ABSTRACT
Plants, as other organisms require certain metals and metalloids at physiological concentrations for the proper functioning of cellular functions. Some metals and metalloids which belong to the category of heavy metals are not required in general by the plants. Some heavy metals are used by plants in small quantities for their physiological function but most of these are harmful and beyond certain limits become toxic to plants and show morpho-anatomical, physio-biochemical symptoms on various processes like seed germination, root-shoot growth, grain yield, induction of reactive oxygen and nitrogen species, etc. To counter the adverse effects induced by heavy metals, plants have evolved diverse mechanisms to minimize the damage caused, such as the exclusion, compartmentalization, uptake reduction, inhibition of long-distance transportation and induction of antioxidant system, detoxification mechanism (storage into the vacuole, chelation, trafficking), heat shock proteins, hormones, and cell signaling molecules. Sugar is one of the key biochemical products of a photosynthetic process that aids in counteracting mechanisms. Soil contaminated with heavy metals is uptaken by plants and affects biochemical processes such as respiration, sugar metabolism, uptake of essential metal ions, and replication. The heavy metals also affect symbiotic N fixation and N metabolism by affecting the bacterial consortium. 126Some heavy metals adversely affect enzyme activities related to protein metabolism, synthesis, modifications, and protease activity, enzymes involved in N assimilation like NR, NiR, GS, and GOGAT. Therefore, it is amply clear that heavy metals have a wide spectrum impact on plant growth and physiology that requires correction to maintain proper functioning of plant and ecosystem health to achieve sustainable growth. This chapter is an attempt to overview the impact of heavy metals on the various physiological processes of plants.
