Annals of Botany

Put another nickel in

Flowering plants as a group are amazingly adaptable, as exemplified by species that grow in serpentine soils containing large amounts of nickel and low amounts of essential nutrients. In terms of nutrition these plants face two key problems. The first is obtaining enough essential macronutrients such as K and N. The second is avoiding the toxicity of Ni, concentrations of which far exceed the plant’s requirement for this micronutrient. It is this latter aspect that has been studied by Léon et al. (New Caledonia and Marseilles, pp. 609–618) focusing on Grevillea exul var. rubiginosa, which grows on the serpentine soils of New Caledonia. Seeds were germinated in the presence of Ni, supplied as three different salts: Ni chloride, sulphate and acetate. At concentrations of Ni up to 50 mg L^–1, there was relatively little effect on germination but at higher concentrations inhibition of germination became very severe, although less so with Ni sulphate than with the acetate or chloride, the latter being especially toxic. However, seedling root growth in those seeds that did germinate was inhibited significantly at concentrations of 10 mg L^–1 and above. Nevertheless, these data reveal a high degree of Ni tolerance. SEM and mineral microanalysis of seeds showed that Ni was nearly all confined to the seed coat except that seeds exposed to Ni chloride, the most toxic of the three salts, exhibited some Ni in the endosperm. In respect of seeds, therefore, G. exul is mainly a Ni excluder. However, the presence of Ni in the seed coat did have some effect on the  distribution of certain other nutrients in the endosperm: the homogeneous distribution in controls was replaced by a patchy distribution, the most intriguing feature of which was the accumulation of Mg, P, K and Mn in the region of the endosperm nearest the embryo axis. The mechanism for this effect of Ni remains to be elucidated.