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Nickel (Ni) is an essential, naturally occurring micronutrient required for various metabolic processes in plants, such as ureolysis, hydrogen metabolism, methane biogenesis and acetogenesis, since it is an important component of the enzymes glyoxalases, ureases, superoxide dismutases, hydrogenases, etc. Ni constitutes the active site of two metalloenzymes, i.e., urease and hydrogenase, in plants, which are involved in nitrogen (N) metabolism. Urease is responsible for the breakdown of urea into ammonia and carbon dioxide, while hydrogenase catalyses the oxidation of molecular hydrogen and recovers the energy for the reduction of enzymes. Elevated levels of Ni (≥20 mg kg⁻¹) in the soil pose a significant threat to the agricultural productivity of crop plants world-wide owing to their extreme toxicity. Consequently, Ni has now been ranked 57th in priority hazardous substances by the American Agency for Toxic Substances and Disease Registry (ATSDR, 2022). The common symptoms of Ni toxicity in plants include reduced germination, declined shoot and root growth, membrane damage, reduced nutrient absorption and abnormal flower shape. All these toxic effects result in reduced photosynthesis and respiration rates, loss of key osmolytes, etc., which cause oxidative stress and ultimately reduce the yield of crops. In addition, excess Ni has a negative impact on legume–rhizobia symbiosis by affecting nodulation potential and N₂-fixation processes, as well as decreasing ureides biosynthesis. However, significant variations in terms of Ni metal-induced sensitivities among various crop plants have been observed at various levels. Therefore, this chapter reviews the updated information regarding Ni stress-induced physiological, biochemical and molecular responses in crop plants, especially legumes.

Symptoms of plant nickel (Ni) disorders occurring on ultramafic soils resemble those of iron (Fe) deficiency, such as chlorosis. Since plants absorb Ni via root iron transporters, it is expected that Ni would compete with Fe acquisition on the root surface in soils with high nickel concentrations, resulting in Fe deficiency in plants. However, experiments using Arabidopsis thaliana confirmed that the addition of Ni to a hydroponic solution induced an Fe starvation response instead. It has also been observed that Fe content in plant roots increases with Ni content, and in fact, plants from ultramafic soils tend to have higher Fe contents than those from other soil types. This phenomenon, in which Fe deficiency occurs even though Fe is acquired sufficiently, can be attributed to the disruption of the Fe transport system in plants. Nicotianamine (NA) is a biological compound that chelates Fe and is responsible for its transport in plants; however, NA has a higher affinity for Ni than Fe. Because of that, it is predicted that NA, which should be used for Fe transport, is instead bound by Ni, resulting in the inhibition of chlorophyll synthesis in plants under high Ni conditions. Experimentally, plants that overexpress NA genes are more tolerant to high levels of heavy metals, especially Ni.

Soil Ni strongly inhibits plant growth during or immediately after germination. This may be because Ni inhibits the supply of Fe, which is essential for chlorophyll synthesis, and thus prevents the initiation of photosynthesis. An adequate supply of NA may be an essential condition for plants to adapt to ultramafic soils.

A detailed knowledge and recent advancements on the sources, speciation, toxicity and chemistry of nickel and its different compounds are discussed in this chapter. Nickel is an essential micronutrient for plant growth, and it is also a component of the enzyme urease, which plays a role in nitrogen metabolism in higher plants. Nickel and nickel compounds are also important for several biological processes in animals and soil/water microbes, and they have many industrial and commercial uses. Nickel is a known heavy transition metal and is found at very low levels in the environment. The vast industrial use of nickel leads to widespread environmental pollution. Higher levels of nickel can affect the photosynthetic function and transpiration of higher plants; it reduces seed germination, root and shoot growth, biomass accumulation and final production. Moreover, nickel toxicity leads to chlorosis and necrosis and may cause oxidative damage in plants. In addition, nickel toxicity degrades soil fertility, which may reduce crop production in the near future. The limited knowledge regarding the mechanisms of nickel tolerance in plants further highlights this fact. Furthermore, it causes many chronic diseases in humans, including allergy, cardiovascular and kidney diseases, lung fibrosis and lung and nasal cancers. The molecular mechanisms of nickel-induced toxicity, which cause the above diseases in humans, are still unknown. Mitochondrial dysfunctions and oxidative stress are mainly considered to play a crucial role in inducing toxicity from this metal. Therefore, we should pay attention in future research to find approachable and prominent ways of minimising the entry of nickel into our environment.