Nickel localization and response to increasing Ni soil levels in leaves of the Ni hyperaccumulator Alyssum murale

Abstract

We have previously developed phytoremediation and phytomining technologies employing Alyssum Ni hyperaccumulators to quantitatively extract Ni from soils. Implementation of these technologies requires knowledge of Ni localization patterns for the Alyssum species/ecotypes of interest under realistic growth conditions. We investigated Ni uptake and localization in mature Alyssum murale ‘Kotodesh’ and ‘AJ9ç leaves. Seedlings were grown in potting mix with an increasing series of NiSO₄ addition (0, 5, 10, 20, 40, 80 mmol Ni kg⁻¹), NiC₄H₆O₄ addition (0, 5, 10, 30, 60, 90 mmol Ni kg⁻¹), in Ni-contaminated soil from metal refining operations, and serpentine soil. Plants at Ni levels 0, 5, 10, 20 mmolkg⁻¹ and in native soils grew normally. Plants at 40 mmolkg⁻¹ exhibited the onset of phytotoxicity, and 60, 80, and 90 mmolkg⁻¹ were demonstrably phytotoxic, but symptoms of phytotoxicity abated within 6 months. Cryogenic complement fractures were made from frozen hydrated samples. High-resolution scanning electron microscope (SEM) images were taken of one half. The other half was freeze-dried and examined with SEM and semi-quantitative energy dispersive x-ray analysis. Ni was highly concentrated in epidermal cell vacuoles and Ni and S counts showed a positive correlation. Trichome pedicles and the epidermal tissue from which the trichome grows were primary Ni compartments, but Ni was not distributed throughout trichomes. Palisade and spongy mesophyll and guard/substomatal cells contained lesser Ni concentrations but palisade mesophyll was an increasingly important compartment as Ni soil levels increased. Ni was virtually excluded from vascular tissue and trichome rays.