Saline and alkaline stress genotypic tolerance in sweet sorghum is linked to sodium distribution

Salt and alkali stress limit crop growth and reduce agricultural productivity worldwide, which have led to increased interest in enhancing salt tolerance in crop plants. Sweet sorghum (Sorghum bicolor (Linn.) Moench) is a monocotyledonous crop species that shows greater tolerance to salt–alkali stress than most other crops, although the underlying mechanisms behind this tolerance remain unclear. Therefore, we investigated the effects of salt and alkali stresses on two sweet sorghum varieties M-81E, which is stress tolerant, and 314B, which is stress sensitive. Namely, we surveyed plant growth parameters, measured Na⁺ and K⁺ distributions at the level of the whole plant as well as in three specific tissues, and then determined the activities of H⁺-ATPase, H⁺-PPase and Na⁺/H⁺ exchange in root vacuole membranes under stress conditions. Following treatment of the seedlings for 3 days with salt or alkali solutions, the plant growth was inhibited and Na⁺ levels in the whole plant, leaves, sheath, and roots were increased in both genotypes. Under alkali stress, K⁺ levels in the whole plant, leaves, sheath, and roots were decreased in both genotypes. M-81E roots accumulated significantly higher levels of Na⁺ than leaves, whereas the opposite was true for 314B. Under salt stress, both the hydrolytic and proton-transporting activities of V-H⁺-ATPase were enhanced and Na⁺/H⁺ exchange activity was dramatically upregulated, whereas V-H⁺-PPase activity was decreased. M-81E showed a greater capacity to compartmentalize Na⁺ within root cell vacuoles and maintain higher levels of K⁺ uptake compared with 314B, resulting in higher K⁺/Na⁺ transport selectivity in this genotype. These results also demonstrated that H⁺-ATPase activity and ionic homeostasis (Na⁺/K⁺) were likely important contributors to the tolerance of saline-alkali stress and crucially important for understanding alkaline stress in both crops and wild plants.