Here we describe and illustrate Dizygostemon riparius (Plantaginaceae, Gratioleae) as a new species from the state of Maranhão, northeastern Brazil. We present morphological and micromorphological descriptions and comparisons, along with illustrations, photographs, and information on its taxonomy, habitat, geographic distribution and conservation status. Dizygostemon riparius differs from D. floribundus, the only other accepted species of the genus, by having the stems sparsely to densely tomentose, the sepals as long as or slightly longer than the corolla tube, the posterior sepal to 1.5 times as wide as any of the other four, the ovary glandular villous, and the leaves larger. The new species is assessed as Endangered (EN) according to IUCN categories and criteria. Additionally, we provide diagnostic features that help to differentiate Dizygostemon from the similar Brazilian genera Achetaria, Stemodia and Tetraulacium.
Citation: Scatigna A. V., Brandão C. M., Colletta G. D., Teles R. de M., Cavalcante K. S. B., Souza V. C. & Simões A. O. 2019: Dizygostemon riparius (Plantaginaceae, Gratioleae), a new species from Maranhão, northeastern Brazil. – Willdenowia 49: 177–186. doi: https://doi.org/10.3372/wi.49.49206
Version of record first published online on 27 June 2019 ahead of inclusion in August 2019 issue.
Dizygostemon Radlk. ex Wettst. belongs to the Gratioleae, a mainly tropical tribe of the Plantaginaceae that comprises c. 25 genera and over 300 species (Albach & al. 2005; Estes & Small 2008; Scatigna & al. 2018). The genus, restricted to the northeastern region of Brazil, is currently composed of a single accepted species and is characterized by the herbaceous habit, the short-pedi-cellate and bibracteolate flowers, and the androecium composed of four fertile stamens, the anterior pair being monothecous and the posterior pair being bithecous with a bi-brachiate connective separating the thecae (Souza & Giulietti 2009).
Dizygostemon was described by Bentham (1846) as a monotypic section of Beyrichia Cham. & Schltdl., represented only by Beyrichia floribunda Benth., and was placed within the tribe Gratioleae. According to Bentham (1846) and Bentham & Hooker (1876), Dizygostemon differs from Achetaria Cham. & Schltdl., the other section of Beyrichia sensu Bentham (1846), by the posterior pair of stamens being fertile (vs sterile) and by the lo-culicidal capsule (vs septicidal). Subsequently, Wettstein (1891) elevated Dizygostemon to the generic level based on these differences and proposed the combination D. floribundus (Benth.) Radlk. ex Wettst. Later, Giulietti (1973) recognized D. angustifolius Giul. as a new species characterized by narrower leaf blades and a more restricted geographic distribution (Giulietti & Wanderley 1973; Souza & Giulietti 1990). More recently, Souza & Giulietti (2009) adopted a broader concept of D. floribundus, including D. angustifolius in its synonymy, and this circumscription is currently followed (BFG 2015, 2018).
The phylogenetic placement of Dizygostemon within the Gratioleae has not been tested, but the anthers with the thecae separated by a bi-brachiate connective was used by Bentham & Hooker (1876) to include Achetaria and Dizygostemon (then under Beyrichia) in the subtribe Stemodiineae (Stemodieae sensu Bentham & Hooker 1876) along with Adenosma Nees, Hydrotriche Zucc., Limnophila R. Br. (these three exclusive to the Old World), Lindenbergia Lehm. (currently in Orobanchaceae; Young 1999; Olmstead & al. 2001), Morgania (= Stemodia L.; Barker 1990), Stemodia and Tetraulacium Turcz. Moreover, Souza & Giullieti (2009) suggested a close relationship between Achetaria (currently at the generic level; Pennell 1940), Dizygostemon and Tetraulacium based on the calyx having the posterior sepal much larger than the other four, and the anterior pair of stamens being monothecous. In a recent phylogenetic study of tribe Gratioleae, Scatigna & al. (2018) recovered Achetaria as closely related to Stemodia in part, but not to Tetraulacium.
In Brazil, the only genera of Stemodiineae (sensu Bentham & Hooker 1876) with native representatives are Achetaria, Dizygostemon, Stemodia and Tetraulacium (Souza & Giulietti 2009; BFG 2015, 2018). In the consulted herbaria, some specimens of Achetaria, Dizygostemon and Tetraulacium were identified as Stemodia, probably due to their resemblance and the fact that the last is the largest genus of the group.
In 2011, one of the authors (R.M.T.) while on a trip back to his birthland, São Benedito do Rio Preto, Maranhão, Brazil, was attracted by the pleasant fragrance of a riparian herb, locally called “melosa”, on the bank of the Rio Preto River. This pleasant aroma aroused interest in plant identification and investigation of its essential oil composition. Specimens were sent for identification to the MAR herbarium, identified there as Stemodia foliosa Benth., and to SLUI herbarium, identified there as S. arizonica Pennell (= S. durantifolia [L.] Sw.). However, preliminary analysis of its essential oils revealed chemo-types that differ substantially from those known in some Stemodia species (Arriaga & al. 2007; Silva & al. 2009); therefore, the plant identity remained uncertain. In 2017, specimens were sent for identification to other two authors (A.V.S. and G.D.C.), both of whom are working on the systematics of Gratioleae, and they concluded that the “melosas” from the Rio Preto River represented an undescribed species of Dizygostemon.
Here we describe this new species of Dizygostemon from the State of Maranhão, northeastern Brazil. We include morphological and micromorphological evidence, along with illustrations, photographs, and information on its taxonomy, habitat, geographic distribution and conservation status. Additionally, we provide diagnostic features that help in differentiating Dizygostemon from Achetaria, Stemodia and Tetraulacium.
Material and methods
Morphological comparisons and the key in this work are based on field observation, direct examination of specimens of Dizygostemon housed in CEPEC, EAC, ESA, HUEFS, K, SLUI and UEC (herbarium codes according to Thiers 2018+), and consultation of digital images of specimens housed in G, IPA, MG, MO, NY and P, complemented with information available in the literature (Bentham 1846; Schmidt 1862; Bentham & Hooker 1876; Wettstein 1891; Pennell 1940; Giulietti 1973; Giulietti & Wanderley 1973; Ronse 2001; Souza & Giulietti 1990, 2009). All cited specimens were directly observed, except where otherwise stated. Additional records of the new species were obtained from the following herbaria: ALCB, BHCB, BHZB, CEN, CGMS, COR, CPAP, CTES, CVRD, DIAM, HCJS, HRB, HRCB, IAC, IAN, ICN, MBM, MBML, OUPR, PACA, R, RB, SP, SPF, UB and UPCB. Line drawings were made by Klei Sousa from fresh, fixed and herbarium specimen material under an Olympus SZH10 stereomicroscope with a drawing attachment, and also digital photographs of specimens in vivo. The distribution map was generated with ArcGIS, a product of the Environmental Systems Research Institute (ESRI; Redlands, California).
Micromorphological study — Fresh samples from stems, leaves, flowers and seeds of the new species were fixed and stored in 70% ethanol, whereas samples from dried material of Dizygostemon floribundus (Melo 3449 [ESA]) were rehydrated with boiling water and stored in 70% ethanol. The material was critical-point dried, mounted on aluminium stubs with double-sided adhesive tape and covered with c. 40 nm of gold in a SCD 050 sputter coat-er (Bal-Tec). Images were observed in a JSM 5800LV scanning electron microscope (Jeol, Japan), at 10kV, and captured with the software SemAfore 5.21.
Conservation status assessment — The conservation status evaluation was based on field observations and followed the IUCN Red List categories and criteria (IUCN 2012) and subsequent guidelines (IUCN 2017); we estimated the extent of occurrence (EOO) and area of occupancy (AOO) using the Geospatial Conservation Assessment Tool (Bachman & al. 2011) with a cell width of 2 km and based on the coordinates provided on specimen labels.
Results and Discussion
Defining the genus
Dizygostemon is one of the four genera of the subtribe Stemodiineae (sensu Bentham & Hooker 1876) that occur in Brazil (BFG 2018). It is similar to Achetaria in having the posterior sepal usually much wider than the other four, which are uniformly narrower, the corolla with a papillate lower lip (Fig. 4H), the corolla tube occluded by the inflated palate of the lower lip, touching the upper lip (Fig. 1D, E, 2A, B, C), and the anterior pair of stamens monothecous (Fig.1H left, 3J; Pennel 1940; Ronse 2001; Souza & Giullieti 2009). Dizygostemon can be confused with Stemodia in general aspect and is similar to this genus in the posterior pair of stamens being fertile. It is also similar to Tetraulacium in the four stamens being fertile and in the unequal sepals.
Dizygostemon differs from Achetaria in having the anterior pair of anthers free (Fig. 1G, 4L; vs adherent) and the posterior pair of the anthers fertile with unequal thecae (Fig. 1H right, 4I, J, L; vs sterile with equal thecae) (Giulietti & Wanderley 1973; Souza & Giulietti 2009). It differs from Stemodia in having the corolla tube occluded by the palate (vs not occluded), the anterior pair of stamens monothecous (vs bithecous) and the posterior pair of anthers with unequal thecae (vs equal). It also differs from Tetraulacium in having the corolla tube occluded (vs not occluded), bracteoles present (vs absent) and the posterior pair of stamens bithecous (vs monothecous).
Identification key to the species of Dizygostemon from northeastern Brazil
1. Sepals as long as or slightly longer than corolla tube; posterior sepal to 1.5 times as wide as any of other four; ovary glandular villous; leaves larger, 1.5–8 × 0.5–4 cm; stems sparsely to densely tomentose; corolla tube 5.2–6.2 mm long D. riparius
– Sepals shorter than half of corolla tube; posterior sepal more than 2 times as wide as any of other four; ovary glabrous to glandular puberulent; leaves smaller, 0.6–1.6 × 0.2–0.7 cm; stems glabrous to strigillose; corolla tube 2.5–6 mm long D. floribundus
Holotype: Brazil, Maranhão, São Benedito do Rio Preto, margem do Rio Preto, 03°19′28″S, 43°31′02″W, 28 m, planta com flor roxa, 30 Sep 2017, Teles s.n. (UEC[accession number]202426; isotypes: B, K, MAR).
Diagnosis — Dizygostemon riparius is characterized by its glandular villous ovary. It differs from D. floribundus in having stems sparsely to densely tomentose (vs glabrous to strigillose); sepals as long as or slightly longer than corolla tube (vs shorter than half of corolla tube); posterior sepal to 1.5 times as wide as any of other four (vs more than 2 times); ovary glandular villous (vs. glabrous to glandular puberulent); and leaves larger, 1.5–8 × 0.5–4 cm (vs 0.6–1.6 × 0.2–0.7 cm).
Morphological description — Subshrubs, to c. 50 cm tall, aromatic, viscid. Stems crawling to erect, subquad-rangular to clearly quadrangular, canaliculate, branched, frequently presenting adventitious roots, sparsely to densely tomentose, densely covered with non-capitate, flexuous trichomes, interspersed with short, capitate trichomes and sessile glands, denser towards apex. Leaves opposite or rarely 3-whorled, sessile or petiole to 1.5 cm long; blade narrowly ovate to elliptic, 1.5–8 × 0.5–4 cm, base attenuate, margin crenate-serrate, apex acute to obtuse; abaxial surface covered with short, capitate trichomes, sessile glands and more abundant, longer, curved, non-capitate trichomes, these concentrated on veins; adaxial surface covered with short, capitate trichomes and sessile glands interspersed with longer, non-capitate, curved trichomes scattered over surface. Flowers axillary, single or geminate, bracteo-late; pedicel 1–3 mm long, densely covered with short, capitate trichomes, sessile glands and abundant, longer, curved, non-capitate trichomes; bracteoles 2, opposite, narrowly elliptic to lanceolate, 2.9–7 × 1.5–1.7 mm, indumentum similar to leaves; sepals subequal, connate at base, lanceolate, 5.9–7.7 × 2–2.8 mm, posterior one slightly longer and wider than others, apex acute, all five slightly curved outwards, external surface indumentum similar to leaves, internal surface covered exclusively with sessile glands; corolla bilabiate, personate, entirely white or lilac with a pair of white patches on throat, tube 5.2–6.2 mm long, occluded by palate of lower lip, base slightly inflated, externally sparsely tomentose, covered with short, capitate trichomes, sessile glands and longer, curved, non-capitate trichomes, internally tomentose, upper lip 2-lobed, 5–6.2 × 4–4.5 mm, apex truncate to slightly emarginate, externally sparsely tomentose, covered with short, capitate trichomes, sessile glands and more abundant, longer, curved, non-capitate trichomes, lower lip 3-lobed, palate inflated, 3.5–5 × 4–6.5 mm, covered with short, non-capitate trichomes on throat, papillate, lobes 1.8–2.5 × 2–2.5 mm, apex emarginate. Stamens 4, all fertile, didynamous, reaching throat, filaments filiform, apex capitate, anterior pair 3.3–3.7 mm long, posterior pair 4.4–4.6 mm long, both with rare, short, capitate trichomes; anthers of anterior stamens with only one theca, 1–1.1 mm long, glabrous or with rare, sessile glands, anthers of posterior stamens with two thecae, slightly subequal, separated by bi-brachiate connective, 0.7–1 mm long, glabrous or with rare, sessile glands; staminode 1, much reduced. Ovary superior, 2-carpellate, 2-locular, ovoid, 2.8–3 × 1.9–2.1 mm, glandular villous, densely covered with long, glandular trichomes and sessile glands; placentation axillary, ovules numerous; style terminal, solitary, filiform, 4.3–4.6 mm long, sparsely glandular puberulent, persistent; stigma obdeltoid, 0.7–0.8 mm long, curved, glabrous. Capsule ovoid, 5.8–6 × 3.3–3.5 mm, with a deep, longitudinal groove on septum line, sparsely covered with long, glandular trichomes and sessile glands, dehiscence not observed; seeds ovoid, 0.45–0.5 × 0.3–0.31 mm, surface reticulate.
Micromorphological study — We observed non-glandular trichomes (type NG) and glandular trichomes (types G1, G2, G3 and G4). The NG type consists of a multicellular, uniseriate trichome with an acute head cell and tiny cuticular granules on the surface (Fig. 3A); its length is variable, reaching 2 mm in Dizygostemon riparius, but scarcely exceeding 0.5 mm in D. floribundus. Type G1 has a broad base, a uniseriate, 2- or 3-celled pedicel, and an apparently 1-celled, round head (Fig. 3B); this type is referred to as capitate throughout the text. Type G2 consists of an obscure, 1-celled pedicel and a wide (60–80 µm in diam.), multicellular head covered with a cuticle layer (Fig. 3C); this type is referred to as sessile glands in the key, diagnosis and descriptions, because this is how it appears under the stereo microscope. Type G3 has a short, 1-celled pedicel and a wide (10–30 µm in diam.), usually clearly 4-celled (sometimes 8- or 16-celled) head (Fig. 3D); this type is also referred to as capitate throughout the text. Type G4 is a variation of the NG type except for the slightly inflated head cell (Fig. 3E). All types of trichomes were found in both compared specimens, but with relatively different distribution and abundance among plant parts.
Occurrence and frequency of each type of trichome on aerial parts of Dizygostemon riparius and D. floribundus. Trichomes: – absent, ± rare, + present, ++ abundant.
Diagnostic characters between Dizygostemon riparius and D. floribundus.
The indumentum and micromorphological aspects of trichomes have been useful for recognizing species within some genera of Gratioleae such as Achetaria (including Otacanthus Lindl.), Philcoxia P. Taylor & V. C. Souza and Stemodia (Ronse 2001; Souza & Giulietti 2009; Scatigna & al. 2017a, 2017b). In this study, these characters were useful in distinguishing Dizygostemon riparius from D. floribundus (Table 1; Fig. 4A–F, O, P).
Both studied specimens were found to possess a reticulate seed surface with smooth to slightly cristate anticlinal walls and more or less papillate external periclinal walls. In Dizygostemon riparius (Fig. 3F, 4N), anticlinal walls are slightly cristate and relatively higher and the periclinal wall is more markedly papillate. In D. floribundus (Fig. 4M), anticlinal walls are smooth and relatively lower and the external periclinal wall is less markedly papillate. In addition, the cells delimited by anticlinal walls are larger and more angu-late in D. riparius.
Phenology — Specimens were observed with flowers and fruits from May to December, especially during the dry season. In São Benedito do Rio Preto, the population was found submerged in the waters of the Rio Preto River from January to June, during the rainy season.
Distribution and ecology — Dizygostemon riparius was recorded in three municipalities of the northeastern portion of Maranhão, Brazil (Fig. 5). Populations grow in periodically waterlogged habitats such as banks and margins of rivers and marshes (Fig. 2E); this riparian life-form is present in some species of Achetaria and Stemodia, which are not described as typical macrophytes (Souza & Giulietti 2009).
Conservation status — Although one of the recorded populations of Dizygostemon riparius is at least partly encompassed by a protected area (Parque Nacional dos Lençóis Maranhenses), we observed strong threats to the quality and extent of its habitat. Threats include inappropriate waste disposal, sand removal, and deforestation of riparian forest, leading to riverbed silting. The species is known from only three locations, with an EOO of 2027 km2 and an AOO of 12 km2. As based on the IUCN (2012, 2017) criteria, D. riparius should be considered Endangered [EN, B1ab(i,ii,iii)+2ab(i,ii,iii)]. New efforts to track populations of the new species along the river basin may provide additional information for a more accurate evaluation.
Etymology — The specific epithet refers to the riparian habit of the new species, which is found in dense patches along watercourse margins (Fig. 2D, E)
Vernacular name — Dizygostemon riparius is known as “melosa” by local people of São Benedito do Rio Preto because of its viscid aspect when rubbed between fingers. Its leaves, stems and roots release a sharp, refreshing and pleasant aroma and are used to scent clothes and in domestic animals' hygiene.
Remarks — The originally neuter terminations of the epithets of Dizygostemon angustifolius and D. floribundus have been corrected to agree with the masculine gender of Dizygostemon (Art. 23.5, 32.2 and 62.2(a) of the Shenzhen Code; Turland & al. 2018).
Based on the description of Dizygostemon floribundus provided by Schmidt (1862) and an examination of photographs of type specimens, Giulietti & Wanderley (1973) differentiated it from D. angustifolius by leaf blade dimensions, shape and margin. Souza & Giulietti (2009) considered these characters to be highly variable, even among type specimens of D. floribundus, and therefore included D. angustifolius under its synonymy. Dizygostemon floribundus, as currently circumscribed (sensu Souza & Giulietti 2009), has a wide range of morphological variation, especially regarding general indumentum (from glabrous to strigillose), leaf shape (linear, linear-lanceolate, ovate, obovate, oblanceolate), ovary surface (glabrous to glandular puberulent) and corolla dimensions. It occurs in the Caatinga domain in four states of the northeastern region of Brazil, namely Bahia, Ceará, Pernambuco and Piauí. Nevertheless, D. riparius has a unique set of morphological traits not encompassed by this variation; it has sparsely to densely tomentose indumentum (Fig. 4B, D, F), much larger leaves (Fig. 1C, 2A), a glandular villous ovary surface (Fig. 1I, 3E, 4P), and is restricted to the state of Maranhão, within the Cerrado domain. We observed two morphotypes in the population of D. riparius from São Benedito do Rio Preto, one with a lilac corolla with white patches on the lower lip (Fig. 1A, B) and the other with an entirely white corolla (Fig. 2C); in D. floribundus, the corolla is reportedly pale lilac (Souza & Giulietti 2009). For additional diagnostic characters, see Table 2.
Giulietti (1973) and Giulietti & Wanderley (1973) used only a few characters to recognize Dizygostemon angustifolius as a distinct species. Conversely, Souza & Giulietti (2009) examined a limited number of specimens when proposing its synonymization. It is possible that the broad concept of D. floribundus covers even more undescribed species. To properly assess this issue, we recommend thorough examination of several specimens, considering both morphological and geographic ranges, and the use of additional characters such as the types of trichomes and their distribution in the plants, as has proven useful within Achetaria, Philcoxia and Stemodia (Ronse 2001; Souza & Giulietti 2009; Scatigna & al. 2017a, 2017b).
Additional specimens examined — Brazil: Maranhão: Barreirinhas, PARNA dos Lençóis Maranhenses, Povoado de Santo Inácio, campos brejosos abertos, 02°34′49″S, 42°47′32″W, 7 m, 16 May 2015, Mota & al. 3099 (MG[digital image]); Primeira Cruz, Rodovia MA-402 margem direita (Barreirinhas-São Luís), zona rural, margem de lagoa temporária, 02°44′04″S, 43°22′57″W, 25 Sep 2014, Mota & Viana 2776 (MG[digital image]); São Benedito do Rio Preto, 10 Aug 2016, Brandão & Soeiro s.n. (SLUI[accession number]4587); idem, margem do Rio Preto, 03°19′28″S, 43°31′02″W, 1 Oct 2017, Planta com flor branca, Teles s.n. (ESA, HUEFS, UEC[accession number]202423); idem, margem do Rio Preto, 03°19′28″S, 43°31′02″W, planta com flor roxa, 1 Oct 2017, Teles s.n. (UEC[accession number]202425); idem, planta com flor branca, amostra de material cultivado, 21 Oct 2017, Brandão s.n. (UEC[accession number]202422); idem, planta com flor roxa, amostra de material cultivado, 21 Oct 2017, Brandão s.n. (UEC[accession number]202424).
A.V.S. and G.D.L. thank the Programa de Pós-graduação em Biologia Vegetal, UNICAMP, and CNPq for Ph.D. scholarships. A.V.S. thanks CNPq for a post-doctoral fellowship (159924/2018-9). C.M.B. thanks the Academic Department of Chemistry, Programa de Pós-graduação em Química, IFMA and FAPEMA for the M.Sc. scholarship. V.C.S. thanks CNPq for a productivity research grant. We thank the crew of the Laboratory of Electronic Microscopy (IB/UNICAMP) for all assistance; Dr. Nara Mota for providing high-resolution digital images of the specimens (paratypes) housed in the MG herbarium; Dr. Georgiana Eurides de Carvalho Marques, Mr. Jose Ubira-ci Pereira dos Santos, Mrs. Maria das Graças Rodrigues Pinto and Mrs. Alianda Pinto dos Santos for field support; Ermenson Martins e Silva for some of the photographs; Raquel Moura Machado for preparing Fig. 2, 3 and 4; Patricia Messias for help with preparing the map (Fig. 5); Klei Sousa for the line drawing (Fig. 1); Dr. Peter W. Fritsch for improving the text; and Dr. Carlos Alexandre Holanda for valuable suggestions on an early version of the manuscript. Finally, we thank Dr. María de las Mercedes Sosa, two anonymous reviewers and Nicholas Turland, whose comments helped to improve the manuscript.