Nickel toxicity in plants: reasons, toxic effects, tolerance mechanisms, and remediation possibilities-a review

Calcium signaling confers nickel tolerance in Cucurbita pepo L

Excessive nickel (Ni) accumulation in edible parts of the plants has become a serious challenge for food security over the past few decades. Therefore, in this study, the role of calcium (Ca²⁺) signaling in imparting Ni tolerance was investigated in zucchini (Cucurbita pepo L. cv Courgette d'Italie). Exposure of zucchini seedlings to Hogland solution containing 0.28 mmol L^-1 Ni(NO₃)₂ reduced plant growth, the content of chlorophyll and carotenoids and the relative water content (RWC) in leaves, increased Ni accumulation that was accompanied to depletion of the essential bivalent cations and induced oxidative stress and proline accumulation in both shoots and roots. Pretreatment with the nutrient solution containing 15 mmol L^-1 calcium chloride (CaCl₂), significantly improved zucchini growth and photosynthetic pigment contents and maintained RWC in leaves under both control and Ni stress conditions. Pretreatment with CaCl₂ reduced Ni accumulation, modified cation homeostasis, increased the activities of peroxidase and catalase enzymes and lowered Ni-induced accumulation of hydrogen peroxide, malondialdehyde and proline in leaves and roots. Pre-exposure of root with Ca²⁺ chelator (ethylene glycol tetraacetic acid) and plasma membrane Ca²⁺ channel blocker (lanthanum chloride) impaired impact of Ca²⁺ on the aforementioned attributes. Outcomes of this study not only highlight the signaling role of Ca²⁺ in regulating defensive responses but also suggest an eco- friendly approach for reducing the Ni contamination in plants that ensure food safety.

Rice straw, biochar and calcite incorporation enhance nickel (Ni) immobilization in contaminated soil and Ni removal capacity

Although rice straw (RS), biochar (BI) and calcite (CC) have proved to be effective immobilizing agents in acidic contaminated soil, we lack up-to-date scientific data regarding nickel (Ni) fractionation in soil and removal capacity in water. Therefore, an incubation study was undertaken to investigate the efficacy of RS, BI and CC with three application rates (0, 1 and 2%) of RS, BI and CC on the immobilization of Ni in polluted soil. Various extraction techniques were carried out: sequential extraction procedure, the European Community Bureau of Reference (BCR), extraction with CaCl_2, and the toxicity characteristics leaching procedure (TCLP) techniques. Additionally, Ni sorption behavior was determined using the Langmuir and Freundlich isotherms. Results showed that adding all amendments into Ni contaminated acidic soil, enhanced soil pH, reduced the exchangeable fraction of Ni by 48%-55%, 59%-71% and 58%-66.3%, when RS, BI and CC were applied at 1% and 2% rates, respectively. According to the Langmuir adsorption isotherm results, the maximum sorption capacity was recorded using 2747 mg kg^-1 in 2% CC amended soil. However, biochar exhibited the maximum Ni sorption capacity (13348 mg kg^-1), due to its porous structure, larger surface area, and having more functional groups. Furthermore, the results of FTIR, SEM and zeta potential techniques confirmed that the immobilization and biochar's capacity to remove Ni were more effective when compared to other immobilizing agents.