Influence of low calcium availability on cadmium uptake and translocation in a fast-growing shrub and a metal-accumulating herb.

Franziska Eller,Hans Brix

doi:10.1093/aobpla/plv143

Abstract

Calcium (Ca) and the toxic heavy metal cadmium (Cd) are antagonistic ions competing for uptake in plants when they co-occur in soil solutions, and high Ca concentrations can reduce the uptake of Cd in plants. However, less is known about the effects of low Ca bioavailability on Cd uptake and translocation in plants. We hypothesized that low Ca availability would enhance Cd uptake and translocation in Sesbania sesban, a fast-growing shrub potentially useful for Cd removal from contaminated soils, and Brassica juncea, a well-known Cd-hyperaccumulator. The two species were grown under controlled conditions for 21 days in hydroponic nutrient solutions with either 0.2 or 2 mM Ca and 0 or 50 µM Cd in a 2 × 2 factorial experimental design. Both species had a lower relative growth rate, final root length and shoot and root biomasses at 50 µM Cd compared with growth without Cd. The negative effects of Cd on both species were exacerbated at low Ca availability. Brassica juncea had higher root Cd concentrations than S. sesban, but the shoot Cd concentrations did not differ between the two species. The low Ca concentration enhanced the uptake of Cd in the roots of both species, but Cd translocation to the shoots was low, especially at 0.2 mM Ca. We conclude that the low Ca concentration enhanced the uptake of Cd into roots of S. sesban and B. juncea and increased the phytotoxicity of Cd. The translocation of Cd to the shoots of the two species was, however, lower at 0.2 mM than at 2 mM Ca, implying that Cd removal from polluted soil cannot simply be increased by adjusting ion concentrations.

Highlights

Cadmium (Cd) is an environmentally and sanitarily harmful heavy metal contaminant, resulting from anthropogenic sources such as industrial effluents, sewage sludge, phosphorus fertilizers and mining (Williams and David 1973; Molina et al 2009; Schipper et al 2011)
This concentration level is within the normal range (1– 3 mM Ca) of the Ca concentrations used in other studies investigating heavy metal uptake in B. juncea and S. sesban (Ishikawa et al 2006; Gudu et al 2009; Mohamed et al 2012; Suthar et al 2013)
Brassica juncea had two to three times higher shoot biomass production than S. sesban across all treatments, but the difference was especially obvious under Cd exposure, where B. juncea produced up to eight times more shoot biomass than S. sesban

Summary

Introduction

Cadmium (Cd) is an environmentally and sanitarily harmful heavy metal contaminant, resulting from anthropogenic sources such as industrial effluents, sewage sludge, phosphorus fertilizers and mining (Williams and David 1973; Molina et al 2009; Schipper et al 2011). Highly mobile and toxic even at relatively low concentrations (Das et al 1997). As several metals are essential nutrient elements, plants have a natural disposition to take up minerals, including heavy metal. Transporters or channels in root membranes for essential ions have been determined as possible entry pathways for metals with unknown biological function, such as Cd, into plants (Clemens 2006).

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Discussion

Conclusion