Soil amendments affecting uptake and storage of nickel in the (hyper) accumulator plants Berkheya coddii R. and Helianthus Annuus L.

Abstract

Phytoremediation provides one of the most environmentally friendly solutions to pollution. However, the long timeframes for plant growth and slow uptake rates are a deterrent for widespread use of this technique. Heavy metal uptake and accumulation in plants is governed by biosynthesis of organic and amino acids. Understanding their inter-relationship may provide insight into potential soil amendments which can be used to increase phytoremediation capacity of plants, and reduce the timeframe of phytoremediation. The aim of this project was to analyse the relationship of the heavy metal Ni with selected amino and organic acids within two plant species of the Asteraceae family: B. coddii R. (B. coddii), classified as a South African Ni-hyperaccumulator, and Helianthus annuus L. (sunflower), a metal accumulator. Amendments applied to investigate uptake changes included addition of Ni, chelidonic acid (ChA), histidine (His) and a Ni/ChA mixture to soil in which these plants were grown. The impact of addition of His and ChA on organic and amino acids within plants has not previously been analysed. Nickel was quantified by ICP-MS, and organic and amino acids by HPLC-UV. Nickel accumulated predominantly in the shoots of B. coddii (1060 –7300 mg kg-1) and sequestration was linked to chelation by chelidonic acid and malic acid. The plants exhibited toxic effects to high concentrations of applied bioavailable Ni. Severe necrosis was observed at a concentration of 7300 mg kg-1, indicating for the first time, a threshold accumulation capacity for B. coddii. Sunflowers accumulated lower levels of Ni (20 –246 mg kg-1), with sequestration occurring in the roots. The addition of bioavailable Ni induced biosynthesis of amino acids which respond to stress factors in the plant. Soil amendments with results most applicable to future phytoremediation work were the application of His in both plants and ChA in sunflower plants. Development of both species was enhanced by His enrichment, with an enhanced biosynthesis and metabolism of amino acids (AA). The potential for enhanced Ni uptake through increasing plant biomass may provide a useful tool in improving the amount of Ni that plants are able to tolerate in shorter periods of time. Addition of ChA negatively impacted development of both species of plants. Amino acids were biosynthesised in shoots of B. coddii in response to addition of chelidonic acid, a stress response. There was no apparent influence on Ni uptake within B. coddii. Sunflowers, however, accumulated ChA, along with Ni from the soil. Malic acid, an organic acid related to metal sequestration, was biosynthesised. This study has shown that nickel hyperaccumulation is dependent on plant development and biosynthesis of chelating ligands. Addition of His into (hyper) accumulators can be useful to enhance growth and development of plants, while ChA can be used to enhance biosynthesis of the metal chelating ligand, malic acid, within metal accumulators