Synergistic effects of melatonin and abscisic acid on salinity tolerance of bell pepper

• ABA + MT treatment improved bell pepper growth and yield under salinity. • Combined ABA + MT reduced Na⁺ accumulation while sustaining K⁺ and Ca²⁺ levels. • ABA + MT co-application enhanced antioxidant activity and lowered oxidative stress. The productivity of bell pepper ( Capsicum annuum L.), a globally important vegetable, is compromised by soil salinity. Both melatonin (MT) and abscisic acid (ABA) have been reported to improve tolerance to environmental stresses; however, their combined influence on pepper has yet to be clarified. This study examined the individual and synergistic effects of 50 µM ABA (ABA50), 100 µM MT (MT100), and their combination (ABA50+MT100) on the salt tolerance of green and red bell pepper varieties under 0 mM (S0) and 100 mM (S100) NaCl. A completely randomized factorial design was used with six replicates. Salinity stress significantly reduced the root biomass, fruit size, firmness, chlorophyll content, and overall yield. However, the ABA50+MT100 treatment notably improved the plant height, fruit diameter, firmness, chlorophyll a content, and total fruit number under saline conditions. This combined application also enhanced total soluble solids (TSS), titratable acidity (TA), vitamin C, magnesium content, and antioxidant enzyme activities (CAT, POD, DPPH), while markedly reducing oxidative stress markers such as MDA, H₂O₂, proline, and electrolyte leakage. Moreover, it maintained potassium and calcium homeostasis and limited sodium accumulation. Heat map analysis confirmed that ABA50+MT100 was the most effective treatment for preserving physiological, biochemical, and quality traits under salinity stress. Red peppers responded more favorably than green ones. Endogenous ABA and MT levels increased with salinity, suggesting a coordinated hormonal interaction that underlies enhanced stress adaptation.