CADMIUM TOXICITY AND TOLERANCE IN VASCULAR PLANTS

Man and the biosphere are currently under an increasing threat of heav y metal pollution. Cadmium, in particular, is extremely toxic to human s as well as plants. Cd flow to humans is more through cereals, fruits , vegetables and other edible plant parts than through meat (muscles). A trend analysis shows that the production and usage of Cd would cont inue unless non-hazardous substitutes are found. Availability of Cd to plants is regulated by pH, Eh (redox potential) and other physico-che mical parameters. Bioconcentrations of Cd by various categories of vas cular plants, i.e. agricultural, horticultural and silvicultural plant s, weeds and aquatic macrophytes are of special concern in the interes t of human welfare. This review considers not only the toxic symptoms of Cd in plants but also the tolerance manifestations being evolved by plants. Excess Cd causes a number of toxic symptoms in plants, e.g. g rowth retardation, inhibition of photosynthesis, induction and inhibit ion of enzymes, altered stomatal action, water relations, efflux of ca tions and generation of free radicals. Plants must therefore attempt t o adapt themselves to environments contaminated with excess Cd. Some o f the prevalent mechanisms of Cd-tolerance are: accumulation, sequestr ation, synthesis of Cd-binding complexes (phytochelatins) and their st abilization by sulphide ions, damage rescue by heatshock proteins and phytochelatin constituting organics. Identification of Cd ligands pres ent in cytosol and vascular tissue emerges as one of the priority area s of investigation. The occurrence of metallothionein genes, genetic e ngineering and development of transgenic plants for acclimation to Cd- stressed environments are also considered.