Heavy metal toxicity to food legumes: effects, antioxidative defense and tolerance mechanisms

Heavy metal stress has emerged as one of the most detrimental for the major food crops due to persistent soil pollution. These metals are known to replace essential metals in pigments or enzymes disrupting their function and have thus proved to be toxic. Their toxicity results in chlorosis, weak plant growth, yield depression accompanied by reduced nutrient uptake, disorders in plant metabolism and, in leguminous plants, a reduced ability to fix molecular nitrogen. Most of the leguminous crops are affected by metal stress present in the soil mainly due to contaminated agrochemicals and sewage sludge. They are known to cause deleterious effect on the cell division of the plants and cause many chromosomal abnormalities which depend upon the concentration and the intensity of the exposure of the same. Uptake and accumulation of metals at higher concentrations cause ultra-structural and anatomical changes in the plant cells. Various plant physiological activities like seed germination, nutrition distribution, enzymatic activity, nitrogen fixation, photosynthesis and pollen function are adversely affected. Uptake into the seeds adversely affects the nutritional quality of seeds. However, the plants possess various defense and tolerance mechanisms to cope up with stresses. In this review, we describe various effects, defense and tolerance mechanisms due to heavy metals, especially for legumes.