Acidic and Alkaline pH Stresses Impair Tomato Seed Germination and Seedling Growth via Disruption of Reactive Oxygen Species and Auxin Homeostasis

Soil pH is a critical environmental determinant of seed germination, seedling establishment, and ultimately crop yield. However, the physiological and molecular mechanisms underlying pH stress-mediated inhibition of germination and early seedling development remain poorly understood. Here, tomato was employed as a model system to systematically evaluate the dose-dependent effects of pH stress (ranging from pH 3.5 to 10.5) on germination performance and post-germinative growth. Our results demonstrate that both acidic and alkaline conditions significantly suppressed germination parameters in a pH intensity-dependent manner. Concurrently, seedling growth was markedly inhibited, root and hypocotyl elongation declined progressively, and total seedling biomass decreased substantially. Exposure to acidic (pH 3.5) or alkaline (pH 9.5) stress reduced seed viability and triggered a robust reactive oxygen species (ROS) burst and cell death. Biochemical assays revealed that acidic and alkaline stress disrupted redox homeostasis by compromising the coordinated activity of antioxidant enzymes, elevating membrane lipid peroxidation, and impairing osmotic adjustment capacity. Furthermore, acid and alkaline stress-induced inhibition of root growth coincided with diminished root cell viability and reduced endogenous auxin accumulation. Gene expression analyses showed that acidic and alkaline stress downregulated auxin biosynthesis genes and cell wall-associated genes involved in extension and modification, including EXPs and XTHs. Notably, IAA priming effectively rescued germination and early seedling growth under alkaline stress. Collectively, these findings elucidate a mechanistic framework linking pH-induced oxidative damage, auxin deficiency, and cell wall remodeling to impaired seed germination and seedling establishment and identify IAA priming as a physiologically grounded strategy to enhance crop resilience in alkali-affected marginal soils.