Abstract
The genus Alyssum (Brassicaceae) contains Ni hyperaccumulators (50), many of which can achieve 30 g kg−1 Ni in dry leaf. Some Alyssum hyperaccumulators are viable candidates for commercial Ni phytoremediation and phytomining technologies. It is not known whether these species secrete organic and/or amino acids into the rhizosphere to solubilize Ni, or can make use of such acids within the soil to facilitate uptake. It has been hypothesized that in fields with mixed plant species, mobilization of metals by phytosiderophores secreted by Graminaceae plants could affect Alyssum Ni, Fe, Cu, and Mn uptake. We co-cropped the Ni hyperaccumulator Alyssum murale, non-hyperaccumulator A. montanum and perennial ryegrass in a natural serpentine soil. All treatments had standard inorganic fertilization required for ryegrass growth and one treatment was compost amended. After 4 months A. murale leaves and stems contained 3600 mg kg−1 Ni which did not differ significantly with co-cropping. Overall Ni and Mn concentrations were significantly higher in A. murale than in A. montanum or L. perenne. Copper was not accumulated by either Alyssum species, but L. perenne accumulated up to 10 mg kg−1. A. montanum could not compete with either A. murale or ryegrass, and neither Alyssum species survived in the compost-amended soil. Co-cropping with ryegrass reduced Fe and Mn concentrations in A. murale but not to the extent of either increasing Ni uptake or affecting plant nutrition. The hypothesized Alyssum Ni accumulation in response to phytosiderophores secreted by co-cropped grass did not occur. Our data do not support increased mobilization of Mn by a phytosiderophore mechanism either, but the converse: mobilization of Mn by the Alyssum hyperaccumulator species significantly increased Mn levels in L. perenne. Tilling soil to maximize root penetration, adequate inorganic fertilization and appropriate plant densities are more important for developing efficient phytoremediation and phytomining approaches.
Highlights
More than 400 plant species are known to naturally accumulate high levels of metals such as Cd, Cu, Co, Mn, Ni, and Zn (Baker et al, 2010; Krämer, 2010; van der Ent et al, 2013)
The compost contained pathogen(s) that both species were susceptible to, and it kept the soil damp longer between waterings. This is desirable in pot studies, A. murale in particular is adapted to semi-arid conditions, and once established survives with watering once per week or less
Our results indicate there is no value with respect to phytomining or phytoextraction in co-cropping A. murale with L. perenne
Summary
More than 400 plant species are known to naturally accumulate high levels of metals such as Cd, Cu, Co, Mn, Ni, and Zn (Baker et al, 2010; Krämer, 2010; van der Ent et al, 2013). The genus Alyssum (Brassicaceae) contains the greatest number of reported Ni hyperaccumulators (50), many of which can achieve 30 g kg−1 Ni in dry leaf biomass (Baker and Brooks, 1989; Reeves and Adigüzel, 2008; van der Ent et al, 2015). The technology employs the Ni-hyperaccumulating species Alyssum murale and A. corsicum to phytoextract Ni from a range of Ni-rich soil types. Alyssum hyperaccumulators accumulate appreciable Mn in the same locations that contain Ni (Broadhurst et al, 2004b, 2009)
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