Silicon and thiourea synergy boosts wheat cultivars’ grain quality under salinity via coordinated ion and antioxidant responses

Salinity stress severely impacts wheat productivity and grain quality. This study investigated the effects of application of plant growth promoting substances (non-primed control (NP), hydropriming (HP), combined priming (1 mM silicon + 250 mg L^{- 1} thiourea) (SiP + TUP), combined TU priming (250 mg L^{- 1} thiourea) + Si foliar application (1 mM silicon) (TUP + SiF), combined TU foliar application (250 mg L^{- 1} thiourea) + Si priming (1 mM silicon) (TUF + SiP) on salt tolerance and grain quality of two wheat cultivars (salt-tolerant Barzegar and salt-sensitive Amin) under three salinity levels (0.5, 4 and 10 dS m^{- 1}). The experiment used a completely randomized factorial design with three replications under greenhouse conditions. Salinity reduced grain yield by 42-60% (more severely in Amin), decreased ash content, moisture, carbohydrates, fiber, and gluten, while increasing grain hardness and protein. The combined TUP + SiF treatment significantly mitigated these effects, particularly in Barzegar, restoring 10.5-39.0% of yield versus 34.4-50.0% in Amin. Mechanistically, the treatment: (1) enhanced K⁺/Na⁺ ratio (1.7-2.2-fold) via upregulation of transporters, (2) reduced oxidative damage (MDA decreased by 33.8-60.9%) through boosted antioxidant enzymes (SOD 12-14%, POD 33%, APX 25-26%, CAT 55-94%), and (3) improved grain quality by increasing gluten (8.8-9.8%) and carbohydrates (6.7-7.5%) while reducing hardness and protein. Silicon's role in epidermal silica deposition reduced Na⁺ uptake, while thiourea's thiol group directly scavenged ROS. Cultivar differences revealed Barzegar's superior ion homeostasis and antioxidant capacity. The TUP + SiF combination effectively counters salinity impacts through synergistic physiological mechanisms, with greater efficacy in tolerant cultivar. These findings provide practical strategies for wheat cultivation in saline soils, though cultivar-specific optimization may enhance results.