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Friday - January 16, 2009

From: Louisville, KY
Region: Southeast
Topic: General Botany
Title: Will lead accumulate in the flower nectar of plants used for phytoremediation
Answered by: Nan Hampton


I'm attempting to phytoremediate lead in my garden with mustard and/or sunflowers. I also keep bees. I understand that lead is sequestered in roots and stalks. Would the nectar also be contaminated? Thanks for your time.


I can't answer your question precisely since I haven't been able to find any studies that address the accumulation of heavy metals in nectar; nor, have I found any studies that address the sequestration of these pollutants in the differents parts of Helianthus spp. or Brassica spp. However, I did find several recent reports for other plant species and I will summarize for you their findings:

In an article by V. Angelova and others ['Heavy Metal Content in Plants from Family Lamiaceae Cultivated in an Industrially Polluted Region', Journal of Herbs, Spices & Medicinal Plants, 11 (2006) pp. 37—46] it was reported that the amount of heavy metals accumulated in the different organs of the plant depended on what plant it was, which element it was, and the surface charactertistics of the plant organ.  Three plant species were tested in polluted soil for accumulation of lead (Pb), zinc (Zn), cadmium (Cd), and copper (Cu).  The results, in decreasing order. for accumulated Pb in the plants' organs were:

Salvia sclarea (clary sage or Europe sage):  Leaves > inflorescences > roots > stems

Mentha x piperita (peppermint):  Roots > leaves > stems > inflorescences

Salvia officinalis (kitchen sage):  Leaves > inflorescences > roots > stems

The result for Zn followed the same order for all three plants.  For Cd the order was the same for clary sage and kitchen sage, but for peppermint the order was: Roots > inflorescences > leaves > stems.  Cu followed the same order for peppermint and kitchen sage, but for clary sage the order was:  Leaves > roots > inflorescences > stems.

The study considered the feasability of growing these plants in the contaminated soil for human use.  They determined that the extracted essential oils had a lower concentration than that of the concentration in the leaves and inflorescences indicating that the pollutants were not transferred in the extraction process.  Additionally, the extracted oils from S. sclarea and M. x piperita resulted in a product with an acceptably low concentration of pollutants and could be used commercially.  However,  the entire leaves of S. officinalis and M. x piperita could not be used for consumption since their concentrations were too high.  They determined that the higher concentration in the leaves was due to contribution by aerosol pollutants of the heavy metals.

In a more recent article ['Metal uptake by medicinal plant species grown in soils contaminated by a smelter'. Environmental and experimental botany, 64 (2008) pp. 207-216] V. Zheljazkov reports testing for accumulation of five heavy metal elements—Pb, Zn, Cd, Cu and manganese (Mn)—using five different plants—Leonurus cardiaca (common motherwort), Marrubium vulgare (horehound), Bidens tripartita (threelobe beggarticks), Melissa officinalis (common balm), and Origanum heracleoticum (winter marjoram)

The plants differed in the amount of overall uptake of the metals with L. cardiaca = M. vulgare > B. tripartita = M. officinalis = O. heracleoticum for Pb

The metal concentration in the plant parts was leaves > roots > flowers > stems for Mn and Zn; while the order  was roots > leaves > flowers > stems for Pb, Cd, and Cu.  And, specifically for Pb, the concentration could be shown as roots >> leaves > flowers > stems.  In the highest contaminated soils, the roots contained the highest concentration of lead (Pb).

This study also found that in the extraction of essential oils, the heavy metals were left behind in the plant residues. Pb concentration in essential oils was below detection and concentration in infusions and/or teas was very low, almost below detection.

One further note,  K. C. Jones [Honey as an indicator of heavy metal contamination, Water, Air, & Soil Pollution 33 (1987) pp. 179-189] investigated the correlation between concentrations of heavy metals (including Pb) in honey from hives in several areas in the U. K. and the levels of these elements in soils where the hives were located.  The sample areas included an area of high mineralization, agricultural areas, an uncontaminated site, sites in heavy metal mining districts, and in London.  Here are his conclusions: 

"Considerable spatial and seasonal fluctuations were apparent. No correlations were observed between honey and soil concentrations for either Cu or Pb. ...

Honey appears to be largely insensitive to spatial differences in contaminant emissions or soil concentrations. The low concentrations of heavy metals in honey and the variability due to differences in such factors as floral source, foraging range, entrapment of atmospheric aerosols by flowers, season and time of year, rainfall, etc., detract from the reliable and sensitive use of honey as an indicator."

So, my assessment of whether you need to worry about Pb concentrations in honey from the nectar of your plants would be that the Pb from the soil will be mainly in the roots; however, the concentration in the flowers could be elevated by aerosol Pb contamination depending on whether there is a continuing source of contamination in your area.  Prior to 1976 when laws were passed to eliminate lead from gasoline, you would have had to worry about proximity to roadways; but that shouldn't be a major concern now.







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