Lamb’s lettuce (Valerianella locusta L. Laterr.) is a leafy green vegetable that is rich in various biological active compounds and is widely used in ready-to-eat salads. The cultivation conditions and growth stage could affect the secondary metabolism in plants and thereby modify their food value. In the present study, the effect of selenium (Se) application in various concentrations (5.0, 10.0, and 20.0 µM) on the contents of Se, phenolic compounds, vitamin C, carotenoids, chlorophylls, and antioxidant activity of hydrophilic and lipophilic extracts of lamb’s lettuce harvested at three growth stages (38, 52, and 66 days after sowing (DAS)) was studied. Se application significantly increased the Se concentration in the shoots (up to 124.4 μg g−1 dry weight), as well as the contents of chlorogenic acid, total flavonoids, total phenolics, ascorbic acid, chlorophyll b, and the antioxidant activity of hydrophilic and lipophilic extracts. A higher content of phenolic compounds and higher antioxidant activity of hydrophilic extracts was observed at the first growth stage (38 DAS). On the contrary, higher contents of lipophilic compounds (chlorophyll a, chlorophyll b, lutein, β-carotene) and higher antioxidant activity of lipophilic extracts were found for shoots harvested at later stages (52 and 66 DAS).
Hyssop (Hyssopus officinalis L.) is an aromatic plant that is rich in various biologically active compounds and is widely used as a natural preservative and flavoring agent in the food and cosmetic industry and as a folk medicine against certain respiratory diseases. Despite the fact that selenium is not an essential trace element for plants, in a certain range of concentrations it can not only improve plant growth, but also increase the content of nutrients and biologically active compounds in plants. In the present study, the effect of treatment type (in a nutrient solution (NS) or by foliar application (FA)) and selenium (Se) concentration (2.0, 5.0, 10.0, 20.0, 40.0 μM) on the biomass, phenolic compounds, essential oil content, and antioxidant properties of hyssop grown in hydroponics was studied. Neither a toxic nor a beneficial effect of Se addition on the plants was observed. Selenium treatment significantly increased Se concentration in hyssop up to 38.35 μg g−1 (NS) and 34.86 μg g−1 (FA). The effect of selenium on the content of phenolic compounds, essential oil, and the antioxidant activity of hyssop depended on the applied selenium concentration. Hyssop treated with 10 μM Se (NS) and with 20 μM Se (FA) had a higher total content of hydroxycinnamic acids and higher contents of rosmarinic and chlorogenic acids, as well as a higher total content of phenolic compounds, higher content of essential oil, and higher antioxidant activity compared to other experimental plants. The results confirm the feasibility of selenium treatment of hyssop without reducing its production in order to enhance its nutritional and pharmaceutical properties.
Selenium is a micronutrient with a wide range of functions in animals, including humans, and in microorganisms such as microalgae. However, its role in plant metabolism remains ambiguous. Recent studies of Se supplementation showed that not only does it increase the content of the element itself, but also affects the accumulation of secondary metabolites in plants. The purpose of this review is to analyze and summarize the available data on the place of selenium in the secondary metabolism of plants and its effect on the accumulation of some plant metabolites (S- and N-containing secondary metabolites, terpenes, and phenolic compounds). In addition, possible molecular mechanisms and metabolic pathways underlying these effects are discussed. It should be noted that available data on the effect of Se on the accumulation of secondary metabolites are inconsistent and contradictory. According to some studies, selenium has a positive effect on the accumulation of certain metabolites, while other similar studies show a negative effect or no effect at all. The following aspects were identified as possible ways of regulating plant secondary metabolism by Se-supplementation: changes occurring in primary S/N metabolism, hormonal regulation, redox metabolism, as well as at the transcriptomic level of secondary metabolite biosynthesis. In all likelihood, the confusion in the results can be explained by other, more complex regulatory mechanisms in which selenium is involved and which affect the production of metabolites. Further study on the involvement of various forms of selenium in metabolic and signaling pathways is crucial for a deeper understanding of its role in growth, development, and health of plants, as well as the regulatory mechanisms behind them.
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