Uptake and translocation of p,p'-dichlorodiphenyldichloroethylene supplied in hydroponics solution to Cucurbita

Abstract

Field studies show shoots of zucchini (Cucurbita pepo L.) accumulate various hydrophobic contaminants from soil, although many other plants do not, including cucumber (Cucumis sativus L.). To investigate the mechanism for this uptake, we presented p,p'-dichlorodiphenyldichloroethylene (DDE) to these two species in hydroponics solution. A mixture of DDE bound to Tenax beads stirred with a solution of water passing through a reservoir provided a flowing solution containing DDE at approximately 2 microg/L for many weeks duration. Approximately 90% of the DDE supplied in solution was adsorbed on the roots of both cucumber and zucchini. Less than 10% of the sorbed DDE was released subsequently when clean solution flowed past these contaminated roots for 9 d. The shoots of both species accumulated DDE, but the fraction that moved from the roots to the shoot in zucchini, ranging from 6 to 27% in various trials, was 10-fold greater than that in cucumber, 0.7 to 2%. The gradient in DDE concentration in zucchini tissues was in the order root more more than stem > petiole > leaf blade, indicating the movement was through the xylem in the transpiration stream. Some DDE in leaf blades might have been absorbed from the air, because the concentration in this tissue varied less with time, position in trough, or species, than did DDE in stems and petioles. The remarkable ability of zucchini to translocate DDE could not be attributed to differences in tissue composition, growth rate, distribution of weight among plant parts, or in the leaf area and rate of transpiration of water from leaves. Some other factor enables efficient translocation of hydrophobic organic contaminants in the xylem of zucchini.