Nickel stress modulates growth dynamics, disrupts redox homeostasis, and induces ultrastructural damage in maize

Heavy metal (HMs) contamination is becoming increasingly critical due to rapid urbanization and unregulated industrialization. Nickel (Ni), although an essential trace element for plants, becomes toxic at high concentrations, where it can accumulate to phytotoxic levels and make plants highly susceptible to its adverse effects. This study investigated the impact of variable Ni concentrations on maize seedlings by analyzing physiological, biochemical, ultrastructural, and molecular responses to understand the stress adaptation mechanisms. The results validate that Ni exposure suppressed plant growth and development by inducing oxidative stress, limiting nutrient uptake, and reducing photosynthetic efficiency. Higher Ni concentrations led to excessive reactive oxygen species (ROS) production in roots and shoots, resulting in oxidative damage as indicated by elevated malondialdehyde (MDA) content and ultrastructural disruptions. In vivo ROS detection using Dichloro-dihydro-fluorescein diacetate (H 2 DCFDA) and dihydroethidium (DHE) staining further confirmed ROS overaccumulation under stress. Antioxidant enzyme activities initially increased with Ni levels but declined sharply at the highest concentration, while ROS levels continued to rise, suggesting a breakdown in redox homeostasis. Furthermore, qPCR analysis revealed changes in the expression of antioxidant-related genes under Ni stress. In summary, Ni exposure disrupted redox balance, triggered oxidative damage, and activated defense responses in maize seedlings. • Nickel stress reduces maize growth, photosynthesis, and nutrient uptake. • It induces ROS overproduction and oxidative damage in roots and shoots. • Antioxidant responses rise at moderate Ni levels but collapse at higher doses. • Structural damage to stomata and chloroplasts confirms toxicity. • Study reveals key redox and ultrastructural disruptions in maize under Ni stress.