0141786 



sulfhydryl groups. Reduced carbon dioxide fixation may result 

 from cadmium substitution for zinc in zinc metal loenzymes and sub- 

 stitution for manganese may cause inhibition of electron flow in 

 plant photosystems (Ernst 1980). 



Plant respiration may be enhanced or inhibited depending upon 

 species-specific carbohydrate metabolism. Cadmium has been shown 

 to cause pronounced swelling of mitochondria, with a resultant 

 decrease in respiration rate (Bittell and Miller 1974). Like 

 numerous other metals, cadmium may have a strong effect on the 

 properties of DNA. It has been demonstrated that cadmium may 

 decrease cell viability, increase single-strand breakage of DNA 

 and inhibit cell division (Mitra and Bernstein 1978) . 



6.2.3 Lead toxicology 



Lead is considered a nonessential element for plant growth. 

 Lead uptake from soils is dependent on many factors, including 

 soil pH, cation exchange capacity (CEC), organic matter, calcium 

 content, plant species and the soluble metal concentration. 

 Climatic conditions such as precipitation, temperature and the 

 length of daylight also influence lead uptake. 



Lead uptake is enhanced by low pH conditions and by soils 

 with little organic matter. Organic matter is known to have a 

 high CEC and tends to adsorb or bind most metal cations. Thus, 

 high CEC or organic matter content renders soil lead less availa- 

 ble to plants. Low pH conditions enhance the solubility of most 

 metals, including lead, making them more available for plant 

 uptake. The addition of phosphate and liming have been shown to 

 reduce lead uptake by plants by forming low solubility compounds 

 such as lead hydroxide, carbonate and phosphate (Demayo et al . 

 1982) . Plant species also differ in their lead uptake. Lead 

 tends to collect in the top layer of soil and, therefore, shallow 

 rooted plants such as annual grasses take up more lead than deep 

 rooted perennials such as alfalfa. 



Absorption of lead by plants is both by root uptake and 

 absorption through foliage of airborne lead fallout. Most of the 

 literature indicates that uptake by roots is the primary means of 



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