Unravelling the protective mechanism: calcium alleviates nickel toxicity in maize (Zea mays) by modulating osmolytes and antioxidant systems

Nickel (Ni) toxicity in agricultural soils, stemming from industrial activities, fertiliser use, and sewage sludge, severely disrupts plant growth by inducing oxidative stress. Calcium-based soil amendments have emerged as promising strategies for mitigating heavy metal toxicity; however, their comparative effectiveness and underlying protective mechanisms remain insufficiently understood. This study evaluated the potential of lime and gypsum to mitigate Ni toxicity in maize (Zea mays). A pot experiment was conducted using Ni-contaminated soil amended with lime and gypsum applied at rates of 0.2%, 0.4%, and 0.6% of calcium. The results showed that both amendments effectively reduced Ni bioavailability in the soil, leading to improved maize growth and physiological performance. Among all treatments, the application of gypsum at 0.6% was the most effective, significantly increasing root and shoot biomass, enhancing chlorophyll content, and improving photosynthetic gas exchange parameters. This improvement was linked to the upregulation of antioxidant enzyme activities and increased osmolyte production, which together alleviated oxidative stress. Concurrently, calcium treatments substantially reduced Ni uptake and accumulation in plant tissues. These findings demonstrate that calcium-based amendments, particularly gypsum, serve as a cost-effective strategy for the remediation of Ni-contaminated soils. It was concluded that applying 0.6% gypsum mitigates Ni toxicity in maize by reducing metal bioavailability and uptake while simultaneously enhancing antioxidant defences and photosynthetic efficiency, thereby offering a practical solution for sustaining crop productivity and ensuring food security in heavy metal-polluted environments.