Chapter 4: Physiological and Biochemical Responses of Crop Plants to Nickel Toxicity

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.