Goniothalamusroseipetalus and G.sukhirinensis (Annonaceae): Two new species from Peninsular Thailand

Abstract Two new Goniothalamus species (Annonaceae), G.roseipetalussp. nov. and G.sukhirinensissp. nov., are described from the southern limits of Peninsular Thailand (Narathiwat and Yala Provinces). Both new species resemble G.macrophyllus, G.scortechinii and G.uvarioides. The addition of these two new species brings the total number of Goniothalamus species in Thailand to 27. Separate identification keys are provided for flowering and fruiting specimens of the Thai species.


Introduction
The genus Goniothalamus (Blume) Hook.f. & Thomson (Annonaceae subfam. Annonoideae tribe Annoneae: Chatrou et al. 2012;Guo et al. 2017) is widely distributed in lowland and submontane tropical forests across Southeast Asia . It is characterised by pendent, protogynous flowers with two trimerous petal whorls, with the inner whorl forming a mitriform dome over the reproductive organs (a 'type III' chamber sensu Saunders 2010). The outer petals are typically larger than the inner and periodically block the apertures between the inner petals, thereby controlling pollinator access and enabling the flower to temporarily trap the pollinating beetles (Lau et al. 2016). The timing of the petal movements that regulate pollinator trapping and release are synchronised with the circadian rhythms of the beetles (Lau et al. 2017;Saunders 2020); this allows the plant to utilise beetles with diverse circadian activities, and also allows the staminate floral phase to be extended to promote pollen deposition and enhance interfloral movement of beetles. These floral characteristics provide a possible biotic explanation for the statistically significant increase in the evolutionary diversification rate recently reported for the genus (Xue et al. 2020).
Goniothalamus fruits are apocarpous, with distinct fleshy 'monocarps' that develop from individual carpels after fertilisation. Two contrasting seed dispersal systems have been inferred, correlated with differences in fruit and seed morphology (Tang et al. 2015a): the species that are dispersed by non-volant mammals typically have ramiflorous or cauliflorous fruits with large (often sessile) monocarps and hairy seeds; whereas the species that are bird-dispersed have fruits that are borne on young growth and have small stipitate monocarps with glabrous seeds.
Discussion. Although G. roseipetalus is yet to be included in a molecular phylogenetic analysis, it shares several morphological similarities with species in a clade (nested within clade 'A1a' sensu Tang et al. 2015a, b) that comprises G. loerzingii R.M.K. Saunders, G. macrophyllus, G. scortechinii, G. uvarioides and G. wrayi King. These species were previously classified by Bân (1974) within Goniothalamus subgen. Goniothalamus sect. Goniothalamus, and are characterised by their essentially glabrous vegetative shoots and petioles, percurrent tertiary leaf venation, generally fused sepals with distinct venation, short inner petals, apiculate staminal connectives, relatively few carpels per flower, thickcylindrical pseudostyles with a broad, hairy stigma, and seeds with a hairy testa. Although G. roseipetalus shares most of these diagnostic characters, its stigmas are glabrous.
Phenology. Flowering and fruiting in February and March (based on limited data). Distribution and habitat. Endemic to Narathiwat Province, Peninsular Thailand (Fig. 4). Growing in shady and moist areas of tropical rainforests; 167-200 m alt.
Key to Goniothalamus species in Thailand (flowering specimens)