First record of Isachne ligulata (Poaceae: Micrairoideae) in Brazil and a key for the Brazilian species of the genus

While performing a taxonomic revision of the Neotropical species of Isachne, analysis of herbarium collections revealed the occurrence of Isachne ligulata in Brazil. We provide a description and the fi rst illustration of I. ligulata, along with comments on its ecology, distribution and morphological affi nities with related South American species. We also provide an identifi cation key for the Brazilian species of the genus.

Isachne R. Br. includes ca. 100 species (Veldkamp 2016), and is the largest and the most widespread genus of the subfamily Micrairoideae.Th e genus has its greatest diversity in tropical and temperate Asia (Judziewicz 1990), but also occurs in Africa, the Americas, Pacifi c Islands and Australia (Clayton et al. 2006).Th irteen species are recorded for the Neotropics (Rodrigues et al. 2016), and are distributed from southern Mexico to Bolivia and southeastern Brazil (Renvoize 1987).Despite the extensive area of occurrence of the genus Isachne in the Neotropics, most of the species are restricted to Mesoamerica, the Antilles and northern South America.
Historically, Isachne was placed in the subfamily Panicoideae, and considered as a distinct genus or as a section of Panicum L., mainly because of the apparent similarity of the spikelets in the members of these two genera.Surprisingly, recent phylogenetic studies resolved Isachne (tribe Isachneae Benth.) as belonging to a clade that also included the tribes Micraireae Pilg.and Eriachneae Eck-Boorsb., within the resurrected and emended subfamily Micrairoideae (Duvall et al. 2007;Sánchez-Ken et al. 2007).Currently, Isachne is included in tribe Isachneae and is artifi cially divided into two sections according to the degree of similarity of the anthecia.Isachne sect.Isachne includes the species with heteromorphic anthecia (i.e., anthecia very dissimilar or completely diff erent in texture and pilosity), whereas Isachne sect.Albentes V. Prakash & S.K. Jain includes the species with homomorphic anthecia (i.e., anthecia similar or almost similar in texture and pilosity) (Iskandar & Veldkamp 2004;Veldkamp 2016).
Isachne comprises annual or perennial plants, usually with trailing or climbing culms, which may vary from being short and delicate to long, robust, rigid, thick and very branched (Renvoize 1987).Th e genus can be recognized by the spikelets disarticulating above the subequal glumes; the anthecia frequently homomorphic (less often heteromorphic); mostly coriaceous to subcoriaceous; the lower staminate, pistillate or bisexual; the upper bisexual or pistillate (Hitchcock 1920;Clayton & Renvoize 1986).
Flowering in January, March, April, June, and from August to November.In the Guyana highlands, according to Judziewicz (1990), the species flowers from September through February.Distribution and ecology: Isachne ligulata is an endemic South American species reported from mountainous areas of the Amazon Basin and the Guiana Shield, at altitudes ranging 1700-2500 m.It was originally described from Colombia but later its distribution was found to extend to Guyana, Peru and Venezuela (Swallen & García-Barriga 1943;Renvoize 1987;Zuloaga & Morrone 2003).Individuals are found along forest edges, especially along streams (Judziewicz 1990), and are sometimes associated with shrubby vegetation.This is the first record of I. ligulata for Brazil, 73 years after its original description (Swallen & García-Barriga 1943).In Brazil this species is known only from the region of Santa Isabel do Rio Negro, in the state of Amazonas, in Pico da Neblina National Park, near the border with Venezuela.The long span of time between the original description and the discovery of the species in the Brazilian Amazon is probably due to the difficulty of accessing the area where it is found and there having been few studies undertaken there, most of them focused on woody rather than herbaceous species, and are conducted in relatively limited and isolated locations (BFG 2015).
Conservation: In Brazil, I. ligulata has only been recorded from a conservation unit, the Pico da Neblina National Park.The lack of information about the species' distribution and population size are barriers to assessing the conservation status of this species in Brazil, and so according to IUCN criteria, it should be considered as data deficient (IUCN 2015).
Morphological affinities: As commonly occurs with other species of Isachne, the culms in I. ligulata rebranch at the tips and produce one inflorescence branch per node (Fig. 1).
Specimens currently accepted under the circumscription of I. ligulata used to be primarily identified as I. arundinacea Griseb., a species recorded for Mexico, Mesoamerica, and northern and northwestern South America (Zuloaga & Morrone 2003;Clayton et al. 2006).
Isachne arundinacea has been traditionally circumscribed as having longer leaf blades, glabrous sheaths, ascending panicles, often with secondary branches and pedicels appressed to the spreading primary branches; the pedicels are extremely short, and the spikelets are 1.4-1.8mm long, globose, and clustered at the branch tips.Micromorphological characters of the leaf blade and spikelets also support I. ligulata and I. arundinacea as distinct species (RS Rodrigues & TS Filgueiras unpubl.res.).
The morphology of the spikelets of I. ligulata closely resembles that of I. rigens Trin., another Meso-and South American species not yet recorded for Brazil, but which has a similar habit and distribution (Zuloaga & Morrone 2003;Clayton et al. 2006).Isachne rigens, however, can be distinguished by the more delicate culms, panicle size, spikelets regularly or loosely distributed along the branches, and pedicels usually inserted directly on the primary branches.
The morphological distinctions between I. ligulata and other species that occur in Brazil are highlighted in the following dichotomous key.

Figure 1 .
Figure 1.Isachne ligulata.A: apex of the culm with flowering branches; B: apex of the leaf-sheath and base of leaf blade; C: apex of the leaf-sheath, ligular region and base of leaf blade; D: detail of the adaxial surface of leaf blade; E: spikelet, lateral view; F: anthecia (note the pilosity at the rachilla between the upper and the lower anthecia); G: lower anthecium, lemma view; H: lower anthecium, palea view; I: upper anthecium, lemma view; J: upper anthecium, palea view.[Based on: A -JJ Wurdack 34138 (RB); B-J -RC Forzza et al. 7275 (SP)].Illustration by Rodrigo S. Rodrigues.