Role of the extractable lipids and polymeric lipids in sorption of organic contaminants onto plant cuticles.

The distinct role of extractable and polymeric lipids in plant cuticle, precursors of SOM, has received scarce attention to elucidate plant uptake and soil affinity with organic contaminants. Sorption of naphthalene and 1-naphthol to fruit cuticular fractions isolated from two species were investigated. The polarity index, physical conformation, and glass transition temperature (Tg) of these cuticular fractions were characterized by elemental analysis, Fourier transform infrared spectroscopy, and differential scanning calorimetry, respectively. Cutin, a polymeric lipid, is the major sorption medium of the cuticle due to its large mass fraction and liquid-like nature (Tg approximately -30 degrees C). Sorption of cutin is suppressed by the extractable lipids (wax, Tg approximately 44 degrees C) acting as an antiplasticizer (enhance cutin's Tg) over nonpolar contributor. Whereas polysaccharide, as a plasticizer (lower Tg value) and polar contributor, regulates affinity of polymeric lipids (cutin and cutan). The contribution of cutin to sorption by bulk cuticle overshadows the role of waxes, and the sorption capability (K(oc)) of cutin overwhelms the octanol-water partition coefficient (K(ow)). Therefore, uptake of organic contaminants by these plants would be seriously under-predicted by their extractable lipid content and compound's K(ow) values. Along with the observed linear relationships of K(oc) with cutin content in these cuticular fractions, we suggest for the first time that the depolymerizable lipid fraction (cutin) is required to accurately predict plant accumulation of organic contaminants.