Plant, crop

Cucurbita pepo L.

Zucchini, pumpkin

A singular case among crops: some cultivars of subspecies pepo take up persistent, highly hydrophobic organic pollutants (DDE, PCBs, dioxins) from soil and move them to the stems, which almost no other plant does. It is at once a phytoextraction option and a food-safety risk.

Rasbak, CC BY-SA 3.0

How it works

Mechanisms: phytoextraction

Contaminants

Contaminant	Evidence	What was shown
DDT and breakdown products	field	Field study with 21 cultivars on soil with weathered p,p'-DDE: mean bioconcentration factors of 7.22 (roots) and 5.40 (stems) for subspecies pepo, against 2.37 and 0.454 for subspecies texana; share of DDE extracted from soil 0.301% against 0.065%, with a maximum of 0.780% (White et al. 2003). White et al., Subspecies-level variation in the phytoextraction of weathered p,p'-DDE by Cucurbita pepo, Environ. Sci. Technol. 37:4368, 2003
Polychlorinated biphenyls	field	A site contaminated with Aroclor 1254 and 1260 (21 µg/g in soil on average), pumpkin grown for two years: in the second year 11 µg/g PCBs in stems and 8.9 µg/g in leaves (5.7 and 3.9 the first year), up to 43 µg/g in lower stems, i.e. bioaccumulation factors up to 2; transfer is by root uptake and translocation (Whitfield Åslund et al. 2008). Whitfield Åslund et al., The effects of repeated planting, planting density, and specific transfer pathways on PCB uptake by Cucurbita pepo grown in field conditions, Sci. Total Environ. 405:14, 2008
Dioxins and furans	greenhouse	The first paper to show that cucurbits, zucchini and pumpkin in particular, transfer PCDD and PCDF from soil to fruit far more than other vegetables (Hülster et al. 1994). Hülster et al., Soil-plant transfer of polychlorinated dibenzo-p-dioxins and dibenzofurans to vegetables of the cucumber family (Cucurbitaceae), Environ. Sci. Technol. 28:1110, 1994

Contaminant

Evidence

What was shown

DDT and breakdown products

field

Field study with 21 cultivars on soil with weathered p,p'-DDE: mean bioconcentration factors of 7.22 (roots) and 5.40 (stems) for subspecies pepo, against 2.37 and 0.454 for subspecies texana; share of DDE extracted from soil 0.301% against 0.065%, with a maximum of 0.780% (White et al. 2003).

White et al., Subspecies-level variation in the phytoextraction of weathered p,p'-DDE by Cucurbita pepo, Environ. Sci. Technol. 37:4368, 2003

Polychlorinated biphenyls

field

A site contaminated with Aroclor 1254 and 1260 (21 µg/g in soil on average), pumpkin grown for two years: in the second year 11 µg/g PCBs in stems and 8.9 µg/g in leaves (5.7 and 3.9 the first year), up to 43 µg/g in lower stems, i.e. bioaccumulation factors up to 2; transfer is by root uptake and translocation (Whitfield Åslund et al. 2008).

Whitfield Åslund et al., The effects of repeated planting, planting density, and specific transfer pathways on PCB uptake by Cucurbita pepo grown in field conditions, Sci. Total Environ. 405:14, 2008

Dioxins and furans

greenhouse

The first paper to show that cucurbits, zucchini and pumpkin in particular, transfer PCDD and PCDF from soil to fruit far more than other vegetables (Hülster et al. 1994).

Hülster et al., Soil-plant transfer of polychlorinated dibenzo-p-dioxins and dibenzofurans to vegetables of the cucumber family (Cucurbitaceae), Environ. Sci. Technol. 28:1110, 1994

Limits

The share removed per season is under 1% of the contaminant present: many decades would be needed. Only subspecies pepo accumulates. The same mechanism carries the contaminant into the fruit: zucchini and pumpkins must not be grown for eating on soils with DDE, PCBs or dioxins. Dense planting lowers both yield and accumulation (Whitfield Åslund et al. 2008).

Where it has been tried

Brescia-Caffaro

PhytoremediationGreenhouse, with soil from the site

Festuca arundinacea, Cucurbita pepo subsp. pepo, Medicago sativa, Phalaris arundinacea

Greenhouse rhizoremediation trial on farm soil from the SIN contaminated with aged PCBs (mg/kg range), in 6 kg pots to approach field conditions. Ten treatments in triplicate: seven species (grasses and trees) and five growing conditions (single species, intercropping, redox cycle by flooding, added compost or ammonium thiosulfate), with unplanted controls, for 18 months. The same experiment produced soil half-lives for PCBs and for dioxins and furans.

Outcome After 18 months total PCBs had fallen by 14–20%. The best treatments (about 20%) were Festuca arundinacea with compost, Festuca arundinacea grown with Cucurbita pepo, and Medicago sativa with Rhizobium and mycorrhizal fungi. PCB half-lives ranged from 1.3 to 5.6 years. For dioxins and furans only Festuca arundinacea gave a significant reduction (11–24% depending on the congener; half-lives 2.5–5.8 years). In a later column test, soil grown with Medicago sativa released about twice as much PCB in leachate, bound to particulate organic carbon.

PhytoremediationGreenhouse, with soil from the site, 2015–2016

Festuca arundinacea, Phalaris arundinacea, Cucurbita pepo subsp. pepo, Medicago sativa, Brassica juncea, Salix caprea, Athyrium filix-femina

The University of Insubria, the University of Milan and ERSAF took about 2,500 kg of soil from the site and grew plants on it in a greenhouse for 18 months (May 2015 – November 2016) in 6 kg pots: ten treatments in triplicate with seven species, intercropping, compost, redox cycles and ammonium thiosulphate, with unplanted controls. It is a semi-field trial, not an open-field one.

Outcome After 18 months total PCBs had fallen by 14–20%. The best treatments (Festuca arundinacea with compost or intercropped with pumpkin; alfalfa with Rhizobium and mycorrhizal fungi) reached about 20%. For dioxins and furans tall fescue alone gave reductions of 11–24% depending on the congener, with half-lives of 2.5–5.8 years. The authors name the short duration, against the persistence of these compounds, as a limit.