Sunday, 10 June 2018

The potential of stinging nettle (Urtica dioica L.) as a crop with multiple uses

Elsevier Industrial Crops and Products Volume 68, June 2015, Pages 42-49 Nicola Di Virgilioa,∗, Eleni G. Papazogloub,1, Zofija Jankauskienec,2, Sara Di Lonardod,3,Marcin Praczyke,4, Kataryna Wielgusze,4aNational National Research Council of Italy, Institute of Biometeorology, Via Piero Gobetti 101, 40129 Bologna, Italy b Agricultural University of Athens, Department of Crop Science, 75 Iera Odos St., 118 55 Athens, Greece c LRCAF Upytė Experimental Station, Linininkų 3, Upytė, Panevėžys District, LT 38-294, Lithuania d National Research Council of Italy, Institute of Biometeorology, Via Giovanni Caproni 8, I 50145 Firenze, Italy e Institute of Natural Fibres and Medicinal Plants, Wojska Polskiego 71 B, 60-630 Poznań, Poland Get rights and content Highlights • We collected updated information on potential uses of stinging nettle. • We fixed main cultivation inputs and the still undefined practices. • In medicine and cosmetic industry stinking nettle demonstrated several added value potential application. • High valuable fibre crop with high cellulose content of approx. 86%. • High nutritive value, source of carotenoids, vitamins, minerals and protein. Abstract Stinging nettle (Urtica dioica L.) is a well-known plant species that is considered a weed in intensive agriculture. This crop has gained the interest both scientifically and commercially because it is the source of many added-value natural products by exploiting all the plant parts (stem, leaves, roots and seeds). The main objective of this article is to describe-along with unpublished data-information that is spread in different sources, giving an updated and comprehensive overview of the potential end-products, covering aspects related to the whole plant production chain, and at the same time, providing unpublished data collected under different projects. The effects of nettle cultivation on the environment are potentially favourable, it being a perennial low-requirement crop (it can reach about 3–12 Mg ha−1 dry stalk yield with low inputs). Stinging nettle has a long history as a textile fibre; its fibre quality has been demonstrated (e.g. cellulose content around 86%) and is highly depending on the extraction method. Furthermore, several studies confirmed the presence of numerous active compounds, especially in nettle leaves (e.g. caffeic acid derivative compounds, ceramides, nine forms of carotenoids, essential fatty acids, vitamins, minerals, phytosterols, glycosides and proteins), with most promising application in the food/feed, medicinal and cosmetic sectors. Although with high market potentials, the products made from nettle are currently more a result of curiosity rather than large-scale industrial production, mostly due to lack in crop and post-harvest management. The definition of a production chain able to exploit the plant biomass as much as possible is a prerequisite to increase income and boost farmers’ adoption, and to attract investors. Previous article in issue Next article in issue Keywords Urtica dioica L. Fibre production Multipurpose crop Cultivation Uses Natural products 1. Introduction Stinging nettle (Urtica dioica L.) is a perennial herbaceous plant belonging to the Urticaceae family. It is a well-known and common species, spread in temperate and tropical zones of Europe, Asia and America, adapted to a variety of climatic conditions. Stinging nettle is a perennial, monoecious plant, flowering and fruiting in summertime. Its stems and leaves are covered by stinging trichomes containing a fluid which causes blistering when entering the skin (Bisht et al., 2012). This species is considered a weed in intensive agriculture as its fast vegetative growth and high densities enable increased spread and soil coverage (Klimešová, 1995), but it is potentially able to act as a biomass source for several added-value products by exploiting all the parts of the plant (stem, leaves, roots and seeds). There are economic and ecological reasons to cultivate stinging nettle. According to Dreyer and Müssing (2000), stinging nettle is a perennial crop with satisfying yields for 10–15 years, has low input requirements, can improve soils overfertilized with nitrogen and phosphate, can promote the biodiversity of local flora and fauna, and can be used to produce new high-quality agricultural raw materials for dyeing, textile and energy sectors. The first attempt to consider nettle as an industrial crop was during the 1940s, when 150–200 ha were cultivated for fibre production in Germany (Bredemann and Garber, 1959), and when a cross breeding programme of wild plants for fibre production was established at the Institute of Applied Botany in Hamburg, which released several clones with high fibre content, still maintained in German and Austrian research institutions (Dreyer and Müssing, 2000; Vogl and Hartl, 2003). Recently, the scientific community showed a renewed interest in stinging nettle and several research projects have been carried out in Austria, Germany, Finland, the United Kingdom and Lithuania mainly to produce natural fibres, as well as to extract active herbal compounds (Table 1). In 2001, the Austrian Institute for Agrobiotechnology in Tulln tried to continue the successful introductory steps to establish fibre-nettle production in Austria (1999–2001 EU programme FAIR-CT98-9615: NETTLE-reintroduction of stinging nettle cultivation as a sustainable raw material for the production of fibre and cellulose) by collecting clones improved for fibre content and spinning quality. Most of the projects aimed to use stinging nettle as an alternative herbaceous fibre and for related-cellulose applications. The phytotherapeutic use of stinging nettle extracts from both roots and aerial biomass, its dyeing potential, the food-non-food attitude of the plant, propagation and field establishment, organic and non-organic farming, fibre extraction and quality assessment, and the environmental impact of stinging nettle cultivation have also been investigated (Table 1). Almost all projects mentioned in Table 1 had only academic partners on board. Not all projects produced a real market for nettle in Europe, only some short-term pioneer examples, mainly on the use of nettle fibre in blends with cotton or silk (see e.g. the STING project, and From Nettle to Textile I + II project in Table 1). Table 1. Main projects including stinging nettle, chronologically ordered. Project Main topic covered Main products Study of alternative fibre plants in Lithuania (part of the long-term LRCAF programme “Biopotential and Quality of Plants for Multifunctional Use”, 2010–2013). Crop yielding capacities, crop density, morphological indices, recommendations to growers. Potential possible uses of stinging nettle. Plant resources for food and non-food use in Tuscany Region: characterization and LCA of some products derived from nettle (supported by Tuscany Region 2010–2011). Life Cycle Assessment of some nettle products to ensure complete traceability of some finished nettle products. Textile and phytotherapeutic uses. PRIN 2009: “Medicinal and dyeing-plants natural extracts: characterization, and innovative poly-use of nettle, daphne, lavender and chestnut tannins” (supported by the Italian Ministry of Research 2010–2012). Evaluation of different methods of aqueous extraction of nettle on natural antioxidants content and the effects on aphids, antimicrobial and antifungal properties (biocides). Textile and phytotherapeutic uses, bioactive compounds for biological farm. ICCOG: “Identification and characterization of some clones of nettle and Spanish broom for textile and phytotherapic use” (supported by Tuscany Region 2008–2009). Identify and characterize local clones with high fibre content and metabolites for the phytotherapeutic sector and cosmetics, antimicrobial and antifungal properties. The investigation of vegetative propagation and growing of fibre stinging-nettle (supported by Lithuanian Ministry of Agriculture, 2008–2009). Vegetative propagation, crop establishment. Fibre, biomass. Scientific reasoning of technological possibilities to grow stinging-nettle in Lithuania (supported by Lithuanian Ministry of Agriculture, 2007). Collection of literature about stinging nettle All potential uses. LaMMA-TEST: “The textile processing chain in Tuscany” (supported by Tuscany Region, 2005–2007). Study of textile production and its environmental sustainability, economic feasibility thought modelling. Textile. NATURAL.TEX: “Natural fibres in textile processing chain of Tuscany” (supported by Tuscany Region, 2006). Introduction of natural textile and manufacturing and feasibility study (economic, technological, environmental and socio-spatial) of a brand for “Made in Tuscany”. Textile, dyeing. STING project: “Sustainable Technology in Nettle Growing” (Department for Environmental Food and Rural Affairs, UK, 2001–2004). Developing a sustainable natural textile fibre with non-food crop potential grown on low-grade agricultural land in the United Kingdom. Textile. From nettle to textile I + II (Agricultural Research Centre of Finland and College/Crafts and Design Department, 1997–2003). Nettle cultivation, fibre processing (biotechnological retting followed by mechanical fibre processing). Production of yarns and nettle cloth. Natural textiles made of nettle-innovative technology and product development for the textile industry (German project, 1999–2002). Nettle cultivation (organic/non organic farming), manufacturing textiles. Manufacturing of cloth and other textiles. Nettle-reintroduction of stinging nettle cultivation a s a sustainable raw material for the production of fibres and cellulose FAIR.ST-8356 and FAIR-CT98-9615 (German and Austrian project, 1999–2001). Nettle cultivation (organic/non organic farming), manufacturing textiles. Stinging nettle has gained both commercial and scientific interest due to its multipurpose character. The main objective of this article is to introduce an updated and comprehensive overview of the studies spread in different sources, as well as to present new data, with particular attention to end-products, with the aim of providing base support for the implementation of new projects and activities for the introduction of this plant into the traditional rotation farming system. 2. The stinging nettle production chain 2.1. Field establishment Stinging nettle is highly variable in morphological characteristics and there are probably several subspecies (Bassett et al., 1974; Hegi, 1981; Tutin et al., 2010). Dreyer et al. (1996) evaluated quantitatively and qualitatively 30 clones of fibre nettles and the most used in more recent experimental fields is the clone 13, for its high fibre content (Bacci et al., 2011; Dreyer and Müssing, 2000). Stinging nettle can be propagated both by seed or vegetatively (Luna, 2001). Its seeds are very small and sowing with a drill is possible but it requires accurate preparation of the soil bed and a sowing depth less than 1 cm. Young plants weakly compete against weeds (Ruckenbauer et al., 2002) and because of a high level of heterozygosis of the parents, resulting plants are consequently not homogeneous (Bacci et al., 2011; Bredemann and Garber, 1959; Dreyer et al., 1996; Hartl and Vogl, 2002; Szewczuk and Mazur, 2004; Vetter et al., 1996; Vogl and Hartl, 2003). Vegetative propagation is the only way to avoid such problems. Multiplication by rhizomes is possible (Luna, 2001; Wurl et al., 2002), but the most rational method is propagation by cuttings (Gatti et al., 2008; Hartl and Vogl, 2002; Ruckenbauer et al., 2002; Vogl and Hartl, 2003). Experiments in Italy showed that the German fibre nettle clone (clone 13) could be successfully vegetatively propagated in 10–12 days by wetting the cuttings with indole-3-butyric acid (30 s in a solution of 2 g l−1) and planting them in a thermostatic box with expanded inert silica (Gatti et al., 2008). After root development, cuttings can be transplanted in the field using conventional cabbage planting machinery (Bacci et al., 2009). In the moderate climatic zone, such as Central Europe, the optimal period for establishing a plantation using vegetative cuttings is in early autumn. Stinging nettle cuttings rooted directly under field conditions are also possible (Ammarellou et al., 2012; Jankauskienė and Gruzdevienė, 2010). In vivo propagation with cuttings permits a large number of plants to be obtained in a short period, but it requires large space. Moreover, the season influences both the rate and the percentage of rooting, even if the mother plant used is growing in a greenhouse. Smaller space, and independence from seasonality may be ensured using in vitro micropropagation, which has been tested with good results by Gatti et al. (2008), defining optimal explants and propagation medium (Table 2). Both in vivo and in vitro propagation are suitable for production of plants even under organic farming (Gatti et al., 2008). Table 2. Component concentrations of in vitro medium for propagation and rooting of stinging nettle. Component Concentration in the propagation medium Concentration in the rooting medium Unit Sucrose 30 30 g l−1 Agar 5.5 5.5 g l−1 MS salts 4.3 2.15 g l−1 Thiamine 0.4 0.4 mg l−1 Myo-inositol 0.1 0.1 g l−1 BAP 3.08 0 μM IBA 0.5 1.5 μM MS, Murashige and Skoog salts; BAP, 6-benzyl-aminopurine; IBA, indole-3-butyric acid. Optimal establishment density is different when using rhizomes or cuttings, also depending on the primary final products (e.g. fibre, medicinal uses). Different densities are reported in the literature for establishment of cuttings: from 60 cm to 1 m between rows and 50 cm or 60 cm within rows, but the most common distances used are 60–75 cm between rows and 60 cm within rows (Bacci et al., 2009; Hartl and Vogl, 2002; Jankauskienė and Gruzdevienė, 2013). Bacci et al. (2009) reported that the plant density (50 cm × 50 cm or 50 cm × 75 cm) did not modify the mean fibre content in stalks and the yield per square metre. When establish a stinging nettle plantation by planting into furrows, rhizomes of about 15–20 cm and at a depth of 6 to 8 cm, can be placed in rows every 40–60 cm and in row spacing of 75 cm to 1 m (Szewczuk et al., 2002). If stinging nettle is grown strictly for harvesting leaves, rhizomes may have 3–5 cm length and planting can be done at a distance of 30 cm × 40 cm or even of 20 cm × 20 cm, which leads to higher leaf yield as early as in the first year of cultivation (Szewczuk et al., 2002). Establishing stinging nettle plantation by sowing seeds involves sowing in rows at approximately 1000 seeds m−2, with a row spacing of 20 cm (Szewczuk et al., 2002). In general, stinging nettle has moderate climatic requirements and can be grown in most European countries (Akgül, 2013), survives winter with its remarkable underground rhizomes, and thus acts as a perennial crop. Some authors recognized that high spring rainfall is necessary (Bacci et al., 2009). In general, stinging nettle prefers growing on loose soil with organic matter rich in nitrogen and high phosphate levels for rapid growth (Bisht et al., 2012); it requires moist soil. But does not tolerate long flooding. When plants are grown on rich brown loessial soils, they accumulate a lot of mineral salts in their biomass (Szewczuk et al., 2002). Szewczuk et al. (2002) and Szewczuk and Mazur (2004) showed the correlation between nitrogen doses and biomass yield and protein content, demonstrating that the stinging nettle is a typical nitrophilous species. As regards soil pH, plant establishment can be carried out within the range of 5.6–7.6, thriving in neutral pH. 2.2. Crop management Stinging nettle is claimed to be a species with low input requirement. However, the biology, crop management and best agronomic practices of this crop have not been well investigated and only a few related articles have been published (Dreyer et al., 1996; Dreyer and Müssing, 2000; Schmidtke et al., 1998). Concerning fertilization, some authors reported quite high rates of chemical N fertilization (e.g. from 160–240 kg N ha−1 yr−1 to 250–300 kg N ha−1 yr−1), which probably may be considered not advisable for environmental reasons (Vetter et al., 1996; Vogl and Hartl, 2003). High N rates may reduce fibre quality as observed in flax (Cheng, 2004), but this needs to be verified in the case of nettle. Bacci et al. (2009) reported high dry stalk yield levels (9.81 to 12.85 Mg ha−1) by applying 200 kg ha−1 of N only before transplanting, while Schmidtke et al. (1998) and Kohler et al. (1999) reached from 2.66 to 5.52 Mg ha−1 dry stalk yield without fertilization. The high yields obtained by Bacci et al. (2009) are related to plot trials and manual harvest, and thus probably do not reflect reliable yield levels of a large field cultivation. Dreyer and Müssing (2000) suggested the addition of 60–80 kg N ha−1, 150–180 kg K2O ha−1 and 40–50 kg P2O5 ha−1. With a low input cultivation approach, legumes and herbs, e.g. fast-growing common vetch Vicia sativa and crimson clover Trifolium incarnatum, can be undersown in a 150 cm wide row spacing. The use of slurry and manure in combination with white clover (Trifolium repens) crops has been tested, reaching an average value of 3.8 Mg ha−1 stalk dry matter yield for nettle (Vogl and Hartl, 2003). As an example, the quantity and frequency of the diluted cattle slurry addition used by Hartl and Vogl (2002) were: 16 kg N ha−1 (26 June 1997); 23 kg N ha−1 (23 April 1998), 34 kg N ha−1 (19 June 1998); 40 kg N ha−1 (27 August 1998, after cutting); 40 kg N ha−1 (4 June 1999). Stinging nettle is considered a water-demanding crop, even if data on water requirement and use efficiency are not available in the literature. The most ideal condition is when the plant receives water uniformly over the growing period, meaning that in some cases, support through irrigation may be required (Vogl and Hartl, 2003), which is necessary anyway during the year of establishment. Bacci et al. (2009) found that with a summer precipitation of 56 mm, plants were able to survive without irrigation. Intensive weed management is mainly required during the implant stage if carried out in April or May (e.g. in Mediterranean Countries such as Italy) as small plants may be suppressed by weeds (Bacci et al., 2009). For this reason, some authors suggests to have root crops and hemp preceding nettle, thanks to their effectiveness in suppressing weeds and frequent harrowing of the seedbed. With an organic control approach, it is recommended to use widely spaced rows (100–150 cm with or without underfeeding) and repeated mechanical weed control (Bredemann and Garber, 1959). A narrow plant spacing under intensive cultivation (50 cm × 50 cm) promotes early cover and suppression of weeds (Vogl and Hartl, 2003). In the literature, there is scant information on serious threats by diseases or pests, and when described, they only refer to a very restricted part of the field. Stinging nettle is highly resistant to agrophages in general, suffering only localized damage. Nettle can be potentially attacked by a series of fungi, caterpillars, aphids, and spider mites generally observed on wild nettle (Shattock, 2005), causing damage to leaves and stems, and in some cases, also affecting fibre quality. Active ingredients such as bitertenol, thiophanate-methyl, and sulphur can be recommended for protection against fungal diseases. 2.3. Harvest Timing and harvest methods are strictly related to the final product, while it is also possible to plan a strategy for multipurpose destinations. For fibre production, stinging nettle should be harvested when the seeds are mature or when the stalks reaching 80% of the aboveground biomass and from the second year plantation (Vetter et al., 1996; Vogl and Hartl, 2003) – during the first year, the stalks are too thin, too ramified and with too many leaves. Delaying harvest does not affect fibre consistency and fibres do not lignify, while harvesting prematurely reduces fibre quality as fibres are overly thin. At harvest, stinging nettle can reach up to 210 cm in height, with a base stem diameter of 9 mm (Bacci et al., 2009). A simple cutting bar can be used for harvest. A mechanical harvest chain specifically developed for stinging nettle is not available in the market. Since stinging nettle is morphologically similar to hemp, the same harvesting and post-processing strategies can be applied for fibre production, e.g. retting on field, mechanical, chemical and controlled microbiological and enzymatic processing methods (Vogl and Hartl, 2003). If the main product is leaves, harvest can be carried out when plants are younger. If harvested early in the summer, stalks re-grow in late summer and autumn, but not enough to produce good quality fibre. In this case, one can imagine cutting plants in autumn for using the biomass for other product destinations (Table 3) instead of fibre. Bredemann and Garber (1959) also proposed a sequence of cuttings during one year for several end products. They suggested a first cutting in April for fodder, medical or other industrial processes such as chlorophyll production; a second cutting at the end of June for fibre production; and a third cutting in September for using leaves (Table 3) (Vogl and Hartl, 2003). Such an intensive approach may result in reduced fibre quality and loss of crop vigour, or reduction of the economical duration of a stinging nettle field. The duration of stinging nettle crops can be variable depending on the extent of agronomic intensification. Even if an average of a four-year plantation is considered as economically viable, Vetter et al. (1996) stated that a duration of more than four years may be possible only when plants are supported by intensive N fertilization. Table 3. Potential uses of stinging nettle. Field of application Use Part of the plant Reference Textile/fibre Ropes and fishing nets, tissues and fabrics, silky fabric, cloth and paper, biocomposites, paper, natural dye (for yarns, eggs, etc.) Fibre tissues of stems. Root and leaf extracts for dyes Bacci et al. (2009, 2011), Bisht et al. (2012), Bodros and Baley (2008), Di Virgilio et al. (2008), Gatti et al. (2008), Vogl and Hartl (2003) Medicine Anaemia, rheumatism, gout, eczema, diuretic, hypoglycaemia, hypotension, benign prostatic hyperplasia, cardiovascular problems, arthritis, allergic rhinitis, antioxidant, antimicrobial, antifungal, antiviral, antiulcer Leaves, seeds, roots, aqueous and alcoholic extracts Bisht et al. (2012), Chrubasik et al. (1997), Guarrera and Savo (2013), Gülçin et al. (2004), Jarić et al. (2007), Leporatti and Corradi (2001), Orčić et al. (2014), Pinelli et al. (2008), Roschek et al. (2009), Upton (2013) Cosmetics Soaps, shampoo, skin lotions No details given Bisht et al. (2012), Szewczuk et al. (2002), Upton (2013), Vogl and Hartl (2003) Food Salads, pies, soups and decocted tea Leaves, young plants Bisht et al. (2012), Guil-Guerrero et al. (2003), Orčić et al. (2014) Forage crop Poultry, cattle, horses and pigs for enhancing yolk yellowness Whole plant Loetscher et al. (2013), Szewczuk and Mazur (2004), Vogl and Hartl (2003) Animal housing Bedding Stem, shives as fibre by-product Harwood and Edom (2012) Bioenergy Biochar Removing stinging nettle requires intensive soil tillage. It is unknown if stinging nettle causes problems to succeeding crops (Vogl and Hartl, 2003). 3. Environmental effects of cultivating stinging nettle Besides the interest related to potential products and uses, the cultivation of stinging nettle can also result in important environmental benefits. Stinging nettle can easily be propagated and cultivated with organic farming techniques while maintaining sustainable yields (Di Virgilio et al., 2008; Gatti et al., 2008; Vogl and Hartl, 2003). Stinging nettle, as a perennial crop requires lower inputs in terms of tillage, which maintains soil fertility and structure. Its cultivation was found to protect the upper layer of soil against erosion, crusting and drying (Szewczuk et al., 2002). Furthermore, it promoted biodiversity, functioning as both habitat and source of food for beneficial insects (e.g. water nymphs). It is a fast growing plant and, thus, it has an advantage over other weeds in water and nutrient uptake (Bacci et al., 2009; Filipek et al., 1999), making the use of chemicals against weeds unnecessary. Leaves are very rich in minerals and when falling down return an important amount of nutrients to the soil. On the other hand, nettle is a nitrophilous plant and thus able to grow in overfertilized fields, reducing eutrophication risks, and, as a perennial, it can be used for phytoremediation (Adler et al., 2008). The environmental impact related to the establishment of a stinging nettle production chain is linked to the agronomic inputs and to the post-harvest processes and uses. Olivieri et al. (2011) presented the critical steps in stinging nettle cultivation using the Life Cycle Assessment approach. The input data were from a field experiment carried out in the Tuscany Region (Bacci et al., 2009; Olivieri et al., 2011). On a hectare basis, the results of the impact assessment [Impact 2002+ impact assessment method (Jolliet et al., 2003)] showed that fertilization represented 43% of the total environmental damage, irrigation 36%, maintenance 10% and transplant 6%. The damage category ‘Human Health’ was the most affected (34%), mainly due to the NOx that is emitted in the air, directly related to N application to crop. The damage on the ‘Depletion of non-renewable resources’ or ‘Resource’ category comprised 29% because of crop water requirement; on the ‘Climate Change’ category 25%, mainly because of CO2 emitted into the air from fossil fuels; and on the ‘Ecosystem Quality’ category 12%, mainly due to zinc emission into soil. This study considered only the cultivation process and gave the possibility of identifying which agronomic practice should be redefined in order to reduce overall environmental impact through process indicators (N fertilization vs. amount of dry matter or plant height; water irrigation vs. dead plants) useful for the mapping of good agricultural practices in the cultivation of this plant species. However, if stinging nettle is grown for textile applications, the main burden comes from the post-harvest treatment (textile processes). For the closely textile-related processes, the large inventory of inputs and outputs database of textile processes built in the EU COST Action 628 (Nieminen et al., 2007) could be taken into account, which also includes a list of indicators to be considered specifically for Life Cycle Assessment/Life Cycle Inventory of textile chains. 4. Products from stinging nettle The added value of stinging nettle cultivation is related to several products and applications obtainable from its biomass (Table 3). Stinging nettle has been used in medicine and the cosmetic industry (Szewczuk et al., 2002). Hippocrates (460–377 BC) reported 61 remedies using stinging nettle (Upton, 2013). His statement “Let food be your medicine” has been incorporated into the traditional concept of food, and stinging nettle is a representative example. Since it has been used for several purposes by different folk traditions, some of these uses have been tested scientifically since a new interest on natural products has been discovered (Baverstock et al., 2010). Verdinelli et al. (2013) confirmed the antifeedant effects of water extracts of stinging nettle on aphids, especially of those obtained from leaves that showed the presence of caffeic acid derivative compounds and ceramides. Early on, the plant was used in textiles, although to a limited extent, especially when other fibrous crops, i.e. flax, cotton and hemp, were introduced (Bacci et al., 2009; Vogl and Hartl, 2003). Stinging nettle fibre was previously used mainly for ropes and fishing nets. Currently, stinging nettle is commonly used in farms for feeding livestock, mainly poultry and pigs (Szewczuk and Mazur, 2004). In this respect, the stinging nettle is a valuable source of vitamins, minerals, phytosterols, and glycosides, as well as of proteins (Szewczuk and Mazur, 2004). The research team in the ICCOG project (Identification and characterization of some clones of nettle and Spanish broom for textile and phytotherapic use, Table 1) found a reliable scheme for using stinging nettle as a multipurpose crop, with fibre as base product. After harvest, they used fresh and dry leaves for the food sector and, together with the resulting water from fibre retting, for extraction of added-value bio-active compounds. 4.1. Use for fibre production Stinging nettle has a long history as a textile fibre substitute for linen, dating back to mediaeval times, and has also been used commercially more recently during both World Wars when other crops, such as cotton, were scarce. In Scotland, stinging nettle was cultivated for making durable linen cloth from the fibre stalk. Fibre can also be used for small-scale papermaking (Bisht et al., 2012). Research is underway to develop the use of stinging nettle for textiles, indicated by several projects already mentioned in Table 1. Stinging nettle fibre has a cellulose content of around 86% (Bisht et al., 2012), with fibre diameter of 18–23 μm (Hartl and Vogl, 2002) and elongation of 2.2–2.5% (Hartl and Vogl, 2002), comparable to those of flax and hemp, and it is far stronger than both flax and cotton, (30–35 cN tex−1vs. 15–20 cN tex−1 and 15.50 cN tex−1, respectively; Hartl and Vogl, 2002), while being comparable with ramie fibre (34 cN tex−1; Angelini et al., 2000; Goda et al., 2006). As in other bast plants, stinging nettle fibres are located between the outer bark (epidermis) and the central woody core, arranged in bundles held together with gummy substances called pectins (Fig. 1). Stinging nettle fibre has good absorbent characteristics, good anti-static, thermoregulatory and transpiration characteristics, non-lignified cell wall, soft and resistant fibres with low specific weight (Guo et al., 2005). For some applications, such as replacement for glass or carbon fibres, as composite in the automotive industry, or in the replacement of asbestos fibres, stinging nettle fibres are superior to flax fibre (Guo et al., 2005). Download full-size image Fig. 1. Cross section of stinging nettle stem (original magnification, 5×). Fixation, paraffin and methacrylate resin embedding. In field trials conducted in Austria, fibre yields ranged from 335 to 411 kg ha−1 in the second year and from 743 to 1016 kg ha−1 in the third year. The upper stalk of stinging nettle has a higher percentage of fibre with respect to the woody core, with a maximum of 16% (Hartl and Vogl, 2002; Vogl and Hartl, 2003). A trial carried out in Tuscany showed a fibre content of 13% in the second year at 205 day of the year (Bacci et al., 2009). Values in the bibliography are very variable and are related to the clone, plant density (increasing plant density increases fibre yield), and harvesting time, while the effect of N fertilization on fibre content and quality is still controversial. Fibre extraction from plant stems in the case of stinging nettle is challenging. Traditionally, two methods of retting are used, dew and water retting (Harwood and Edom, 2012). Dew retting consists of laying stinging nettle stems on the ground after they have been cut, allowing the action of moisture, bacteria and fungi to break down the pectins. Due to its dependence on weather conditions, this technique is somewhat unreliable. Water retting is usually preferred: stalks are put in tanks filled with well-water, where a pectinolytic community develops. In this context, the use of pure bacterial cultures retted fibres to a higher yield and quality than natural microflora (controlled microbiological retting; Table 4). In general, microbiological retting produces fibres of better quality because it reduces both the lignin content and fibre diameter, without any negative effect on important mechanical properties, such as tensile strength and elongation (Bacci et al., 2010; Table 4). However, the disadvantages of water retting (e.g. the malodour from fermentation by anaerobic bacteria, environmental problems caused by effluent discharge, and the resulting high labour costs) still remain. Other methods such as steam explosion, application of enzyme formulations along with the chelator EDTA on decorticated fibres without previous water retting have been studied (Bacci et al., 2011, 2010; Dreyer and Müssing, 2000; Dreyer et al., 2002). Hemp and flax processing methods could be adapted to stinging nettle for fibre production, paying attention to the fact that stinging nettle stalks are prone to over-retting (Bacci et al., 2010; Dreyer and Müssing, 2000). The above mentioned methods could be mixed and, depending on their sequence and the degree of retting, the fibre quality could be defined (Bacci et al., 2011; Table 4). Table 4. Chemical composition (cellulose, hemicelluloses and lignin), morphological (diameter and length) and mechanical (tensile strength and elongation) properties of nettle fibres extracted with different methods: decortication (D), chemical retting (CR), water retting (WR), microbiological retting (MR), enzymatic treatment (ET), chelating agents (CA) (modified from Bacci et al., 2011). Treatment D CR WR WR + D D + WR MR D + MR ET ET + CA Cellulose (%) 65 81 78 85 83 78–84 75–85 80–82 81–83 Hemicellulose (%) 5 6 9 6 13 9–10 5–7 11–12 11–12 Lignin (%) 3 2 3 4 2 2–5 3–4 2–3 2–3 Diameter (μm) 23–47 23–37 37–41 29–43 40–46 24–31 16–40 30–40 25–35 Length (mm) 25–58 38–62 41–49 35–55 38–58 41–55 33–60 42–52 42–51 Tensile strength (cN tex−1) 70–182 38–81 8–94 23–71 41–83 33–65 7–98 21–72 32–76 Elongation (%) 2–3 4–7 2–4 1–2 1–3 2–4 0–2 3–6 3–6 An adequate degree of separation between fibres and shives was obtained by mechanical scutching applied on stalks stored for one year, probably resulting from natural retting processes occurring during the storage (Bacci et al., 2011). The resulting tensile modulus depended directly on environmental relative humidity and on the natural bacteria and fungi formed (Davies and Bruce, 1998). For all these characteristics, stinging nettle is currently used in the production of a silky fabric known as ‘ramic’. Rope, paper and composites are also produced from the fibre of this plant (Bisht et al., 2012). The ‘waste’ material produced during fibre extraction should not be disregarded. A significant amount of shive is produced as a by-product of decortication which has the potential to be utilized as horse bedding or biochar. This coarse material could also open a market for cattle and poultry bedding (Harwood and Edom, 2012). 4.2. Use of stinging nettle in the food and feed sector Stinging nettle has been used for centuries as a leafy vegetable for salads, pies, soups and decocted tea (Bisht et al., 2012; Guil-Guerrero et al., 2003; Orčić et al., 2014). U. dioica L. has a high nutritive value, containing several beneficial compounds such as: vitamins A, D and C, proteins, minerals calcium, iron, potassium, manganese, choline, amines, anti-oxidant chlorophyll, and 5-hydroxytryptophan (Bisht et al., 2012; Upton, 2013). The carotenoids are another important nutrient group present in stinging nettle. Nine carotenoids were identified in the leaves, with the major one being β-carotene; the total amount of carotenoids from fresh leaves has been reported as 29.6 mg 100−1 g dry weight (Guil-Guerrero et al., 2003; Upton, 2013). Furthermore, stinging nettle contains essential fatty acids which are important sources of energy for humans and animals because, when metabolized, they yield large quantities of ATP. As found by Rafajlovska et al. (2001), Stinging nettle plant extracts contained 6.8% palmitic, 1.1% stearic, 3.6% oleic, 20.2% linoleic, and 12.4% linolenic acid, respectively. According to Guil-Guerrero et al. (2003), young leaves have higher nutritional value than seeds containing higher quantities of n-3 fatty acids and carotenoids. Since stinging nettle grows in the wild and is palatable to animals, it could be a part of their food, providing them all aforementioned nutrients. In periods of forage shortage, e.g. during the two World Wars, stinging nettle was used fresh, dried, milled or as silage for feeding poultry, cattle, horses and pigs (Vogl and Hartl, 2003). The use of stinging nettle as a forage crop has been investigated with promising results. As reported by (Bisht et al., 2012), it is possible to increase vitamin intake by 60–70% and protein intake by 15–20% by adding stinging nettle into poultry feed, while the green feed requirements can be reduced by 30%. Moreover, stinging nettle supplementation of layer diets resulted in enhancing yolk yellowness and, therefore, this plant species could be regarded as a suitable natural and economic option for yellow saturation of the egg yolk, without risking unfavourable side-effects (Loetscher et al., 2013). When stinging nettles replaced ryegrass silage in the diet of dairy cows, rumen health was promoted by stabilizing the rumen environment with respect to pH, even though rumination was reduced; however, milk production was not affected (Humphries and Reynolds, 2010). 4.3. Use of stinging nettle in the cosmetic/pharmaceutical sector Since ancient times, stinging nettle (both leaf and root) has been used for treating a wide range of ailments, mainly related to its urtication property (Alford, 2008). The stinging action of the hairs in leaves is due to the compounds histamine, serotonin, acetylcholine and leukotriene (Czarnetzki et al., 1990; Upton, 2013). Recent studies have justified the numerous medicinal uses of stinging nettle and have attributed its biological activity to distinctive constituents. In fact, the presence of flavonoids (Farag et al., 2013; Upton, 2013), phenylpropanoids (Bucar et al., 2006; Farag et al., 2013; Pinelli et al., 2008), fatty acids (Guil-Guerrero et al., 2003; Rafajlovska et al., 2001) and carotenoids (Guil-Guerrero et al., 2003; Upton, 2013) has been demonstrated in leaves and roots, as well in water extracts of aerial plant parts. Some studies applied mutagenesis with gamma irradiation to in vitro stinging nettle cultures, increasing their antioxidant content. Mutants exhibited great differences in phenolic content: these increased mainly in two principal classes, hydroxycinnamic acid derivatives and flavonoids, ranging from 0.7 to 8.4 mg g−1 of fresh weight (data not published). The aqueous and alcoholic extracts have been used for hundreds of years for the treatment of anaemia (Leporatti and Corradi, 2001; Pinelli et al., 2008), rheumatism (Chrubasik et al., 1997; Jarić et al., 2007), gout and eczema (Orčić et al., 2014; Pinelli et al., 2008), and treatment of urinary, bladder and kidney problems (Guarrera and Savo, 2013; Orčić et al., 2014). Beneficial effects have also been reported on inflammation, hypoglycaemia, hypotension, benign prostatic hyperplasia, cardiovascular problems, arthritis, and allergic rhinitis (Bisht et al., 2012; Guarrera and Savo, 2013; Pinelli et al., 2008; Roschek et al., 2009; Upton, 2013). Furthermore, stinging nettle exhibits antioxidant, antimicrobial, antifungal, antiviral, and antiulcer activity (Gülçin et al., 2004; Orčić et al., 2014; Upton, 2013). The use of stinging nettle in the cosmetic sector is well known. Several commercial products are currently present in the market, e.g. soaps, shampoo to improve the condition of hair and to control dandruff, and lotions to clean skin. The main constituents giving stinging nettle its cosmetic attributes are the anti-oxidant chlorophyll; proteins; vitamins A, D and C; the minerals calcium and potassium; iron; choline; amines; and 5-hydroxytryptophan (Bisht et al., 2012; Upton, 2013; Vogl and Hartl, 2003). 5. Critical aspects and barriers to scaling-up stinging nettle production As shown in the previous section, stinging nettle is a species able to provide several products, but is still of marginal importance today, in particular on the industrial side. Even currently, when interest in manmade and other natural fibres are increasing, the products made from stinging nettle fibres are marketable mainly due to curiosity rather than being a large-scale industrial brand. There is a great demand by the Italian textile industry, but no raw material for the spinning industry is available at present. Imports from China fall far from the quality requirements of the European textile manufacturers because of the huge amounts of pesticides used for cultivation and due to issues of labour exploitation (Ruckenbauer, 2004). Several reasons may explain the lack of large-scale industrial processes for stinging nettle. In the field phase, establishment costs are currently high when compared with the corresponding cost of other bast fibre crops. Although vegetative propagation with cuttings is simple, the process is very labour intensive, particularly in the case of large-scale production. Seed propagation should form the primary basis for further research in order to reduce establishment costs. For large-scale production, mechanization is a basic requirement, in particular for harvest. The possibility of using hemp or flax mechanization chains for stinging nettle has not yet been well investigated, especially important if multipurpose exploitation of biomass is demanded. Although stinging nettle plants produce good fibre, the commercial extraction of fibres fine enough for high quality manufacture has not yet been achieved. Enzymatic retting has been attempted but further development is needed. When enzyme is used in too high concentrations or for too long, the cellulose begins to dissolve and the fibre strength is lowered. Important factors to be considered in this process are therefore the enzyme concentration, temperature, pH value and duration of treatment. Retting in water has proven successful but is not always practical (Bacci et al., 2011; Dreyer and Müssing, 2000; Vogl and Hartl, 2003). 6. Conclusion Stinging nettle could represent a valuable biomass source for several natural products, with most promising application in the food/feed, medicinal, cosmetic and fibre sectors. The presence of several active compounds in stinging nettle has been demonstrated, giving scientific justification to traditional folk uses of the plant. If technical problems related to harvest mechanization, optimal crop nutrient and water requirements, and post-processing (e.g. extraction, spinning and weaving for fibre) are solved, the stinging nettle has great potential as a widely distributed biomass source for increasing the income of rural communities. Slope and marginal areas could be exploited with stinging nettle cultivation, focusing on alternative natural products for food, medicine and cosmetics. On the other hand, more specialized and mechanized agricultural areas may use more intensive cultivation for fibre production for both long fibre and pulping, which can represent a useful alternative or complementary products to other natural fibres such as hemp, linen and cotton. The definition of a production chain able to exploit as much as possible all the biomass is a prerequisite for increasing income and boosting farmer adoption. There is a lack of comparative studies on stinging nettle cultivation among different areas and climatic zones since most research and results reported so far was of a local character. This review shows that valuable uses are possible and that stinging nettle represents feasible new environmentally and economically sustainable opportunities for rural areas, which should be the stimulus to boost further studies on the domestication of this plant. 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