Cadmium phytotoxicity

Cadmium transport within plant and cadmium phytotoxicity... 

Czuba, M., and D. P. Ormrod. Effects of cadmium concentration on ozone-induced phytotoxicity in cress. Plant Physiol. Ann. Suppl. 1974 31. (abstract)... 

John, M. K., Van Laerhover, J., and Chauh, H. H. (1972). Factors affecting plant uptake and phytotoxicity of cadmium added to soils. Environ. Sci. Technol. 6, 1005—1009. 

A procedure Ls developed to determine the upper critical level of the concentration of heavy metals in plants and solids. Experimentally it has been sliown that the soil type affects the upper critical (phytotoxic) level of zinc (Zn) and cadmium (Cd) for maize and spinach. 

Cadmium is accumulated by all plants. The extent of Cd accumulation, however, varies markedly with species and variety. Soil pH is the most important factor controlling Cd uptake by plants, with lower pH favoring its uptake. Tobacco plants have been shown to absorb high levels of Cd from the soil (Bache 1985). Phytotoxicity of Cd is manifested by stunting, chlorosis,... 

Fertile soils supply plants with all of the trace elements essential for growth, believed at the present time to be Fe, Mn, Zn, B, Cu, Mo, and CL These seven elements are called the micronutrients, a term that indicates the small quantities needed by plants but not necessarily the concentrations found in soils. Deficiencies can occur in soils either because they contain extremely low concentrations of these elements or because the elements are present in very unavailable (insoluble) forms. Conversely, many trace elements, including ail of the micronutrients, can reach concentrations in soils that are toxic to plants and microoiganisms. Some of the most toxic are mercury (Hg), lead (Pb), cadmium (Cd), copper (Cu), nickel (Ni), and cobalt (Co). The first three are particularly toxic to higher animals. The last three are more toxic to plants than animals and are termed phytotoxic. From the standpoint of potential hazard to human health, an extended Ust of priority metals has been established. This list presently consists of ... 

Some authors explain the phytotoxicity of heavy metals at their ability to induce cells reactive oxygen species, leading to oxidative stress [21], From this viewpoint, due to the ability of a plant organism under the influence of the metal to keep activity of antioxidant enzymes (catalase, peroxidase), the resistant plants are at a certain level [18], We have shown earlier that white mustard plants, the seeds of which were pne-treated by the bacteria when exposed to high concentrations of cadmium, had higher growth and activity of catalase and peroxidase at the same time there was a simultaneous decrease in the intensity of lipid peroxidation [22], We assume that the anti-stress effect of bacilli in the rye plants when exposed to cadmium ions is also related to the preservation of the activity of antioxidant enzymes. 

Clearly, effects which have to be studied are the extent to which the enhanced levels in the contaminated soils now characteristic of urban areas, lead to increased trace-element levels in plants grown for human or animal consumption, the extent to which increased trace-element uptake can lead to phytotoxic effects and the possibility of toxic effects on livestock and human beings. The possible consequences of the permanent presence of enhanced levels of elements such as cadmium and mercury in food chains are so far-reaching and unpredictable that there is everything to be said for preventing the development of such a situation. 

Normal levels of cadmium in plants grown in uncontaminated soil are probably less than 1 ppm in dry matter, but John [309] has quoted a level of 668 ppm cadmium in the dry matter of lettuce leaves from plants grown in cadmium-contaminated soil. This level is certainly exceptionally high, but it is clear that in cadmium, we have an element which is both highly toxic and readily taken up by plants to produce quite high concentrations in the plant without the appearance of phytotoxic symptoms, so that an apparently normal crop may be unsafe for human or animal consumption. It is evident from the results of these experiments that even a small addition of cadmium to uncontaminated soil can produce striking increases in the cadmium contents of a number of widely different kinds of crops. Cadmium is therefore an element which can readily enter mammalian food chains at a dangerous level. 

The results in Table 28 indicate that for lettuces, at any rate, the level at which phytotoxic effects begin to appear is very much lower than this, at around 20 ppm acetic acid>extractable cadmium. We can therefore conclude that levels greater than 20 ppm are generally undesirable for crop production in that there is some possibility of phytotoxicity. The precise biochemical mechanism within the plant associated with cadmium toxicity still appears to be the subject of controversy, but phytotoxicity has been ascribed to competition of cadmium with zinc for active sites without functional substitution [312]. 

Unfortunately, quite high levels of cadmium (up to 60 ppm dry matter) may be present in lettuce leaves before either visual phytotoxic symptoms or any significant reduction in yield are encountered, so that an apparently normal crop may be unsafe for human or animal consumption. 

STEWART I. 1963. Chelation in the absorption and translocation of mineral elements. Annual Review of Plant Physiology, 14, 295-310. STRICKLAND R.C., CHANEY W.R. and LAMOREAUX R.J. 1979. Organic matter influences phytotoxicity of cadmium to soybeans. Plant and Soil, 5, 393-402. 

Abstract Cadmium is an important poiiutant in the environment, toxic to most organisms and a potential threat to human heaith Crops and other plants take up Cd from the soil or water and may enrich it in their roots and shoots. In this review, we suimnarize natural and anthropogenic reasons for the occurrence of Cd toxicity, and evaluate the observed phytotoxic effects of plants growing in Cd-supplemented sou or nutrient solution. Cd-induced effects include oxidative stress, genotoxicity, inhibition of the photosynthetic apparatus, and inhibition of root metabolism. We explain proposed and possible interactimis between these modes of toxicity. WhUe discussing recent and older studies, we further emphasize the environmental relevance of the experiments and the physiological response of the plant. 

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