to replace one another chemically, but not functionally. Such 

 substitution of arsenate for phosphate may cause decoupling of 

 oxidative phosphorylation in mitochondria and inhibit leaf uptake 

 of chemicals. Further, as arsenate is translocated throughout the 

 plant it may interfere with cell organelles such as chloroplasts 

 in which phosphorus plays an important role (NRC 1977). Porter 

 and Sheridan (1981) noted reduction in the nitrogen fixing 

 activity at low levels (1 mg/L of added arsenic) and inhibition 

 of photosynthesis and respiration at very high levels (100 mg/L). 



6.2.2 Cadmium toxicology 



Cadmium is an element serving no apparent essential biologi- 

 cal function, yet it is often readily taken up, translocated and 

 accumulated by plants. It is found in very low concentrations in 

 natural soils and generally only reaches phytotoxic levels due to 

 anthropogenic activities. Plant uptake occurs both through roots 

 and leaves. Uptake of soil-cadmium is influenced by several 

 factors including pH, CEC, plant species and varieties and age 

 (Jastrow and Koeppe 1980, Boggess et al. 1978). Recently, added 

 chloride was shown to increase the level of soluble soil-cadmium 

 (Bingham et al. 1984). A study of cadmium uptake and transloca- 

 tion from solution has shown most of the cadmium to be retained 

 in plant roots (Jarvis et al. 1976). Symptoms of cadmium toxicity 

 include stunting and chlorosis. While much is known about the 

 tox icological effects of cadmium, little has been discovered con- 

 cerning the biochemical basis for plant toxicity. 



Cadmium is chemically allied with zinc and often substitutes 

 for zinc in plant metabolic activities; this substitution may.be a 

 reason for its phytotoxici ty . Vallee and Ulmer (1972) proposed 

 that cadmium toxicity is in part due to the replacement of zinc by 

 cadmium at certain enzyme sites. Root et al. (1975) stated that 

 excess cadmium may cause chlorosis in corn leaves due to decreased 

 zinc uptake and subsequent changes in the Fe:Zn ratios. Cadmium 

 interference with zinc uptake and translocation in beans was 

 documented by Hawf and Schmid (1967). In contrast, added cadmium 

 levels significantly increased the zinc concentration of tomato 



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