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

Hughes, Colin E., Ringelberg, Jens J., Lewis, Gwilym P. & Catalano, Santiago A., 2022, Disintegration of the genus Prosopis L. (Leguminosae, Caesalpinioideae, mimosoid clade), PhytoKeys 205, pp. 147-189 : 147

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

Prosopis sect. Algarobia DC. Prodr. 2: 446. 1825.

Mitostax Raf., Sylva Tellur.: 120. 1838.

Algarobia (DC.) Benth., Pl. Hartw.: 13. 1839.

Prosopis sect. Monilicarpa Ruiz Leal ex Burkart, J. Arnold Arbor. 57(3): 230. 1976.

Type.

Neltuma juliflora (Sw.) Raf. [= Mimosa juliflora Sw.].

Description.

Spiny, erect to prostrate subshrubs, shrubs and small trees, (0.1-) 4-10 (-20) m high, usually with a short trunk to 40-60 (->100) cm diameter, branching lax with a spreading rounded or flat-topped crown, twigs cylindrical, flexuous, often arched downwards, glabrous, green or reddish, often with rather long internodes, armed with uninodal axillary, solitary or paired, straight, strong, cylindrical, subulate spines (Figs 2 and 3E ), these not necessarily at all nodes, 0.2-15 (-33) cm long × 0.2-1.4 cm in diameter and sometimes thicker than the subtending twig, or with spinescent rigid straight cylindrical branchlets 8-50 cm, brachyblasts congested, blackish. Stipules small, triangular and dry. Leaves with 1-3 (-8) pairs of pinnae, the petiole (0.2-) 2-7.5 cm long, the pinnular rachises (0.2-) 4-19 (-24.5) cm long, with (1-) 2-30 (-50) pairs of opposite leaflets, these linear, ovate-oblong, oblong-linear or lance-ovate, more or less acute, palmately pinnativeined or almost without veins, (0.15-) 2.5-10 × 0.05-3.5 cm, puberulous to scarcely ciliolate or glabrous, or sometimes aphyllous or subaphyllous ( N. sericantha , N. kuntzei ), the leaves small and soon falling off the young developing shoots which become spinescent. Inflorescences axillary, solitary or fascicled, spicate, (1.5-) 3-15 cm long with 20-250 flowers on short 1.6 mm pedicels. Flowers white, yellow, greenish-yellow or occasionally red, often perfumed, sometimes some functionally male flowers; calyx 1-2 mm long; corolla 3-5 mm long, the petals almost free, pubescent, usually villous within; stamens and style exserted, anthers with a minute caducous incurved claviform gland arising from the connective. Fruits linear moniliform or compressed turgid (Figs 6 and 7H-I ), straw yellow, sometimes tinged reddish-maroon or black, 1-several per infructescence, indehiscent, glabrous, mostly straight to subfalcate, S- or C-shaped or annular with 1-3 very lax open spirals, acuminate, (2-) 5-29 cm in length × 0.5-2.6 cm diameter, margins often thickened and undulate, valves striate corrugate or smooth, exocarp crustaceous, mesocarp thin or more usually thick and pulpy, mealy or spongy, dry, usually sweet, endocarp hard and bony or coriaceous, with convex faces and acute extremities, segmented in longitudinal or transverse subquadrate closed seed chambers. Seeds brown, compressed ovate, 5-10 × 3-6 mm. See also Johnston (1962).

Geographic distribution.

Potentially up to 43 species, but probably somewhat fewer (see below). Widespread across seasonally dry tropical and arid regions of the Americas with a pseudo-amphitropical bicentric pattern of greatest species diversity in the Mexican-Texan and Argentinian-Chilean-Paraguayan regions, especially diverse and abundant in the Chaco, with an outlying disjunct occurrence of Neltuma ruscifolia of questionable nativity in the Caatinga in north-east Brazil ( Burkart 1976; Oliveira & Queiroz 2020) and extending into warm and some colder temperate areas in Texas and Nevada in the north and Patagonia in the south, where N. denudans Benth. reaches 48 °S (Fig. 8 ).

Habitat and uses.

Dominant across large tracts of the Gran Chaco in mixed sub-xerophyllous woodland, also in Monte vegetation, open desert forests in quebradas along seasonal rivers, in Stipa -dominated pampas and semi-desert shrub steppe with hot summers and cold winters in Patagonia as far as 48 °S, some species capable of surviving extreme drought; spanning a wide range of substrates and edaphic conditions including stony and sandy mesas, coastal and inland sand dunes and deep black seasonally inundated, sometimes saline, clay vertisols. Some species weedy and invasive, both within their native ranges and where introduced (see Introduction). The wood generally hard, dense, durable and flexible and widely used for fence posts, parquet flooring, barrels, firewood and charcoal and the fruits are eagerly consumed by all forms of livestock (see Introduction).

Etymology.

Possibly derived from the common name Mulla Thumma in the Dravidian language Teluga in the Indian states of Andhra Pradesh and Telangana, where Neltuma juliflora is introduced.

Affinities.

Neltuma is sister to, but deeply divergent from, the combined Strombocarpa + Xerocladia clade (Fig. 1 ).

Thirteen species of Prosopis have been described since the publication of Burkart’s (1976) monograph. One of these, Prosopis bonplanda P.R. Earl & Lux, was already placed in synonymy under P. glandulosa by Palacios (2006). All of the rest can be confidently placed in Neltuma (= Prosopis sect. Algarobia + Prosopis sect. Monilicarpa ), based on morphological descriptions and illustrations from their respective protologues. We provide new combinations in Neltuma for all these names, listing potentially up to 43 species for the genus, but we suspect that some of these new species may be no more than regional variants of the widespread and taxonomically difficult N. pallida / N. juliflora species complex. Given the difficulties of species delimitation across parts of Neltuma , we suggest that a detailed molecular study with complete sampling of species and dense sampling of multiple accessions, representing intraspecific diversity, is needed to properly re-assess species boundaries and possible hybridisation. The Mimobaits gene set of Koenen et al. (2020) would be an ideal tool for such a study.

Figures

Figure 1. A Phylogeny of the Caesalpinioideae showing the placement of the Prosopis grade (boxed in red) within the subfamily, based on analyses of DNA sequences of 997 nuclear genes (Ringelberg et al. 2022) B the part of the phylogeny that includes all elements of Prosopis s. l. Genera recognised in the new generic system presented here are in bold. Pie charts show the fraction of gene trees supporting that bipartition in blue, the fraction of gene trees supporting the most likely alternative configuration in green, the fraction of gene trees supporting additional conflicting configurations in red and the fraction of uninformative gene trees in grey. Numbers above pie charts are Extended Quadripartition Internode Certainty (Zhou et al. 2020) scores. Branch lengths are expressed in coalescent units and terminal branches were assigned an arbitrary uniform length for visual clarity, see Ringelberg et al. (2022); the root is not drawn to scale C, D the two most likely alternative tree topologies which would allow for a monophyletic Prosopis s. l., either without (C) or with (D) Xerocladia and Indopiptadenia. In C and D numbers above pie charts = number of gene trees supporting the species tree, numbers below pie charts = number of gene trees conflicting with the species tree C lack of gene tree support (just 69 gene trees) for the alternative species tree topology where sections Algarobia + Monilicarpa (≡ Neltuma) are sister to section Strombocarpa (≡ Strombocarpa) vs. 573 genes supporting a sister group relationship between Strombocarpa and Xerocladia (as shown in D) D lack of gene trees (zero gene trees) supporting a monophyletic Prosopis s. l.

Figure 2. Variation in armature of Prosopis, Strombocarpa, Neltuma and Xerocladia A Neltuma denudans (nodal spines on a zig-zag stem) B N. humilis (paired striate spine-tipped branches) C Prosopis cineraria (scattered internodal prickles) D Neltuma sericantha (spine-tipped stems) E Strombocarpa burkartii (stipular spines) F Neltuma argentina (single nodal axillary spine) G N. kuntzei (spinescent shoots) H Strombocarpa ferox (stipular spines) I S. strombulifera (stipular spines) J Neltuma elata (variation in paired nodal spines on one specimen) K N. alba (paired nodal spines) L N. velutina (paired nodal spines) M Prosopis farcta (scattered internodal prickles) N Neltuma ruscifolia (single nodal axillary spine) O Xerocladia viridiramis (recurved, deflexed stipular spines) (5 cm scale bar). All specimens at K A drawn from Seijo 1489 B Tweedie s. n. C Willcox 299 D MERL 8792 E Acosta & Rosas 748 F Guaglianone et al. 1762 G Nee & Coimbra 35556 H Atahuachi et al. MA 1147 I Hunziker 2036 J Legname & Cuezzo 10396 (large and small spines from same specimen) K Hughes & Forrest 2312 L Harding & Balsinhas 140 M Guest et al. 17463 N Wood & Mamani 14063 O Kolberg & Tholkes HK 2493. Drawn by Andrew Brown, July 2021.

Figure 3. Variation in armature across Prosopis s. l. and allies A stipular spines of Strombocarpa strombulifera B internodal prickles on shoots and branches of Indopiptadenia oudhensis which it shares with its sister group, Prosopis s. s. illustrated in C; C internodal prickles of Prosopis farcta D stipular spines of Xerocladia viridiramis which it shares with its sister group, the genus Strombocarpa illustrated in A; E axillary nodal spines of Neltuma juliflora F spinescent straight cylindrical shoots of the subaphyllous Neltuma kuntzei. Photos courtesy of Guillermo Debandi (A) (see https // www. inaturalist. org / taxa / 78750 - Prosopis-strombulifera / browse _ photos), Dr. Omesh Bajpai and Dr. Lal Babu Chaudhary (B), Zeynel Cebeci (C) (see https // commons. wikimedia. org / wiki / FileProsopis _ farcta _ - _ Syrian _ mesquite _ 01), N. Dreber (D) (see http // www. southernafricanplants. com /), Colin Hughes (E, F).

Figure 6. Fruits of Neltuma A Neltuma alba B N. argentina C N. kuntzei D N. denudans E N. laevigata F N. nigra G N. articulata H N. ruscifolia. (5 cm scale bar). All specimens at K A drawn from Hughes & Forrest 2312 B Guaglianone et al. 1762 C Nee & Coimbra 35556 D Seijo 1489 E Manriquez & Tenorio 6563 F Arenas 3123 G Hughes et al. 1559 H Wood & Mamani 14063. Drawn by Andrew Brown, July 2021.

Figure 7. Variation in fruits across Prosopis s. l. and allies A indehiscent pods of Anonychium africanum with thick pulpy mesocarp collected as fodder for livestock B plano-compressed pods of Indopiptadenia oudhensis lacking a thickened mesocarp and dehiscent along both sutures C indehiscent fruits of Prosopis farcta with a thick pulpy mesocarp D tightly coiled indehiscent screwbean fruits of Strombocarpa strombulifera E indehiscent pods of Strombocarpa ferox with a thick pulpy mesocarp F indehiscent fruits of Strombocarpa palmeri G small reniform to flabellate, flattened, indehiscent, 1 (- 2) - seeded, winged fruits of Xerocladia viridiramis which are unique within mimosoid legumes H indehiscent fruits of Neltuma articulata with a thick mesocarp and a hard bony segmented endocarp which remains closed I. Unripe indehiscent pods of Neltuma kuntzei with a thick pulpy mesocarp, these turning dark blackish-brown when ripe, reminiscent in colour to fruits of Anonychium. Photos courtesy of Marco Schmidt (A) (see Dressler et al. 2014), Dr. Omesh Bajpai and Dr. Lal Babu Chaudhary (B), Zeynel Cebeci (C) (https // en. wikipedia. org / wiki / Prosopis _ farcta), Dick Culbert (D) (see https // eol. org / pages / 640506, Colin Hughes (E, F, H, I), and Herta Kolberg (G) (see Plants of Namibia https // herbaria. plants. ox. ac. uk / bol / namibia).

Figure 8. The distributions of Indopiptadenia, Prosopis s. s., Anonychium, Xerocladia, Neltuma and Strombocarpa, based on 6,469 quality-controlled species occurrences from GBIF (www. gbif. org), DryFlor (www. dryflor. info), SEINet (www. swbiodiversity. org / seinet) and several other data sources (Ringelberg et al., in prep.). Map created using R packages ggplot 2 (Wickham 2016), sf (Pebesma 2018) and rnaturalearth (South 2017). The eight occurrence records, mapped in Bahia Brazil, are of Neltuma ruscifolia which is considered potentially native to that region (Burkart 1976 Oliveira & Queiroz 2020), while records of N. juliflora from Bahia, which is introduced and naturalised in that region, have been eliminated.