A threatened new species of Ipomoea (Convolvulaceae) from the Brazilian Cerrado revealed by morpho-anatomical analysis

Abstract A new species of Ipomoea, endemic to the Cerrado domain in Maranhão, Brazil, is described. Ipomoea maranhensis D.Santos & Buril, sp. nov. has been misidentified as I. burchellii Meisn. in several herbaria. Even though both species have oblong, pubescent leaves, they can be distinguished by morpho-anatomical characters. We present a diagnosis, complete description, illustration, taxonomic comments, conservation status and distribution map.


Introduction
Ipomoea (Convolvulaceae) comprises approximately 700 species (Staples 2015) and is widely distributed throughout the world, but is predominantly pantropical and absent in Mediterranean areas and temperate climates (Staples and Brummitt 2007). In Brazil, it is represented by 149 species distributed in all regions and phytogeographic domains (Flora do Brasil 2019). The genus can be distinguished by its echinate pollen (Simão-Bianchini 1998). Recently, knowledge of Ipomoea has been increased with the publication of several new species (e.g. Wood et al. 2015;Vasconcelos et al. 2016;Wood et al. 2017a, b;Wood and Scotland 2017a, b), many of which are endemic to Brazil (Wood et al. 2017c;Santos et al. 2019;Santos et al. in press).
Among Brazilian phytogeographic domains, the Cerrado stands out as a center of diversity and endemism for several plant groups (Simon et al. 2009). Despite being one of the 34 global biodiversity hotspots (Mittermeier et al. 2004), this domain has been intensely devastated in recent decades due to the expansion of agriculture and livestock (Cunha et al. 2008). Of the 12,113 Cerrado angiosperm species (BFG 2018), 366 are considered endangered (Martinelli et al. 2014). The genus Ipomoea is represented by 92 species in the Cerrado biome, which corresponds to 22% of the family's diversity in the country (Flora do Brasil 2019). Of these, I. macedoi Hoehne, I. maurandioides Meisn. and I. sobrevoluta Choisy are considered endangered and do not occur in protected areas (Martinelli and Moraes 2013), and may become extinct if we do not take any conservation initiative.
While analyzing Ipomoea collections from CEN, HST, HUEFS, and SLUI (acronyms follow Thiers 2019), we found a morphotype collected in the municipality of Carolina, Chapada das Mesas National Park in the State of Maranhão, that was dubiously identified as I. burchellii Meisn. When we analyzed the protologue and the type collection of I. burchellii, we noted significant differences in the morphology and indumentum of sepals between this species and the morphotype. After analyzing several I. burchellii specimens, we found that these differences are consistent.
We consulted literature related to climbing and subshrubby species of Ipomoea from South America (O'Donell 1948;O'Donell 1959a, b;Simão-Bianchini 1998;Léon 2006;Krapovickas 2009;Wood et al. 2015;Wood et al. 2017a, b, c;Wood and Scotland 2017a, b), type specimens from online database (http://plants.jstor. org), and Brazilian herbaria through SpeciesLink network (http://www.splink.org.br) in order to investigate similiar species to this morphotype. However, the combination of morphological features found in the morphotype did not match those of any known species.
To support the morphological delimitation between this morphotype and I. burchellii, we performed a comparative anatomical analysis of the leaf, as this has been used to support the morphological delimitation of species in various plant groups (Lersten 1974;Gomes et al. 2005;Rio et al. 2005;Oliveira et al. 2011;Thadeo et al. 2014), as well as in Convolvulaceae (Ketjarun et al. 2016;Traiperm et al. 2017). Thus, considering the taxonomic alleged consistency of the anatomical characters analyzed in Convolvulaceae (Metcalfe and Chalk 1979), along with the morphological discontinuities found, we describe this morphotype as a new species.

Morphological analysis
We analyzed specimens from the following herbaria: CEN, HUEFS, SLUI (acronyms follow Thiers 2019), and HST (unindexed herbarium from the Universidade Federal Rural de Pernambuco). Morphological terminology followed Harris and Harris (1994). The specimen collected in the Carolina municipality, Chapada das Mesas National Park, Maranhão state, was preserved according to standard taxonomic techniques (Mori et al. 1989) and deposited in SLUI. The main diagnostic characters of the species were illustrated based on the type specimen.

Comparative anatomical analysis of the leaf
For comparative anatomical analyses, three I. burchellii specimens and two vouchers of the new species were included in this study (Table 1). Three leaves from the fourth node and parts of the petiole of each specimen were rehydrated according to Smith and Smith (1942). Then, the samples were placed in 2% potassium hydroxide solution at room temperature for two hours. Subsequently, the material was washed with distilled water three times. The samples were progressively dehydrated from 10% alcohol until their final storage in 70% ethanol (Johansen 1940). The median region of blade and petioles were free-hand sectioned and clarified with 50% sodium hypochlorite and stained with safrablue (safranin and Astra-Blau, Bukatsch 1972). The slides were prepared with glycerin and fixed with enamel (Kraus and Arduin 1997). The slides were deposited into the Plant Anatomy Laboratory (LAVeg) at the Universidade Federal de Pernambuco. The analysis and documentation were performed under a Leica DM500 microscope. The anatomical terminology followed Metcalfe and Chalk (1979).
Anatomical description -petiole and leaf blade. Petiole epidermis uniseriate with juxtaposed cells, parenchyma with isodiametric cells, prominent/concave main rib shape, U-shaped vascular bundles. Leaf epidermis uniseriate, mesophyll dorsiventral with crystallized idioblasts (druses), palisade parenchyma with one to two layers of elongated     cells more evident on the adaxial surface, spongy parenchyma with three to four layers of round cells with sinuous anticlinal walls; glandular trichomes on both leaf surfaces. According to the anatomical analysis, we observed that I. maranhensis and I. burchellii share mesophyll with crystallized idioblasts (druses) and petiole parenchyma with isodiametric cells. However, I. maranhensis is distinguished from I. burchellii by a prominent/concave main rib shape (vs. convex/flat main rib shape in I. burchellii), Ushaped vascular bundles (vs. V-shaped), glandular trichomes spread on both leaf surfaces (vs. only on abaxial one) and dorsiventral mesophyll (vs. isobilateral) (Fig. 3) (Table 2).
Phenology. Collected with flowers in April. Distribution and habitat. Ipomoea maranhensis is known only from two disjunct populations between the municipality of Mirador, in the Mirador State Park (area of 4370 km 2 ) and the municipality of Carolina, in the National Park Chapada das Mesas (CMNP, area of 1600 km 2 ) (Fig. 4). In both areas this species grows on quartzite soils associated with Cerrado vegetation at 186-345 m elevation, average temperature of 26 °C and annual precipitation between 1250-1500 mm (Alcântara 2004;IBAMA 2013). The occurrence of this new species in the National Park Chapada das Mesas and in the Mirador State Park emphasizes the importance of these protected areas for the preservation of this taxon in the Cerrado domain.
Etymology. The specific epithet refers to Maranhão state, where the type specimens were collected.
Conservation status. We categorized this species as Endangered (criteria EN) according to B1 (EOO < 5000 km 2 ) and B2ab (ii, iii) (AOO < 500 km 2 ) from IUCN (2019). Although populations of I. maranhensis occur in protected areas, this species is threatened due to the reduction of its habitat because of anthropogenic fires lit by small communities who remove vegetation with fire for subsistence agriculture in the Cerrado sensu stricto and Cerradão formations (Ribeiro and Walter 2008). According to Estivalet (1997), prostrate or climbing plants seem to be more prone to burning than upright species, whose growth points are more protected inside the clump.
Identification key for Ipomoea species from the Chapada das Mesas National Park and Mirador State Park

Discussion
The new species has been confused with I. burchellii because they share oblong, discolorous and sericeous leaves, flowers arranged in a dichasium and peduncle 2-3 mm long. However, according to the analysis of the type specimens (Burchell 8738 deposited in K [K000612855]) and protologue of I. burchellii, the new species is morphologically different from I. burchellii by its habit, shape, apex, and indumentum of sepals. Misidentifications probably occurred because both species have oblong, discolorous leaves, 2-3 mm long peduncles and inflorescence arranged in dichasium, as well as because they occur in the Cerrado domain. Anatomical analysis revealed that the morphological delimitation of these species is supported by the shape of the vascular bundles, distribution of glandular trichomes in the leaf and type of mesophyll. These characters are considered consistent (Metcalfe and Chalk 1979) and useful for Convolvulaceae taxonomy (e.g. Ketjarun et al. 2016;Traiperm et al. 2017). Ipomoea maranhensis can also be compared to I. langsdorffii Choisy, an endemic species from the Southeastern region of Brazil occuring in the Cerrado and Atlantic forest, due to its oblong leaves and flowers in dichasia. However, I. maranhensis can be distinguished from this species by its acute leaves (vs. obtuse in I. langsdorffii), lanceolate (vs. ovate), long-acuminate (vs. acute), and densely sericeous (vs. hirsute) sepals. These morphological characters present great taxonomic value for Ipomoea (Simão-Bianchini 1998;Ferreira and Miotto 2009;Wood et al. 2015).
One of the barriers that can hinder access to knowledge about the diversity of the genus Ipomoea is that several of its new species have been described based on the morphology from one or two specimens without the without using a tool to support the morphological delimitation (Wood et al. 2017a, b). Such a limited number of specimens can lead to misinterpretation about the consistency of characters, making it difficult to recognize these species. In these cases, investigating other sources of characters is important for preventing the proliferation of names that cause taxonomic confusion and nomenclatural instability. Anatomical studies have been used to support the morphological delimitation of species in various plant groups (Smith and Smith 1942;Lersten 1974;Gomes et al. 2005;Rio et al. 2005). Among the leaf anatomical characters used to support such delimitation are mesophyll type, main vein shape, and vascular bundle type (Gomes et al. 2005;Gomes et al. 2008;Zini et al. 2016).
In Convolvulaceae, these characters have also been consistent and informative, such as the type of mesophyll that was used to delimit three species of Evolvulus (Ketjarun et al. 2016), the shape of the main vein and vascular bundles used to clarify the relationship between morphologically similar Argyrea species (Traiperm et al. 2017). Furthermore, anatomical characters strongly supported species identification in an investigation of Merremia section Xanthips (Pisuttimarn et al. 2013). Such anatomical information has been used in these studies because it has proven to be useful and informative for taxonomic identification of plants (Thadeo et al. 2014).