reports the effects of salt stress on Sorghum bicolor during germination and the role of calcium (Ca²⁺) to ameliorate some of the effects of salt. To this end, sorghum seeds were germinated in the presence and absence of different NaCl (200 and 300 mM) and Ca²⁺ (5, 15, or 35 mM) concentrations. Salt stress delayed germination, reduced growth, increased proline, and hydrogen peroxide (H₂O₂) contents. Salt also induced the expression of key antioxidant (ascorbate peroxidase and catalase) and the Salt Overlay Sensitive1 genes, whereas in the presence of Ca²⁺ their expression was reduced except for the vacuolar Na⁺/H⁺ exchanger antiporter2 gene, which increased by 65-fold compared to the control. Ca²⁺ reversed the salt-induced delayed germination and promoted seedling growth, which was concomitant with reduced H₂O₂ and Na⁺/K⁺ ratio, indicating a protective effect. Ca²⁺ also effectively protected the sorghum epidermis and xylem layers from severe damage caused by salt stress. Taken together, our findings suggest that sorghum on its own responds to high salt stress through modulation of osmoprotectants and regulation of stress-responsive genes. Finally, 5 mM exogenously applied Ca²⁺ was most effective in enhancing salt stress tolerance by counteracting oxidative stress and improving Na⁺/K⁺ ratio, which in turn improved germination efficiency and root growth in seedlings stressed by high NaCl.