Seed priming with salicylic acid and seaweed extract enhances salt tolerance in Glycine max varieties from Bangladesh by enhancing antioxidant defense

Soil salinity is a growing threat to soybean production in coastal and marginal agricultural areas. This study evaluated whether seed priming with salicylic acid (SA) and foliar application of a red seaweed extract (SWE) from Gracilaria verrucosa applied individually and in combination (SA + SWE) could alleviate salinity stress in three improved Bangladeshi soybean varieties: BARI Soybean-5, BARI Soybean-6, and BARI Soybean-7. Plants were grown under greenhouse conditions and exposed to three salinity levels (0, 3, and 6 dS m⁻¹) in a fully factorial randomized complete block design (n = 3). Growth traits (plant height, root length), photosynthetic pigments (chlorophyll a, chlorophyll b, carotenoids), soluble protein, proline, and oxidative stress markers (malondialdehyde, MDA; hydrogen peroxide, H₂O₂) were measured at 40 days after sowing. Salinity at 6 dS m⁻¹ reduced plant height by ~ 23% and suppressed chlorophyll a, soluble protein, and root growth across all varieties. Both biostimulant treatments significantly mitigated these effects, with SA + SWE being consistently superior to SA alone. SA + SWE increased plant height by up to 112% over the untreated control under severe salinity in BARI Soybean-7, and raised chlorophyll a by ~ 90% relative to controls at 6 dS m⁻¹. MDA and H₂O₂ were reduced by ~ 32% and ~ 46%, respectively, under SA + SWE at 6 dS m⁻¹, reflecting reduced oxidative damage. Proline responses were variety-dependent: SA + SWE suppressed salinity-induced proline accumulation in BARI Soybean-5 and BARI Soybean-6 but sharply elevated it in BARI Soybean-7, pointing to distinct osmoprotective strategies among genotypes. BARI Soybean-7 showed the strongest growth recovery under treatment, while BARI Soybean-6 maintained greater biochemical stability under stress. These findings indicate that the combination of SA seed priming and Gracilaria verrucosa foliar spray is an effective, eco-friendly approach to improve soybean performance under saline conditions, and that the choice of variety remains important for maximizing the benefit.