Role of Nickel in Plants Growth and Regulation: Essential in Trace and Toxic in Excess

Nickel (Ni) is essential heavy metal (HMs), it is required for plant growth and metabolism in trace amount, but it becomes toxic threat for plants in higher concentration. Ni is indeed a transition element that mainly exists in +2 oxidation state (Ni²⁺) and occurs in various salt forms such as NiSO₄, NiCl₂, and Ni(II)-EDTA, which exhibiting different levels of toxicity. Nickel contamination in the environment originates from both natural and anthropogenic sources; however, industrial activities such as mining, smelting, and alloy production are the primary contributors. These processes have led to widespread nickel pollution, posing a significant global environmental threat due to the extensive use of nickel in industrial and consumer products. Ni is a part of metabolically important enzyme urease, but in excess leads to its toxicity. Excess Ni shows some visible symptoms like chlorosis, wilting and bleaching (after long exposure of excess Ni) and it checked plant growth, decreased photosynthesis, and altered enzyme activity, initially due to its competition with other elements (such as Fe) for uptake. In some research it is found that Ni initially accumulates in plant roots, also found in low concentration in plant stems and leaves. Understanding the impact of Ni on plant physiological and biochemical processes—especially its role in nutrient imbalance and oxidative stress—is crucial for developing strategies to mitigate its detrimental effects on plants, the environment, and agricultural productivity.