Hybanthus floribundus

Heavy metal toxicity in plants is infrequent (143). In many cases, metal concentrations in plant parts show poor correlation with soil concentrations of the element (147). Plants tend to exclude certain elements and readily accept or concentrate others. Lisk (148) reported natural plant soil concentration ratios of 0.05 or less for As, Be, Cr, Ga, Hg, Ni, and V. Cadmium appears to be actively concentrated and selenium appears to be easily exchangeable. Indicator plants are capable of markedly concentrating specific elements, e.g., Astragalus spp. for selenium (138) and Hybanthus floribundus for nickel (149). Plants growing on mine wastes have been shown to evolve populations which exhibit metal-specific tolerances (150). 

Certain plants accumulate897 very high levels of nickel in their tissues Hybanthus floribundus contains up to 1.3% nickel on a dry mass basis, while Psychotria douarrei has up to 44% nickel in the plant ash. AUysum is also known as a nickel accumulator. A large part of the nickel in these plants is water soluble, and in the last case is associated with malic and malonic acids.898 In Hybanthus floribundus, the nickel is largely accumulated in the leaves, where it is associated with pectic carbohydrates. 

A more complex extraction scheme than those of Hewitt, Bowen, and Reilly described above, was used to show that 65Zn is primarily associated with pectin in the root residue (Peterson, 1968). This was modified and used to indicate the association of copper in Armeria maritima (Mill) Willdenow (Farago et al., 1980) and Ni in Hybanthus floribundus (Farago and Mahmoud, 1983). The scheme is shown Fig. 10-1, and some typical results are shown in Table 10-1. In these last two investigations radiotracers were not required, since the plants accumulated Cu and Ni, respectively, and metal determinations were carried out by flame atomic absorption spectrometry on the individual fractions. 

Table 10-1. Sequential extraction of leaves of Hybanthus floribundus from Western Australia, showing percentages of Ni in original dried sample (13000 ug g 1). Extraction scheme as in Fig. 10-1 (from Farago and Mahmoud, 1983).

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