Submitted to: New Phytologist
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/11/2006
Publication Date: 2/10/2007
Citation: Zhang, L., Angle, J.S., Chaney, R.L. 2007. Do High-nickel Leaves Shed by the Ni-hyperaccumulator Alyssum Murale Inhibit Seed Germination of Competing Plants?. New Phytologist. 173:509-516. Interpretive Summary: Unusual plants hyperaccumulate over 100-times more metals to their shoots than needed to cause phytotoxicity and death in normal crop plants. Researchers have wondered how this characteristic evolved, and several possible explanations have been proposed, including defense against insects and microbial disease organisms, and allelopathy which would limit other plants from growing near the hyperaccumulator plants. A number of papers have supported the defense hypothesis, but a valid test was needed of the Ni allelopathy hypothesis. Based on preliminary studies, we tested the effect of adding both low and high Ni Alyssum biomass (and equivalent soluble Ni salt) to a low Ni sandy loam from Maryland, and a high Fe oxide serpentine soil rich in Ni from Oregon. Seeds of a number of weed and crop species were sown and germinated measured. The results showed that the serpentine soil bound Ni so effectively that the added biomass or Ni salt did not harm germination of the weed seeds, while addition of biomass, and especially of the soluble Ni salt to the MD soil caused severe Ni phytotoxicity to many of the tested plant species. The MD soil had little ability to bind Ni, so the added Ni could be toxic. Where toxicity was observed, the addition of biomass-Ni was much less adverse than soluble Ni salt. The conclusion was that hyperaccumulators growing on natural serpentine soils were unlikely to obtain any benefit by the allelopathic effect of dropping high Ni leaves to the soil surface.
Technical Abstract: Elemental allelopathy suggests that nickel (Ni)-rich leaves shed by hyperaccumulators inhibit the germination and growth of nearby plant species. Here, the germination of eight herbaceous species following addition of Alyssum murale biomass or Ni(NO3)2, with the same Ni level added to soil, was assessed. The distribution of Ni in soil was tested by determining Ni phytoavailability and speciation over time. Phytoavailable Ni in soil amended with biomass declined rapidly over time due to Ni binding to iron (Fe)/manganese (Mn) oxides in the soil. No significant effects on seed germination were observed. Unlike the Ni complex in Alyssum biomass, more Ni remained soluble and phytoavailable in soil amended with Ni(NO3)2, thus significantly inhibiting seed germination. High-Ni leaves shed by hyperaccumulators did not appear to create a 'toxic zone' around the plants and inhibit germination or growth of competing plants. The lack of an allelopathic effect was probably related to low Ni availability.