Zinc, lead and cadmium accumulation and tolerance in Typha latifolia as affected by iron plaque on the root surface

Abstract

The effects of iron plaque on the growth of Typha latifolia L. and its accumulation of zinc (Zn), lead (Pb) and cadmium (Cd) were investigated under field conditions and in nutrient solution cultures in the laboratory. In the field, Zn concentrations (but not Cd) on the root surface were positively related to Fe concentrations on the root surface. In the laboratory, seedlings with and without iron plaque on their roots were exposed to 2.0 and 1.0 μg ml⁻¹ Zn for 3 and 72 days and to 10.0 μg ml⁻¹ Pb and 0.2 μg ml⁻¹ Cd for 48 days. Zn accumulation was similar in seedlings with and without plaque when exposed to 2.0 μg ml⁻¹ Zn for 3 days. Over the longer growth periods there was no significant difference in root and shoot dry weights when seedlings with and without plaque were treated with control or metal solutions. Root lengths both in the control and the Cd solution were significantly shorter in seedlings with plaque than in those without. Zn was absorbed fairly equally on root surfaces with and without plaque; however, roots with plaque absorbed more Pb, but less Cd, than those without. The presence of plaque did not alter Zn, Pb and Cd translocation in seedlings in the nutrient solutions. Most of the Pb and Cd taken up was retained in the roots, whereas most of the Zn taken up was retained in the shoots, suggesting that root tissue rather than the root surface or plaque is the main barrier for Pb and Cd transport. However, plaque may act as an effective Fe reservoir to increase Fe ion concentrations in active cells and then ameliorate metal toxicity.

Introduction

Coatings of iron oxides or hydroxides deposited on roots of plants are called `iron plaque.' Plaque can form on roots of submerged, emergent or terrestrial plants subjected to flooding (Crowder and St.-Cyr, 1991). Field observations have shown that wetland plants which can survive in metal-contaminated soils frequently have iron plaque on their roots (Taylor et al., 1984; Otte et al., 1989, Otte et al., 1991; St.-Cyr and Crowder, 1990; Greipsson and Crowder, 1992). The question therefore arises whether iron plaque improves their performance in some way. It is suggested that iron plaque may act as a barrier to toxic substance uptake, mainly metals, owing to adsorption and immobilization of metals by iron plaque (Taylor and Crowder, 1983a; Otte et al., 1987). It has been reported that the formation of plaque can improve growth and ameliorate copper (Cu) and nickel (Ni) toxicity under mildly toxic conditions of Cu and (or) Ni exposure (Greipsson and Crowder, 1992; Greipsson, 1994). However, different results have been reported when seedlings of Typha latifolia with and without plaque were exposed to Cu and Ni solutions (Ye et al., 1997b).

Typha latifolia L. has colonized a wide range of wetland habitats, including heavy metal polluted areas (McNaughton et al., 1974; Taylor and Crowder, 1983b; Ye et al., 1997a), and can produce iron plaque under field and laboratory conditions (Taylor et al., 1984; Macfie and Crowder, 1987; Ye et al., 1994). The aims of the present experiments were to determine the effect of iron plaque on growth, and zinc (Zn), lead (Pb) and cadmium (Cd) immobilization and accumulation in T. latifolia and to test the hypothesis that plaque can ameliorate metal toxicity. Zn, Pb and Cd were chosen for the study as they are common pollutant heavy metals, affecting plants adversely at fairly low concentrations. The novelty of the work presented here is that all experiments were performed using seedlings of T. latifolia, a plant which hitherto had been extremely difficult to culture from seed. Earlier work on iron plaque and metal effects in Typha have employed plantlets regenerated from rhizome fragments where `carry-over' effects may be significant, and where only adventitious roots have thus been involved (e.g. Taylor and Crowder, 1983b; Taylor et al., 1984).