6544

17: Fibre plants

Hibiscus sabdariffa L.

Sp. pl.: 695 (1753).

MALVACEAE

2n = 4x = 72

Hibiscus digitatus Cav. (1787).

Roselle, Siam jute (En). Roselle (Fr). India, Bangladesh: mesta. Indonesia: rosela (general), gamet walanda (Sundanese), kasturi roriha (Ternate). Malaysia: asam susur. Philippines: roselle (Tagalog), kubab (Ifugao), talingisag (Subanon). Cambodia: slök chuu. Laos: sômz ph'oox dii. Thailand: krachiap, krachiap-daeng (central), phakkengkheng (northern), paw keao. Vietnam: day nh[aaj]t, b[uj]p gi[aas]m, c[aa]y d[aas]m.

Roselle is probably of tropical African origin and was domesticated in ancient times first for its edible seeds and subsequently for its leaves, young shoots and flower parts as well. Both roselle and kenaf (Hibiscus cannabinus L.) may have been domesticated as early as 4000 BC in Sudan. As a vegetable (Hibiscus sabdariffa cv. group Sabdariffa) it reached the Americas and Asia in the 17th Century. Its use as a fibre plant (Hibiscus sabdariffa cv. group Altissima) is of more recent date. Hibiscus sabdariffa cv. group Altissima was unknown outside Africa until 1914, when seeds from Ghana were received in the Philippines, where its potential as a source of fibre was recognized. Experimental plantings started in Cuba in 1919. In Asia, Hibiscus sabdariffa cv. group Altissima was introduced into Java in 1918 and experimental work soon started, to be followed in other countries, including Malaysia (1921), Sri Lanka (1923) and India (1927). It remained a relatively unimportant fibre crop in most countries, except Thailand where it expanded after 1958 into a large bast fibre industry with considerable annual exports. Roselle was introduced into Vietnam in 1957. Nowadays roselle is widely distributed in the tropics and subtropics, usually in cultivation as a fibre plant or vegetable, sometimes as an escape.

The fibre extracted from the bast of roselle has similar applications to that of jute (Corchorus spp.), including gunny bags and hessian cloth. In Indonesia bags made from roselle were formerly extensively used for packing sugar. Roselle is also used, alone or with jute, in the manufacture of carpet and linoleum backing, cordage, tapes, upholstery, wall coverings, cable cores and interior car panelling. In the handicraft industry the fibre is made into cordage, carpets, rugs and handbags. The woody central core material or 'stick' (also often referred to as 'hurd' or 'shiv') left after decorticating can be used for paper making and serves for fencing and window shutters and as fuel. Whole stems have been processed to some extent into pulp for paper making in Thailand, Indonesia, Bangladesh and India.
The fleshy calyces of cv. group Sabdariffa are made into beverages, jams and jellies. Young leaves and tender shoots are eaten raw or cooked as a vegetable or condiment. In Africa the seeds of Hibiscus sabdariffa are eaten roasted or ground into meal. The leaves are considered emollient and leaf poultices are applied on abscesses and ulcers. Decoctions of the whole plant, and especially the calyx, are considered diuretic, tonic and antiscorbutic.

World production of jute-like fibres, a group consisting of kenaf, roselle, Congo jute (Urena lobata L.), Abutilon theophrasti Medik. and other Malvaceae, and sunn hemp (Crotalaria juncea L.), was 450 000 t per year from 370 000 ha averaged over the period 1997—2001. Separate statistics are unavailable but roselle is estimated to make up only about 10% of this total. In Thailand roselle is an important bast fibre crop, constituting 85% of total bast fibre production there. Roselle production in Thailand reached its peak of 660 000 t retted fibre per year in 1966, with exports of 475 000 t of raw fibre and gunny bags to over 60 countries, at a value of over US$ 810 million, making roselle the second largest foreign exchange earner of the country. About 70 000 t of the fibre was used locally by 10 gunny bag mills. However, production in Thailand gradually decreased to 120 000 t in 1994 and 35 000 t in 2000. Only 10% of Indonesia's bast fibre production is from roselle and during 1995—1999 the average production of roselle was 700 t from 440 ha. Roselle fibre is sometimes traded under the name kenaf, but this name is normally restricted to the fibre derived from Hibiscus cannabinus.

Roselle produces a bast fibre similar to jute (Corchorus capsularis L. and Corchorus olitorius L.), except that it is whiter and somewhat coarser. The raw (retted and dried) fibre makes up about 5% and the dry wood 18% of the freshly harvested and defoliated green stems. The commercial fibre has a length of up to 2.1 m. The ultimate bast fibre cells are (1.2—)1.9—3.1(—6.3) mm long and (10—)12—25(—44) µm wide, with maximum length and width in the middle portion of the stem. Lumen width and cell wall thickness vary from 3—15 µm and (4—)8—15 µm, respectively. Most fibre cells have tapering rounded ends. The wood consists of fibre cells 0.5—1.0 mm long and about 24—26(—32) µm wide, with a lumen width of 9 µm and a cell wall thickness of 3—7 µm. The chemical composition of roselle stems is not as well known as that of kenaf stems. The bast fibres contain about 32% a-cellulose, 10—15% lignin and 1% ash.
Roselle seeds contain 17—20% oil. The main fatty acids are palmitic acid (9—25%), oleic acid (26—38%) and linoleic acid (35—46%), but sometimes a combination of a much higher oleic acid and much lower linoleic acid content is found. The oil also contains 3—5% cyclopropenoid acids, which are toxic. The oil has shown antibacterial and antifungal activity. The press cake has a nitrogen content of 4.7% or the equivalent of 29% crude protein. The defatted seed contains antinutritional factors such as protease inhibitors, phytic acid and gossypol, making proper processing necessary before use in human nutrition. The flowers contain anthocyanins with, among others, hypotensive, diuretic and chloretic effects, whereas the extract of dried flowers showed antioxidative properties. Calyx extracts have shown hypotensive and antimutagenic activity.
The 1000-seed weight is 15—25 g.

Roselle fibre is coarser than jute and hence less valued. For many purposes, such as coarse packaging fibres and cordage, jute, kenaf and roselle may be substituted for each other. However, as kenaf and roselle are coarser and cheaper, they are considered more as a substitute of jute than the other way round. In some jute-producing countries the traders mix roselle with jute fibres in an effort to obtain higher prices. Bast fibres which can serve as substitutes for jute, kenaf and roselle include those of Abroma augusta (L.) L.f., Helicteres isora L., Malachra capitata (L.) L. and Urena lobata.

An erect, annual herb, up to 5 m tall. Taproot well developed, 18—30 cm deep, lateral roots few, occasionally with adventitious roots. Stem terete, beset with prickles or bristles, entirely green, green with red nodes, green with red patches or entirely red. Leaves alternate, polymorphic; stipules filiform, 5—13 mm long; petiole 0.3—12 cm long, green to red; blade 2—15 cm x 2—15 cm, entire to deeply 3—5(—7)-palmately lobed, base cuneate to truncate, margin finely to coarsely serrate, apex obtuse to acuminate, lobes lanceolate, middle one longest, subglabrous to pubescent with simple hairs, midrib with numerous mucilaginous glands on lower surface and ribs sometimes bearing prickles. Flowers solitary, axillary, bisexual, 5-merous; pedicel 5—20 mm long, articulating near the base to halfway between the base and the epicalyx, glabrous to hispid; epicalyx with 8—12 persistent bracts, fused at base, the free parts subulate to triangular, narrow, up to 7 mm long, apex pointed, green or red, leathery and with short bristles (cv. group Altissima) or fleshy and glabrous (cv. group Sabdariffa); calyx persistent, campanulate, 5-lobed, lobes triangular to ovate, up to 2.2 cm long in fruit, leathery and bristly (cv. group Altissima) or fleshy and glabrous (cv. group Sabdariffa), the nectaries inconspicuous, green, red or whitish; corolla bell-shaped and spreading with 5 free petals; petal asymmetrically obovate, up to 3 cm x 2 cm, base narrow, fleshy, apex rounded, glabrous to pubescent dorsally, yellow or yellow with deep red inner centre; stamens arranged in a column 7—20 mm long, yellowish-green to pink or red, free part of filaments 1 mm long, anthers most densely at the apex of column, dorsifixed, reniform and unilocular, pollen spiny and yellowish; pistil with superior, ovoid to globose ovary, 4—6 mm in diameter, densely silky hairy, 5 chambered with many ovules in each chamber, arranged in 2—3 rows, style included in staminal column, 5 branched, each branch ending in a capitate, hairy, red or yellow stigma. Fruit an ovoid capsule, 13—22 mm x 11—20 mm, deeply embedded within calyx, dehiscing by 5 valves when ripe with 30—40 seeds per fruit. Seed subreniform, 3—5 mm x 2—4 mm, reddish-brown with many small yellowish-brown warty spots, hilum reddish-brown. Seedling with epigeal germination.

Roselle seeds germinate within a few days after sowing in warm and moist soil. The young seedlings have a taproot, a hypocotyl of 5—9 cm and petiolate, cordate cotyledons. The first true leaves are petiolate and entire, oval-shaped, but subsequent leaves become increasingly lobed to palmate. Total duration from sowing to first flowering varies from 150—210 days, depending on photoperiod and temperature. Flowers of roselle are short-lived, remaining open for not more than three hours, closing at about noon of the same day they open. Roselle is almost entirely self-pollinating, though some cross-pollination by insects may occur. The stigmatic lobes remain drooped over the staminal column, and the anthers shed their viscous balls of pollen shortly before the flowers open so that any movement of the flower by wind or insects will transfer the pollen to the stigmatic surface. Fruit ripening takes 2—3 months from pollination. Roselle fruits are dehiscent as the fruit wall bursts at maturity and the seeds are scattered.

Hibiscus L. comprises 200—300 species, found mainly in the tropics and subtropics, many of which are grown as ornamentals. The estimated number of species varies because opinions differ about inclusion or exclusion of several related groups of species in the genus. Roselle belongs to Hibiscus section Furcaria, a group of about 100 species which have in common a pergamentaceous calyx (rarely fleshy) with 10 strongly prominent veins, 5 running to the apices of the segments and bearing a nectary, and 5 to the sinuses. The section Furcaria has x = 18 as basic chromosome number. Hibiscus sabdariffa is an allotetraploid (2n = 4x = 72) with Hibiscus asper Hook.f. as one of the likely parental species, and perhaps Hibiscus mechowii Garcke as the other. Within Hibiscus sabdariffa two main cultivar groups are distinguished:
— cv. group Altissima (synonym: Hibiscus sabdariffa L. var. altissima Wester): plants with single stem, usually branching only at the top, 3.5—5 m tall; epicalyx and calyx leathery and strongly hispid or bristled, enlarging little after anthesis, usually inedible; cultivated for its bast fibre.
— cv. group Sabdariffa (synonym: Hibiscus sabdariffa L. var. sabdariffa): plants branching profusely, 1.5—2 m tall; epicalyx and calyx fleshy and glabrous, enlarging considerably after anthesis; edible; cultivated mainly as a vegetable.
In Africa forms, usually branched and with bristly or aculeate plant parts, occur which do not seem to be part of either of these 2 groups. Some are cultivated for their seeds, whereas others seem wild.

Roselle is cultivated between 7°S (Java, Indonesia) and 23°N (Bangladesh). Climatic requirements during the growing period are mean monthly temperatures of 25—30°C, a rainfall of 140—270 mm per month and high air humidity (>70%). Although the crop requires abundant rainfall during the vegetative period for maximum fibre yield, roselle is also grown in areas with lower monthly rainfall since the crop is known to be drought-resistant once established. A drier period is also required for flowering and seed production. Roselle, like kenaf, is sensitive to frost, which is the main constraint to its cultivation in temperate zones. Roselle is a short-day plant, requiring a photoperiod of 12—12.5 hours for flowering and fruiting. In Java, for instance, no flowering is typically observed during the period December-March. The length of the vegetative period and fibre yield can thus be manipulated through the sowing date. Roselle can be grown on many soil types, provided these are deep, light textured and have good drainage. It tolerates both highly acid and moderately alkaline soils, but is intolerant of waterlogging.

Roselle is usually propagated by seed, but can also be grown from stem cuttings. Seed rate and planting distance vary from country to country. In Indonesia 12—14 kg/ha is recommended for line sowing, at 20 cm between and 15 cm within row plant distances, and 18—20 kg/ha for broadcast sowing. In Thailand most farmers drill roselle at 5—10 seeds per hole (to be thinned to 4—5 plants per hill) at 40 cm between and 30 cm within row spacing, using about 10 kg/ha, even though higher yields of fibre are obtained by single plants at closer (30 x 10 cm) spacing, using 18 kg/ha. For leaf or calyx production, plants are spaced more widely, e.g. 60 cm x 100 cm and 120 cm x 90 cm, respectively. Deep cross-ploughing and thorough tillage are important for unrestricted growth of the taproot and good crop establishment in general. The proper time for planting roselle as a fibre plant is October-November in South Kalimantan (Indonesia) and May in Thailand and Vietnam.
Roselle is mostly a smallholder crop and usually part of a cropping system with rice or maize. In north-eastern Thailand it is cultivated as a sole crop in upland areas. Because it occupies the land for a large part of the year, roselle is often not popular with farmers.

A single round of weeding and thinning to final plant density is usually performed 20—30 days after emergence of roselle. However, if weed infestation is high farmers carry out a second weeding. In Indonesia supplementary water for irrigation is often available, whereas in Thailand roselle is mainly a rainfed crop. The uptake of major nutrients by one ha of roselle producing 48 t green plants (2.1 t dry retted fibre) is estimated at about 106 kg N, 52 kg P, 148 kg K, 154 kg Ca and 35 kg Mg. As with kenaf, part of this is usually returned to the soil when stems are left to defoliate on the field after harvesting. The recommended rate of fertilizer application for irrigated roselle in Indonesia is 40—120 kg N per ha and for rainfed roselle 90 kg N, 50 kg P and 60 kg K per ha. In Thailand the recommended fertilizer dose is 15 kg N, 15 kg P and 15 kg K per ha in a single application, 20—30 days after seedling emergence.

The most important disease of roselle in South-East Asia is collar rot (Phytophthora nicotianae var. parasitica), which attacks plants at all growth stages. It becomes particularly serious in densely planted crops and during heavy rains. Roselle cultivars with red stem are more resistant than those with green stem. Phytosanitary measures help to reduce incidence of the disease. Vascular wilt caused by Fusarium oxysporum is an important disease of roselle in Malaysia.
Roselle has many pests in common with other malvaceous crops such as cotton (Gossypium spp.) and okra (Abelmoschus spp.). In Indonesia and Thailand, the main pest is the jassid leaf hopper (Amrasca biguttula). It attacks the plants by piercing the stems and then sucks the cell sap, ultimately killing the plants. Severe infestations occur when there is a long dry spell. Early planting can minimize the damage, because older plants are more tolerant to attacks. Seed treatment with systemic chemicals can protect the seedlings. The farmers usually do not use insecticides so it is important to breed insect-resistant cultivars. Other common pests are cotton stainer bugs (Dysdercus superstitiosus), bollworms (Earias biplaga, Earias insulana) and flea beetles (Podagrica spp.).
Root knot nematodes (Meloidogyne spp.) can cause serious crop losses. Attack by this nematode is usually followed by secondary infection with fungal diseases caused by Fusarium, Rhizoctonia and Sclerotium spp. Roselle genotypes are available with resistance against Meloidogyne incognita and Meloidogyne javanica.

Opinions vary with respect to the optimal time for harvesting, ranging from before the onset of flowering to the full flower stage. An important consideration is that, unlike in kenaf, the quality of roselle fibre deteriorates rapidly after initial flowering. In Thailand it is usually harvested between 140—160 days after sowing, before flowering has started. In Indonesia the crop is harvested within 150—160 days. Roselle plants may be harvested by hand using a sickle to cut the stems near ground level, but in sandy soils whole plants may be pulled up. Sometimes the root ends are removed before retting, or the crop may be retted with the roots remaining, with the root material removed afterwards. The harvested crop is usually sorted into thick- and thin-stemmed plants before bundling. In Thailand the plants are shaken to defoliate; elsewhere the plants are left on the ground to defoliate or they are tied into loose bundles, which are placed upright against each other for 2—3 days. When grown as a vegetable, the leaves or young shoots are harvested from the third month onwards and the calyces at about 15—20 days after flowering.

National average yields of dry roselle fibre over 1997—2001 were 1.9 t/ha for India, 1.6 t/ha for Thailand and 1.4 t/ha for Indonesia. Maximum yields obtained experimentally on-farm or on research stations were 4 t/ha in Bangladesh, 3.5 t/ha in Indonesia, 3 t/ha in India and 2.5 t/ha in Thailand. In experiments in India dry stalk yields up to 17 t/ha have been obtained. Seed yields may reach 1.7 t/ha. For roselle grown as a vegetable, leaf yields up to 10 t/ha have been recorded, whereas calyx yields vary from 5—15 t/ha.

After defoliation the roselle plants are tightly bundled and transported to the retting pool. Not more than 10 kg of stems should be in one bundle. Larger-sized bundles ret irregularly and produce poor quality fibre. The water in the retting pools should not be deeper than about 1 m; it should be clean and preferably flowing slowly to accelerate the retting process. The bundles of stems are arranged in 3—5 layers, each at right angles to the other, and tightened into a raft or 'jak'. The rafts are then submerged using weights, generally logs of woods or concrete blocks, but they should not touch the bottom of the pool, as this leads to discoloration of the fibre. Aquatic plants such as water hyacinth (Eichhornia crassipes (Martius) Solms) may also be used to cover the rafts. The time required for retting depends on factors such as maturity and thickness of the stems, temperature and pH of the water and amount of inoculum present. Usually it takes 8—17 days, but more time is needed in the case of fresh and non-defoliated stems. Where water is plentiful or flood water is present during the retting period, roselle is stripped and washed directly at the site of the retting pools. Otherwise, the retted bundles are removed from the pool and taken to a nearby stripping area, as is common practice in Thailand. The bundles are opened and the bottom ends beaten with a heavy stick or mallet to loosen the bast from the woody stem. The fibre is partly peeled from individual stems, after which the fibre is stripped in a single operation from 10—15 stems together, and rolled up into a hank. The washing operation consists of holding the hank at the bottom and jerking it through the water, and then reversing the process holding the tip ends. A skilled worker can strip and wash the equivalent of 35—45 kg of dry retted fibre per day. Drying of the washed hanks over bamboo poles takes 1—2 days in warm, sunny weather. Drying must be thorough with no damp spots in the fibre. Thai roselle growers prepare highly uniform bales ('barrels') by hand weighing 50—80 kg each. In Thailand retted roselle fibre is graded according to length, colour, softness and purity.
Nowadays, a considerable amount of roselle is ribboned directly after harvesting, especially when retting facilities are far from the growing area or when the supply of retting water is limited. The raw ribbon is about 30% of the green weight of stem. Freshly harvested green roselle plants are easier to ribbon than kenaf.
The bark, core and whole stems have been pulped using the soda-process, giving yields of 52—54%. The strength properties (breaking length, burst factor and tear factor) were better for bark pulps than for core and whole stem pulps. Cores as well as whole stems have been pulped using the soda-anthraquinone process, giving 47—54% pulp, with whole-stem pulp having better mechanical properties.

In South-East Asia the existing cultivars of roselle have a narrow genetic base and are constrained by low adaptability to agro-ecological conditions and susceptibility to several diseases and pests. Through mediation of the (former) International Jute Organization (IJO) a total of 140 accessions of Hibiscus sabdariffa of African and other origins have been added to the Centralized Germplasm Repository (CGR) in the Gene Bank of the Bangladesh Jute Research Institute (BJRI) in Dhaka, Bangladesh. Maintenance and characterization of these accessions is duly performed by the BJRI in Dhaka and all IJO member-countries have unrestricted access to these genetic resources.

Cultivar improvement in roselle follows breeding methods commonly applied to self-pollinating crops, such as line and pedigree selection after intervarietal crossing and backcrossing. Crossing techniques include emasculation in the afternoon and pollination the following morning. In addition to conventional breeding objectives of higher yields and better quality fibre, much attention is now also given to adaptation to abiotic and biotic stress factors. This is necessary because roselle is being pushed increasingly into more marginal environments due to pressure from food and other crops. Selection criteria include shorter growth duration to fit roselle into multiple cropping systems, tolerance to soils with low pH and high aluminium and iron content, host resistance to diseases (e.g. collar rot) and pests (the jassid leaf hopper in particular). Roselle cultivars for paper pulp require selection for high total biomass production, without too much emphasis on fibre yield or quality.

Like jute and kenaf, roselle is being pushed onto marginal land under the pressure of food and other crops and is also under strong competition from synthetic fibres, but prospects for increased production for both fibre and whole stems are promising, in view of growing concerns about environmental pollution and decreasing forest resources. Roselle fibre is biodegradable and an environment-friendly raw material suitable for many applications (non-woven fabrics, semi-rigid and laminated sheets for packaging and panelling, geotextiles). Whole stems seem to have potential as raw material for paper making, though much less research has been carried out on pulping of roselle than of kenaf.

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Shamsuddin Ahmad & H.A.M. van der Vossen

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