The effect of 24-epibrassinolide on growth rates, level of oxidative stress and photosynthetic pigments in little duckwood (Lemna minor L.) after exposure to heavy metals

One of the mechanisms for reducing the toxic effect of excess heavy metals is the production of plant phytohormones brassinosteroids (Mandava et al., 1988). Exogenous use of 24-epibrassinolide had a positive effect on the growth and development of various plant species under the action of abiotic factors. In this study, we studied the possibility of 24-epibrassinolide to mitigate phytostress in duckweed with excessive intake of copper, cadmium, zinc, and strontium. Preliminary cultivation of a laboratory culture of little duckweed in the medium with 24-epibrassinolide allowed to increase the growth rate and reduce the share of damaged plants in experiments with zinc (6.3-79 µmol/l), copper (12.6 µmol/l) and cadmium (5, 12.6 µmol/l). The range of effectiveness in exogenous intake of brassinosteroids to reduce the toxicity of heavy metals for duckweed is as follows: Zn > Cd > Cu > Sr. Against the background of improved growth and morphometric parameters, the reduction of oxidative stress occurred only in experiments with a high concentration of strontium (1580 µmol/l). The level of malondialdehyde (MDA) in plants when combined with 24-epibrassinolide and copper ions was higher than only when adding metal (p ≤ 0.05). There was a change in the content of carotenoids and chlorophylls. The use of brassinosteroids allowed to reduce the loss of chlorophyll and carotenoids under the action of 12.6 µmol/l of cadmium and zinc (p = 0.05). However, at certain concentrations of heavy metals (for example, 12.6 µmol/l of copper), the content of photoassimilating pigments and carotenoids in plants pretreated with 24-epibrassinolide was lower than in untreated plants (p ≤ 0.05).