Senegalia Raf., Sylva Tellur.: 119. 1838.

Bruneau, Anne, de Queiroz, Luciano Paganucci, Ringelberg, Jens J., Borges, Leonardo M., Bortoluzzi, Roseli Lopes da Costa, Brown, Gillian K., Cardoso, Domingos B. O. S., Clark, Ruth P., Conceicao, Adilva de Souza, Cota, Matheus Martins Teixeira, Demeulenaere, Else, de Stefano, Rodrigo Duno, Ebinger, John E., Ferm, Julia, Fonseca-Cortes, Andres, Gagnon, Edeline, Grether, Rosaura, Guerra, Ethiene, Haston, Elspeth, Herendeen, Patrick S., Hernandez, Hector M., Hopkins, Helen C. F., Huamantupa-Chuquimaco, Isau, Hughes, Colin E., Ickert-Bond, Stefanie M., Iganci, Joao, Koenen, Erik J. M., Lewis, Gwilym P., de Lima, Haroldo Cavalcante, de Lima, Alexandre Gibau, Luckow, Melissa, Marazzi, Brigitte, Maslin, Bruce R., Morales, Matias, Morim, Marli Pires, Murphy, Daniel J., O'Donnell, Shawn A., Oliveira, Filipe Gomes, Oliveira, Ana Carla da Silva, Rando, Juliana Gastaldello, Ribeiro, Petala Gomes, Ribeiro, Carolina Lima, Santos, Felipe da Silva, Seigler, David S., da Silva, Guilherme Sousa, Simon, Marcelo F., Soares, Marcos Vinicius Batista & Terra, Vanessa, 2024, Advances in Legume Systematics 14. Classification of Caesalpinioideae. Part 2: Higher-level classification, PhytoKeys 240, pp. 1-552 : 1

Senegalia Raf., Sylva Tellur.: 119. 1838.

Figs 189 , 190 , 191

Manganaroa Speg., Bol. Acad. Nac. Ci. [ Córdoba] 26(2): 227. [12 Oct.] 1922. Lectotype (designated by Pedley 1986): Manganaroa monacantha (Willd.) Speg. [≡ Acacia monacantha Willd. (≡ Senegalia monacantha (Willd.) Seigler & Ebinger)]

Dugandia Britton & Killip, Ann. New York Acad. Sci. 35(3): 137. 1936. Type: Dugandia rostrata (Humb. & Bonpl. ex Willd.) Britton & Killip [≡ Acacia rostrata Humb. & Bonpl. ex Willd. (≡ Senegalia rostrata (Humb. & Bonpl. ex Willd.) Seigler & Ebinger)]

Lectotype

(designated by Britton and Rose, N. Amer. Fl. 23: 106. 1928). Senegalia triacantha Raf., nom. illeg. [≡ Mimosa senegal L. (≡ Senegalia senegal (L.) Britton)]. Note: Ross (1975c) did not typify Senegalia as stated by Pedley (1986), but he did select a neotype for Mimosa senegal L.

Two sections are currently recognised:

Senegalia Raf. sect. Senegalia

Acacia subg. Aculeiferum Vassal, Bull. Soc. Nat. Hist. Toulouse 108: 138. 1972; Vassal, Trav. Lab. Forest. Toulouse Tome 1, Vol. 8, Art. 17: 15. 1972. Type: Acacia senegal (L.) Willd. [≡ Mimosa senegal L. (≡ Senegalia senegal (L.) Britton]. Note: The subgenus was based on Acacia ser. Vulgares Benth., London J. Bot. 1: 322. 1842, not on Acacia ser. Vulgares and ser. Parkia Filicinae Benth. as given by Vassal (1981) in Polhill and Raven (1981: 170).

Senegalia sect. Monacanthea (Vassal) Maslin, Pl. Diversity 41: 371. 2019.

Acacia subg. Aculeiferum sect. Monacanthea Vassal, Bull. Soc. Nat. Hist. Toulouse 108: 139. 1972; Vassal, Trav. Lab. Forest. Toulouse Tome 1, Vol. 8, Art. 17: 15. 1972. Type: Acacia ataxacantha DC. [≡ Senegalia ataxacantha (DC.) Kyal. & Boatwr.]

Description.

Armed (with prickles) lianas, shrubs or trees (rarely taller than ca. 12 m); bark usually grey to brown and hard, sometimes yellowish and papery or corky when young; brachyblasts sometimes present; prickles present on branchlets (1, 2 or 3 at leaf nodes, or scattered irregularly or in rows along internodes; sometimes absent from some individuals and/or herbarium specimens) and often on underside of petiole and/or rachis. Stipules usually small and caducous, infrequently large and ± persistent, not spinescent. Leaves bipinnate, not sensitive; extrafloral nectaries present on petiole and/or rachis (sessile or sometimes stipitate) and normally on the pinnae (sessile); pinnae 1-40 (60) pairs; paraphyllidia present or absent; leaflets opposite or rarely alternate [e.g., S. comosa (Gagnep.) Maslin, Seigler & Ebinger, sect. Stryphnodendron Monacanthea s.s.], petiolulate or sometimes sessile, (1) 3-80 (100) pairs per pinna. Inflorescences comprising pedunculate heads, spikes or rarely spiciform racemes that are solitary or in fascicles in leaf axils or arranged in compound racemes or panicles; peduncles lacking a multi-bracteate involucre. Flowers all hermaphrodite or sometimes staminate and hermaphrodite within the one inflorescence, uniform, 5-merous, with a basal nectariferous disk, sessile to sub-sessile or sometimes pedicellate, commonly white to cream, sometimes yellow or red; perianth connate, valvate, not scarious; stamens numerous (ca. 30+), free; anther glands present or absent; pollen normally comprising 16-grained polyads, normally porate and lacking pseudocolpi, exine surface psilate (often with circular depressions) or variously rugulate, exine lacking columellae (or rarely columellae very short); ovary sessile to stipitate. Fruits normally dehiscent, rarely indehiscent [e.g., S. pentagona (Schumach. & Thonn.) Kyal. & Boatwr., sect. Stryphnodendron Monacanthea p.p.] or breaking into 1-seeded articles (e.g., S. rostrata , S. monacantha , sect. Senegalia Monacanthea p.p.), clearly flattened or rarely elliptic in cross-section, valves normally chartaceous to coriaceous, infrequently crustaceous to sub-woody [e.g., S. rugata (Lam.) Britton & Rose, sect. Stryphnodendron Monacanthea p.p.]. Seeds not winged, exarillate; pleurogram U-shaped (open at hilar end), occasionally circular or elliptic, rarely absent [e.g., S. pedicellata (Benth.) Seigler & Ebinger, sect. Stryphnodendron Monacanthea p.p.].

Chromosome number.

2 n = 26 (known for 25 species, including those of sect. Stryphnodendron Senegalia and sect. Stryphnodendron Monacanthea p.p.), 2 n = 39, 52 [ S. laeta (R. Br. ex Benth.) Seigler & Ebinger, sect. Stryphnodendron Senegalia ], 2 n = 40 [ S. galpinii (Burt Davy) Seigler & Ebinger, sect. Stryphnodendron Senegalia ] and 2 n = 26, 52 and 104 ( S. ataxacantha , sect. Senegalia Monacanthea s.s.) ( Hamant et al. 1975; Goldblatt and Johnson 1979 -; Ross 1979; Rice et al. 2015; all as Acacia ).

Included species and geographic distribution.

A total of 219 species accommodated in three infrageneric groups (see Notes below): sect. Senegalia Senegalia (51 species), sect. Senegalia Monacanthea s.s. (four species) and sect. Senegalia Monacanthea p.p. (164 species). The genus is distributed pantropically in the Americas (99 species), the African region (Africa and Madagascar, 68 species), Asia (Arabian Peninsula to East and South East Asia, 57 species) and north-east Australia (two species) (Fig. 191 ). Centres of species richness include Brazil (63 species), Mexico (30 species), East Asia (China, 22 species) and East Africa (e.g., Somalia, 21 species; Mozambique, 20 species). At the infrageneric level: sect. Senegalia occurs in Africa and Asia, sect. Senegalia Monacanthea s.s. in Africa and sect. Senegalia Monacanthea p.p. is pantropical, occurring in the Americas, Africa and Madagascar, Asia and north-east Australia. Distribution maps showing areas of species richness, and further details of species numbers are provided in Terra et al. (2022).

Ecology.

The majority of Senegalia species occur in seasonally dry tropical habitats, especially seasonally dry tropical forests, scrublands and savannas. Despite this predilection for seasonally dry biomes, the genus shows wide adaptability, with some species occurring in wetter lowland tropical vegetation types, which accounts for its almost cosmopolitan distribution across the tropics. Details of habitats for many species of Senegalia are provided in Maslin et al. (2019a) and Bai et al. (2021) for East Asian species, Nielsen (1981b, 1985a, 1985b, 1992) for South East Asian species, Ross (1979) for African species, Du Puy and Villiers (2002) for Madagascan species, and Barros and Morim (2014) and Rico-Arce (2007) for American species.

Etymology.

The genus name refers to Senegal, a country in West Africa where the lectotype S. senegal , was collected.

Human uses.

The most valuable commercial species of Senegalia is the widespread African S. senegal (sect. Senegalia Senegalia ), which is the classical source of Gum Arabic. This water-soluble gum is also derived from a variety of other sources, including Vachellia seyal (Delile) P.J.H. Hurter, where it is harvested from trees both in the wild and in plantations. Gum Arabic is used primarily in the food and soft-drink industries as a stabilizer but also has applications in the production of paint, glue and cosmetics, in textile industries and for viscosity control in inks. Also in Africa, the two sect. Senegalia Senegalia species S. nigrescens and S. caffra (Thunb.) P.J.H. Hurter & Mabb. are sources of tannin ( Mabberley 2017).

In Asia, the most commonly utilised species is S. catechu (L. f.) P.J.H. Hurter & Mabb. (sect. Senegalia Senegalia ). In Pakistan for example, its very durable and termite-resistant wood is used for house construction and for making agricultural implements; it is also an excellent source of firewood and is regarded as one of the best woods for charcoal production; tannin extracted from the wood is used for tanning ( Ali 1973). The soft new shoots of S. pennata subsp. insuavis (Lace) Maslin, Seigler & Ebinger (sect. Stryphnodendron Monacanthea p.p.) are commonly used in Asian cooking and are sold in some local markets in countries including China and Thailand; plants are sometimes grown as live fences in Thailand and northern Australia ( Maslin et al. 2019a) and extracts from its roots are used in traditional medicine in Laos to help combat anaemia ( Nielsen 1981b). The fruits of S. rugata (sect. Stryphnodendron Monacanthea p.p.) are also used for culinary purposes and as a traditional shampoo in Nepal ( Singh 2016). Senegalia modesta (Wall.) P.J.H. Hurter (sect. Stryphnodendron Senegalia ) from Afghanistan, northern India, Pakistan and Myanmar is a source of Amritsar Gum ( Mabberley 2017).

In the Americas many species of Senegalia are used locally for firewood; the wood is often converted to charcoal, but this is not usually economically important. Furthermore, almost all species of Senegalia that are large enough are used for making small tools, small instruments, furniture, and fence posts. The larger species are sometimes used for lumber [e.g., Senegalia aristeguietana (L. Cárdenas) Seigler & Ebinger], but most do not have significant commercial value. Rico-Arce (2007) noted that American Senegalia are sometimes grown as live fences [e.g., S. bonariensis (Gillies ex Hook. & Arn.) Seigler & Ebinger] or ornamentals [e.g., S. occidentalis (Rose) Britton & Rose] and used in traditional medicines [e.g., S. greggii (A. Gray) Britton & Rose, this species is also sometimes used as a coffee substitute]. Senegalia berlandieri (Benth.) Britton & Rose is an important honey source in Texas ( Mabberley 2017), even though the foliage is toxic to livestock, especially cattle, on account of the alkaloids it possesses.

Notes.

Prior to the recent fragmentation of the former, broadly defined genus Acacia , species now regarded as Senegalia had been referred to Acacia subg. Aculeiferum as defined by Vassal (1972). This subgenus corresponded to series Vulgares that Bentham (1842b) had erected, and which was maintained in his magnum opus ( Bentham 1875) and was in use at the time of Vassal’s (1972) publication. Senegalia had been described by Rafinesque (1838) but had been overlooked or ignored until it was adopted for a short period by Britton and Rose (1928) in their Flora of North America. Within the context of a reassessment of the classification of Acacia s.l., Pedley (1986) resurrected Senegalia again. Although this classification was not widely adopted at that time, from 2000 onwards after phylogenetic evidence clearly showed that Acacia s.l. was polyphyletic, Senegalia was once again resurrected and four small New World genera were segregated from it. Regardless, Senegalia as currently defined is most likely not monophyletic (fide Terra et al. 2022).

Vassal’s (1972) Acacia subgenus Aculeiferum included two major infrageneric groups, section Acacia Aculeiferum Aculeiferum which comprised species having cauline prickles located at the nodes, and section Acacia Monacanthea comprising species with cauline prickles scattered between the nodes; the type of this latter section was nominated as Acacia (Senegalia) ataxacantha . Over the past decade several phylogenetic analyses have included species of Acacia subgenus Aculeiferum (now Senegalia ). Whether based on plastid DNA sequences ( Bouchenak-Khelladi et al. 2010; Kyalangalilwa et al. 2013; Boatwright et al. 2015) or plastid loci combined with nrDNA ITS sequences ( Terra et al. 2017), all recovered two well supported clades. One contains species with nodal prickles that are now included in the Afro-Asian Senegalia section Senegalia (= section Senegalia Aculeiferum as recognised by Vassal 1972) plus species of the small African S. ataxacantha group that possessed internodal prickles (because this group included the type of section Senegalia Monacanthea , it was designated as section Senegalia Monacanthea sensu stricto by Terra et al. 2022). The second clade contains a pantropical group of species with internodal prickles, but as this group did not include S. ataxacantha it was designated section Senegalia Monacanthea pro parte by Terra et al. (2022). This same three-group topology was also revealed by phylogenomic analyses of large numbers of nuclear genes ( Ringelberg et al. 2022), albeit with very sparse taxon sampling.

In summary, the present concept of Senegalia is based on a combination of genetic and morphological evidence as detailed by Terra et al. (2022), and synoptically, the genus comprises the following three major groups:

Senegalia sect. Senegalia : cauline prickles 1, 2 or 3 at the nodes, flowers almost always in spikes; 51 species in Africa and Asia (Fig. 189D-J ).

Senegalia sect. Monacanthea s.s.: cauline prickles internodal, flowers in spikes; four species in Africa (Fig. 189A-C ).

Senegalia sect. Monacanthea p.p.: cauline prickles internodal but sometimes also at the nodes in some American species; flowers in heads or spikes; 164 species in Australia, Asia, Africa and the Americas. The name Manganaroa is available if this group is ever treated as a distinct genus (Fig. 190 ).

The pollen description above is based on Guinet (1969), Guinet and Vassal (1978), Nielsen (1992), Caccavari and Dome (2000) and Duarte et al. (2021). Where species of Senegalia are included in those works, they are described as having 16-grained polyads. However, in Guinet (1981b, Table 1 ) Senegalia was included in Acacia group one which was described as having (24-), 16-, 12-grained polyads. Because it is not known what species were included within that group, it is not possible to verify this range of variation for pollen grain number in Senegalia .

Taxonomic references.

Ali (1973); Bai et al. (2021); Barros and Morim (2014); Bentham (1842b, 1875); Britton and Rose (1928); Kyalangalilwa et al. (2013); Maslin et al. (2019a); Nielsen (1981b, 1985a, 1985b, 1992); Pedley (1986); Rafinesque (1838); Ross (1979); Terra et al. (2017, 2022); Vassal (1972).

Figures

Figure 189. Morphological features of A-C Senegalia sect. Monacanthea s. s. and D-J Senegalia sect. Senegalia A-C Senegalia ataxacantha (DC.) Kyal. & Boatwr. A lianescent shrub habit B branch showing internodal prickles C spicate inflorescence, Pretoria National Botanical Garden, South Africa D, E S. modesta (Wall.) P. J. H. Hurter D papery peeling bark on branch, living collection at Singapore Botanic Gardens E branch showing two prickles at nodes and leaves with few pinnae, Asia F S. catechu (L. f.) P. J. H. Hurter & Mabb. spicate inflorescence, South China Botanic Garden, Guangzhou G S. laeta (R. Br. ex Benth.) Seigler & Ebinger habit H S. senegal (L.) Britton three cauline prickles at nodes I S. goetzei (Harms) Kyal. & Boatwr. two cauline prickles at nodes and spicate inflorescence J S. polyacantha subsp. campylacantha (Hochst. ex A. Rich.) Kyal. and Boatwr. habit. Photo credits A P Birnbaum B S Piry C E Koenen D, E B Maslin F Y Chen G M Schmidt H A Dreyer I C Boucher Chisale J E Faust. A, B, G-J from African plants - A Photo Guide (www. africanplants. senckenberg. de).

Figure 190. Morphological features of Senegalia sect. Monacanthea p. p. A Senegalia x emoryana (Benth.) Britton & Rose habit, New World B S. micrantha (Benth.) Britton & Rose leaf, New World C S. grandistipula (Benth.) Seigler & Ebinger large foliaceous stipules (such stipules not especially common in Senegalia) (Terra 715), New World D Senegalia x emoryana internodal prickles, New World E S. subsessilis Britton & Rose thin-textured fruits, New World F S. sakalava (Drake) Boatwr. globose scarlet inflorescence (scarlet flowers are rare in Senegalia but are found in several species from Madagascar) (Koenen 215), Madagascar G S. pruinescens (Kurz) Maslin, Seigler & Ebinger lianescent shrub habit and terminal paniculate inflorescences (insert of head showing calyx red in upper part) (Maslin 11023), Asia H S. pennata subsp. insuavis (Lace) Maslin, Seigler & Ebinger liana habit (Maslin 11016), Asia I S. menabeensis (Villiers & Du Puy) Boatwr. spicate inflorescences (Du Puy M 359), Madagascar J S. rugata (Lam.) Britton & Rose hard-textured pods, Macau, China K S. tonkinensis (I. C. Nielsen) Maslin, Seigler & Ebinger branch showing internodal prickles and two glands on petiole (uncommon in Senegalia) (Maslin 11041), Asia. Photo credits A, B, D, E, H B Maslin C V Terra F E Koenen G, K L Bai I D Du Puy J L-x Yuan.

Figure 191. Distribution of Senegalia based on quality-controlled digitised herbarium records. Note that the Indian subcontinent was only sparsely sampled for this map. Details of species distribution, based on states of India and other countries of the subcontinent, are provided in Deshpande et al. (2019). See Suppl. material 1 for the source of occurrence data.