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of arsenic from soils and the arsenic levels in natural soils are 

 rarely high enough to cause phy totox ici ty . Aerial deposition of 

 arsenic from smelters, or long-term application of arsenical 

 pesticides may elevate soil values to phytotoxic levels. Plant 

 toxicity to arsenic occurs when: 1) abnormally high arsenic 

 levels are produced in soil, either deliberately or accidentally 

 by man's activities; 2) a change in soil chemistry increases 

 arsenic availability; and 3) plant foliage is sprayed with 

 arsenical compounds (Wauchope 1983). Symptoms of arsenic toxicity 

 include wilting of new-cycle leaves, followed by retardation of 

 root and top growth (Liebig 1966) . 



Arsenite is 4 to 100 times more toxic and its compounds are 

 more available to plants than arsenate (Wauchope 1983). However, 

 in most cases arsenite is rapidly oxidized to arsenate in the 

 soil. Arsenic phytotoxicity is a four-stage process: 1) absorp- 

 tion onto plant surfaces; 2) movement to the plant interior; 3) 

 translocation to the site of action; and 4) a biochemical reaction 

 that is toxic (Wauchope 1983). Both arsenate and arsenite are 

 rapidly and intensely adsorbed to plant roots, resulting in very 

 high concentrations in the root vicinity (Machlis 1974). Because 

 of its extremely high toxicity to cell membranes, very limited 

 translocation of arsenite occurs once the chemical has penetrated 

 the cuticle and entered the apoplast phase of the plant system. 

 Membrane degradation is the result of arsenite oxidation by 

 sulfhydryl groups, causing cessation of root functions and foliar 

 necrosis upon contact (Speer 1973). Internal injury of this type 

 is manifested as wilting due to loss of turgor. 



Arsenate is less toxic and therefore is more readily trans- 

 located. If sub-lethal concentrations are present in the soil, 

 substantial accumulation may occur in foliage (Liebig 1966). 

 Translocation occurs both intra- and extracellularly , including 

 xylem and phloem transport. Arsenate does not react with sulfhy- 

 dryl groups, nor does it degrade cell membranes like arsenite. 

 Its main toxic effects are apparently due to its disturbance of 

 phosphorus metabolism in plants. Studies have shown that the 

 chemistry of arsenate and phosphate is very similar and they tend 



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