0141767 



lead absorption (Zimdahl and Arvik, 1973). Translocation of lead 

 from the root system to other parts of the plant is poor, with 

 roots generally accumulating the highest lead concentration. The 

 translocation is predominantly apoplastic in nature (Holl and 

 Hampp 1975). Indirect evidence suggests transport is via sieve 

 tubes which are part of the phloem (food) transport system in 

 plants. Some lead may be precipitated in root dictyosomes, 

 possibly due to phosphatase enzymes (Haque and Subramanian 1982) . 

 The dictyosome vesicles contain cell wall precursors and as the 

 dictyosomes move to the cell walls and fuse to it, the lead may be 

 bound at that site. Translocation of lead is apparently enhanced 

 when the soil solution is deficient in other nutrients. Many 

 researchers have found increased lead levels in all plant tissues 

 growing in a nutrient solution containing lead. The fruiting and 

 flowering parts of plants have been found to accumulate the least 

 amount of lead (NRC 1972). 



The toxicosis of lead in plants is expressed by reduced 

 growth and vital processes such as photosynthesis, mitosis and 

 water absorption. Lead accumulates in tissues with high mitotic 

 activity and appears to be bound to polyuronic acids of the cell 

 walls (Holl and Hampp, 1975). High concentrations of lead are 

 found in organelles such as mitochondria, chloroplasts and also in 

 nuclei. The lead is apparently bound to certain phosphate groups 

 in cells . 



Roots that are in contact with lead degenerate because of a 

 decrease in cell division in root meristems. The photosynthetic 

 process is hindered by diminished CO2 fixation by chloroplasts and 

 by the disturbance that lead causes in the transport of electron 

 between the site of primary electron donor and water oxidation 

 (Holl and Hampp 1975). The activity of many enzymes is inhibited 

 due to blocking by lead of sulfhydryl groups in proteins due to 

 changes in the phosphate levels of living cells. 



6.2.4 Zinc toxicology 



Zinc is an essential element in plant metabolism. Zinc 

 deficiency in crops is the most common micronutr ient deficiency in 



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