Arthrocereus grandiflorus D.R. Gonzaga & Menini Neto, 2021

Gonzaga, Diego Rafael, Fernandez, Eduardo Pinheiro, Moraes, Lucas, Neto, Luiz Menini & Peixoto, Ariane Luna, 2021, A giant among the giants: Arthrocereus grandiflorus (Cactaceae, Cactoideae), a new species from the Brazilian Cerrado, Phytotaxa 510 (2), pp. 179-186 : 180-185

Arthrocereus grandiflorus D.R. Gonzaga & Menini Neto , sp. nov. ( Fig. 1A–L ).

Type:— BRAZIL. Minas Gerais: Diamantina, distrito de Mendanha / Inhaí , 18°0’8’’S, 43°34’7’’W, ca. 740 m a.s.l., 22 January 2018, fl., Gonzaga & Campanharo 1133 (holotype RB!, isotype HDJF!) .

Diagnosis:— Arthrocereus grandiflorus resembles A. melanurus subsp. odorus from which differs by its larger cladodes (50–80 × 1.4–3.5 vs. 20–50 × 1.8–2.5 cm), larger flowers (14.5–16 × 8–9 vs. 12–14 × 8.5 cm), filaments (2.0–6.2 vs. 1.2–3 cm) and stamens strongly divided into two series (vs. one series).

Description (macromorphology):—Terricolous to rupicolous plants, 50–80 cm high, light green cladodes, 1.4–4 cm diameter, branched only at the base, ribs 9–13, ca. 2 × 3 mm, areole 1.5–3.0 mm diameter, spaced 5–10 mm apart, spinescent, yellowish to grayish spines, central 2.7–5.0 cm long, radial 3–9 mm long. Brown floral buds, hirsute, grayish-brown trichomes. Flowers 14.5–16.0 × 8–9 cm, white, sub-apical to lateral, of nocturnal anthesis; pericarpel 1.5–1.6 × 1.1–1.2 cm, pubescent, reduced leaves 1.0–1.5 × 0.5–1.0 mm, triangular, floral tube 9.5–11.0 × 1 cm, brownish-green, both endowed with areole with grayish-brown trichomes and reduced, triangular to lanceolate leaves 3–5 × 1–2 mm; segments of the perianth 55–62, the external 1.2–3.5 × 0.15–0.40 cm, lanceolate, greenish-brown, the internal 3.6–6.0 × 0.5–1.1 cm, lanceolate, white; stamens 224-269, divided into two series, filaments 2.0– 6.2 cm long; style 11.0–11.1 × 0.15 cm length, yellowish-white, stigma ca. 8 mm length, 10–12 lobed, lobes ca. 8 mm; fruit imature, globose.

Micromorphology of pollen grain: —The pollen grains is large (50–100 µm in diameter), isopolar, 3-colpate, oblate spheroidal, polar area is small, subtriangular and subcircular amb, long colpi, widest colpi (ca. 3.6 µm wide), sexine less thick, exine microechinate, perforate with annular perforations conspicuous ( Fig. 2 ). Palynological evidence shows the separation of pollen grains from the nearest species, mainly in the pollen variables of equatorial diameter, polar view equatorial diameter, length, and width of the bodies; however, they are similar to the number of apertures, presenting three ( Gonzaga et al. 2019) and morphometric analyses using vegetative and reproductive flower variables ( Gonzaga 2020).

Etymology: —The specific epithet refers to the large flowers.

Phenology: —Flowering time in January. The flowers present nocturnal anthesis and start opening around 7 p. m., and around 9 p. m. are already fully open. The flowers have a pleasant odor.

Distribution and habitat: — Arthrocereus grandiflorus was found in two subpopulations in Diamantina municipality, Minas Gerais state in Espinhaço Range ( Fig. 3 ). They are geographically separated by about 50 km. The first is in the vicinity of the Parque Nacional das Sempre Vivas ( Diamantina Plateau ), being accessed by the district of Mendanha-Inhaí, at elevation of 740 m ( Fig. 4A–B ). This subpopulation is sympatric with other cactus species such as Cereus jamacaru Candolle (1828: 467) , Cipocereus minensis (Werderm. 1933: 93) F.Ritter (1979: 57) and Pilosocereus aurisetus (Werderm. 1933: 103) Byles & G.D.Rowley (1957: 66) . The second subpopulation is in the district of Senador Mourão, BR 451, 16 km, at elevation of 998 m. ( Fig. 4C–D ), in a small rocky area surrounded by plantations of Eucalyptus spp. A specimen of this second subpopulation was collected sterile in 2016 and remains under cultivation in the Cactarium of Jardim Botânico do Rio de Janeiro but has not bloomed. Both subpopulations were located by Mr. Gerardus Olsthoorn. This collector has a sample of this same growing population in his private collection, located in the municipality of Holambra, São Paulo state, and flowered in cultivation in December 2018.

Notes: — Arthrocereus grandiflorus was cited as Arthrocereus sp. by Herm et al. (2001) who published a photograph of a specimen from Mendanha district from Diamantina municipality. Taylor & Zappi (2004) identified this specimen as A. melanurus subsp. odorus N.P. Taylor & Zappi (1997: 7) , mentioning that it is a sample of HU 1555 in cultivation, once they did not saw the flowers in the field, and no herbaria samples were found.

The Parque Nacional das Sempre Vivas (PNSV) is located in the Serra do Espinhaço and was created on 13 December 2002. Its main vegetation is the Cerrado, occupying 124,154.47 hectares, covering the municipalities of Bocaiúva, Buenópolis, Diamantina, and Olhos-D’Água, in Minas Gerais state. Arthrocereus grandiflorus can be considered as a microendemic species according to Rappini et al. (2008). In another part of the PNSV occurs A. rondonianus Backeberg & Voll (1943: 62) , however, it differs by vegetative size, the coloring of spines, and flowers with magenta perianth segments.

A comparison with the subspecies of A. melanurus (K.Schum. 1890: 200) Diers, P.J. Braun & Esteves (1987: 314) , shows that A. grandiflorus has the following distinguishing characteristics:. Arthrocereus melanurus subsp. melanurus is the most widely occurring taxon of the genus, occurring in the states of Minas Gerais and São Paulo, has shorter style (5 vs. 11.1 cm), more stigma lobes (20 vs. 12), more stamens (587 vs. 269) and shorter filaments (0.8–1.6 vs. 2.8–6 cm). Arthrocereus melanurus subsp. magnus N.P. Taylor & Zappi (1997: 7) is endemic and restricted to the Atlantic Forest phytogeographic domain, presenting the largest vegetative dimensions (1.5–2 vs. 0.5–0.8 m), smaller pericarpel (0.9 × 1 vs. 1.6 × 1.2 cm), shorter style (9.5 vs. 11.1 cm), more stigma lobes (16 vs. 12), fewer stamens (230 vs. 269).

Conservation status:— Arthrocereus grandiflorus is known from only two subpopulations located in areas of campos rupestres of the Cerrado domain in Diamantina municipality, which is embedded within the Espinhaço Range at Diamantina Plateau. Once is known only from two geographically separated subpopulations it does not have an associated EOO polygon and has an estimated AOO of 8 km ². Several threats may affect both subpopulations, like private mining ( Morais et al. 2016; Freitas 2017), agriculture ( Fernandes et al. 2005), and fires ( Ávila & Souza 2012). Furthermore, the species was found occurring very near eucalypt plantations, which also poses a major threat. Species of this genus are also subject to predatory plant collections by humans for ornamental purposes, which may lead to sharp declines in the number of mature individuals. Both known subpopulations are outside protected areas, which might increase species’ vulnerability in face of severe anthropogenic events. Considering the intensity of the described stress vectors, one single location is considered. No information on population size and trends is known and so the species could not be assessed based on its population size and reduction rate, although it is a rare species and it is suspected that might be declining based on the incidence of the threats mentioned above. Therefore, based on the minimum values of EOO and AOO, combined with one single location and an estimated continuing decline in its EOO, AOO, extend and quality of habitat, the number of locations, and possibly in the number of mature individuals, the species is here assessed as CR [criterion B2ab(i,ii,iii,iv,v)], due to its narrow distribution area associated to multiple threats that may affect the species habitat and individuals.

Selected specimen examined:— Arthrocereus grandiflorus . BRAZIL. Minas Gerais, Diamantina, distrito de Mendanha / Inhaí, 18°0’8’’S, 43°34’7’’W, ca. 740 m a.s.l., 22 January 2018, fl., D. R . Gonzaga & W. A . Campanharo 1132 ( RB!); distrito de Senador Mourão, ca. 998 m a.s.l., December 2018, G . Olsthoorn 208 ( RB!, paratype) .

Arthrocereus melanurus subsp. odorus . BRAZIL. Minas Gerais, Jaboticatubas, Distrito de Almeida , ca. 801 m a.s.l., 29 September 2015, fl., R. C .C. Reis et al. 2275 ( RB!); Estrada de Acesso a Portaria Areias , Parque Nacional Serra do Cipó , ca. 804 m a.s.l., 30 October 2016, fr., D. R . Gonzaga et al. 791 ( RB!); Estrada de acesso para o Parque Nacional Serra do Cipó , ca. 804 m a.s.l., 23 September 2017, fl., D. R . Gonzaga et al. 1002 ( RB!) .

Figures

FIGURE 1. Arthrocereus grandiflorus D.R. Gonzaga & Menini Neto.A. Habit; B. Branch detail; C. Branch in cross-section, detail of the number of ribs; D–E. Floral buds: lateral view; F. Floral bud in pre-anthesis; G. Flower: lateral view; H. Flower: front view; I. filaments in two externally visible series; J. Dry flower; K. Pubescent pericarp; L. Fruit. Photos by D.R. Gonzaga (A-K) and G. Olsthoorn (L).

FIGURE 2. Photomicrographs and electromicrographs of pollen grains of Arthrocereus grandiflorus. Polar view:A. optical section (LM), B. general aspect (SEM); Equatorial view: C. optical section (SEM), D. general aspect (SEM); E. ornamentation detail (SEM). Scale bars: A,C: 5 μm; B,D: 10 μm; E: 2 μm.

FIGURE 3. Geographic distribution of Arthrocereus grandiflorus and A. melanurus subsp. odorus in Minas Gerais state.

FIGURE 4. Areas of occurrence of Arthrocereus grandiflorus. A–B Mendanha-Inhaí, C–D Senador Mourão, boths in Diamantina municipality. Photos by D.R. Gonzaga.