﻿Three new species of Mitrephora (Annonaceae) from Thailand

﻿Abstract Three new Mitrephora species (Annonaceae), M.langsuanensissp. nov., M.sirindhorniaesp. nov. and M.sukhothaiensissp. nov., are described from Thailand. Mitrephoralangsuanensis resembles M.macclurei, but its leaves have more secondary veins, and its flowers have more carpels, with yellow-and-pink striped outer petals with a margin that becomes undulate with age. Mitrephorasirindhorniae resembles M.tomentosa, but has larger sepals and petals, longer flowering pedicels, and shorter monocarp stipes, with monocarps that have a longitudinal ridge. Mitrephorasukhothaiensis is distinct from its Thai congeners in having outer petals that reflex at maturity and inner petals with a pair of appendages on the margin adaxially. The addition of these three new taxa raises the total number of Mitrephora species in Thailand to 14. An identification key for Thai species is provided.


Introduction
Mitrephora Hook.f. & Thomson (Annonaceae subfam. Malmeoideae tribe Miliuseae: Chatrou et al. 2012;Guo et al. 2017) is a horticulturally important genus of small tropical and subtropical trees that often bear attractive, flamboyant flowers. The only comprehensive taxonomic revision of the genus (Weerasooriya and Saunders 2010) recognised 47 species, although three additional species have since been described (Okada 2014;Damthongdee et al. 2019;Saunders and Chalermglin 2019).
The flowers are pendent and have two petal whorls, each of three parts. The inner whorl forms a mitriform dome ('type III' sensu Saunders 2010) that forms a floral chamber that is likely to function in secondary pollen presentation, capturing pollen that is released from the anthers and retained by hairs on the inner surface of the floral chamber (Saunders 2020). As with most Annonaceae species, the flowers are bisexual: although Mitrephora species are self-compatible, self-fertilisation is largely avoided by protogyny (Weerasooriya and Saunders 2010;Pang and Saunders 2014). Mitrephora species are likely to be pollinated by small beetles (Weerasooriya and Saunders 2010). The flowers are apocarpous and give rise to fruits comprising separate 'monocarps' that are derived from individual fertilised carpels.

Mitrephora langsuanensis
Phenology (in cultivation). Flowering between May and June; fruiting in May. Distribution and habitat. Endemic to Chumphon Province, Peninsular Thailand (Fig. 3). Growing on limestone hill in tropical rain forest; ca. 100 m alt.
Phenology (in cultivation). Flowering in April and fruiting in July. Distribution and habitat. Endemic to Bueng Kan Province, North-Eastern Thailand (Fig. 3). Growing on sandstone hill in dry dipterocarp forest; ca. 200 m alt.
Etymology. Named after Her Royal Highness Maha Chakri Sirindhorn, in honour of her project on plant germplasm conservation in Thailand.
Discussion. Mitrephora sirindhorniae is characterised by its sparsely hairy abaxial leaf surface, flowers with long outer petals (40-60 mm) that are broadly oblanceolate, and by its densely pubescent fruits with a longitudinal ridge. The species is morphologically most similar to M. tomentosa Hook.f. & Thomson, from which it differs as its leaves are sparsely hairy abaxially (vs densely hairy), with flowers with larger sepals (8-10 by 10-12 mm vs 5-9 by 5-9 mm), outer petals Diagnosis. Mitrephora sukhothaiensis is distinct from all other species in having a combination of outer petals that reflex at maturity as well as inner petals that have inwardly folded marginal protrusions at the midpoint adaxially.
Phenology (in cultivation). Flowering between December and March to June; fruiting between May and July.
Distribution and habitat. Endemic to Sukhothai Province, Northern Thailand (Fig. 3). Growing in mixed deciduous forest; ca. 360 m alt.
Etymology. From the name Sukhothai province.  Discussion. Mitrephora sukhothaiensis is easily distinguished from all other species in two key characters: its outer petals that reflex at maturity, and its inner petals that have inwardly folded marginal protrusions at the midpoint adaxially. Mitrephora sukhothaiensis also resembles M. tomentosa in the appearance of the fruit (monocarp shape and surface) and seeds, but differs in its shrubby habit, growing to 2 m in height (vs small to medium trees to ca. 20 m), sparsely to moderately hairy leaf indument abaxially (vs densely hairy), outer petals that reflex at maturity and without undulate margins), inner petals with inwardly folded marginal protrusions, and narrower seeds (5-6 mm vs ca. 8 mm). Flower pedicels 4.5-9 mm long; sepals 3-4 mm long; outer petals yellow, 6.5-9.5 mm wide; inner petals yellow with pink margins, 7.5-9.5 × 4.