Cadmium(II) and Zinc(II) Ions Effects on Maize Plants revealed by Spectroscopy and Electrochemistry

Toxicity of cadmium has been described in numerous papers and includes especially ability to generate reactive oxygen species (ROS), which are responsible for damaging of biomolecules and loss of their biological functions, however, mutual effect of zinc(II) and cadmium(II) ions is still unclear. Due to this fact, we focused on studying of joint effect of cadmium(II) and zinc(II) ions (0 µM Zn2+ + 0 µM Cd2+; 100 µM Zn2+ + 0 µM Cd2+; 0 µM Zn2+ + 100 µM Cd2+; 10 µM Zn2+ + 100 µM Cd2+; 50 µM Zn2+ + 100 µM Cd2+; 75 µM Zn2+ + 100 µM Cd2+ and 100 µM Zn2+ + 100 µM Cd2+) on experimental plants represented by Zea mays cv. CE220. Growth parameters (root/shoot length, fresh and dry weight), and stress markers and enzymes (oxidized/reduced glutathione, phytochelatins, total content of thiols, activities of glutathione-S-transferase (GST), alanine transaminase (ALT), aspartate transaminase (AST), and alkaline phosphatase (ALP)) were monitored during the treatment. In addition, total content of zinc(II) and cadmium(II) ions was determined in plant tissues electrochemically using differential pulse anodic stripping voltammetry. Moreover, content of reduced (GSH) and oxidized (GSSG) glutathione, and phytochelatin2 and phytochelatin5 were quantified using high performance liquid chromatography with electrochemical detection (HPLC-ED). Firstly, we studied the influence of metal ions on the growth parameters, i.e. fresh/dry weight and root/shoot length. Application of both metals led to the reduction of root as well as shoot biomass compared to control untreated plants. These adverse effects were found also in length of shoot and roots as well as dry weight. Content of both cadmium and zinc was determined in roots and shoots of experimental plants using Differential Pulse Anodic Stripping Voltammetry (DPASV). The enhanced content of both zinc(II) and cadmium(II) ions in roots and shoots was identified. In comparison of cadmium and zinc, plants taken up higher amounts of zinc(II) ions. It was found that the plants treated with 100 µM Zn2+ + 0 µM Cd2+ took less Zn in the second collection day on average 0.8 - 6 times and in the tenth day of collection 0.1 - 1.1 times in aboveground parts and in the second day of sampling averaged 0.3 - 4.8 times and in the tenth day of collection 0.7 - 2.2 times in roots. Antioxidant activity was determined in roots and shoots of control and experimental maize plants exposed to cadmium(II) and/or zinc(II) ions. To determine the antioxidant activity, we chose three methods as FRAP, DPPH and ABTS, and correlated the obtained results. The monitored enzyme activities (AST, ALT and ALP) were also correlated and we found that the activity of these enzymes enhanced with the increasing dose of both ions. In addition, we monitored GST activity correlation with total content of thiol compounds (-SH) measured spectrometrically using Ellman´s method. Amount of thiol compounds as well as GST activity was two times higher in roots compared to shoots. Aiming our attention on content of particular thiols, significant increasing of total content of GSH was observed at both 2nd and 10th day of the treatment. The most significant changes were evident in the case of the highest joint concentrations of zinc(II) and cadmium(II) ions - 100 µM Zn2+ + 100 µM Cd2+. Treatment with both cadmium(II) and/or zinc(II) ions led to the increase of PC2 and PC5 in experimental plants. Higher amount of phytochelatins in shoots compared to roots indicates their role in chelatation and transport of both heavy metals to aerial parts.