Enhancing harvestable nickel yield in Alyssum argenteum through chemical, hormonal and biological amendments: implications for nickel agromining

Nickel agromining harnesses hyperaccumulator plants to recover critical metals from metal-rich soils, offering a low-impact alternative to conventional mining and a pathway for the valorisation of contaminated or marginal land. However, the economic viability of this approach remains constrained by limited nickel yield in aboveground biomass, motivating the development of strategies to enhance plant metal uptake. This study evaluated a broad range of agronomic strategies to enhance shoot Ni accumulation in the Ni hyperaccumulator Alyssum argenteum under controlled glasshouse conditions.

Plants were grown in a substrate spiked to 600 mg kg⁻¹ Ni and subjected to three categories of treatments applied independently: (i) the biodegradable chelating agent EDDS and a set of low-molecular-weight organic acids (citric, oxalic, malic, and acetic acids), (ii) plant growth regulators representing auxin (IAA, NAA), cytokinin (BAP) and gibberellin (GA) classes, and (iii) plant growth-promoting bacteria (Pseudomonas fluorescens, P. putida, P. protegens, and a consortium). At harvest, nickel yield was quantified as shoot Ni per plant (leaf + stem). In parallel, physiological and biochemical responses were assessed, including photosynthetic pigments (chlorophylls, carotenoids, and anthocyanins), histidine, hydrogen peroxide (H₂O₂), and reduced glutathione (GSH).

All three treatment classes produced substantial positive effects on shoot Ni yield. Among phytohormones, IAA and GA produced the strongest enhancements, with mean shoot Ni increases typically exceeding +50% vs control and, under the highest application levels, surpassing +100% vs control. The strongest overall responses were observed for the chemical amendments, with malic and oxalic acids, followed by citric acid and EDDS, frequently increasing shoot Ni by >+100% vs control, depending on dose. In the biological treatments, inoculation with P. putida and the consortium consistently enhanced shoot Ni yield, with increases of up to ~+50% vs control.

Enhanced shoot Ni accumulation was accompanied by treatment-specific changes in pigment composition, histidine concentrations, and redox markers (H₂O₂ and GSH), indicating that improved Ni yield was associated with modulation of photosynthetic performance, metal complexation, and oxidative stress responses rather than simple biomass effects. While more detailed statistical analyses are pending, these results demonstrate that targeted chemical and biological amendments can markedly enhance harvestable Ni yield in A. argenteum, providing a strong experimental basis for optimising nickel agromining systems