Optimizing Planting Density for p,p′ -DDE Phytoextraction by Cucurbita pepo

Abstract

A field experiment was conducted to determine the optimum planting density of Cucurbita pepo subspecies pepo (zucchini) for the phytoremediation of weathered p,p′-dichlorodiphenyldichloroethylene (p,p′ -DDE). In previous greenhouse investigations, preliminary data indicated significant increases in remedial potential at higher planting densities. Three zucchini cultivars were grown in DDE-contaminated soil at cultivation densities of 1, 4, 8, or 16 plants per square meter (0.25 m depth)or approximately 270 kg of soil. At all densities, zucchini accumulated large amounts of the weathered contaminant, with root and stem bioconcentration factors, the dry weight ratio of DDE content in tissue to that in the soil, ranging from 8.5-14 and 6.8–10, respectively. The total plant biomass of the three cultivars, as well as the mass of individual tissue compartments, increased significantly as planting density rose from 1 to 4 plants/m². However, further increasing cultivation density to 8 or 16 plants did not result in additional biomass increases. The root and stem DDE BCFs increased by nearly 30% as the planting density was raised from 1 to 4 plants, but contaminant accumulation at densities higher than 4 plants were unaffected. The overall amount of DDE phytoextracted, which is the ratio of absolute contaminant amount in the plant to that in 270 kg of soil, also increased significantly with cultivation density. At a density of 1 plant, 0.20% of the DDE was removed from the soil but at 4, 8, and 16 plants, the amount phytoextracted rose to 0.85, 0.80, and 1.4%, respectively. DDE phytoextraction by zucchini is a clearly complex process; although the highest phytoremedial value is associated with the greatest planting density, the contaminant extraction process is clearly dependent on factors other than biomass alone.