Cadmium toxicity and tolerance in vascular plants

Man and the biosphere are currently under an increasing threat of heavy metal pollution. Cadmium, in particular, is extremely toxic to humans 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 continue unless non-hazardous substitutes are found. Availability of Cd to plants is regulated by pH, Eh (redox potential) and other physico-chemical parameters. Bioconcentrations of Cd by various categories of vascular plants, i.e. agricultural, horticultural and silvicultural plants, weeds and aquatic macrophytes are of special concern in the interest 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. growth retardation, inhibition of photosynthesis, induction and inhibition of enzymes, altered stomatal action, water relations, efflux of cations and generation of free radicals. Plants must therefore attempt to adapt themselves to environments contaminated with excess Cd. Some of the prevalent mechanisms of Cd-tolerance are: accumulation, sequestration, synthesis of Cd-binding complexes (phytochelatins) and their stabilization by sulphide ions, damage rescue by heatshock proteins and phytochelatin constituting organics. Identification of Cd ligands present in cytosol and vascular tissue emerges as one of the priority areas of investigation. The occurrence of metallothionein genes, genetic engineering and development of transgenic plants for acclimation to Cd-stressed environments are also considered.