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Nickel (Ni) is the 22nd abundant element in the earth crust and holds a special place among the
heavy metals. Ni is an essential micronutrient for plant growth and it is also a component of the
enzyme urease which is required for nitrogen metabolism in higher plants. Nickel and nickel
compounds have many industrial and commercial use s, and the progress of industrialization has led
to increased emission of pollutants into the environment. Ni is absorbed and redistributed in plants
via cation and/or metal –ligand complex transport system.
Ni is strongly phytotoxic at higher concentration. In several plants Ni induces change in activity of
antioxidant enzymes like superoxide dismutase (SOD), Ascorbate peroxides (APX) and Catalase
(CAT). The most common symptoms of nickel toxicity in plants are inhibition of growth, induction of
chlorosis, necrosis and wilting. Nickel strongly influences metabolic reaction in plants and has the
ability to generate Reactive Oxygen Species (ROSs) which may cause oxidative stress.
Elevated levels of Ni can inhibit cell division at root meristem in non- tolerant plants and decreases
plant growth. Studies have shown that Ni has a negative effect on photosynthesis and respiration.
High uptake of Ni induces a decline in water content of dicot and monocot plant species. The
decrease in water can act as an indicator for Ni toxicity in plants. Ni is associated with proteins
inhibition germination and chlorophyll synthesis.
Nickel received very little attention due to its dual character and complicated electronic chemistry
which acts as barrier to reveal the toxicity mechanism in plants. The objective of this review paper is
to summarize the overview of the sources, essentiality, uptake and toxicity in plants.
Nickel pollution is a serious environmental concern which led to research on phytoremediation.
However, studies are needed to know the details at both biochemical and molecular levels to
understand the Ni tolerance of Ni hyper accumulators.
Keywords: micronutrient, phytotoxic, chlorosis, necrosis, toxicity mechanism, hyper accumulators.