Novelties in the genus Viridantha Espejo (Tillandsioideae, Bromeliaceae)

Abstract Based on morphological evidence, we propose to raise Tillandsia mauryana forma secundifolia to species level with the name Viridantha secundifolia (Ehlers) Hern.-Cárdenas, Espejo & López-Ferr. Viridantha secundifolia can be readily distinguished by the falciform rosettes, the broadly oblong to square, 1–1.2 × 0.8–1.1 cm leaf sheaths and by the 1.8–2 × 0.7–1.2 cm floral bracts. Additionally, we describe and illustrate Viridantha uniflora Hern.-Cárdenas, Espejo & López-Ferr., from the state of Oaxaca, Mexico. The new species is morphologically similar to Viridantha boqueronensis, but differs by the nearly square leaf sheaths, 1.3–1.5 × 0.4–0.5 cm spikes and by the presence of only one flower per spike. A key to the taxa, morphological descriptions, list of specimens examined, illustrations and a distribution map of the described taxa are included.


Introduction
The genus Viridantha Espejo (Tillandsioideae, Bromeliaceae) is endemic to Mexico and includes 14 species (Hernández-Cárdenas et al. 2018). Due to its morphological characteristics, Smith and Downs (1977) Gardner (1986) included Viridantha in Tillandsia subgenus Tillandsia and Till (2000) included it in Tillandsia subgenus Allardtia, but as a separate group from the rest of the species in that subgenus. Espejo-Serna (2002), based on morphological characteristics, considered that this group of species constituted a distinct genus from Tillandsia L. and called it Viridantha.
Viridantha species are herbaceous plants with leaves arranged in acaulescent rosettes; protandrous flowers with petals dark green towards the apex and white towards the base; stamens equal in length, included, with filiform filaments and sub-basifixed anthers and simple-erect type style branches (Espejo-Serna 2002). The last taxonomic revision for Tillandsioideae subfamily, based on multi-loci DNA sequences phylogeny, proposed to circumscribe Viridantha and the Tillandsia tectorum E. Morren complex as Tillandsia subgenus Viridantha (Espejo) W. Till & Barfuss (Barfuss et al. 2016). However, some of the systematic changes proposed by Barfuss et al. (2016) were not supported by molecular and morphological data (Gomes-da-Silva and Souza-Chies 2017).
It should be mentioned that, in all the phylogenies reconstructed so far, Tillandsia emerged as polyphyletic (Gardner 1986;Terry et al. 1997;Horres et al. 2000;Barfuss et al. 2004Barfuss et al. , 2005Barfuss 2012;Barfuss et al. 2016;Gomes-da-Silva and Souza-Chies 2017) or paraphyletic (Terry and Brown 1996;Benzing et al. 2000;Givnish et al. 2007;Donadío et al. 2015) and, consequently, its validity as a formal taxonomic entity (genus) is unacceptable. On the other hand, Viridantha has always been monophyletic and related to the Tillandsia tectorum complex (Barfuss et al. 2004(Barfuss et al. , 2005(Barfuss et al. , 2016Barfuss 2012). The members of Viridantha have morphological, ecological and geographical coherence, all the species are easily recognised by their vegetative and reproductive characteristics and can be distinguished from the rest of the species of Tillandsia s. l. Furthermore, Viridantha species are mostly saxicolous and all are endemic to Mexico. Moreover, the species of the Tillandsia tectorum complex are clearly distinct from Viridantha by the presence of caulescent rosettes and petals purple towards the apex and white towards the base, besides the taxa of T. tectorum clade being endemic to northern Peru and southern Ecuador (Hromadnik 2005). By the above mentioned reasons, we maintain Viridantha as a genus. It becomes necessary to carry out more studies using a larger number of species and/or characteristics to propose a more precise and objective classification of Tillandsia s.l. and their relatives.
As a result of botanical explorations for the project Phylogeny of the genus Viridantha Espejo (Tillandsioideae; Bromeliaceae), we collected specimens of two different populations of Viridantha: the first one in the vicinity of Tolantongo, in the municipality of Metztitlán, state of Hidalgo, corresponds to Tillandsia mauryana L. B. Sm. forma secundifolia Ehlers. This form can be readily distinguished from the typical form by the falciform rosettes, the broadly oblong to square, 1-1.2 × 0.8-1.1 cm leaf sheaths, by the 1.8-2 × 0.7-1.2 cm floral bracts and other morphological characters, so we propose to raise it to species level. The second one comes from the municipality of Santos Reyes Tepejillo, in the state of Oaxaca. Initially, we thought that these specimens could correspond to Viridantha boqueronensis (Ehlers) Hern.-Cárdenas, Espejo & López-Ferr.; however, after a careful and detailed revision of living and herbarium specimens, including types, we conclude that these populations correspond to an undescribed taxon.

Viridantha secundifolia
Habitat and ecology. Viridantha secundifolia is only known from the state of Hidalgo in the western and eastern regions of Metztitlán and Tolantongo municipalities, respectively, where it grows on vertical walls in xerophilous scrubs at elevations between 1100 and 1900 m a.s.l. (Figs 1, 2).
Phenology. Blooming in January and February.
Observations. Tillandsia mauryana was described by Lyman B. Smith (1937), based on specimens from the canyon of Metztitlán, in the state of Hidalgo, Mexico. Espejo-Serna (2002) transferred the species to Viridantha. Ehlers (2009) described T. mauryana L. B. Sm. forma secundifolia, differentiating it from the typical form only by the secund disposition of its leaves. Besides, Ehlers (2009) mentioned that its populations grow separated from those of T. mauryana. The detailed analysis of the morphological characteristics of living and dried specimens of T. mauryana forma mauryana and T. mauryana forma secundifolia allowed us to detect that, in addition to the characteristic mentioned by Ehlers (2009), there are other differences in the plants of both populations such as: the shape of the rosettes (falcate vs. spherical); the length (1-1.2 cm vs. 1.5-2.5 cm) and the shape (broadly oblong to square vs. broadly elliptic) of the leaf sheaths; the length of the floral bracts (1.8-2 cm vs. 1-1.5 cm), the presence or not of a keel on the floral bracts (absent or visible only in the apex vs. present along the bract); and the colour of the anthers (pale to dark green vs. black).
Viridantha grandispica ( (Table 1). Viridantha secundifolia differs from V. grandispica in the shape of the rosettes (falcate vs. spherical); in the shape of the leaf sheaths (broadly oblong to square vs. narrowly oblong); in the presence or not of a keel on the floral bract (absent or visible only in the apex vs. present along the bract); and in the shape of the sepals (lanceolate vs. ovate). Viridantha secundifolia differs from V. rzedowskiana in the shape of the rosettes (falcate vs. spherical); in the size of the leaf sheaths (1-1.2 cm × 0.8-1.1 cm vs. 1.8-2 cm × 1.5-1.7 cm); in the keel of the floral bract (absent or visible only in the apex vs. present along the bract); and in the shape of the sepals (lanceolate vs. ovate). Viridantha secundifolia differs from V. teloloapanensis in the shape of Diagnosis. Viridantha uniflora is similar to V. boqueronensis but differs in the shape (square vs. ovate to triangular) and the width of the leaf sheath (0.7-0.8 cm vs. 1 cm); the width (0.4-0.5 cm vs. 0.7-1.3 cm) of the spikes, the number of flowers per spike (always 1 vs. 2-5); and in the shape of the floral bract (ovate vs. elliptic).
Habitat and ecology. Viridantha uniflora is only known from the boquerón of the Santos Reyes Tepejillo municipality, located in the Sierra Madre del Sur in the northwest region of the state of Oaxaca, where it grows on vertical walls in dry oak forests and tropical deciduous forests. The plants of V. uniflora grow in colonies, between 1700 and 1900 m a.s.l., on the cliffs of the boquerón amongst other saxicolous herbs. (Figs 2, 3).
Phenology. The plants of Viridantha uniflora bloom in April and May. Etymology. The specific epithet refers to the presence of one flower per spike, condition only known in the proposed taxon.
Observations. Plants of Viridantha uniflora had previously been collected by J.I. Calzada 20057 (MEXU), but had been wrongly identified as V. atroviridipetala (Matuda) Espejo. However, V. uniflora differs from V. atroviridipetala in the outline shape of the rosettes (falcate vs. spherical); in the shape of the leaf sheaths (square vs.  Hernández-Cárdenas et al. 2156, UAMIZ). V. boqueronensis C spike (voucher: K. and R. Ehlers EM7851,MEXU). V. penascoensis D spike (voucher: Hernández-Cárdenas and Sarabia 2116, UAMIZ). Photographs A-B by E. Negri Lavín; C-M by R. Hernández-Cárdenas. oblong to ovate); in the shape of the floral bracts (ovate vs. lanceolate to narrowly triangular) and in the number of flowers per spike (1 vs. 2-5). Viridantha boqueronensis and V. penascoensis grow in nearby locations to the type locality of V. uniflora, but without overlapping its distributions. These species share the saxicolous habit and the falcate rosettes in outline. However all these species are easily distinguishable from the newly proposed taxon (Table 2). Viridantha uniflora differs from V. penascoensis in the inflorescence (branched vs. simple); in the length and in the colour (green vs. red-pink) of the spikes (1.3-1.5 cm vs. 2-3 cm). Viridantha uniflora differs from V. boqueronensis in the shape of the leaf sheaths (square vs. ovate to triangular); in the number of flowers per spike (1 vs. 2-5); in the colour of the spikes (green vs. green with red-pink); in the shape of the floral bracts (ovate vs. elliptic); and in the presence or not of a keel on the floral bracts (absent or visible only in the apex vs. present along the bract. Additional specimens examined (paratypes). MEXICO, Oaxaca: Distrito Santiago Juxtlahuaca, municipio de Santos Reyes Tepejillo. 3 km al N de Santos Reyes Tepejillo rumbo a Corral de Piedra (17°27'N, 97°57'W), 1770m a.s.l., 20 July 1995; en los alrededores del boquerón de Santos Reyes Tepejillo (17°26'58"N, 97°56'29"W), 1960m a.s.l., 18 March 2017, R. Hernández-Cárdenas, F. Gómez y A. González 2120.
To facilitate the identification of the species of Viridantha, we include an artificial key for all representatives of the genus.