Phytotoxic Effects of Selected Herbal Extracts on the Germination, Growth and Metabolism of Mustard and Oilseed Rape

The use of plants that have high allelopathic potential as natural herbicides in the form of aqueous extracts is gaining popularity in environmentally friendly agriculture. Usually, their effect on the germination and growth of weeds is investigated. However, less attention is paid to the effect of the allelopathic compounds from extracts on cultivated plants. Therefore, the aim of this study was to evaluate the impact of herbal extracts that have allelopathic properties on selected physiological and biochemical processes of two plants of great economic importance—white mustard (<i>Sinapis alba</i> L.) and oilseed rape (<i>Brassica napus</i> L. var. <i>oleifera</i>). The extracts were prepared from mountain arnica (<i>Arnica montana</i> L.), ribwort (<i>Plantago lanceolata</i> L.), hypericum (<i>Hypericum perforatum</i> L.), common milfoil (<i>Achillea millefolium</i> L.), sunflower (<i>Helianthus annuus</i> L.) and sage (<i>Salvia officinalis</i> L.). The germination of white mustard and oilseed rape was most inhibited by the extracts that were prepared from sage and sunflower. Additionally, in the germinating plants, the sunflower extracts increased the membrane permeability, which indicates membrane injuries. The metabolic changes in the plants were monitored using isothermal calorimetry and FT-Raman spectroscopy. The total heat production, which provided information about the metabolic activity of the white mustard and oilseed rape, was decreased the most by the sage extract but generally all of the tested extracts disturbed the shape of the heat emission curves compared to the water control. The impact of the allelopathic compounds that are present in the herbal extracts on the metabolism of the seedlings was clearly visible on the FT-Raman spectra—in the fatty acids and flavonoids range, confirmed using a cluster analysis. In conclusion, the herbal extracts from medicinal plants that have herbicidal activity could be used as a natural herbicide for weed control, but since they may also have negative impacts on cultivated plants, preliminary tests are advisable to find the extract from the species that has the least negative effect on a protected crop.