Mesosetum aureum Anádria, A.R.O. Ribeiro & R.C. Oliveira, 2025

Silva, Anádria Stéphanie Da, Ribeiro, André Rodolfo De Oliveira, Reis, Priscila Alves Dos, Buso, Glaucia Salles Cortopassi & Oliveira, Regina Célia De, 2025, Mesosetum aureum (Poaceae: Paspaleae: Arthropogoninae), an annual new species with reddish golden trichomes from the northern South America, Phytotaxa 702 (2), pp. 166-174 : 167-172

Mesosetum aureum Anádria, A.R.O. Ribeiro & R.C. Oliveira , sp. nov. ( Figs. 1–4 )

Type:— FRENCH GUIANA: Mont Saint-Marcel, Southeastern zone of the massif, 300 m, 2°23’00”N, 53°00’20”W, 22 July 2002, fl. and fr., J.J. de Granville, L. Aliker & C. Sarthou 15423 (holotype: CAY! [CAY037442]; isotypes: P! [P02541087], B! [B100144248], K!, MO, U, US)

Diagnosis:— Mesosetum aureum is distinguished from all other known species in Mesosetum sect. Mesosetum by having annual life form (versus perennial), racemes ending in a meristematic branch (versus racemes ending in a developed spikelet), pedicels 1.5‒2.0 mm long (versus 0.5‒1.0 mm long), and the pilosity of lower and upper glume, and upper anthecium 2.0‒ 2.4 mm long (versus ≥ 3.0 mm long).

Description:— Caespitose annual grass. Culms 25‒40 cm tall, slender, decumbent to erect, no branching; nodes glabrous. Leaves membranaceous, mostly clustered in basal portion. Leaf sheaths shorter than internodes, pilose at the margin, the younger papillose-hirsute. Ligules ± 1 mm long, ciliate. Blades 3.5‒6(‒10) cm long, 2‒3 mm wide, lanceolate, flat, widely acuminated, pilose on both surfaces, with long marginal tuberculous trichomes; base slightly broader than the sheath. Peduncles 6.5‒10 cm long, terete, glabrous. Synflorescence composed of a single raceme 3‒6 cm long; rachis ± 0.2 mm wide, glabrous, apex ending in a meristematic branch; pedicels 1.5‒2.0 mm long, glabrous, apex oblique. Spikelets solitary, 3.0‒ 4.5 mm long, 0.8‒1 mm wide, imbricate in 2 series, dorsiventrally compressed, with trichomes around at base; golden to ferrugineous trichomes abundant, whitish trichomes absent or sometimes sparse; callus attached to the pedicel upon spikelet disarticulation. Lower glumes 1.8‒2.5 mm long, 0.4‒0.7 mm wide, flat, apex acute, muticous, 5-nerved, membranaceous, pilose in the proximal portion on the back, the trichomes reaching 2 mm long. Upper glumes 3.0‒ 4.5 mm long, 0.7‒1.0 mm wide, apex strongly acuminated, 5- nerved, membranaceous, silky-pilose on the back along the nerves. Lower anthecium neuter; lower lemma 2.5‒3.1 mm long, 0.6‒1 mm wide, apex acuminated, 5‒7-nerved, membranaceous, silky-pilose on the margins with a few thicker trichomes in the middle portion; lower palea lacking. Upper anthecium bisexual, 2‒2.4 mm long, 0.5‒1 mm wide, narrowly ellipsoid, glabrous; lodicules 2, minute; stamens 3, anthers ± 1 mm long, purple. Caryopsis 1.2‒1.5 mm long, 0.5 mm wide, elliptical, hilum linear.

Etymology:— The specific epithet refers to the abundant golden or ferrugineous trichomes on the glumes and lower lemma surface, which give a golden appearance to the synflorescence ( Fig. 2 , 3 ).

Distribution and habitat:— Mesosetum aureum is presently known from a few localities in the territory of French Guiana, near the border with Amapá State, Brazil ( Fig. 5 ). The new species occurs in inselbergs, rocky savannas and humid areas, and grows on soils rich in organic matter. In the Neotropics, inselbergs are elevations with a special vegetation type surrounded by rain forest ( Sarthou & Villiers 1998, Barbosa-Silva et al. 2022).

Conservation status:— Mesosetum aureum presents a restricted distribution with the Extent of Occurrence (EOO) equal to 6,862 km 2 and Area of Occupancy (AOO) of 16 km 2, falling in the threshold of a threatened category according to criterion B. However, the species occurs in a protected area (Guiana Amazonian Park) and it is not possible to infer either a continuing decline or extreme fluctuation. According to Plouvier et al. (2012), French Guiana has the highest forest cover in the world, with only 2% of its area suffering forest loss. Also, the average annual deforestation during the period between 2009 and 2018 has decreased in French Guiana from around 2000 ha to 700 ha per year ( Rahm et al. 2021). Considering that the species does not meet any of the required B subcriteria, it is assessed here as Least Concern (LC). To confirm that the species remains well conserved, research action is recommended, such as increased fieldwork, monitoring the population trend, habitats, and potential local threats.

Additional material examined (paratypes):— FRENCH GUIANA. Inselbergs du haut Marouini—A: Inselbergs de la D.Z., 220 m, 02°36’30”N, 54°01’45”W, 21 June 2004, J.J. de Granville & F. Crozier 16271 (CAY[CAY059419], P[P02541086], MO, not seen, US, not seen). Inselbergs de la haute Wanapi: zone du dôme central, 200 m, 02°31'00"N, 53°49'20"W, 14 April 2004, J.J. de Granville, F. Crozier & C. Sarthou 15916 (CAY[CAY057857], P[P02541078]). Inselbergs Roche-Dachine, 200 m, 03°28’N, 53°13’W, 5 April 1997, J.F. Villiers & C. Sarthou 6042 (CAY [CAY012423], P[P02541077]). Frontière Guyane-Brésil, borne 4, 13 April 1983, Feuillet 1005 (CAY[CAY200662], P [P02541075]).

Micromorphology of the upper anthecium:— Mesosetum aureum has a lanceoloid upper anthecium with an acuminate apex and a rounded base ( Fig. 4–A ). The upper lemma does not have simple papillae, but has compound papillae on the entire abaxial surface (including the germination lid surface), with one compound papilla per cell ( Fig. 4–A–D ). The coalescence between the larger papillae is rarely present ( Fig. 4–B ). The cruciform silica cells are absent. The unicellular macrohairs are present only on the distal surface ( Fig. 4–B ). The bicellular microhairs are present on the distal surface and rarely present on the middle surface ( Fig. 4–B, D ). The germination lid is present on the base of the lemma and is narrower than the lemma width ( Fig. 4–C ). The germination lid is bilobed with connate lobes ( Fig. 4–C ). The abaxial surface of the upper palea has a similar micromorphology, except for the absence of macrohairs and microhairs.

Comments:— Among the species belonging to Mesosetum sect. Mesosetum , M. aureum is more closely related to M. rottboellioides and M. cayennense since all three species share the oblique apex of pedicel, golden to ferrugineous spikelets, lower lemma with a few thicker trichomes in the middle portion than the other trichomes of lower lemma, and upper anthecium up to 3.5 mm long ( Silva et al. 2023). Therefore, only these species were included in Table 1.

Mesosetum aureum is distinguished from M. cayennense and M. rottboellioides by its annual life form (versus perennial), racemes ending in a meristematic branch (versus racemes ending in a developed spikelet), pedicels 1.5‒2.0 mm long (versus 0.5‒1.0 mm long), lower glume 1.8‒2.5 mm long (versus 2.8‒3.5 mm long) with trichomes along the marginal nerves (versus two tuffs of trichomes on the middle portion), the apex of the upper glume strongly acuminate (versus acute or 2-dentate), and upper anthecium 2.0‒ 2.4 mm long (versus 3.0‒ 3.2 mm long) ( Table 1).

In Figure 6 , the morphological comparison of the glumes and lower lemma highlights the distinguishing characteristics of Mesosetum aureum in relation to M. cayennense and M. rottboelioides . Notably, M. aureum exhibits unique patterns and textures in these structures, supporting its recognition as a distinct species.

Figures

FIGURE 1. Mesosetum aureum. A. Habit. B. Detail of dorsal view of rachis showing pedicels with the spikelet callus attached to the pedicel upon spikelet disarticulation. C. Spikelet, dorsal view. D. Spikelet, lateral view. E. Detail of dorsal view of lower lemma showing the apex strongly acuminate. F. Upper anthecium, dorsal view. G. Upper anthecium, ventral view. Illustration by Darli Nuza from the holotype.

FIGURE 2. Photographs of the synflorescence of Mesosetum aureum. A. Synflorescence segment showing the rachis and spikelets, J. J. Granville et al. 16271 (CAY). B. Apex of the synflorescence ending in a meristematic branch, J. J. Granville et al. 16272 (CAY). C. Rachis segment showing pedicels with the spikelet callus attached to the pedicel upon spikelet disarticulation, J. J. Granville et al. 16249 (CAY).D. Detail of the rachis segment showing spikelet pedicel with an oblique apex, J. J. Granville et al. 16271 (CAY). Scale bars:A–D: 1 mm. Photos by Anádria Stéphanie da Silva.

FIGURE 3. Photographs of the spikelet of Mesosetum aureum. A–B. Spikelet, J. J. Granville et al. 16271 (CAY). A. Ventral view. B. Lateral view. C. Lower glume, dorsal view, J. J. Granville et al. 16271 (CAY). D. Upper glume, dorsal view, J. J. Granville et al. 15423 (CAY). E. Lower lemma, dorsal view, J. J. Granville et al. 15423 (CAY). F. Upper anthecium in dorsal view showing the upper lemma, holotype J. J. Granville et al. 15423 (CAY). G. Caryopsis showing the linear hilum, J. J. Granville et al. 16271 (CAY). Scale bars: A–G: 1 mm. Photos by Anádria Stéphanie da Silva.

FIGURE 4. Micrographs taken by Scanning Electron Microscopy (SEM) from the upper anthecium of Mesosetum aureum, J. J. Granville et al. 15423 (CAY). A. Upper anthecium in dorsal view. B–D. Abaxial surface of the upper lemma. B. Apex surface showing unicellular macrohairs (Ma) and bicellular microhairs (Mi). C. Proximal portion showing the germination lid. D. Detail of the middle surface of the lemma. Scale bars: A: 500 μm; B–C: 100 μm; D: 50 μm.

FIGURE 5. Distribution map of Mesosetum aureum in the Guiana Amazonian Park, French Guiana.

FIGURE 6. Comparison of the glumes and lower lemma of new species (Mesosetum aureum) with related species (M. cayennense and M. rottboellioides). A–C. M. aureum. A. Lower glume, dorsal view, J. J. Granville et al. 16271 (CAY). B. Upper glume, dorsal view, J. J. Granville et al. 15423 (CAY). C. Lower lemma, dorsal view, J. J. Granville et al. 15423 (CAY). D–F. M. cayennense. D. Lower glume, dorsal view, P.A. Reis et al. 181 (UB). E. Upper glume, dorsal view, P.A. Reis et al. 181 (UB). F. Lower lemma, dorsal view, P.A. Reis et al. 181 (UB). G–I. M. rottboellioides. G. Lower glume, dorsal view, P.A. Reis et al. 331 (UB). H. Upper glume, dorsal view, P.A. Reis et al. 331 (UB). I. Lower lemma, dorsal view, P.A. Reis et al. 331 (UB). Scale bars: 1 mm.