Exogenous application of calcium to 24-epibrassinosteroid pre-treated tomato seedlings mitigates NaCl toxicity by modifying ascorbate-glutathione cycle and secondary metabolites

Abstract

The present study tested the efficacy of 24-epibrassinolide (EBL) and calcium (Ca) for mediating salinity tolerance in tomato. Salinity stress affected the morphological parameters of tomato as well as leaf relative water content (LRWC), photosynthetic and accessory pigments, leaf gas exchange parameters, chlorophyll fluorescence and the uptake of essential macronutrients. The salt (NaCl) treatment induced oxidative stress in the form of increased Na⁺ ion concentration by 146%, electrolyte leakage (EL) by 61.11%, lipid peroxidation (MDA) 167% and hydrogen peroxide (H₂O₂) content by 175%. Salt stress also enhanced antioxidant enzyme activities including those in the ascorbate-glutathione cycle. Plants treated with EBL or Ca after salt exposure mitigated the ill effects of salt stress, including oxidative stress, by reducing the uptake of Na⁺ ions by 52%. The combined dose of EBL + Ca reversed the salt-induced changes through an elevated pool of enzymes in the ascorbate-glutathione cycle, other antioxidants (superoxide dismutase, catalase), and osmoprotectants (proline, glycine betaine). Exogenously applied EBL and Ca help to optimize mineral nutrient status and enable tomato plants to tolerate salt toxicity. The ability of tomato plants to tolerate salt stress when supplemented with EBL and Ca was attributed to modifications to enzymatic and non-enzymatic antioxidants, osmolytes and metabolites.

Figure 1

Pretreatment of seeds with EBL (10⁻⁷ M) and foliar application of Ca (50 mM) enhanced the activity of (A) SOD (B) CAT, and (C) GST in tomato seedlings under NaCl stress. Data presented are means ± SE (n = 5). Different letters indicate significant differences at P ≤ 0.05.

Figure 2

Pretreatment of seeds with EBL (10⁻⁷ M) and foliar application of Ca (50 mM) enhanced the activity of (A) APX (B) GR (C) MDHAR and (D) DHAR in tomato seedlings under NaCl stress. Data presented are means ± SE (n = 5). Different letters indicate significant differences at P ≤ 0.05.

Figure 3

Pretreatment of seeds with EBL (10⁻⁷ M) and foliar application of Ca (50 mM) maintained (A) AsA, (B) GSH, (C) GSSG, (D) GSH/GSSG ratio and (E) flavonoid content in tomato seedlings under NaCl stress. Data presented are means ± SE (n = 5). Different letters indicate significant differences at P ≤ 0.05.