Chroesthes (Acanthaceae) in Peninsular Malaysia, including a new species from Kelantan and a new record from Terengganu

Abstract Chroesthes is a small genus that includes three species from Peninsular Malaysia: Chroesthes faizaltahiriana Siti-Munirah sp. nov., C. lanceolata (T. Anderson) B.Hansen and C. longifolia (Wight) B.Hansen. Chroesthes faizaltahiriana, recently discovered in the State of Kelantan, is described and illustrated. This species is similar to the common species C. longifolia, but is distinguished mainly by its inflorescence type, calyx shape and its flowers being bright orange instead of dark purple internally. Chroesthes lanceolata is a new record for Peninsular Malaysia and has only been collected once. Following the IUCN Red List Categories and Criteria, these three species are assessed (national scale assessment) as Critically Endangered (C. faizaltahiriana and C. lanceolata) and Least Concern (C. longifolia).


Introduction
The small genus Chroesthes Benoist (1927), Acanthaceae, currently consists of three species distributed in South China, Indo-China and Malaysia (Hansen 1983;Govaerts 1999;POWO 2019). In the current classification of the family Ranunculaceae, it is placed within the putative members of tribe Barlerieae (Scotland and Vollesen 2000;McDade et al. 2008;Champluvier and Darbyshire 2012). This genus can be distinguished from other genera of Acanthaceae by the structure of the calyx and corolla, the presence of bicalcarate anthers, i.e. anther thecae spurred at the base or awned anther thecae and are diagnostic within Barlerieae, for which quincuncial aestivation is diagnostic and seeds are shortly pubescent (Hansen 1983;Hu et al. 2011;Darbyshire et al. 2019). The three species of Chroesthes are distributed as follows: Chroesthes lanceolata (T. Anderson) B. Hansen occurs in Myanmar, N Thailand, N Laos, N Vietnam SW China (Yunnan) and Peninsular Malaysia (Hansen 1983;Ummul-Nazrah et al. 2011); Chroesthes bracteata (J.B. Imlay) B.Hansen occurs in SE Thailand (Hansen 1983); and Chroesthes longifolia (Wight) B.Hansen is endemic in Peninsular Malaysia (Hansen 1983).
The Forest Research Institute Malaysia (FRIM) launched the Peninsular Malaysia Flora Project (FPM) in 2005, which aims to provide a more complete and up-to-date account of the national flora (Middleton et al. 2019). To achieve this goal, the FRIM flora team continues to conduct vegetation surveys throughout Peninsular Malaysia, especially in unexplored and ecologically-significant areas. Through this fieldwork, the number of species encountered within the Flora region has increased. A few new species are discovered every year and published in journals. For example, many new fairy lantern species (Thismia)  During a recent botanical survey at Berangkat Forest Reserve (FR), Kelantan, an upright shrub with an unusual bright orange corolla colour was encountered growing in a patch of forest, under shade beside an old logging road. Its morphological characteristics, including its stamen type, indicated that it belongs to the genus Chroesthes, but is unmatched amongst the three species currently recognised. This new species is described here as Chroesthes faizaltahiriana Siti-Munirah, which brings the total species for Chroesthes in the world to four. It is also an additional endemic species in Peninsular Malaysia. With this recent discovery, an account of the genus Chroesthes in Peninsular Malaysia is provided, including the key to Chroesthes of the world; however, only the Malaysian species are considered in the remainder of the treatment.

Materials and methods
Specimens of Chroesthes species from Peninsular Malaysia, held in the herbaria at Kepong Herbarium (KEP) and Singapore Botanic Gardens Herbarium (SING), were examined. All cited specimens were observed by the author. The study of the new species was based on material collected by the author on 26 February 2020 from the only populations found during a botanical trip to Berangkat FR, Gua Musang, Kelantan. The specimens were pressed as herbarium specimens and an inflorescence was also preserved as a spirit collection. Morphological characters were studied using a stereomicroscope and high-resolution macro photography. Measurements were taken from living plants and herbarium material. The specimen details were compared with original drawings and descriptions given in the protologues for each species of the genus Chroesthes and also with information gathered from the relevant literature (Wight 1850;Clarke 1908;Ridley 1923;Hansen 1983, Hu et al. 2011. The conservation status assessments were made with reference to the Categories and Criteria of IUCN (2019).
Ecology. Chroesthes faizaltahiriana is found in upper hill dipterocarp forests under shade at 822 m elevation. It was found flowering in February in patches of unlogged forest beside a logging road (Fig. 4).

Etymology.
Chroesthes faizaltahiriana is dedicated to Mohd Faizal Mat Tahir (known as Faizal Tahir), the husband of Siti-Munirah for his strong support in many ways towards the author's botanical work.
Conservation status. Critically Endangered B2 ab(ii,iii). Following the IUCN Red List Categories and Criteria (IUCN 2019), this species is assessed as critically endangered because it is only known from one locality and AOO is less than 10 km 2 . Furthermore, about eight individuals were observed (all at the flowering stage). The collection locality is within the forest reserve, in an area of disturbed forest near a logging road at an elevation of 822 m. The status of the population is uncertain because the active selective logging activity within the forest reserve is ongoing. It is certainly not within a Totally Protected Area. During the 3-day visit to Berangkat FR, no other populations of C. faizaltahiriana were found. A further survey is needed to obtain more information that could estimate and determine the population size of this species.
Notes. Based on the general morphology of this plant, C. faizaltahiriana is close to C. longifolia, which was previously the only known species of Chroesthes in Peninsular Malaysia. However, a detailed comparison has shown that its inflorescence type is entirely different (Fig. 5, Table 1). Additionally, its phytogeography is also different when compared to C. faizaltahiriana, which is endemic to Gunung Berangkat (Kelantan) in upper hill dipterocarp forest (822 m a.s.l.), while C. longifolia is a widely distributed species inhabiting lowland dipterocarp forest. Based on herbarium specimen collections of C. longifolia at Kepong Herbarium (KEP) and the Singapore Herbarium (SING), it has been recorded in Kedah, Perak, Kelantan, Terengganu, Pahang, Selangor, Negeri Sembilan, Melaka and Johor. C. faizaltahiriana is a distinctive      And., J. Linn. Soc., Bot. 9: 524. 1867. Type: Myanmar, Pegu, Thaungyin, Brandis s.n. (holotype CAL).
Conservation status. Critically Endangered B2 ab(ii). Following the IUCN Red List Categories and Criteria (IUCN 2019), this species is assessed as critically endangered at a national level in Peninsular Malaysia because it is currently known from only one specimen in one locality. It is certainly a very rare species. The forest area is lowland dipterocarp forest which was previously logged in the past and it is not a Totally Protected Area. Globally, its conservation status possibly lists it as least concern (LC), by its wide range distribution.