Toxicity in Higher Plants

Each element can act as a poison - either when its content is too high, or if it causes a diminution of the efficacy of another. The following elements have very low toxicity thresholds As, Al, B, Cd, Co, Cr, Cu, F, 

The most important causes of toxic effects include (i) replacement of an essential element in enzymes or intermediates without fulfilling the functions of the original constituent (ii) chemical modification of essential cell components (hi) changes in membrane permeability and 

The replacement of S by Se in S-contain-ing amino acids is an example of the substitution of one essential element by another (Eustic et al. 1981, Brown and Shrift 1982). After its uptake, selenate is activated by ATP under the formation of adenosine phos-phoselenate (APSe). Following some conversions inclusive reduction, Se appears as sele-nocysteine and later in selenomethionine. 

The selenoamino acids are incorporated into proteins. These do either not or at least much less function as enzymes than the original S-containing proteins. This means that Se cannot accomplish the function of S in this case. 

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