Macrolobium aracaense Farroñay, 2018

Farroñay, Francisco, Adrianzén, Marisabel U., Perdiz, Ricardo De Oliveira & Vicentini, Alberto, 2018, A new species of Macrolobium (Fabaceae, Detarioideae) endemic on a Tepui of the Guyana Shield in Brazil, Phytotaxa 361 (1), pp. 97-105 : 98-103

Macrolobium aracaense Farroñay , sp. nov. ( Figs. 1 , 2 )

Type: — BRAZIL. Amazonas: Mun. Barcelos, Plateau of Serra do Aracá, 0°56’54” N, 63°23’20” W, 1261 m.a.s.l., 27 August 2001, A. Vicentini & R. C. Mesquita 1886 (holotype INPA!, isotype MO!).

Macrolobium aracaense is similar morphologically to M. longipes . Both species have bifoliolate leaves, with leaflets of similar shape (oblong-ovate to oblong-elliptic) and a rounded apex. Also, they share a papillary epidermis on the abaxial surface. They differ by the leaflets with strongly revolute margin and always bifoliolate in M. aracaense (vs. 1–3 pairs of leaflets with margin flat or slightly revolute), the bracteoles are green (vs. bracteoles red), and the flowers with a single white petal (vs. red petal), 4 sepals (vs. 5 sepals), stamens with sparsely pubescent filaments (vs. stamens with filaments glabrous), and ovary villose at the margin (vs. ovary totally glabrous). In addition, M. aracaense occurs only above 1000 m.a.sl., while M. longipes is a lowland species.

Treelet 1– 4 m. Branchlets brownish, essentially glabrous. Stipules early caducous. Leaves always bifoliolate; petioles 2.5–4.4 mm long; leaflets sessile to subsessile, 4–6.8 × 1.8–3.4 cm, coriaceous, oblong-ovate, oblong-elliptic, margin strongly revolute, moderately inequilateral at the base, apex rounded or retuse, both surface glabrous and discolour, adaxially lustrous, abaxially opaque with papillary epidermis; midvein slightly impressed above, prominent and glabrous below, secondary venation discernible on both surfaces. Inflorescences terminal and axillary, racemose, 5–9 cm long; peduncles 1.2–3.1 mm long. Bracteoles 10.6–12.7 × 5.6–7.9 mm, glabrous, persistent during fruit development, greenish. Flowers with pedicels 7.2–9.6 cm long; hypanthium cupuliform 3.1–3.7 mm long, 2.7–3.4 mm diameter, glabrous; sepals 4, elliptic to oblong-ovate, 5.6–7.7 × 5.5–6.6 mm, green with apex minutely ciliate; single petal, undulate, 8.5–11.2 × 6–9.7 mm, basally obtuse, apically rounded, glabrous, white with green lines; stamens 3, filaments 11.8–16.8 mm long, sparsely pubescent along the basal third; gynoecium slightly sigmoid; ovary 3.2–3.8 × 1.9–2.2 mm with 3–4 ovules, villose at the margin; gynophore villose 3–3.5 mm long; style glabrous, stigma capitellate. Fruit 8–8.8 × 3.9–5 cm, green, acumen 1 mm long. Seed not observed.

Distribution and habitat: — Macrolobium aracaense is so far only known from two samples collected on the plateau of the Serra do Aracá, at 1200–1286 m high, in open scrublands ( Figure 2 E–F ). This vegetation type grows in areas where white sand has accumulated together with the rocks. According to Prance & Johnson (1992), the common woody species on this vegetation type are Humiria balsamifera Aublet (1775: 564) , Calliandra tsugoides Cowan (1958: 143–144) , Calyptranthes clusiifolia (Miquel: 1849: 533) O. Berg (1857: 39), Ilex costata Edwin (1965: 137) and Tetrapterys cordifolia Anderson (1987: 107) .

Flowering and fruiting: —Both flowering and fruiting plant specimens were collected in August.

Conservation status: — Macrolobium aracaense is known from only two collections. Due to incomplete information about its distribution, we assign the new species to the category of Data Deficient (DD), according to IUCN (2017) criteria.

Etymology: —The specific epithet “aracaense ” alludes to the type locality on the Serra do Aracá, the only place where this species was found.

Discussion: —According to the taxonomic revision of the genus Macrolobium ( Cowan 1953) , this species should be included in Macrolobium section Vouapa Aublet (1775: 25) , because of the cupuliform hypanthium, flowers with 4–5 sepals and an unguiculate petal. Within this section, there are only 12 species with bifoliolate leaves like M. aracaense , and M. longipes is the most similar, both are also species from white-sand vegetation. However, Macrolobium . aracaense differs from M. longipes in many morphological characteristics ( Table 1). The presence of papillary epidermis on the abaxial surface of the leaflets of M. longipes and M. aracaense ( Figure 4 C–D ) is associated with epicuticular wax and makes the leaf surface non-adherent and free of contaminants ( Neinhuis & Barthlott 1997). Waxes form a protective barrier against water loss due to excessive perspiration, pathogen action, solar radiation and chemical inputs ( Domínguez et al. 1998). Other species of Macrolobium from savannas in Roraima also present leaves with papillary epidermis ( Ferreira & Flores 2013).

Macrolobium aracaense has a restricted distribution and is apparently endemic to the plateau of the Serra do Aracá (1020–1260 m elevation), while M. longipes occurs at low elevations (100–150 m) in the Upper Rio Negro and Upper Orinoco regions ( Cowan 1957, Félix-da-Silva et al. 2016; Figure 3 ). Although they are allopatric and occurr at different elevations, hence reproductive isolation between these species could not be inferred, the large morphological and ecological differences indicate that it is an independent lineage that deserves to be formally recognized as a new species.

Additionally, Macrolobium aracaense is sympatric on the plateau of the Serra do Aracá with two other taxa of Macrolobium - M. discolor Bentham (1870: 222) var. discolor and and M. gracile Spruce ex Bentham (1870: 223) var. confertum ( Gleason 1931: 371) Cowan (1953: 274) . M. aracaense is morphologically different from M. discolor var. discolor by possessing 1 pair of leaflets and glabrous inflorescence (vs. 3–7 pairs of leaflets and tomentose inflorescence), and from M. gracile var. confertum by having 1 pair of glabrous leaflets (vs. 18–30 pairs of tomentose leaflets).

Characters Macrolobium aracaense M. longipes

Stipules Early caducous 2 × 0.6 mm

Petiole 2.5–4.4 mm long 4–15 mm long

Pairs of leaflets Only 1 1–3 pairs

Leaflet apex Rounded, obtuse or truncate Rotund, truncate

Black spots on abaxial face Absent Present

Leaflet margin Strongly revolute Sometimes slightly revolute

Inflorescence 5–9 cm long 8–12.5 cm long

Petal White, 8.5–11.2 mm long Red, 9–15 mm long

Ovary Villose Glabrous

Filament Sparsely pubescent Glabrous

Habitat Sandstone of Serra do Aracá Savanna and White-Sand Forest

Additional specimens examined (paratypes): — BRAZIL. Amazonas: Mun. Barcelos, Serra do Aracá State

Park, 0°86’35” N, 63°33’16” W, 1020 m.a.s.l., August 2017, F. Farroñay. & R. Oliveira 178 (INPA!).

Figures

FIGURE 1. Macrolobium aracaense. A. Flowering branch. B. Stamens. C. Detail of an anther. D. Flower. E. Gynoecium. F. Hypanthium and pedicel. G. Single petal. H. Bracteole. I. Sepal. J. Fruit. (A–J from Farroñay & Oliveira 178, K from Vicentini & Mesquita 1886). Drawing by Marisabel U. Adrianzén.

FIGURE 2. Macrolobium aracaense. A. Fertile specimen. B. Lateral view of flower. C. Frontal view of flower. D. Frontal view of flower bud. E. Lateral view of flower bud. F. Vegetation of white-sand rocky scrub. G. Panoramic view of Plateau of Serra do Aracá. Each white line represents 1 cm. Plant images from Farroñay & Oliveira 178.

FIGURE 3. Distribution map of Macrolobium aracaense Farroñay (circles) and M. longipes R.S. Cowan (triangles).

FIGURE 4. Comparison of Macrolobium aracaense with M. longipes. A–B. Fruits. C–D. Papillary epidermis. E–F. Leaflets. G–H. Abaxial surface. (A, C, E, G. Macrolobium aracaense from Vicentini & Mesquita 1886. B, D, F, H. Macrolobium longipes from Huber & Medina 5849).