Growth and metal accumulation of an Alyssum murale nickel hyperaccumulator ecotype co-cropped with Alyssum montanum or perennial ryegrass in serpentine soil

Authors: BROADHURST, C - Former ARS Employee; Chaney, Rufus

Submitted to: Frontiers in Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/22/2016
Publication Date: 4/8/2016
Citation: Broadhurst, C.L., Chaney, R.L. 2016. Growth and metal accumulation of an Alyssum murale nickel hyperaccumulator ecotype co-cropped with Alyssum montanum or perennial ryegrass in serpentine soil. Frontiers in Plant Science. 7:451. doi: 10.3389/fpls.2016.00451.

Interpretive Summary: Nickel hyperaccumulator plants can accumulate over 1% Ni in their dry shoots and the harvest of nickel from the aboveground biomass can achieve an alternative to traditional mining. Growing hyperaccumulators to produce biomass for sale as Ni ore is called "phytomining." For sub-economic ore grades (most serpentine soils), phytomining is the only way to obtain Ni from the soil material that is cost effective. Phytomining offers an alternative crop for Ni mineralized or Ni contaminated soils and opportunity for farmers that is much more profitable than traditional crops on these very infertile soils. We have previously domesticated Ni hyperaccumulator species and bred improved cultivars for phytomining, and characterized the fertilizer requirements and production practices. Research continues to better understand how these plants achieve hyperaccumulation from essentially insoluble minerals in serpentine soils. One hypothesized mechanism is that roots secrete organic compounds which increase the solubility of soil Ni and then moves to the roots with transpiration flow increasing Ni uptake potential. Another hypothesis is that growing plant species known to secrete metal chelating compounds in mixed plant culture could increase Ni hyperaccumulation. Grass species are known to secrete phytosiderophores, non-protein amino acids, which chelate ferric iron to support iron uptake by grasses, but these compounds are not highly selective and can also chelate nickel, copper, and other trace elements in serpentine soils. The present experiment tested whether growing a typical grass species (perennial ryegrass, Lolium perenne) with roots intermingled with those of Alyssum murale (hyperaccumulator) or Alyssum montanum (non hyperaccumulator) affected yield or nickel hyperaccumulation. Results showed that growing Alyssum murale with roots co-mingled with those of Alyssum montanum or perennial ryegrass did not influence either yield or nickel concentration in the Alyssum murale shoots. Further, in practical phytomining, co-planted grasses would use nutrients and water and reduce yields of the phytomining crop. Thus co-planting is not a useful method to increase the value of phytomining.

Technical Abstract: More than 400 plant species naturally accumulate high levels of metals such as Cd, Cu, Co, Mn, Ni, and Zn. The genus Alyssum (Brassicaceae) contains the greatest number of reported Ni hyperaccumulators (50), many of which can achieve 3 wt% Ni in dry leaves. 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 mixed fields, mobilization of metals by phytosiderophores secreted by Graminaceae plants could affect Alyssum Ni, Fe, Cu and Mn uptake. We mono- and co-cropped the Ni hyperaccumulator Alyssum murale, non-hyperaccumulator A. montanum and perennial ryegrass in a natural serpentine soil. The soil is infertile and high in Ni, but is not phytotoxic and supports native vegetation. All treatments had standard inorganic fertilization 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 at normal quite low levels in the 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 with Alyssum Ni hyperaccumulators than organic soil amendments or co-cropping.