Features of the effect of copper and strontium ions on the laboratory culture of duckweed (Lemna minor L.)

The purpose of this study is to investigate the morphometric changes and the level of oxidative stress in the duckweed when exposed to strontium and copper ions. The laboratory culture of the duckweed was provided by the Institute of Biology, Komi Science Centre, Ural Division of the Russian Academy of Science (Russia, Syktyvkar). The plants were cultivated on Steinberg medium (Steinberg, 1946). We determined the following morphometric parameters: specific growth rate, damage to the fronds (chlorosis and necrosis), the area of the fronds. Strontium and copper were added to the different media as filter-sterilized solutions of Sr (NO3)2 or CuCl2∙2H2O in a final concentration ranging from 0 to 1.58 mM for strontium and 0 to 12.6 μM for copper. As controls, plants grown on Steinberg medium were used. The area of the fronds was calculated from photographs before exposure and seven days after it (OECD, 2006). The images were analyzed using the Image J software (NIH, USA). To determine the level of malondialdehyde (MDA), a TBA test was used. Duckweeds were previously contained in an experimental solution for four days (Uruç Parlac, Demirezen, 2012). As expected, both copper and strontium induced a dose-dependent decrease in specific plant growth (p ≤ 0.05). Strontium caused the inhibition of the specific growth rate of duckweeds as compared with controls since 0.6 mmol/l and copper - since 3.15 μmol/l. When exposed to 0.63 mmol / liter of strontium solution, the plants growth rate was reduced by 15.33 %, 3.15 μmol/l of copper solution - by 30 % as compared with controls. The maximum used concentrations caused a reducing growth rate in comparison with controls by 73.3% for 1.58 mmol/l of strontium solution and by 80 % for 12.6 μmol / l of copper solution. Based on this 7-day growth inhibition test, it was concluded that copper was more toxic to L. minor than strontium. The level of damaged plants increased with increasing metal concentrations (p ≤ 0.05). Damage in the form of chlorosis appeared at 0.3 mmol/l of strontium and 0.211 μmol/l of copper. Over 60 % of damaged plants were at 0.948 mmol/l of strontium and 5 μmol/l of copper. At high concentrations of copper and strontium ions, all plants are necrotic damaged. High concentrations of the heavy metals caused the surface area of the fronts to decrease. After seven days of exposure the area decreased at a strontium concentration 0.316 mmol/l, copper - of 0.63 μmol/l compared to control plants. Univariate analysis of variance showed that when the concentration of strontium and copper in solution increased, the MDA level rose (p ≤ 0.05). Strontium and copper provoke the development of oxidative stress in plants. The level of MDA in plants when exposed to strontium is statistically significantly higher than in controls, beginning with 0.63 mmol/l, and copper - 5 μmol/l. An increase in the intracellular level of heavy metals causes a disturbance in the oxidation-reduction balance in the plant cells and the accumulation of active oxygen species. Because of its greater reactivity, copper is more toxic than strontium. A high level of oxidative stress is accompanied with a significant increase in the proportion of plants with chlorosis and necrosis, a reduction in the area of fronds, and a low specific growth rate of plants.