Changes

The average annual world production in 1997-2001 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 about 450 000 t, harvested from 335 000 ha. Separate statistics are not available, but kenaf is estimated to make up about 90% of the total. Important kenaf fibre producing countries are India, China and the Russian Federation. In South-East Asia sizeable quantities of kenaf fibre are produced in Indonesia and Thailand. Almost all kenaf fibre is processed domestically and there is very little international trade. The fibre is traded under the name kenaf, but sometimes this name is also used for roselle fibre.

Kenaf produces a bast fibre similar to jute ( ''Corchorus'' spp.), except that it has a greater tensile strength and is somewhat coarser and more brittle. Kenaf fibre can be spun alone, or in admixture with jute, on machinery for jute spinning. The raw (retted and dried) bast fibre makes up 5-7% and the wood 15-21% of the freshly harvested and defoliated green stems. On a dry weight basis the bast fibre content of the stem ranges from 21% in wild accessions to 36% in modern cultivars. The ultimate bast fibres are (1.5-)2-3(-12) mm long and (7-)15-25(-41) μm wide, with an average cell wall thickness of 4-9 μm and lumen width of 7-13 μm. In cross-section they are polygonal to round or oval, with a varying lumen width and cell wall thickness. In longitudinal section the fibre cells are cylindrical. The fibre ends show great variation, but most often they taper into a blunt point. The fibre cells in the woody core are (0.4-)0.5-0.9(-2.4) mm long and 18-33(-37) μm in diameter, with an average cell wall thickness of 4-8 μm and a lumen width of 16-23 μm. The length of both bast and core fibre cells increases from the base to the top of the stem. The fibre strands of commerce, consisting of fibre cells cemented together by pectin and hemicelluloses, are 1.5-3.0 m long. Kenaf fibre contains 44-62% α-cellulose, 14-20% hemicelluloses, 4-5% pectin, 6-19% lignin and 0-3% ash.

*An erect annual herb, up to 2 m tall in the wild, up to 5 m in cultivars. Taproot well developed, up to 25 cm deep with lateral roots spreading horizontally to 1 m and adventitious roots on lowest stem section. *Stem slender, cylindrical, in cultivation unbranched and glabrous, prickly on wild accessions, entirely green, green with red or purple pigmentation, or red, sometimes lower half green and upper half pigmented. *Leaves alternate; stipules filiform, 5-8 mm long, pubescent; petiole 3-30 cm long, finely pubescent on the adaxial surface and bristled on the abaxial surface, green to red; blade 1-19 cm × 0.1-20 cm, very shallowly to very deeply palmately 3-7-lobed on lower stem, often unlobed on upper stem or even bract-like near the apex, base cuneate to cordate, margins serrate or dentate, apex acuminate, upper surface glabrous but with a prominent, 3 mm long nectary at the base of the midrib, lower surface hairy along the veins. *Flowers axillary, solitary or sometimes clustered near the apex, bisexual, 5-merous, 7.5-10 cm in diameter; pedicel 2-6 mm long, articulated at the base; epicalyx of 7-8 linear bracteoles, 7-18 mm long, persistent; calyx campanulate with 5 acuminate to subcaudate lobes 1-2.5 cm long (up to 3.5 cm in cultivars), persistent, green, bristly and with a characteristic white, woolly, arachnoid tomentum especially near the base and margins, with a prominent nectary gland on each midrib; corolla large and showy, usually cream to yellow with red inner base, sometimes blue or purple; petals free, usually spreading, twisted clockwise or anticlockwise, obovate, 4-6 cm × 3-5 cm, outer side stellate-pubescent; staminal column epipetalous and surrounding the style, 17-23 mm long, dark red, with numerous 1-2 mm long filaments and 1-celled yellow or red anthers; pollen spiny, spherical; pistil with superior, 5-locular, ovoid, pointed, villose ovary, each locule containing many ovules arranged in 2 vertical rows, a single red style, branching into 3-5, hairy arms 2-4 mm long, each branch ending in a capitate stigma. *Fruit an ovoid, beaked capsule, 12-20 mm × 11-15 mm, densely appressed pubescent, the beak 1 mm long, containing 20-25(-35) seeds. *Seed subreniform to triangular with acute angles, 3-4 mm × 2-3 mm, ash grey or brown-black with light yellowish spots, hilum brown. *Seedling with epigeal germination.

''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. In a recent revision (2001), for instance, 22 ''Hibiscus'' spp., formerly grouped under ''Hibiscus'' sect. ''Azanzae'' DC., have been excluded from ''Hibiscus'' and placed in the segregate genus ''Talipariti'' Fryxell. These include some trees of which the bark is made into good-quality rope and other cordage items in South-East Asia, such as ''T. macrophyllum'' (Roxb. ex Hornem.) Fryxell (synonym: ''H. macrophyllus'' Roxb. ex Hornem.) and ''T. tiliaceum'' (L.) Fryxell (synonym: ''H. tiliaceus'' L.), both treated in PROSEA 5 "Timber trees". ''H. floccosus'' Mast., the bast of which is made into rope in Peninsular Malaysia, has not been removed from ''Hibiscus'' .

Kenaf 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. Interspecific hybridization has been attempted with varying success between ''H. cannabinus'' and other species within the same section, e.g. ''H. acetosella'' Welw. ex Hiern, ''H. diversifolius'' Jacq., ''H. radiatus'' Cav., and ''H. sabdariffa'' . ''H. cannabinus'' can easily be distinguished from the related species ''H. radiatus'' and ''H. sabdariffa'' by the white, arachnoid tomentum on the calyx. In the Philippines, ''H. cannabinus'' as listed by Merrill has to be referred to ''H. radiatus'' , since the specimens quoted belong to that species.

''H. radiatus'' , possibly an allotetraploid of ''H. cannabinus'' and ''H. surattensis'' is grown as an ornamental, vegetable and medicinal herb in South-East Asia and as a fibre plant in Brazil. ''H. surattensis'' L. is sometimes grown for its fibre.

Kenaf has a wide range of adaptation to climate and soil, and is grown between 45N 45°N and 30S30°S. Kenaf plants are tolerant of daily temperature variation between 10°C and 50°C, but are killed by frost. It grows best where mean daily temperatures during the growing season are higher than 20°C and average monthly rainfall is 100-125 mm during the growing season. These conditions are met during the rainy season in the tropics and the wet summer season of the subtropics. Kenaf is a short-day plant: regardless of the time of planting, most cultivars remain vegetative until the daylight period falls below 12.5 hours. Cultivars planted at a latitude of 20N 20°N will, therefore, not start flowering before early September. At higher latitudes flowering commences progressively later, whereas at the equator plants flower early and attain insufficient height, except when the grown cultivar is photo-insensitive. Kenaf can be grown on a wide range of soils, but thrives best on free-draining sandy loams of alluvial or colluvial origin, with pH 6-6.8. It is salt-tolerant, but sensitive to waterlogging.

Kenaf is mostly a rain-fed crop sown directly in the field at the start of the rainy season. Time of planting is very important, as the crop should grow during long days. The longer the period of vegetative growth before first flowering, the higher the yield of biomass and fibre and also the better the fibre quality. In Java, Indonesia (8S8°S) kenaf is sown during October-November with the onset of the rainy season and flowers in March-April, while in north Thailand (18N18°N) the cropping season is between April and September. A deep cross-ploughing is usually desirable for thorough tillage, followed by harrowing to subdue weeds and to obtain a fine state of tilth. Most kenaf is grown by smallholders, who plant by broadcasting at seed rates of 15-25 kg/ha, followed by harrowing or laddering to cover the seed with 1-2 cm soil. Such plots require thinning by hand to reduce plant density to about 400 000 plants per ha. Row plantings require less seed and result in more uniformity and higher yields at lower establishing and maintenance costs (e.g. by mechanical drilling and weeding). Recommended spacings for row planting vary between countries. They are generally similar to those for jute, i.e. 20-30 cm between and 5-10 cm within rows, when grown for fibre, but a somewhat wider spacing to produce pulp for paper making. Seeds can be produced on plants left in the field after harvesting the main crop. However, the amount and quality of seeds is much higher on special seed plots sown late in the season, as profuse flowering is induced by declining daylengths.

The economic importance of kenaf diseases varies among countries. Stem and seedling rots caused by ''Macrophomina phaseolina'' and leaf spot by ''Cercospora hibisci-cannabini'' are serious kenaf diseases in several Asian countries. ''Fusarium'' sp. is a major disease in Indonesia causing damping-off in young seedlings and black or brown stem lesions resulting in lodging and death of older plants. Collar-rot caused by ''Phytophthora nicotianae'' var. ''parasitica'' is the most serious soil-borne disease of kenaf in Thailand, in particular during the peak growing period. Other diseases in kenaf in Asia are a leaf spot caused by ''Phoma'' sp., anthracnose ( ''Colletotrichum'' spp.), virus A (leaf curl) and virus B (mosaic).

Kenaf harbours numerous insects, but only a few cause serious damage to the crop. Flea beetles ( ''Podagrica'' spp.) and the jassid leaf hopper ( ''Amrasca biguttula'' ) are among the most important insect pests in Asia. Other insect pests of kenaf in Asia include the spiral borer ( ''Agrilus acutus'' ), cotton bollworm ( ''Helicoverpa armigera'' ), black cutworm ''(Agrotis ipsilon'' ), the cotton stainer ( ''Dysdercus'' spp.) and stink bug ( ''Tectocoris'' spp.).

Kenaf is particularly susceptible to root-knot nematodes ( ''Meloidogyne'' spp.), which may cause severe damage, especially in light-textured soils. Flooding of the land prior to kenaf cultivation and regular crop rotation help to reduce nematode attacks. Some difference in tolerance to nematodes exists among kenaf cultivars.

Kenaf stems are usually retted in water for a period of 10-15 days at warm (>30°C) temperatures to liberate the fibres from the bark tissues by enzymatic action of micro-organisms. The bundles are steeped in shallow water in 2-3 layers, which are kept submerged by the weight of bundles of water hyacinth ( ''Eichhornia crassipes'' (Martius) Solms) or rice straw, bricks or concrete slabs. The retting progress is monitored towards the end of the process by testing a few stems removed from the centre of the bundles. Retting efficiency depends on factors such as cultivar, maturity, water quality, microbial activity, and day and night temperatures. Retting should be carried out in clear and slow moving water at the optimal proportion of 1:20 for plant biomass to water quantity. In case of stagnant water, addition of urea (0.01% of the green weight of kenaf plants) may enhance the retting process and removal of brown colour. When retting is complete, the fibres are stripped manually from the stem, washed thoroughly in clean water and dried well in dust- and sand-free conditions. As in jute, the fibre quality is very much determined by correct retting, cleaning and drying. Ribbon retting, whereby the bark is stripped from freshly harvested stems by manual or machine decorticators ("ribboning") and only the ribbons are steeped in water, requires much less water and reduces the retting time by half. It is a method generally applied in areas with chronic water shortage during the harvesting season, but it also reduces handling and transport costs from the field to the retting pools and tends to produce fibre of more uniform and higher quality. The dried fibres are transported in crude bales to local centres for initial grading and packing in low compression bales of 60-150 kg before transportation to the spinning mills. Whole kenaf stems may also be transported directly from the field to pulp- and paper-making factories.

Methods of selection and breeding common to self-pollinating crops are applied to kenaf, mostly line selection within landraces or after intervarietal crosses and backcrosses. Higher yield is an important breeding objective in kenaf, but in South and South-East Asia the on-farm yields of improved cultivars are often less than half the on-station (potential) yields. This gap is caused partly by inadequate cultural practices and low inputs at farm level, but also by the fact that the available cultivars are often inherently poor achievers in marginal ecosystems. Breeding for new cultivars with the potential of high yields under suboptimal conditions is all the more urgent because the kenaf crop is being pushed increasingly into marginal environments due to pressure from food and other crops. Earlier maturing and photo-insensitive kenaf cultivars also fit better in systems of multiple cropping. Other breeding objectives are plants without prickly stems and bristly capsules to facilitate manual harvesting, and host resistance to diseases, insect pests and nematodes. Resistance to diseases such as anthracnose ( ''Colletotrichum'' spp.) and stem rot ( ''Macrophomina phaseolina'' ) is available from ''H. cannabibus'' accessions, but adequate levels of resistance to root-knot nematodes ( ''Meloidogyne'' spp.) are mostly restricted to related species, such as ''H. acetosella'' , ''H. rostellatus'' Guill. & Perr., ''H. sabdarrifa'' , and ''H. surattensis'' . Introgression of target characters into kenaf from these species is being attempted by conventional as well as advanced techniques (e.g. protoplast fusion) of interspecific hybridization.