Toxicities and Tolerance of Mineral Elements Boron, Cobalt, Molybdenum and Nickel in Crop Plants

Abstract

The minerals boron (B), cobalt (Co), molybdenum (Mo) and Nickel (Ni) are beneficial to plant in trace amounts, but excess levels of these cause toxicity limiting crop production. An attempt was made to review the phytotoxicity symptoms, effects on growth and physiology and tolerance and amelioration of these toxicities in crop plants. Though, chlorosis and necrosis of leaves are the common expression of toxicities of these minerals and except B the critical toxic concentration of Co, Mo and Ni in soil has been worked out only for a few crops, the toxicity responses of these minerals in soil and plant tissues vary considerably across the soils and crop genotypes. These toxicities reduce chlorophyll, affect cell metabolites and enzymes specially antioxidant and lipid peroxidation, alter nutrient transport and have negative effects on cellular functioning, these all result in reduced growth and yield. Existence of genetic variation among the crop genotypes highlight the differences in tolerance and scoring for toxicity symptoms and biomass at early growth stages can be considered as reliable criteria for screening for tolerance to toxicity. The Bo1 gene provides a major source of B toxicity tolerance. The restriction of uptake and transport and internal tolerance mechanisms are the two important criteria which plants employ to combat high external concentrations and hence tolerance could be attributed to the lower B, Co, Mo and Ni content of seed and lower uptake or accumulation of these in the root and shoot and high yield in toxic soils. Ameliorating high-mineral soils using soil amendments is expensive and extremely difficult. Use of tolerant crop genotypes, phytoremediation by tolerant crops, and inoculations of beneficial microorganisms are the solutions.