It is assumed that the seed coat is able to reduce the amount of Ni entering the seed, and that a high level of Ni induced the mobilization of macronutrients.
Grevillea exul var exul (Proteaceae), a tree species native to serpentine soils in New Caledonia, is a reported manganese accumulator. Since the metal tolerance of this species remains unknown, its growth and metal accumulation were studied for seven heavy metals under controlled conditions. Brassica juncea, a popular species for metal phytoremediation, was used as a reference. G. exul seedlings were more tolerant to Cr, Zn, Ni, and Cu than B. juncea. There were no differences in Hg, and Cd tolerance between both species. B. juncea seedlings concentrated more Cd, Hg, and Cr in their shoot than G. exul seedlings, while Ni, Zn, and Mn levels were similar for both species. Comparison then focused on tolerance at toxic doses of Ni and Mn using older individuals of both species. No growth inhibition for G. exul plants was observed, whereas the growth of B. juncea was significantly inhibited at the higher metal concentrations. Shoot Mn and Ni concentrations were again lower in G. exul plants as compared to B. juncea, suggesting a mechanism of partial Ni and Mn exclusion in G. exul. In a subsequent study, 1-year-old G. exul plants favored Ni accumulation in roots while Mn accumulated preferentially in shoots.
Plants growing on serpentine soils are usually assumed to be nickel (Ni)-tolerant species. The objective of the work reported here was to find out whether Cunonia macrophylla Brongniart & Gris, an endemic species common on Ni-rich soils in New Caledonia, is indeed Ni tolerant. In a greenhouse experiment, plants were watered with solutions containing Ni chloride (5 to 1000 mg Ni L −1 ) for 15 months before their morphological characteristics were measured, and the levels of nitrate, protein, and nutrients in the plants were determined. The plants grown with 5, 10, 50, and 100 mg Ni L −1 had more leaves, and there was a tendency toward greater stem thickness and fresh shoot biomass at 50 mg Ni L −1 . Manganese (Mn), copper (Cu), and zinc (Zn) concentrations were also higher in the presence of 50 mg Ni L −1 . One thousand mg Ni L −1 induced symptoms of Ni toxicity, and this effect may account for the lower concentration of potassium (K) in the shoots and roots of plants exposed to this concentration, as well as for inducing higher concentrations of soluble protein in the shoots. Calcium (Ca) and iron (Fe) levels were lower in roots exposed to higher concentrations of Ni. Nickel concentrations in the plant as a whole increased with the level of Ni applied, and the Ni gradient decreased from roots to stem and from stem to leaves. It was concluded that Cunonia macrophylla is a Ni-tolerant species that accumulates Ni without any impairment of growth when exposed to levels of up 500 mg Ni L −1 .
Endemic plant species have been considered as one of the best means of ecological restoration of exploited mine sites in New Caledonia. These plants have the adaptability characteristics that allow them to thrive on serpentine soils. This study has investigated the physiological effects of nickel concentrations at different developmental stages on Grevillea exul var. rubiginosa an endemic New Caledonian species. The first two stages of a plant life cycle germination and post germination under controlled conditions were studied. The experiment involved Petri dish germination and growth of seeds and seedlings respectively particularly root development and length in different salts of nickel (acetate, chloride and sulphate) with 0�500 ppm concentrations. Decrease in both studied parameters: germination rates and root lengths were observed with increasing concentrations of nickel solutions supplied to seeds. A minimum concentration of 5 ppm of nickel in the medium had a positive effect on germination rate as well as root length. Results suggest that it is the concentration of nickel used in the present investigation to test the germination rate and root elongation in Grevillea exul var. rubiginosa which are essential for experimental work in the field rather than the form (salt) of nickel. These results will be help-full in re-vegetation efforts of nickel mining sites in New Caledonia.
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