Phytoaccumulation of lead by sunflower (Helianthus annuus), tobacco (Nicotiana tabacum), and vetiver (Vetiveria zizanioides)

Abstract

The ability of three plant species: Helianthus annuus, Nicotiana tabacum, and Vetiveria zizanioides for phytoaccumulation of Pb was studied. Plants were grown in hydroponic solution containing Pb(NO3)2 at concentration of 0.25 and 2.5 mM Pb in the presence or absence of chelating agents (EDTA or DTPA). Lead (Pb) transport and localization within the tissues of the plant species was determined using scanning electron microscope equipped with energy dispersive X-ray spectrometers (SEM-EDS). The addition of chelators increased Pb uptake as compared to plants grown in solution containing Pb alone. Lead taken up by the plant species were concentrated in both leaf and stem at the region of vascular bundles with greater amounts in the leaf portion. Lead granules were also found in the H. annuus root tissue from the epidermis layer to the central axis. After four weeks of growth a 23-fold increase in shoot Pb content for H. annuus and N. tabacum and 17-fold increase in shoot Pb for V. zizanioides resulted from plants grown in the 2.5 mM Pb-EDTA treatment. The higher Pb treatment (2.5 mM Pb containing EDTA) resulted in higher concentrations of Pb in plant tissue at the fourth week of exposure as compared to Pb treatment containing DTPA. Overall, Pb accumulation potential of H. annuus was greater than that of N. tabacum and V. zizanioides as indicated by the bioconcentration factor (171, 70, and 88, respectively). The highest measured Pb concentrations were found in H. annuus roots, stems, and leaves (2668, 843, and 3611 microg/g DW, respectively) grown in the 2.5 mM Pb-EDTA treatment. The addition of chelators caused some reduction in plant growth and biomass. Results showed that the three plant species tested have potential for use in phytoaccumulation of Pb since the Pb was concentrated in leaf and stem as compared to control plants. H. annuus however best meet the prerequisites for a hyperaccumulator plant and would have the potential for use in the restoration of abandoned mines and factories sites contaminated with elevated Pb levels in the soil.