(Riordan and Vallee 1976, Chesters 1978). The toxic effects of 

 excessive zinc include disrupting bone mineralization (by depress- 

 ing calcium and phosphorus levels and by decreasing the cal- 

 cium: phosphorus ratio), interference with copper metabolism 



(lessened activity of cytochrome oxidase and catalase), and 

 reduced iron concentrations in some tissues (iron deficiency 

 anemia and reduced hepatic iron stores) (NRC 1979) . 



Zinc chloride has been shown to induce testicular tumors when 

 injected into the active gonads of some fowl, but there is no 

 evidence that zinc is carcinogenic when ingested. Some studies 

 suggest zinc supplements may inhibit tumor growth. 



Zinc is antagonistic to cadmium and can reduce many of the 

 adverse effects produced by cadmium when the diet is supplemented 

 with zinc. Animals receiving both zinc and lead exhibit lower 

 lead in bones but higher levels of lead in kidneys and liver. 

 The neurologic dysfunction associated with high lead intake has 

 been absent in the presence of supplemented zinc in the diet. 

 Zinc is antagonistic to copper and may produce copper deficiencies 

 at elevated levels (Eamens et al . 1984). Zinc also disrupts 

 levels of calcium, phosphorus and iron, as indicated above. 



6.2 Toxicology Mechanisms of Metals for Plants 



The toxicology of metals in plants may involve different 

 biochemical mechanisms in different species and varieties (Foy et 

 al. 1978). Numerous other factors also influence the toxicity of 

 heavy metals. These factors and plant toxicology mechanisms are 

 presented in the following sections. 



6.2.1 Arsenic toxicology 



Wh-ile elemental arsenic is not toxic, many of its compounds 

 are toxic. Chief among these are arsenate (AsC>4~ 3 ) and arsenite 

 (As02"2). Other common forms are methanearsenate and dimethyl- 

 arsenate, which are commercially prepared as post-emergence 

 herbicides, but may also be synthesized in trace amounts in the 

 soil by microorganisms. Plants take up relatively small amounts 



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