Detection of nickel in maize roots: A novel nondestructive approach by reflectance spectroscopy and colorimetric models

Abstract

In this paper, a novel method capable of assessing nickel (Ni) uptake in intact plant roots is proposed and validated. Dimethylglyoxime (DMG) was used to stain roots of seven-day-old maize (Zea mays L.) seedlings grown in solutions containing 0, 0.025, 0.1, 0.2, and 0.5mM Ni2+. A nondestructive approach by using reflectance spectroscopy and CIE XYZ and CIE L*a*b* color systems was used to investigate their optical properties. The maximum intensity in reflectance spectra at 545nm (A545) was used to monitor the development of DMG–Ni complexes. The values of A545 were polynomially related to the concentration of Ni2+ in the growing solution (R2=0.98), and significant correlations were also observed between A545 and the two indicators of phytotoxicity, namely, root growth inhibition (I) and variations in the shoot-to-root ratio (S/R). The variation of dominant wavelength (λd) and excitation purity (Pe) graphically derived from the x, y color gamut in the CIE XYZ system paralleled the two highest Ni concentrations. Most of the color parameters in the CIE L*a*b* system were closely related to both Ni2+ concentrations in plant growing medium and the values of (I) and (S/R) used here as markers of Ni toxicity. The revealed dependences confirm that the prelaminar method proposed here is capable of nondestructive estimating of Ni concentrations in intact root tissue. The values of A545 in the reflectance spectra of stained roots and color parameters, namely, opponent redness–greenness (a*), integral color difference (ΔE)*, lightness color difference (ΔL)*, and hue angle difference (ΔH)*, showed the highest potential for diagnostic purposes.