Bound and unextractable pesticidal plant residues: chemical characterization and consumer exposure

Abstract

Plants are well known to incorporate pesticides into bound and unextractable residues that resist solubilization in common laboratory solvents and are therefore not accessible to standard residue analysis. A characterization of such residues has been proposed for incorporation rates above trigger values of 0.05 mg kg(-1) parent pesticide equivalents, or percentage values of 10% (United States Environmental Protection Agency, 1995) or 25% (Commission of the European Communities, 1997) of the total radioactive residue. These trigger values are often exceeded. The present review describes the current status of the chemical characterization and animal bioavailability of bound and unextractable residues that may be xenobiotic in nature or result from natural recycling of simple degradation products. The latter case represents a mechanism of detoxification. Bound residues have been shown to be covalent or non-covalent in nature. With regard to the plant matrix molecules involved, incorporation into proteins, lignins, pectins, hemicelluloses and cutins has been demonstrated, and four covalent linkage types are known. Animal feeding experiments have revealed cases of low as well as high bioavailability. Many of the studies are limited by experimental uncertainties and by results only being reported as relative percentage values rather than absolute exposure. A preliminary value of absolute exposure from bound and unextractable residues is derived here for the first time from eight case studies. The mean exposure (ca 1.5 mg kg(-1) pesticidal equivalents) exceeds some of the existing maximum residue levels (MRLs) of residual free pesticides that are typically in the range of 0.05-1 mg kg(-1). A mathematical framework for the correction of current maximum residue levels is presented for cases of highly bioavailable bound residues. As bound pesticidal residues in food plants could represent a source of significant consumer exposure, an experimental test scheme is proposed here. It consists of basic chemical characterization, model digestibility tests and, in exceptional cases, animal bioavailability and additional toxicological studies.

The fate of the herbicide propanil in plants of the littoral zone of the Three Gorges Reservoir (TGR), China

“…Although lower amounts of applied radioactivity were taken up by C. dactylon, absorbed propanil equivalents were similar as in the other two plants due to the higher concentration and the smaller biomass of the plant. All propanil equivalents in the experiments exceeded previous proposed trigger values (0.1-0.5 mg/kg, dry commodities) of the herbicides, similar to the findings summarized by Sandermann (2004).…”

Application of plant tissue cultures in phytoremediation research: Incentives and limitations

“…In other work, an altered distribution of bound metabolites between pectins and hemicelluloses in suspended cells compared with plant stems was explained by the lack of secondary wall development in cultured plant cells (Laurent and Scalla, 1999). There is evidence that bound residues accumulate with time in plants as the amounts of free and soluble metabolites decrease (Sandermann, 2004); this suggests that measurements of bound components in plant tissue cultures may underestimate the levels characteristic of whole plants. Yet, in other studies, cultured plant cells have been found to generate levels of bound residues similar to those found in whole plants Schmidt et al, 1993).…”

“…The nonextractable or bound residues of plant cells have attracted considerable interest in recent years because of concerns that the persistence of chemical residues in edible plants may allow toxic components to enter the food chain (Sandermann, 2004). Nonextractable residues are associated primarily with cell walls: incorporation of metabolites in covalent and noncovalent linkage with proteins, lignin, pectin, polysaccharides, cellulose, hemicellulose, starch, and cutin has been reported (Sapp et al, 2003).…”

“…Nonextractable residues are associated primarily with cell walls: incorporation of metabolites in covalent and noncovalent linkage with proteins, lignin, pectin, polysaccharides, cellulose, hemicellulose, starch, and cutin has been reported (Sapp et al, 2003). Plant cell suspensions have been used to investigate the properties of bound residues (Sandermann et al, 1983;Sapp et al, 2003); the advantages of in vitro cultures for these studies include the elimination of microbial effects and measurement artefacts due to photosynthetic refixation of 14 C into natural nonextractable cell components (Sandermann, 2004). However, there is the question of whether plant cells in culture incorporate metabolites into bound residues in the same way or to the same extent as in plants.…”

“…In particular, wheat plants and plant cell cultures both tend to accumulate high levels of bound residues accounting for up to 72% of the 14 C applied in xenobiotic compounds Scheel et al, 1984): this has been attributed to the high lignin content of wheat (Schmidt et al, 1994). In any case, plant tissue cultures have been recommended for initial experiments on bound residues to minimize the expense of greenhouse or field trials (Sandermann, 2004).…”