The use of plants for phytoextraction of heavy metals from contaminated soil is limited by the ability of the plants to grow on these soils and take up the target metals, as well as by the availability of the metals for plant uptake in the soil solution. The hypotheses of this study were that the growth-promoting phytohormone auxin (indole-3-acetic acid, IAA) can alleviate toxic effects of metals on plants and increase metal phytoextraction in combination with the biodegradable chelating agent ethylene diamine disuccinic acid (EDDS). To test these hypotheses we performed two sets of experiments with sunflowers (Helianthusannuus L.) in hydroponic solution. In the first set of experiments, five IAA concentrations (0, 10(-12), 10(-11), 10(-10), 10(-9)M) were applied in combination with Pb (2.5 microM) or Zn (15 microM). In the second set of experiments we applied combinations of IAA (0 or 10(-10)M) and EDDS (0 or 500 microM) to Pb or Zn-stressed sunflowers. Root and shoot growth of metal-stressed plants were most effectively increased with 10(-10)M IAA, and also the extraction of both metals was significantly increased at this treatment level. IAA reduced the negative metal effects, such as reduced shoot and root dry weight, root length, root volume and root surface area. EDDS significantly decreased metal uptake by the plants, thus reducing metal stress and promoting plant growth. The combined application of IAA with EDDS significantly increased Zn uptake in comparison to EDDS only treated plants. The experiments indicate that IAA can alleviate toxic effects of Pb and Zn on plant root and shoot growth and can in combination with chelants such as EDDS increase the phytoextraction potential of these plants.
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