Dose effect of cadmium ions on the functional state of Echinacea purpurea (L.) Moench plants: from hormesis to oxidative stress

The use of medicinal plants for the rehabilitation of metal-contaminated natural environments is one of the modern directions of phytoremediation. Such plants are characterized by increased accumulation of specialized metabolites with antioxidant and chelating properties, which lead to metal detoxification and increased tolerance. Echinacea purpurea (L.) Moench (purple coneflower) is considered among promising plants with significant potential for phytoremediation. Some important aspects are neglected in studies of the effect of metals on E. purpurea, including the dose effect on growth processes and the accumulation of protective metabolites, including the stages of oxidative stress and stimulation at low doses (hormesis). The results of the study of the dose effect of cadmium ions as a representative pollutant among metal compounds on E. purpurea plants in a model experiment are presented. A more significant toxic effect on the inhibition of root growth compared to the aerial part was established, which is due to a greater accumulation of Cd in the root system of plants. As a marker of Cd-induced hormesis, the accumulation of phenolic metabolites, which participate in adaptive mechanisms due to their antioxidant and chelating properties, was found. Such a tendency was manifested in the form of an inverted U-shaped curve, which corresponded to the regression equation of the second degree polynomial, and was observed up to a certain limit concentration of Cd2+ depending on the degree of toxic effect on plant organs. For roots, in the case of a more significant toxic effect of Cd2+, the effect of hormesis in the accumulation of phenolic metabolites was observed at a lower concentration of the metal ion compared to the aerial part. As a comparative indicator of a more significant decrease in the content of these metabolites, the maximum intensity of plant tissues upon selective excitation of the fluorescence of phenolic compounds is proposed. The dependence of growth indicators of vegetative organs as a parabolic function on the content of protective metabolites against the background of different concentrations of metal ions was revealed, which confirmed the coordination of growth processes and adaptive metabolic changes. With a decrease in the tolerance index against the background of an increase in the concentration of Cd2+, there was a gradual change in trends: a decrease in the hormesis effect and an increase in oxidative stress. An increase in the number of test plants with brown root pigmentation and the relative length of this zone was found as markers of increased oxidative stress. Since in the case of cadmium-induced phytotoxicity, the function of this polymeric pigment is to block the entry of the metal into the cytoplasm, it can be argued that there is a coordinated metabolic adaptation depending on the dose of the toxicant. To identify the brown pigment, which is formed in roots de novo as a result of the oxidative modification of phenolic metabolites, the characteristics of the reflectance spectra in the visible range are proposed. The obtained results expand the understanding of the phenomenon of hormesis in the adaptation of plants to the stressful action of metals, the coordinated change of adaptive strategies, protective mechanisms of metal resistance. The used spectral methods can be applied to research the functional state of plants and improve the standardization system of medicinal raw materials of purple coneflower.