What makes mulberry leaves so special that the fiber derived from them is of highest quality among all natural fibers?
It is still not fully investigated why the silkworms prefer mulberry. However, it is certain that the protein substances produced in silkworm bodies in 70% come from mulberry leaves. Previously, some authors (Memon et al. 2010, Radojkovič et al. 2012, Kim et. al. 2014, Flaczyk et al. 2013) studied the quality, nutritional potentials and chemical composition of different Morus species. Morus alba leaves contain proteins, carbohydrates, calcium, iron, ascorbic acid, β -carotene, thiamine, folic acid and vitamin D and variety of phenolic compounds.
Phenolic compounds such as flavonoids, phenolic acids, anthocyanins, stilbenes, tannins, lignans, and lignins, are important for normal plant growth and development as well as a defense against infection and injury. These compounds are commonly found in plants, and they have been reported to have multiple biological effects, including antioxidant activity (Urbanek Krajnc et al. 2014).
Phenol compounds, as antioxidants, phenolics represent a more important multi-tiered system and may affect in various ways: direct reaction with free radicals, scavenging of free radicals, growing dismutation of free radicals to the compounds with much lower reactivity, chelatation of pro-oxidant metals (mainly iron), delaying or strengthening many enzymes. Phenolics can act as antioxidants by donating electrons to guaiacol-type (class III) peroxidases that have been implicated, for example, in lignification and pathogen defence. The phenoxyl radicals that are formed can then be reduced by the ubiquitous cell antioxidant, ascorbate (Sakihama et al. 2002). However, phenoxyl radicals may also exhibit pro-oxidant activity under conditions that prolong the radical lifetime (e.g. microbial attack) and catalyse cellular DNA degradation in the presence of transitive metals, such as iron (Bhat et al. 2007). Most of the phenolics contribute to resistance indirectly. For instance, some low molecular weight phenolic compounds may function as precursors for other defensive compounds (e.g. lignin; Bonello and Blodgett 2003), or they may confer resistance as a group, rather than as individual compounds (Wallis et al. 2008). Catechin is a building block of condensed tannins for which antimicrobial activity via protein precipitation and iron depletion has been suggested (Witzell and Martin 2008).
The main phenolic compounds in mulberry leaves were identified as chlorogenic, gallic, protocatechuic, p-hydroxybenzoic, vanillic, caffeic, p-coumaric, ferulic, and sinapic acids. The flavonols fraction contained rutin, quercetin 3-β-D-glucoside, and kaempferol 3-β-D- glucopyranoside (Flaczyk et al. 2013). Among the flavonoids the leaves of white mulberry contain higher amount of quercetin which is responsible for reduction of oxidation process in vivo and in vitro (Enkhmaa et al., 2005; Katsube et al., 2006; Chen and Li, 2007; Butt et al., 2008; Iqbal et al., 2012; Memon et al. 2010). Furthermore, M. alba leaves contains oxyresveratrol and 5,7-dihydroxycoumarin 7-methyl ether which scavenge superoxide and have antioxidant potential (Oh et al., 2002). Mulberroside A is a major stilbene glycoside of M. alba and it showed inhibitory effects against lipid peroxidation (Chung et al., 2003, Zafaret. al. 2013, Memon et al. 2010). Similarly to M. alba, the dominant phenolic components in the M. nigra samples were rutin and chlorogenic acid. Along with these compounds, black mulberry has been found to contain carotenoids (Radojkovič et al. 2012).
Plant phenolics are mainly involved in the defence mechanisms of plants against fungal pathogens and insects. Most plants respond to insect feeding by increasing the synthesis of particular phenolic compounds. Phenolics in the plant are key factors in deterrence and it is the accumulation of phenols in particular parts of the plant which represent a feeding barrier. Many flavonoids (quercetin, kaempferol, isorhamnetin) and isoflavonoids can act as that are thought to serve as antifeedants to phytophagous insects at relatively low concentrations.
In addition to phenolics, the research priority of this project is to determine the efficiency of the low molecular weight thiols (cysteine, glutathione) in mulberry leaves and their significance in ascorbate–glutathione cycle, phenolic metabolism and nutrition of silkworms. In our previous studies on antioxidant system in trees (Urbanek Krajnc et al. 2014), phenolic metabolism has been recognised to integrate ascorbate–glutathione system as a redox hub in defence against pathogens and pests.
The sulphur (S) amino acids are of great significance in the structure, function of proteins and enzymes. One of the most important S containing peptides is glutathione (GSH), which has many important functions in plants and animals including protection against oxidative stress. The ascorbate-glutathione-cycle is considered to be the main pathway for ROS removal. GSH further provides protection against heavy metals, herbicides, and xenobiotics. It is a key regulator of redox signaling and buffering and plays key roles in plant defense through the activation of defense genes and is correlated with the development of sulphur-induced resistance (SIR) (Noctor & Foyer 2011).
Mulberry leaves play a crucal role in the cultivation of silkworms. Recently, Thirumalaisamy et al. (2009) has screened biochemical content of total sugars, protein and phenols in leaves of six mulberry varieties to find out nutritively richer one. Furthermore, silkworms feeded with nutritively richer variety were taken for different biochemical and bioassay, such as protein estimation in larval, pupal haemolymph and silkgland as well as enzymatic (amylase, protease) analysis of different (II to V) instars larvae. Likewise fibroin sericin ratios in cocoon were also done. The all the above parameters of silkworm feeded with nutritively richer variety was found to be high and silk productivity was also significant higher.
As mulberry can grow in a wide range of climatic, topographical, and soil conditions, this may affect the chemical composition and nutritional status of plant. The nutritional value of mulberry leaves grown in Hungary and Slovenia has not been a subject of previous studies. Therefore, the aim of the proposed project will be to investigate the antioxidant properties (phenolics, ascorbate, glutathione) in leaves of different local mulberry varieties (Morus alba L. (white mulberry) and Morus nigra L. (black mulberry)).
The obtained results may be useful in the evaluation of dietary for silk worm as well as fruits as new dietary supplements and food products.