Agave sisalana Perrine ex Engelmann

Agave amaniensis Trel. & Nowell, Agave sisalana var. armata Trel., Agave sisalana f. armata (Trel.) Trel., Agave rigida var. sisalana (Perrine) Engelm., Agave rigida var. sisalana Perrault ex Engelmann, Agave rigida var. sisalana Baker, Agave segurae D. Guillot & P. Van der Meer

Asparagaceae

Agave, Century Plant, Hemp Plant, Mescal, Sisal, Sisal Agave, Sisal Hemp

The origin of Agave sisalana is in Central America, probably in southern Mexico based on the strength of traditional local usage (Gentry 1982). In the nineteenth century, sisal cultivation spread to Florida, the Caribbean islands, Brazil, parts of Africa notably Tanzania, Kenya and Madagascar, Asia, the Pacific Islands and Australia where it has become naturalized. Agave has become an invasive weed species in some countries like Australia where it develops dense infestations which can prevent the regeneration of trees and exclude understorey species in native bushland.

Sisal is a robust, hardy tropical species found growing from near sea level to a 1,800 m altitude as in tropical Africa. In its native and naturalized range, it can be found in bushland, roadsides, savanna and along drainage lines. It thrives in full sun, in areas with a maximum temperature of 27–32 °C and minimum temperatures of 16 °C or higher and daily fluctuations not exceeding 7–10 °C. It grows best in regions with an average annual rainfall of 1,000–1,250 mm but is often grown with less rain as in semiarid areas. Excessive rainfall is detrimental to the plant. Under dry conditions or at low average temperatures, it forms fewer leaves per year and has a longer life cycle. Sisal prefers sandy-loam soils but can be grown on a range of soils, provided they are rich in bases, especially calcium, and well drained. Sisal is intolerant of waterlogging. Optimum pH is between 5.5 and 7.5, though sisal has been grown on soils with pH 4–5.

Five major parts of the Agave are edible: the flowers, the leaves, the stalks or basal rosettes, the sap (called aguamiel—honey water) (Davidson 2006; Deane 2012), and the roots (Deane 2012). Each Agave plant will produce several pounds of edible flowers during the summer. The flower stalks before flower opening can be harvested, roasted and eaten; they are sweet, like molasses. During inflorescence development, the base of the young flower stalk is rich in sap and yields a syrup (also called Agave nectar) when tapped, which is used as an alternative to sugar in cooking, and is promoted as a healthy alternative or fermented into alcoholic beverage, pulque or mescal. The leaves may be collected in winter and spring—when the plants are rich in sap—boiled and eaten. In Java, the heart of new shoot is eaten (Tanaka 1976). The root is caustic, but once cooked for a couple of days it is sweet and can be eaten.

A robust, monocarpic herbaceous perennial with a short thick stem 120 cm by 20 cm, with a basal rosette of numerous (50–>150) leaves, 1.5–2 m high (Plates 1 and 2). Leaves glaucous when young, margin minutely spiny, later dark blue-green, ensiform, linear-lanceolate, straight 100–150 cm long by 10–15 cm wide, succulent, adaxially concave, abaxially convex, margin not spiny, apex straight and tipped with a red-brown spine 2–3 cm. Inflorescence a panicle on a long peduncle, 2–8 m tall, branching at the upper half, branches widely spreading, 30–100 cm by 2 cm, apically 5–6 times branched trichotomously, bearing about 40 flowers per branch and bearing numerous bulbils on the inflorescence branches after anthesis (Plate 2). Flowers strongly odorous, erect, protandrous; pedicel short; perianth tubular, 6-lobed, 5–6 cm long, pale yellowish green, tube 1–2 cm long, lobes obovate-oblanceolate, on inner side of the top with a tuft of hairs; stamens 6, attached above the middle of the perianth tube, 6–8 cm long; ovary inferior, 3-loculed, style 6–7 cm long, stigma 3-lobed. Fruit (rarely produced) an ellipsoid capsule, green becoming black when matured containing about 150 seeds. Seeds rounded-deltoid, thin, flat and black.

Agave sisalana juice was found to be acidic (pH = 5.42) and to contain abundant water 93.73 %, 11.56 % crude protein, 1.11 % total soluble sugar, 1.48 % ash and 8.7 % inulin content (stored juice), while fresh juice had 20.87 % inulin (Sharma and Varshney 2012). Wilson (1971) reported that sisal fibre contained 78 % cellulose, 8 % lignin, 10 % hemicelluloses, 2 % waxes and about 1 % ash by weight. Rowell (1992) reported sisal to contain 43–56 % cellulose, 7–9 % lignin, 21–24 % pentosan and 0.6–1.1 % ash. Joseph et al. (1996) reported sisal to contain 85–88 % cellulose. Agave sisalana was found to contain 77.3–84.4 % cellulose and 6.9–10.3 % hemicelluloses, lignin 7.4–11.4 (Martin et al. 2009). Sisal leaf was reported to contain about 4 % by weight of extractable hard fibre, with the remains from the leaf still possessing up to 20 % by weight of extractable pulp and short fibres, the remainder being water and soluble sugars and the stem (bole) and the pole contained abundant solid pulp and liquid juice with a high concentration of soluble sugars (Bisanda and Enock 2003). Sisal pulp was found to have a crude protein of 7.3 %, crude fibre of 15.2 % and NFE (nitrogen free extract) 59.6 %(Gebremariam and Machin 2008). Sisal plant was reported to contain three types of fibres: structural (mechanical) occurring in the periphery of the leaf (most useful fibre); ribbon, longest fibres, occurring in association with the conducting tissues in middle of the leaf; and xylem fibres with thin-walled cells in the conducting vascular bundles (Bisanda and Ansell 1992; Li et al. 2000).

Agave sisalana and kenaf (Hibiscus cannabinus) were found to contain highly gamma-acylated lignins with acetate groups (del Río et al. 2008). The structures of all these highly acylated lignins were characterized by a very high syringyl/guaiacyl ratio, a large predominance of β-O-4′ linkages (up to 94 % of all linkages), and a strikingly low proportion of traditional beta-beta′ linkages. The occurrence of beta-beta′ homocoupling and cross-coupling products of sinapyl acetate in the lignins from sisal and kenaf indicated sinapyl alcohol to be acetylated at the monomer stage and that, therefore, sinapyl acetate should be considered as a real monolignol involved in the lignification reactions. The acetylated heteroxylan O-acetyl-(4-O-methylglucurono)xylan with a molecular weight (Mw) of 18 kDa was isolated from Agave sisalana (Marques et al. 2010). The heteroxylan backbone was composed of (1 → 4)-linked β-d-xylopyranosyl units (Xylp) partially branched with terminal (1 → 2)-linked 4-O-methyl-α-d-glucuronosyl (MeGlcpA, 9 mol%) and a small proportion of α-d-glucuronosyl (GlcpA, <1 mol%) residues. Roughly 61 mol% of Xylp residues were acetylated.

Agave sisalana was also reported to contain sapogenins and saponins. The distribution of the steroid sapogenin constituents of Agave sisalana at various phases of growth was studied by Dawidar and Fayez (1961). It was suggested that in the bulbils, gitogenin (as first generation in sapogenin biogenesis ) afforded tigogenin (as second generation) which was transformed during the course of the long life of the plant to hecogenin and neotigogenin (as third generation). At the end of the life cycle, neotigogenin and hecogenin of the old leaves were transformed by a reverse mechanism to tigogenin in the flowering top and then to gitogenin. They found that hecogenin was most abundant (0.235 %) in the leaves of the old plant. The major sapogenins (hecogenin, 9(11)-dehydrohecogenin and tigogenin) occurring in the Agave species including A. sisalana were analyzed by reversed-phase high-performance liquid chromatography (Higgins 1976). Sarsasapogenin, 9(11)-dehydrotigogenin and diosgenin were also analyzed by this rapid and accurate method. Rockogenin was found to be formed from hecogenin during processing of A. sisalana leaves (Blunden et al. 1977); hecogenin 3β-hydroxy-(25R)-5α-spirostan-12-one and tigogenin (25R)-5α-spirostan-3β-ol were isolated from A. sisalana leaf and leaf juice, crude saponin concentrates known as ‘coffee grounds’ (Cripps and Blunden 1978). In East African samples, the tigogenin proportion of the total sapogenin content is usually about 10 %. Leaves of Tanzanian, Kenyan and Angolan A. sisalana plants were found to have 0.34, 0.47 and 1.16 % hecogenin and 0.2, 0.2 and 0.1 % tigogenin, respectively (Sitton et al. 1982).

Barbourgenin a steroidal sapogenin, rockogenin and chlorogenin were isolated from coffee grounds produced from the acid hydrolyzed juice of the leaves of A. sisalana (Blunden et al. 1986). Three steroidal saponins, dongnosides C–E, were isolated from the methanol extracts of the fermented residues of leaf juices of Agave sisalana and their structures elucidated as tigogenin-3-O-β–d-xylopyranosyl(1 → 2)[β-d-glucopyranosyl(1 → 3)]β-d-glucopyranosyl(1 → 4)β-d-galactopyranoside, tigogenin-3-O-β-d-xylopyranosyl(1 → 3)β-d-xylopyranosyl(1 → 2)[β-d-glucopyranosyl (1 → 3)]β-d-glucopyranosyl(1 → 4)β-d-galactopyranoside and tigogenin-3-O-α-l-rhamnopyranosyl (1 → 4)β-d-xylopyranosyl(1 → 2)[β-d-glucopyranosyl (1 → 3)]β-d-glucopyranosyl(1 → 4)β-d-galactopyranoside, respectively (Ding et al. 1989).

Two major steroidal saponins, named dongnosides B and A, were isolated from A. sisalana leaf juice, and their structures characterized, respectively, as tigogenin 3-O-alpha-l-rhamonpyranosyl-(1 → 4)-β-d-glucopyranosyl-(1 → 2)-[β-d-glucopyranosyl-(1 → 3)]-β-d-glucopyranosyl-(1 → 4)-β-d-galactopyranoside and 3-O-α-l-rhamnopyranosyl-(1 → 4)-β-d-glucopyranosyl-(1 → 2)-[β-d-xylopyranosyl-(1 → 3)-β-d-glucopyranosyl-(1 → 3)]-β-d-glucopyranosyl-(1 → >4)-β-d-galactopyranoside (Ding et al. 1993). Two furostanol saponins were isolated from Agave sisalana leaves, and their structures established as (25S)-26-(β-d-glucopyranosyl)-22 xi-hydroxyfurost-12-one-3β-yl-O-α-l-rhamnopyranosyl-(1 → 4)-β-d-glucopyranosyl-(1 → 3)-O-[O-β-d-glucopyranosyl-(1 → 2)]-O-β-d-glucopyranosyl-(1 → 4)-β-d-galacto-pyranoside and (25S)-26-(β-d-glucopyranosyl)-22xi-hydroxyfurost-5-en-12-one-3β-yl-O-α-l-rhamno-pyranosyl-(1 → 4)-β-d-glucopyranosyl-(1 → 3)-O-[O-β-d-glucopyranosyl-(1 → 2)]-O-β-d-glucopyranosyl– (1 → 4)-β-d-galactopyranoside (Zou et al. 2006). Three flavonoids 5,7-dihydroxyflavanone (1), kaempferol 3-rutinoside-4′-glucoside (9) and kaempferol 3-(2G-rhamnosylrutinoside) (10) and seven homoisoflavonoids 7-O-methyleucomol (2) [9], 3′-deoxysappanone (3), (±)-3,9-dihydroeucomin (4), dihydro-bonducellin (5), 7-hydroxy-3-(4-hydroxybenzyl)chromane (6) 5,7-dihydroxy-3-(4′-hydroxybenzyl)-4-chromanone (7), and 5,7-dihydroxy-3-(3′-hydroxy-4′-methoxybenzyl)-4-chromanone (8) were isolated from methanol extraction of Agave sisalana leaves (Chen et al. 2009).

A furostanol saponin, sisalasaponin C (1), and a spirostanol saponin, sisalasaponin D (2), were isolated from the fresh leaves of Agave sisalana, along with three other known steroidal saponins and two stilbenes (Yu et al. 2011). Their structures were identified as (3β,5α,6α,22α,25R)-3,26-bis[(β-d-glucopyrano-syl)oxy]-22-hydroxyfurostan-6-yl β-d-glucopyranoside (1); (3β,5α,25R)-12-oxospirostan-3-yl 6-deoxy-α-l-mannopyranosyl-(1 → 4)-β-d-glucopyranosyl-(1 → 3)-[β-d-xylopyranosyl-(1 → 3)-β-d-glucopyranosyl-(1 → 2)]-β-d-glucopyranosyl-(1 → 4)-β-d-galactopyranoside (2); (3β,5α,6α,22α,25R)-22-methoxyfurostane-3,6,26-triyl tris-β-d-glucopyranoside; cantalasaponin-1; polianthoside D; (E)-2,3,4′,5-tetrahydroxystilbene 2-O-β-d-glucopyranoside; and (Z)-2,3,4′,5-tetrahydroxystilbene 2-O-β–d-glucopyranoside.

The most predominant compounds identified in the lipophilic extract of A. sisalana fibres were fatty acids (30 % of total lipids) including α- and ω-hydroxy fatty acids, fatty alcohols (20 %), free sterols (11 %), alkanes (11 %) and a series of ferulic acid esters of long-chain alcohols and ω-hydroxy fatty acids (10 %) (Gutiérrez et al. 2008). Additionally, steroid hydrocarbons and ketones, monoglycerides, aldehydes, waxes and sterol glycosides were also found together with minor amounts of diglycerides and sterol esters. d-mannitol was isolated from an ethanol extract from the liquid residue of Agave sisalana (Branco et al. 2010). d-mannitol was isolated from an ethanol extract from the liquid residue of Agave sisalana leaf waste (Branco et al. 2010). Agave sisalana juice waste aqueous extract was found to contain saponins, glycosides, phlobatannins, terpenoids, tannins, flavonoids and cardiac glycosides (Ade-Ajayi et al. 2011).