Factors affecting the phytoaccumulation of weathered, soil-borne organic contaminants: analyses at the ex Planta and in Planta sides of the plant root

Abstract

Persistent Organic Pollutants (POPs) in the soil–plant system were tracked from their origin in the bulk soil, into the rhizosphere soil pore water, to the xylem sap, and up to the aerial plant tissue. Specifically, the profiles of both chiral and achiral components of technical chlordane along this continuum were examined in detail for members of the Cucurbitaceae family: Cucurbita pepo L. subsp. pepo (“Black Beauty” true zucchini), Cucurbita pepo L. intersubspecific cross (“Zephyr” summer squash), and Cucumis sativus (“Marketmore” cucumber). The experiments were based on the use of mini-rhizotrons for collection and analysis of rhizosphere soil pore water for organic pollutants, as well as for low molecular weight organic acids (LMWOAs). In addition, the xylem sap and aerial plant tissue for intact, homografted, and heterografted C. pepo “Black Beauty” and C. sativus “Marketmore” plants were compared. The data indicate that profiles of the chlordane components in the pore water show no alteration in chiral patterns from those in the bulk soil and may be interpreted by physicochemical partitioning coefficients. Low molecular weight organic acids (LMWOAs) in the rhizosphere were observed to have a minor impact on bioavailability of the pollutants. However, once the pollutants cross the root membrane, major distinctive uptake and enantioselective patterns are apparent in the xylem sap, which are maintained in the aerial tissue. These in planta patterns are based on plant genotype. Specifically, grafting experiments with compatible heterografts of C. pepo and C. sativus establish that the chiral patterns are fully dependent on the plant root. The genotypic dependence of the data suggests possible mechanisms for phytoaccumulation.