Dimethylglyoxime (DMG) staining for semi-quantitative mapping of Ni in plant tissue

Determination of the nickel (Ni) distribution in tissues of hyperaccumulator plants aids in understanding the strategies and mechanisms used by these plants to take up Ni from soils. Commonly used methods for measuring Ni distribution in plant tissues require expensive equipment and complex sample preparation. We tested a suite of staining methods consisting of dimethylglyoxime (DMG) dissolved in a range of solvents for the mapping of Ni distribution in the Ni hyperaccumulator Berkheya coddii Roessler. The best solution was DMG (10 g l⁻¹) dissolved in borax (25 mM) and KOH (30 mM). Plant tissue cross-sections were imaged under a microscope immediately after DMG application. A Karhunen-Loeve transformation was applied to the images to minimize interference from colours of other origin, e.g. from chlorophyll. The distribution of Ni could be determined at the cellular level and consistent patterns were obtained for replicates. Staining of Ni dissolved in agar at various concentrations was used to calibrate the method. Concentrations as low as 50 mg kg⁻¹ (fresh weight) could be detected. Averaged over several cross-sections the DMG method systematically gave lower concentrations than ICP-OES analysis of the respective plant part, indicating that not all Ni in the tissue reacted with DMG, but only Ni that is readily available. The DMG method may be used in conjunction with spectroscopic methods to resolve biologically active Ni.