In mid-1600 s, the Reunion Island was first settled by French colonist. They bring slaves from Madagascar, Africa India, and Asia to work first in cane and coffee plantation. Slaves brought with them some medicinal and other plants traditionally used in their country. Progressively, many new plant species were introduced by people from all walks of life. People bring some traditional knowledge and know-how notably in the field of dyeing textile from Madagascar [23, 24], and some dye-producing plants were probably introduced at that time. The evolution of indigenous species, far from their region of origin, led to the emergence of several endemic species [25]. In this study, the assessment of the Reunion Island’s plant biodiversity through the “PLANTIN” project (co-funded by European Regional Development Fund) allowed us to establish the first ethnobotanical inventory of plants growing on Reunion Island which may have promising properties as a new alternative source of dyes or pigments for the industries.
Local people from Reunion Island has accumulated traditional knowledge of dyeing plants and their uses over the years. This approach had been discontinued in Reunion, in comparison with Madagascar, where a rich ancestral knowledge of dye plants has been described and transmitted from generation to generation [6, 14, 24]. The oldest trace of the uses of dye plants in Reunion Island seems to come from Indigofera plantations in the north of the island, which was abandoned a long time ago. The uses of Curcuma longa locally called “saffran péï” have been cited several times during the survey. Considering that this plant is very present in the Indian culture, it is not surprising to see the extent of this plant in different uses in Reunion Island including natural dyeing as his history is attached in great part to the Indian culture. The third most famous dye plants in the island is the famous endemic species Weinmannia tinctoria Sm., locally called “Tan rouge” (red “tan”) [26]. While bark of this last specie was cited several times in the literature review and during the ethnobotanical surveys as used traditionally for tanning application in Reunion, no scientific data are really available on its utilization for tanning applications over the world. Furthermore, there is evidence that this natural dyeing with plant-based textile dyes has not been abandoned uniformly in Reunion Island, particularly in recent years. Craftsmen and small companies working in textiles have tried to reintroduce dye plants from some alien-invasive species in Reunion Island into their dyeing process (i.e., for the formulation of traditional dye baths in hot water). For instance, “WHOLE,” “Kouleur local,” and “Nuance de Couleurs” are examples of companies that are recently working on natural dyeing from some Reunion Island’s dyestuffs of plant origin. As the reuse of alternative sources of colorants including dye plants has grown considerably these last decades, this new trend can contribute to the conservation of some species as the example of Weinmannia tincoria. Actually, W. tinctoria was famous for its dyeing properties and known by most of the local population. Due to its non-use, it has been replaced by other species and has been considered as an endangered species lastly by the International Union for Conservation of Nature (IUCN). This a great example of the loss of some traditional knowledges in Reunion Island and the importance of its conservation as it can also preserve some species to not disappear. If it will be used as dyeing sources or for further uses in the following years, it’s sure that it will be planted again by local population or even in industrial scale.
From the literature review and ethnobotanical surveys focused on uses of plants traditionally used in dyeing and conducted from local elderly informants between January 2021 to November 2022, a list of 194 plant species (72 botanical families, and 157 plant genera) with diverse botanical status (endemic, native, introduced or alien-invasive species) have been inventoried. The entire list of 194 plant species can be found as Additional file 1: Table S1. It demonstrated the potential of these plant species as new sources of water-soluble dyes or pigments, including the endemicity, the IUCN (International Union for Conservation of Nature) and/or the protected status of the species in Reunion Island, the accessibility, the cultivability, the plant organs used for the extraction process, the colors that may be extracted, the scientific knowledge, the industrial interests and current uses of these plant species, amongst others. Dye plants inventoried here include 34 endemic species, 22 native and cryptogenic species, and 138 introduced species (including 37 alien-invasive species) which may have promising properties as new alternative crops for producing bioactive dyes and pigments for the industries.
The uses of endemic plant species in Reunion Island potentially rich in dyes or pigmentsA total of 13 endemic plant species to Mascarenes, 12 plant species strict endemic species to Reunion Island have been mentioned as having potential traditional uses as sources of dyestuff materials. The botanical information and the coloring properties of all inventoried endemic plant species in Reunion Island potentially rich in dyes or pigments are described in Table 2. Some of them were mentioned in the literature as plants formerly used in textile dyeing, e.g., tanins extracted from wood of Indigofera ammoxylum [27] and Foetidia mauritiana [28], tanins from barks of Weinmannia tinctorial [26, 29, 30], Mimusops balata [26], and Terminalia bentzoe subsp. bentzoe [26, 30] bluish-brown colors from fruits of Bertiera borbonica and Bertiera rufa [21], and pigments from stem and leaves of Psychotria borbonica [22, 26, 30].
Table 2 Endemic plant species in La Réunion Island potentially rich in dyes or pigmentsOn the other hand, the traditional uses of some endemic plants as textile dyestuffs have only been identified from the ethnobotanical survey conducted here, such as the uses of bark of Ochrosia borbonica (locally called “bois jaune”—yellow wood), the barks and stems of Erythroxylum laurifolium, the flowers of Hypericum lanceolatum, the leaves of Latania lontaroides, the bark of Monimia ovalifolia, and Sophora denudata, amongst others.
Some of these endemic species are classified by the International Union for Conservation of Nature (IUCN) as Vulnerable (VU), Endangered (EN), or Critically Endangered (CR). For example, eight endemic species inventoried here are protected in Reunion Island by local environmental legislation, e.g., Indigofera ammoxylum (CR), Foetidia mauritiana (CR), Ruizia cordata (CR), Latania lontaroides (CR), Terminalia bentzoe subsp. bentzoe (CR), Aloe macra (EN), Sophora denudata (EN), and Ochrosia borbonica (VU). Interestingly, L. lontaroides and T. bentzoe subsp. Bentzoe—both classified as CR by IUCN—are two species widely available and easily cultivated in Reunion Island. Thus, they may have promising interests as local sources of natural dyes or pigments for the industries. Edible fruits of L. lontaroides contain oil seeds with very similar composition to that of the edible palm oil [31] but the coloring properties of the species have never been described in the literature. Barks of T. bentzoe subsp. bentzoe have interesting biological properties like antioxidant and cytotoxic activities [32]. The barks of this plant are rich in tannins and can be used to dye natural fibers with a black or yellow color, depending on the mordant used during natural dyeing [18, 26, 30, 33].
On the other hand, the bark of the endemic species O. borbonica has been exploited without a sustainable policy and almost led to the extinction of this species in the Mascarenes. This plant species has been used by local population for its medicinal properties against malaria and fever many years ago. Only a few trees have been recently inventoried in Reunion Island and the species are locally protected. The health benefits of O. borbonica have been proven by the most recent pharmacological research [34,35,36,37]. Thus, the bark of O. borbonica can produce a yellow dyestuff rich in bioactive alkaloids with therapeutic activities. It would be interesting to relaunch the study on public and/or private partnership reforestation projects with this promising but endangered plant, O. borbonica. It may led to the conservation of this highly-valued species rich in bioactive yellow dyes.
Furthermore, even if the species W. tinctoria is classified as endangered in some parts of the world by IUCN, this tree is not locally protected and very common in Reunion Island (in the wild and private gardens). It is used as an ornamental plant. The plantation of this species has resumed recently to support the specific honey production from Reunion Island called “miel vert” (green honey). The bark of W. tinctoria has been used for its dyeing properties to dye leather in red color a long time ago. It is a source of tannins [29]. However, very limited information is available on the coloring properties and dye composition of the dyestuffs extracted from bark of W. tinctoria, and it may become an important and interesting topic for future research on natural dyeing from tropical dye plants.
Finally, other recorded endemic species, not protected and widely available in the island (Table 2), such as fruits of Olax psittacorum (locally called “corce rouge”), Bertiera rufa, Erythroxylum laurifolium, Hypericum lanceolatum, Monimia ovalifolia, Weinmannia mauritiana, etc., also represent an interesting group of alternative crops for natural dyeing application, if they will be exploited in a sustainable way by local population.
The uses of native plant species in Reunion Island potentially rich in dyes or pigmentsAll the inventoried native and cryptogenic plant species in Reunion Island potentially rich in dyes or pigments are described in Table 3. Among the 23 species listed below, many of them are also growing in Madagascar [6, 9, 24] and inventoried as dye-producing plants, as the example of Thespesia populnea Danais fragrans [9], and Morinda citrifolia [48,49,50]. The cryptogenic species T. populnea (Malvaceae; locally called ‘Porché’) is widely and easily cultivated in Reunion Island, and should be used as an efficient alternative source of reddish natural dyestuffs [6, 51,52,53,54]. D. fragrans (locally called ‘Liane jaune’—yellow liana) and M. citrifolia (noni, or locally called ‘malaye’) are native plant species from the Rubiaceae family. The uses of roots as sources of anthraquinonoid dyes are common in this botanical family (like the case of the famous Rubia tinctorum) [55]. The species M. citrifolia is widely available and easily cultivated in Reunion Island, whereas D. fragans is a relatively rare species in the wild, but the roots of these two Rubiaceae plants could be used for natural dyeing properties (yellowish to orangish-red) [6, 49].
Table 3 Native and cryptogenic plant species in La Réunion Island potentially rich in dyes or pigmentsThe native species Leea guineensis (Vitaceae; locally called ‘Bois de sureau’) is a tree that is very common in wetlands in Reunion. Its rapid growth and strong regeneration capacities make it an invasive plant in the wild. The barks of this species are rich in flavonoids, tannins and terpenes [6], and they could produce red dyes with tinctorial property. The same applies to terminal bud and leaves of the native cultivated species Pandanus utilis (Pandanaceae; locally called ‘Vacoa’) which contain bioactive components [56] and could produce yellowish and greenish textiles dyes according the ethnobotanical data recorded here.
Furthermore, other inventoried native species (today not cultivated in Reunion, but easily accessible in wilderness area and not protected) should also be promising for natural dyeing application, such as liana of Cassytha filiformis (Lauraceae) for orangish-yellow colors; flowers and roots of Mussaenda arcuata (Rubiaceae) for orangish-red shades; barks of Pemphis acidula (Lythraceae) and Securinega durissima (Phyllanthaceae) for reddish shades; barks of Antirhea borbonica (Rubiaceae), leaves of and Phymatosorus scolopendria (Polypodiaceae) for greenish colors; or leaves and fruits of Pteridium aquilinum (Dennstaedtiaceae) for black shades. However, additional studies are necessary to confirm the pigment properties and tinctorial strength on natural fabrics of these uncharacterized tropical plants.
Introduced and alien-invasive species in Reunion Island potentially rich in dyes or pigmentsActually, Reunion Island contains several microclimates where the pedological properties and the climates vary from one area to one another. It explains that a multitude of species and botanical family of plants with well-known dyeing properties are able to acclimatize to this island. Nevertheless, unlike the history behind dye plants sectors in Madagascar, this field of natural colorants and pigments have never been really developed in Reunion island. Traces of industrial plantation and uses of Indigofera tinctoria have been quoted during the ethnobotanical survey in the north of the island. Unfortunately, it has been replaced by other crops because at that time, natural colorants didn’t have the current importance in the world market. 138 introduced species potentially rich in dyes or pigments have been listed, and most of them are cultivated in the island (see Table 4). Most relevant examples of the introduced cultivated plants rich in dyes or pigments are Curcuma longa [69, 70], Selenicereus undatus (syn. Hylocereus undatus; called ‘Pitahaya’) [71], Lawsonia inermis [72], Terminalia catappa [73, 74], Haematoxylum campechianum [75], and Casuarina equisetifolia [76, 77], amongst others. The species C. longa and S. undatus are widely cultivated and used in the island for local foods [69, 70]. Interestingly, according to the ethnobotanical data recorded here, a large number of alien-invasive plants in Reunion are potentially rich in dyes or pigments, such as Leucaena leucocephala [78], Antigonum leptopus [79], Acacia dealbata [80], Casuarina equisetifolia [77], etc.
Table 4 Examples of alien cultivated and invasive plant species in La Reunion Island potentially rich in dyes or pigmentsFirst selection and characterization of some promising dye plants in Reunion IslandBased on different criteria, like endemicity, scientific knowledge, accessibility and availability, cultivability, plant organs used for the extraction, and the industrial interests and other known applications of the species, we established a classification method to finally select the most interesting and widely available dye plant species listed above. From this first collection of Reunion Island’s dye plants, we valued endemic and native dye plants because of their value to the extension and conservation of the Island plant biodiversity, and also for their potential as a locally available resource that can lead to regional economic development. Thus, 15 endemic and 13 native species (i.e., 58% of the endemic and native species inventoried below) have been harvested from different growing areas in Reunion Island. The results of our laboratory experiments in terms of extraction yield and coloring properties (shade, solubility and stability) of the colors that have been extracted from each dye plant harvested, as ranked by their ‘dye score,’ are described in Table 5. Some colors are more interesting than others in terms of industrial dyeing applications, thus we valued rare/special colors (e.g., blue, magenta, pink, cyan, mauve, black…) as well as yellow, orange and red shades which are sought in the dyeing industries to replace synthetic azo dyes. Moreover, the colors extraction yield from the plant organs, and the capacity to obtain stable water-soluble dyes extracted easily and efficiently using eco-extraction method at low temperature with water or aqueous ethanolic solution as solvents are also two main factors contributing to the success of the industrial use of the considered plant-based dyestuffs.
Table 5 Colors obtained from endemic and native dye plants in Reunion island as ranked by their dye scoreThe species Thespesia populnea sets itself apart by its high dye score of 60 (from a scale of 1 to 100). This is the highest dye score obtained among all the endemic and native plant species inventoried in this study (Table 5). First, a value of 9 was attributed to the endemicity index (criteria C1*C2; Tab.1) considering that T. populnea is a cryptogenic species in Reunion and few information is available on the coloring properties of the flowers and fruits of this species [51, 54, 68]. However, only little information is available on the dyeing properties of the barks analyzed here, and the nature of the pigments from the red barks is not established. The cultivability index (criteria C3*C4; Tab.1) was scored at 100 because T. populnea is easily cultivated in the island, and the industrial extrapolation index (criteria C5*C6; Tab.1) presents the value of 49 because the barks are used to extract the dyes, and this plant species has other industrial interests in Reunion island (e.g., medicinal uses). Then, the coloring strength index, based on the criteria C7 (color and stability of dyes and pigments extracted) and C8 (yield and difficulty of color extraction), is the highest among all the plants investigated here. Indeed, T. populnea is characterized by the reddish bloody color of its bark extract (with the highest red a*-value of + 55.1 in the CIELab system among all the color extracts analyzed) with a promising extraction yield of 12.8 kg per 100 kg of barks (dry matter; dm.) obtained after a solid/liquid eco-extraction using ethanol and water mixture (70:30, v/v), and a high pH and temperature stability of the extract containing reddish water-soluble dyes (Table 5).
Red color is one the most used color in the industries. Even if sources of reddish colors are already available in the market like carminic acid from Dactylopius coccus or bixin from Bixa orellana, the research of other natural sources are still in demand because the actual colorants in the market don’t really offer the bright red color that their artificial counterparts offer. Thus, our results demonstrated that barks of T. populnea, as well as the dyestuff extracted from barks of Weinmannia tinctoria (a*-value of + 47.8; yield of 16.9 kg per 100 kg dm.; dye score of 56), barks of Leea guinensis (a*-value of + 42.4; yield of 16.8 kg per 100 kg dm.; dye score of 50), barks of Pemphis acidula (a*-value of + 30.7; yield of 11.6 kg per 100 kg dm.; dye score of 49), barks of Erythroxylum laurifolium (a*-value of + 48.7; yield of 6.1 kg per 100 kg dm.; dye score of 48). Roots of Morinda citrifolia (orangish-red color; a*-value of + 35.5; yield of 13.8 kg per 100 kg dm.; dye score of 44) offered promising color extracts containing water-soluble dyes which give a bright red or orangish-red coloration at low concentrations (from 2 to 8 mg/mL) (Table 5). The wavelength of maximum absorbance obtained around 500 and 550 nm in visible confirms the reddish color given by these selected plant-based dyestuffs. These red colors may come from different chemical classes of natural dyes like tannins, proanthocyanidins, flavonoids, alkaloids, anthraquinones, etc. (Tables 2, 3). Furthermore, the reddish colors of T. populnea, W. tinctoria, L. guinensis, P. acidula, E. laurifolium and M. citrifolia extracts can be considered as highly stable considering that they are able to support a wide range of pH (from 4 to 10 in water buffer solutions) and temperature (from 25 to 95 °C) with no significant variation; only an instability (discoloration) to acidic pH was observed for the extract of M. citrifolia roots.
Natural source of orangish and yellowish colorants are the second most used colorants in the dye industries. With red and blue, yellow is classified as one of the primary colors. Even if many sources of yellow colors are proposed in the market like curcumin or carotenoids (lutein…), the main limits of these colorants are their low stability. Our results indicated that many dye plant species from Reunion Island contain orangish and yellowish dyes. These colored molecules can be alkaloids, flavonoids, quinonoids, etc. Three plant-based dyestuffs, namely barks and stems of Ochrosia borbonica (b*-value of + 94.7; yield of 15.5 kg per 100 kg dm.; dye score of 46), roots of Danais fragrans (b*-value of + 93.4; yield of 15.1 kg per 100 kg dm.; dye score of 44), and barks of Terminalia bentzoe (b*-value of + 67.4; yield of 31.4 kg per 100 kg dm.; dye score of 40) give a bright yellowish-orange color (yellow at low concentration and orange in concentration of 8 mg/ml or greater) (Table 5). Stability characterization performed on these color extracts demonstrates a high stability toward temperature and pH of O. borbonica and T. bentzoe extracts, and a high stability specially toward temperature for D. fragrans extract; interestingly, this last extract turns red in alkaline pH (Table 5). Compared to commercial Reseda luteola extract, a common dye plants used in European country as a yellow colorant, the color stability of these above interesting sources of yellowish-orange colorants from Reunion Island is very promising for dyeing applications. This current study offered a real choice of new natural sources of yellowish-orange dyes that remain stables.
Then, greenish-brown, orangish-brown, yellowish-brown, beige, or pale beige-orange colors were obtained by the others endemic and native species harvested in this study (Table 5). For instance, the leaves of Pandanus utilis (yield of 12.7 kg per 100 kg dm.; dye score of 44) produce a pale greenish extract at low concentration, but give a bright greenish-dark color at a concentration of 8 mg/ml. The color remains greenish a few days later both in acidic and alkaline medium. This property is very uncommon for pigments extracted from green leaves and it will be of interest to analyze the chemical classes of the pigments extracted. The leaves of Talipariti tiliaceum (yield of 13.5 kg per 100 kg dm.; dye score of 34) also give a greenish-dark color extract of interest for dyeing application. Interestingly this greenish-brown color turns red in alkaline pH, and the same applies to leaves of the species Pteridium aquilinum which give an intense red in alkaline pH. Furthermore, from a mixture of barks and stems of the native species T. tiliaceum, an original pinkish-buff shade was obtained.
Finally, a selection of 15 introduced cultivated or alien-invasive species in Reunion Island, among the 138 alien species inventoried below (Table 3 and Additional file 1: Table S1), have also been harvested and characterized in this part of the study. Main results in terms of extraction yield and coloring properties (shade, solubility and stability) for these introduced species are summarized in Table 6. Water-soluble dyes with various shades were obtained according to the dye plants, like pink-violet from fruits of Selenicereus undatus (cultivated), yellowish-orange shade for rhizomes of Curcuma longa (cultivated), leaves of Terminalia catappa (cultivated), stems of Antigonum leptopus (invasive), barks of Casuarina equisetifolia (invasive) or fruits of Coccoloba uvifera (cultivated), and greenish shade for leaves of Leucena leucocephala (invasive). The Reunion’s forest is a remarkable reserve of plant biodiversity, and the most promising dye plant species (endemic, native, alien cultivated or invasive) may be used in dyeing application for the industries subject to their successful application for coloring textiles, food or cosmetics.
Table 6 Colors obtained from some alien cultivated or invasive dye plants in Reunion island (as ranked by their dye score)Bath-dyeing applications on natural fibers of dye plants from Reunion IslandTextile industry is a huge industry and this is crucial to change the way textiles are designed to an eco-friendlier production by using natural textile dyes [10]. Proven toxicity and environmental burdens caused by artificial azo dyes and related hazardous substances applied for textile dyeing, especially in Asia–Pacific region (China and India), have motivated consumers and industrials to turn to natural and eco-friendly fabric dyeing alternatives [2]. In this study, for the first time to our knowledge, the most promising plant-based dyestuffs from Reunion island rich in water-soluble dyes have been investigated for textile dyeing application on natural fibers (wool and cotton). For cotton dyeing, it is usually necessary to perform a mordanting step to improve the dye strength fixation on the fabrics. Choice of mordant is important insofar each complex mordant-textile may give a specific color and light. Thus, the dyeing property of the plant extracts has been checked for bath-dyeing with different mordants (alum, aluminum acetate, tannins, and ferrous sulphate, either alone or in combination). Main results and shades obtained after the bath-dyeing process using the reddish color extracts from T. populnea, W. tinctoria, E. laurifolium, M. citrifolia, and L. guinensis are presented in Fig. 2. Similarly, results obtained after the bath-dyeing process utilizing the yellowish-orange color extracts from O. borbonica, D. fragrans, T. bentzoe, T. cattapa, and C. equisetifolia are described in Fig. 3. These experiments have demonstrated very successful and very positive results. By using the adequate dye plant and mordant, it is possible to color the fabrics in a variety of different colors, like bright red with M. citrifolia extract, reddish with W. tinctoria, terracotta and pinkish-orange color with T. populnea and E. laurifolium (turn dark-green with iron mordant), sandy (pale yellow, cream) color with L. guinensis, bright yellowish-khaki color with O. borbonica (turn orangish pink with tannins added, and into dark-green with tannins and iron mordant), ochre-yellow and vibrant auburn colors with D. fragrans (turn ochre-red color in alkaline pH), yellow/beige color with T. bentzoe and T. cattapa (turn dark-blue or black with iron mordant), sandy color with C. equisetifolia (turn deep-blue with iron mordant, and into terracotta in alkaline pH), yellow and pinkish color with Coccoloba uvifera, etc.
Fig. 2Natural wool and cotton dyeing by bath-dyeing process utilizing the reddish color extracts from T. populnea, W. tinctoria, E. laurifolium, M. citrifolia, and L. guinensis. Six different mordants were added in the dye-bath (1–6)
Fig. 3Natural wool and cotton dyeing by bath-dyeing process utilizing the yellowish-orange color extracts from O. borbonica, D. fragrans, T. bentzoe, T. cattapa, and C. equisetifolia. Six different mordants were added in the dye-bath (1–6)
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