Soils can be purified from excessive metal ions using "accumulator" plants, which are capable of actively absorbing metal ions and accumulating them in large quantities in above-ground parts. This technique is called phytoremediation and is regarded as promising, environmentally safe and economically beneficial. However, not any plant species can serve as a metal ion "absorber". Prospective phytoremediator species should be resistant to excessive metal concentrations and be able to grow well on metal contaminated soils in the given region. Plant physiologists from the Institute of Biology of the Karelian Research Center RAS have been searching for plant species suitable for phytoremediation of zinc-contaminated soils in northern regions, as well as for options for their subsequent processing. The researchers studied the phytoremediation potential of two important oilseed crops — brown mustard and white mustard, in the North. The study was supported by a Russian Science Foundation grant #22-24-00668
— Based on data from the literature, we selected two plant species: brown mustard and white mustard, both well-known as green manures, but with different metal accumulation strategies. Brown mustard accumulates metal ions in aboveground biomass, while white mustard - in roots. A special experiment was conducted with their mixed sowing. We supposed this variant could be even more effective. These crops are widely used in agriculture to improve soil structure, enrich it with nitrogen and suppress weed growth. In addition, both mustard species are raw materials for the food and pharmaceutical industries, — told Natalya Repkina, the research leader, Senior Researcher at the Laboratory for Ecological Physiology of Plants, Institute of Biology KarRC RAS.
Natalya Repkina, research leader, Senior Researcher at the Laboratory for Ecological Physiology of Plants, Institute of Biology KarRC RAS.
The scientists especially focused on options for processing the crops grown on zinc-enriched substrate. Since zinc is a vital trace element, the harvested aboveground biomass can be used as an additive for animal feed, and the roots can be composted.
— We found that the two plant species do not differ in the accumulation strategy, as previously thought, but accumulate zinc in the aboveground parts in sufficiently high concentrations already in the stooling stage, 3-4 weeks after sowing, while the metal content in the substrate decreases perceptibly. This soil purification effect was not improved by longer cultivation. This is an important practical consideration, since there is no need to grow plants for a full growing season, and 4 weeks suffices for the desired effect. In addition, it is in this growth stage that the plants are harvested for animal feed. We also found that cultivating the two mustard speices together is inexpedient, because this does not result in more effective accumulation of metal ions in plants and reduction of its content in the soil. Based on the results of our research, we can recommend using brown mustard as a supplement for animals to supply zinc and unsaturated fatty acids, and using white mustard aboveground parts as raw material for biofuel, — the physiologist explained.
The results of the study were published in the international scientific journals Plants and Horticulturae. The results also indicate that the seeds of brown and white mustard grown on soils heavily contaminated with zinc can be processed to produce oil.
— When excessive zinc ions enter plant cells, a cascade of biochemical reactions is triggered in the plant to protect the cell and neutralize the metal ions. At the whole plant level, defense mechanisms are designed to prevent excessive influx of metal ions to seeds. This is confirmed by our analysis of the plants' chemical composition; zinc content in seeds of both mustard species was significantly lower than in the above-ground parts. Owing to the transport channels on the membrane, metal ions are not retained in the lipid bilayer and freely penetrate the cell, where they bind to low-molecular compounds. When oil is pressed out, the lipid and protein fractions are separated. The bound metal ions remain in the protein fraction. Consequently, the isolated "pure" lipid fraction from brown mustard seeds can be used in oil production, – Natalya Repkina outlined.
Since the aim of the study is multifaceted and extensive, some of the tasks were tackled in collaboration with colleagues from the Laboratory of Ecological Biochemistry of the Institute of Biology KarRC RAS, where the lipid and fatty acid composition of the plants was analyzed.
— Besides, seeds of brown mustard grown on zinc-contaminated substrates were richer in unsaturated fatty acids, which is important for the food industry. White mustard seeds, on the contrary, acquire a significantly higher content of saturated fatty acids, which makes this feedstock less valuable for the food industry. In this case, however, considering that we additionally detected a high content of triacylglycerols, such seeds can be used e.g. for producing biofuels with physicochemical properties that can be adjusted according to their intended use, — explained Svetlana Murzina, Head of the Laboratory of Ecological Biochemistry, Institute of Biology KarRC RAS.
