Indigenous dye plants of the Kingdom of Eswatini, traditional uses and new prospects

Eswatini is rich with a diverse assortment of many indigenous plant species with great potential of economic importance. Natural dye producing plants are among those plants which remain in the wild unexploited, while synthetic dyes used in the textile industries are imported. The aim of this study was to investigate indigenous natural dye producing plants found in Eswatini with potential economic use in the handicraft and textile industries. Plant species were identified with potential to produce natural dyes of economic importance. Important species identified include Berchemia discolor, Syzigium cordatum, Kigelia africanum and Sclerocarya birrea. The species, local names, plant parts used and the color produced are presented. Dye extraction, characterization techniques and application technologies are discussed. Coincidentally many dye plants were found to have medicinal properties. This paper attempts to address the environmental concerns possibly arising from exploitation of dye plant species and presenting some recommendations on new prospects.


INTRODUCTION
Traditionally certain plants in Eswatini have been used as source of colourants to dye mats, hats, ropes, baskets and other home made handicrafts for a long time.
Recently natural dyes of plant origin have gained economic advantage over synthetic dyes because of their non-toxic, non-carcinogenic and biodegradable nature (Bhuyan and Saikia, 2008;Samanta and Agarwal, 2009;Wanyama et al., 2011). There is recognition internationally that commercialization of non-timber forest products is increasing (Arnold, 1995: Cunningham, 2001. Taxonomists and ecologists have been concerned with the impact of non-timber forest products (NTFP) extraction on forest structure, composition and sustainability (Daniels et al., 1996;Murali et al., 1996;Cunningham, 2001). Economists are concerned with benefit/cost analysis of exploitation of forest species. Some of the indigenous species have been used for income, social and food security especially by poor households (Chambers, 1983;Campbell, 1987;Manyatsi et al., 2010;Zwane et al., 2011). Indigenous plants have historically been associated with cultural values, traditions and even religious values. Anthropologists and sociologists have been concerned with tenure, traditions, institutions and rules governing the use of indigenous plant resources (Mukamuri et al., 1998;Manyatsi et al., 2010). Previous studies indicated the institutions and traditional rules have weakened over years (Nhira and Fortman, 1992;Braedt and Standa-Gunda, 1998;Manyatsi et al., 2010).
Dyes from indigenous plants apart from being mostly non-toxic they are bio-degradable and thus environmentally friendly (Mkhonta et al., 2014). There has been promotion on use of natural dye plants species in recent years. Work has been done pertaining to dye color measurements, analysis and structure determination (Deo and Desai, 1999;Bhuyan et al., 2002;Debajit and Tiwari, 2005). Colourimetric and spectroscopic methods have been employed to successfully characterize dyes from plants (Kenneth, 1973;Bhuyan et al., 2002;Young-Hee and Han-Do, 2004). More research has to be done to increase cultivation and awareness of crucial importance of natural dyes, which is why this research was undertaken.
The country is currently facing unprecedented economic challenges. Diversification of the country's industrial base is urgently needed. The economic use of indigenous dye plants for local purposes as well as export of value added products containing indigenous plant dyes or dyes themselves will help alleviate current economic challenges.

METHODOLOGY
Information was obtained from literature review and informal surveys in the various agro-ecological zones of the country.

Site description
Eswatini formerly Swaziland is located in Southern Africa. The country has four agro-ecological zones with distinct topography, geology, soils, vegetation and climatic patterns (Figure 1). In the west is the Highveld, which is mountainous and has a vegetation of mainly commercial forests with the bulk of the land used for subsistence farming (Thompson, 2017;Government of Swaziland, 2007). It experiences a temperature range of 4.5 to 33°C (Table 1) (Dlamini and Lupupa, 1995). It has rivers, waterfalls and gorges with protected natural areas including Malolotsha, Hawane and Phophonyane (Government of Swaziland, 2005). The Middleveld is characterized with temperatures ranging from 2.5 to 37.2°C (Dlamini and Lupupa, 1995). This region has fertile valleys which favour intensive farming. It has the most diversely cultivated and heavily populated area in the country (Thompson, 2017). Protected nature reserve areas include Mantenga and Milwane (Government of Swaziland, 2005). To the east, there is the Lowveld with the largest area of 40% of the country and is drought prone. There is the Western Lowveld which is underlain by sandstone/ claystone and the Eastern Lowveld which is underlain by basalt (Dlamini and Lupupa, 1995). It has a vegetation of shrubs, and mean temperature ranges from 2.6 to 41.8°C with the bulk of commercial farms growing crops under irrigation. The nature reserves in the area are: Mlawula, Hlane, Shewula, Mbuluzi, Simunye and Nisela game reserves (Thompson, 2017;Government of Swaziland, 2007). The fourth region is the escarpment called Lubombo plateau with an altitude of 600 m above sea level and climatic conditions similar to the Middleveld. Only one eighth of the Lubombo plateau land is arable and the rest is for animal grazing (Dlamini and Lupupa, 1995).

FINDINGS
Several dye plants are found in the four agro-ecological zones of the Kingdom of Eswatini (Table 2). It must be Masarirambi et al. 9  emphasized that it is in the natural reserve areas where there is abundancy of dye plants in their natural habitat.

Dye extraction
Natural dye extraction conditions, temperature, M:L ratio, time, extraction solvent and pH are critical in maximizing the dye yield and dye strength in all the methods that are employed for extraction. One of the limitations of natural dyes is the use of substantial dye source material to produce a small amount of dye, hence the tinctorial strength of the dye source ought to be considered as trees take longer to replenish. Consequently the use of existing timber byproduct as dyestuff preserves the environmental compared to harvesting trees or tree components for dye material (Doty et al., 2016). The optimum extraction conditions are determined by varying the extraction parameters (Samanta and Konar, 2011). Marigold natural dye colour strength values increase with the increase in temperature from room temperature to boiling point, an increase in extraction time and an increase in the M:L ratio. This is attributed to cell wall ruptures as a result of the high temperature and ultimately more dye release (Farooq et al., 2013;Ahmadi and Houjeghan, 2017). The extraction yields of quercetin increase with an increase in pH due to the effective swelling of the plant material which helps to increase the surface area for solute-solvent contact (Kumar et al., 2014).  Innovative dye extraction methods are being explored and developed to address the substantial use of energy and water in light of global water scarcity which is expected to increase with the rise of global warming. To achieve more sustainable patterns of natural dye extraction requires responsible and efficient use of natural resources, hence the need for more efficient extraction techniques and efficient management of the waste water as articulated by the sustainable development goals. Currently the quantity of plant material required to replace synthetic dyes is high (Kechi et al., 2013). In addition to the commonly used aqueous method, the alkali or acid extraction, microwave and ultrasonic assisted extraction, fermentation, enzymatic extraction, alcoholic or solvent extraction and super critical fluid extraction are used (Grover and Patni, 2011;Prabhu and Bhute, 2012). These emerging approaches and advances are assisting towards the standardization of the extractions processes therefore counteracting the lack of precise technical knowledge on extraction.
The choice of the method of extraction is heavily dependent on the dye plant source as the dye component may be derived from roots, leaf, bark, trunk fruits, or flowers of the selected indigenous plants ( Anjali and Deepali, 2012). The barks of Ficus religiosa L. easily discharge colour in aqueous extraction (Saravanan and Chandramohan, 2011), natural dye is efficiently extracted from Madhuca longfolia leaves at 95°C at pH 10 for 60 min in aqueous extraction (Swamy et al., 2015) gamma ray treatment of 15kGy is an effective absorbed dose for dye extraction from red calico leaves, The natural dye source is dried to reduce the moisture content hence increase shelf life and ground for more surface area during extraction. The challenge with the different extraction methods often employed is to achieve absolute selectivity of the dye component from the extract hence to address this concern solid phase extraction is employed (Kumar et al., 2014).

Characterization techniques for natural dyes
Natural dye extracts usually contain a number of coloured compounds. The characterization of natural dyes involves a combination of techniques, with separation into respective coloured moites done first followed by the characterization of the separated compounds. There are a number of techniques that can be used to identify and characterize natural dyes, and these include chromatographic analysis, ultraviolet visible spectroscopic (Uv-Vis) analysis, Fourier transform infrared analysis (FTIR), differential scanning calorimetry (DSC), X-ray powder diffraction (XRD) and X-ray fluorescence (XRF) (Samanta and Agarwal, 2009;Vankar, 2017). To better understand new dye extracts these techniques are normally used in combination rather than separately as they characterize different aspects of the dye extract (Alemayehu and Teklemariam, 2014).
Thin layer chromatography (TLC) is one of the techniques that has found wide spread use in natural dye extract characterization. This has been used to isolate and identify various coloured compounds found in natural dye extracts. Singh et al. (2017) characterized a dye extract from Butea monosperma (Lam) Kuntze flower petals (palash) using a combination of TLC, FTIR and UV Visible spectroscopic analysis and identified the palash flower dye extract to contain chalcone, butein, orange yellow needles and flavanone, butein and its glycoside butrin. In another study, a combination of spectroscopic and chromatographic analysis was used to characterize dye extracts from a natural mineral dye and TLC showed presence of four different coloured compounds containing aromatic groups as well as Ar-NH2, -CONH2, C=C, C-C and metal-carbon chelate rings, which were confirmed through FTIR and UV Visible analysis (Adebayo et al., 2007).
High performance liquid chromatography (HPLC) is another technique that is used to analyze natural dye extracts (Karapanagiotis et al., 2006). Silva et al. (2018) characterized a dye extract from eucalyptus leaves using HPLC and mass spectrometry. The extract was found to contain a mixture of fourteen characteristic phenolic Masarirambi et al. 11 compounds. Analysis of dye extracts using UV-Vis spectroscopic analysis results in the identification of the most prevalent dye compounds in a particular dye extract (Kusumawati et al., 2017). Depending on the region that the dye absorbs light important information like the light fastness properties of the dye can also be established. Other important characteristics of dyes like solubility and mordanting power of dyes which are influenced by the chemical constituents as well as functional groups in the coloured compounds can be determined through FTIR analysis, as this is capable of identifying functional groups present in dyes (Samanta and Agarwal, 2009). Sanjay et al. (2018) in their study were able to confirm through FTIR the presence of anthocyanin, carotene and chlorophyll molecules in the dyes extracted from leaves of Peltophorum pterocarpum and Acalypha amentacea for use as sensitizers for ZnO based dye sensitized solar cells. UV-Visible analysis of both dye extracts further showed intense absorption peaks in the UV-Visible region showing the presence of anthocyanin, carotene and chlorophyll.

Applications of indigenous dye plants
Plants contain colourants used in a number of applications including textile dying (Sava, 2007). In the past, plant dyes were used mainly in the textile and cosmetic industry (Yusuf et al., 2017). Research and Development (R and D) has seen the expansion of the use of plant dyes to other industries like the medical, food, leather and pharmaceuticals (Cardon, 2010;Grover and Patni, 2011;Yusuf et al., 2017;Kunene and Masairambi, 2018). However, the development of organic chemistry in industrialised countries brought about the use of synthetic dyes and pigments obtained from fossil resources: coal-tar and oil which are not only cheaper but have simpler application procedures (Cardon, 2010) than natural dyes. Although other parts of the world retained their knowledge of colouring properties of their indigenous plants, they slowly lost it as they adopted the western lifestyle over the years (Yusuf et al., 2017) which include the use of synthetic dyes. These synthetic dyes are imported into developing countries like the Kingdom of Eswatini for use mainly in the textile industry.
The Sustainable Development Goals (SDGs) on nature conservation advocate for the use of eco-friendly technologies. The increase in the cost of fossil resources, its scarcity as well as the public's awareness on the effects of synthetic materials on the environment, aquatic and ecosystem, has sparked an interest to revive the use of safe natural colorants from indigenous plants as alternative to synthetic dyes (Cardon, 2010;Yusuf et al., 2017). The technological advancement around the world calls for combining the traditional, current and new technologies and techniques in the applications of indigenous dye plants.

Textile applications
The most common use of indigenous dye plants is the extraction of colourants used in textiles. Natural dyes on their own have limitations which include inadequate degree of fixation, low colour fastness and narrow shade range and therefore require the addition of mordants (Yusuf et al., 2017). A good mordant will produce the desired colour yield at minimal cost without altering the fabric properties, producing toxic substances during processing or carcinogenic substances during use (Arora et al., 2017). Effectiveness of the dyeing process and colour shade are influenced by the fibre content of the dyed fabric, mordanting technique and reaction between the dye, the mordant and substrate (Samanta and Argarwal, 2009;Arora et al., 2017). Currently aluminium and iron mordants are the other preferred mordants over metallic ones that result in toxic metal ion residue in waste water which negatively impacts the environment and also cause allergenic reactions to humans (Yusuf et al., 2017). With the shift from the use of environmentally harmful synthetic substances in dyeing, studies on alternative new plant extracts recommend Aloe vera and lemon (Citrus limon) as comparable alternatives to the conventional mordants (Yusuf et al., 2017;Zubairu and Msheli, 2015). In addition to textile dyeing, plant colorants may also be used to incorporate functional properties in textiles. These include introducing antimicrobial finishes, ultraviolet protection, deodorising finishes and moth and insect repellent finishes (Yusuf et al., 2017).
Textiles retain oxygen, moisture and nutrition which make them susceptible to microbial and dust mite activity. Transmission of micro-organism may occur when the bacteria found in human skin cells are deposited on textiles. Under these conditions together with human perspiration, microorganism may produce odour. This then necessitates the need for anti-microbial and deodorising finishes especially on underwear to inhibit microbial activity and counteract bad odour respectively (Yusuf et al., 2017). Some natural dyes may also have antimicrobial properties which may also be useful in protecting patients and hospital employees from cross transmission of microbes. Natural anti-microbial and deodorizing finishes are preferred over synthetic ones that produce hazardous substances into the environment.
Some natural colorants from plants introduce ultraviolet protection properties on textiles e.g. wool treated with extracts from eucalyptus leaves, silk dyed with natural yellow dyes and indigo dyes used on a number of fabrics. Protein fibers like silk and wool are prone to moth and insect attack. Common chemical moth repellents are harmful to the environment (Yusuf et al., 2017) there is therefore a need to explore indigenous plants for natural colorants with the same effect.

Other prospects of indigenous dye plants
The applications of indigenous dye plants is not only limited to textiles, other uses include natural food colourants that may replace synthetic food colourings due to the many health benefits associated with the use of natural substances and the increased consumer awareness on green consumption (Yusuf et al., 2017).
Other future prospects of indigenous dye plants application that may be explored in the Kingdom of Eswatini include their use in leather, cosmetic, pharmaceutical and paint industries.

Indigenous dye plants have medicinal properties
Dyes are secondary products of plant metabolism, by their nature they are often found to have medicinal properties. The same plant part used to extract dyes is usually the same part used to extract medicinal substance. Indigenous dye plants with medicinal properties are shown in Table 3. Work on indigenous medicinal plants has previously been reported (van Wyk et al., 1997;Adeniji et al., 2000;van Wyk, 2002;Masarirambi et al., 2011;Kunene and Masarirambi, 2018;Kunene et al., 2018). There is need to continue work on indigenous dye plants with medicinal properties to keep pace with modern day technologies.

CONCLUSION AND RECOMMENDATIONS
Indigenous dye plants of the Kingdom of Eswatini and the region have been reviewed. The common names, scientific names, siSwati names, part used colours obtained and conservation, status have been documented. The active ingredients of indigenous plant dye extracts are secondary metabolites most of which apparently have medicinal properties. The indigenous knowledge (IK) pertaining to economically useful yet endangered plant species has been passed on from one generation to another using oral tradition. Oral tradition changes from one generation to another and hence the need for documentation as we go into the future while striving to attain SDGs for the people and the planet. Conservation of biodiversity is crucial for the planet and the people, especially in this era of climate change, severe weather events and uncertainties. It is therefore strongly recommended going forward that all stakeholders do work on indigenous dye plants using modern technologies in order to reach the next upper level of commercialization and to subsequently document that work in leading scientific fora.
It may be concluded that it is important to document indigenous dye species and to raise awareness of local people, governments and industrial entrepreneurs about the species. There is need to protect the wealth so that it  can be utilized by the next generations.