Speciation of Trace Inorganic Contaminants in Plants and Bioavailability to Animals: An Overview

The movement of both essential and pollutant trace elements through agricultural food chains is a complex problem, influenced in part by plant‐mediated changes in trace element speciation that may alter the rate and extent of gastrointestinal absorption after ingestion of plant tissues by animals. An overview is provided of a series of investigations undertaken to describe the processes governing the chemical behavior of several cations (Pu, Ni, Cd, and Fe) and anions (Tc, Se, and S) during transport in the plant xylem and after deposition in plant tissues. Cations were generally present in xylem exudale as organic acid, amino acid, and peptide complexes specific to the element, whereas anions tended to remain in the inorganic forms initially absorbed by the root. Once deposited in tissues, both cations and anions were associated with more complex, but soluble, plant metabolites. The cations (Pu, Ni, Cd) were directly associated with >10 000 molecular weight proteins, whereas the anions (Se and Tc) appeared to undergo chemical reduction and incorporation into proteins using enzymatic processes functioning for S metabolism. The importance of these plant processes on gastrointestinal absorption was demonstrated for Pu and Tc, which differed markedly in their behavior in plants and animals.