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Stinging nettle (Urtica dioica L.) is a perennial plant species that is often perceived as a weed, but in recent years it has attracted interest for its medicinal and nutritional properties. Wild nettle is primarily a variable plant material of unexplored composition, so recent research aims to the importance of its cultivation. Optimal yield and nutritional quality of the final product are influenced by a number of abiotic, biotic and anthropogenic factors, which are difficult to control due to fluctuating environmental conditions during field cultivation. Modern greenhouse hydroponic cultivation techniques, such as floating hydroponics, can eliminate the problems of field cultivation and allow earlier harvesting, higher yields and better quality of plant material. Optimal conditions for plant growth are achieved by proper management of the abiotic factors of air and nutrient solution during cultivation. Since different plant species differ considerably in their uptake of certain nutrients, it is necessary to determine the optimum composition of nutrient solution for each species depending on the cropping season and the purpose of the final product. The aim of this study was to determine the specialized metabolites and antioxidant properties of fresh nettle leaves depending on the content of two different nutrient solutions (EC 1.8 mS cm -1 and EC 3.0 mS cm -1) in floating hydroponics. The nutrient solution with the lower amount of nutrients resulted in higher levels of total phenolics (238.4 mg GAE 100 g-1) and lower levels of vitamin C (80.3 mg 100 g -1) and antioxidant capacity (2413.8 μmol TE L -1).

Nettle is a highly valued medicinal plant that is still largely neglected, both in terms of nutrition and use for pharmacological purposes. Tinctures, i.e., alcoholic extracts, are becoming increasingly popular nettle products, mainly because they allow better availability of phytochemicals and their stability over a longer period of time. The production of alcoholic extracts is a chemically demanding process that is still usually carried out using conventional techniques, which have numerous drawbacks. The use of green technologies such as ultrasound-assisted extraction (UAE), which is characterized by high efficiency of phytochemical extraction, shorter treatment time, and a much lower environmental footprint, is a suitable and sustainable solution. Therefore, the aim of this study is to determine the influence of the extraction method, conventional and ultrasound (by varying two ultrasound equipment systems), time and ethanol concentration on the extraction of specialized metabolites from nettle powder. Ultrasonic extraction using a probe system significantly contributed to increase the ascorbic acid yield, polyphenolic compounds, and antioxidant capacity of nettle extracts compared to conventional extraction. In addition, when a probe system was used during UAE, significantly less time was required for isolation of individual specialized metabolites compared to ultrasonic extraction in the bath. Ethanol concentration (50 and 80% v/v) also proved to be an important factor in the efficiency of extraction of specialized metabolites, with 80% ethanol being more effective for the isolation of ascorbic acid and pigment compounds (chlorophyll and carotenoids), while 50% v/v for the extraction of polyphenolic compounds. It can be concluded that extraction with the ultrasonic probe system is much more efficient in obtaining higher yields of specialized metabolites from nettle powder in a shorter time (average process duration 5–10 min) both compared to UAE in the bath and classical extraction. However, optimization of the key factors of time, solvent type, and ultrasonic power is necessary to maintain the nutritional quality of the nettle extract in order to obtain a final product with a high specialized metabolites content, antioxidant capacity, and functional value. The future application of alcoholic nettle extracts is based on the fact that these products have significant potential as functional foods and pharmacological preparations for the treatment of a number of but also to strengthen the immune system, mainly due to the rich nutritional composition and high content of various specialized metabolites. The prepared extracts can be safely taken orally by diluting the tinctures with water immediately before ingestion.

Innovative agricultural practices are increasingly focused on conserving resources, reducing negative environmental impacts, and producing food with high nutritional quality and high content of health-promoting phytonutrients. Proper selection of cultivating factors, especially balanced nutrition, can significantly increase nutritional value and result in foods with significant biological and functional properties. The innovative techniques for cultivation of plant species, such as floating hydroponic systems, can also ensure a uniform chemical composition and high specialized metabolites (SM) content through nutrient solution management. Although it has been neglected in human diet, stinging nettle has a rich chemical composition (vitamins, minerals, pigments, phenols and other SM) and can be consumed as a green leafy vegetable. The aim of this study was to determine the influence of different corrections (B1, B2 and B3) of the nutrient solution (NS) during three harvest periods on the SM content and antioxidant capacity of nettle leaves. The B1 and B2 treatments were initially filled with NS prepared independently of the water’s chemical composition and B3 was filled with NS adjusted according to the chemical analysis of the water. After each harvest, the corrections of NS were made as follows: B1 was filled with water, B2 with NS, and B3 with nutrients according to the chemical analysis of NS. Of the nutritional and functional properties, total phenolics (TP), ascorbic acid (AsA), pigment content and antioxidant capacity were analyzed. The highest TP (377.04 mg GAE/100 g), total flavonoid (279.54 mg CTH/100 g), AsA (112.37 mg/100 g) and pigment content, as well as the highest antioxidant capacity (3546.91 µmol TE/L) were recorded in the third harvest, B2 treatment. The results show a significant positive influence of multiple harvesting and NS treatment on the SM content of nettle, confirming this type of hydroponic cultivation as an innovative technique for the production of novel food with high nutritional value.

Microgreens are becoming increasingly popular with consumers, not only for their exceptionally intense flavor and color, but also as a functional food with high nutritional value and strong antioxidant activity. Growing microgreens in greenhouses, especially in winter, presents a number of challenges, most notably a lack of light. Light is one of the key factors for optimal plant growth and development, for obtaining adequate yields, but also for the accumulation of various phytochemicals. Quality (wavelength) and photoperiod (duration) are the main light factors that directly affect the morphological characteristics of plants and a number of changes in metabolic functions related to nutrient composition and synthesis of specific metabolites (SM). Photosynthetically active radiation (PAR) includes wavelengths in the range of 400 - 800 nm, with specific photoreceptors of plant cells absorbing red, dark red, blue, and ultraviolet (UV-A) wavelengths. Optimization of the key factors of light, quality and quantity, can significantly positively affect the synthesis of various SM of plant, such as vitamins, polyphenolic compounds, pigments and other compounds of high antioxidant activity. The aim of this study was to determine the influence of red (620 nm) and blue (450 nm) spectra and different photoperiod (12 and 16 h) of supplemental light- emitting diode (LED) illumination on the composition of some SM and antioxidant capacity of mung bean microgreens (Vigna mungo (L.) Hepper)). The study was conducted in a climate chamber with controlled ecological conditions (25 °C, 60% humidity) under supplemental illumination by LEDs. Microgreens mung beans were manually cut at the base of the hypocotyl in cotyledon phenophase at a height of 12 cm and the following SMs were analyzed in the fresh plant material: ascorbic acid content (AsA), total phenolic content (TPC), and antioxidant capacity (by ABTS). The highest levels of AsA (61.74 mg/100 fw) and TPC (223.96 mg GAE/100 g fw) in mung bean microgreens were determined by treatment with LEDs in the red spectrum at a photoperiod of 16 hours. During the photoperiod of 12 hours, higher levels of AsA (50.56 mg/100 g body weight) and TPC (191 mgGAE/100 g body weight) were determined by blue spectrum illumination. Quality and photoperiod had no significant effect on antioxidant capacity. Regardless of the different treatments, antioxidant capacity values were generally high, with an average value of 2374.09 µmol TE /L, indicating that mung bean microgreens represent plant material with high nutritional and functional value. The obtained results indicate that proper management and optimization of quality, i.e. light intensity (selection of appropriate spectrum and photoperiod), can significantly affect the nutritional quality of the plant material and improved synthesis of SMs. From all this, it is clear that further research on specific light factors is needed to optimize the cultivation of plant material with adequate yield and high functional properties.