RHODODENDRON ASTROPHORUM (ERICACEAE, RHODODENDRON SECT. SCHISTANTHE ), A NEW MICRO-ENDEMIC SPECIES FROM CENTRAL PALAWAN, PHILIPPINES

A new micro-endemic species of Rhododendron is described and illustrated from botanical expeditions to the Victoria-Anepahan Mountain Range on the island of Palawan. It closely resembles Rhododendron wilkiei but differs in having leaves with laminae that are narrowly elliptic-oblanceolate, have adaxially indistinct venation, and are arranged in 4–8 active and mostly terminal pseudowhorls along the length of the stem; bracts without scales on the margin; yellow flowers with a red corolla tube interior that is densely hairy towards the base; and hairy anther filament bases. Morphological and geographical data place the species in Rhododendron sect. Schistanthe . The new species, together with a first record of Rhododendron vidalii on Palawan as part of this research, represent the tenth and eleventh members of this genus recorded from the island. A revised key to the Palawan species of Rhododendron is provided.


Materials and methods
The species description is based on in situ observations, photographs, herbarium specimens, and spirit collections preserved in Copenhagen solution.Observations were carried out with a TM4000Plus II low-vacuum scanning electron microscope (Hitachi, Tokyo, Japan) using an accelerating voltage of 5 kV, a secondary electron detector and a 30 Pa vacuum.To prevent cell collapse under vacuum, wet samples were flash frozen to -20°C with a Deben Coolstage (Deben, Bury St Edmunds, UK).General measurements were made with a Leica M80 microscope (Leica Microsystems, Fremont, CA, USA) and a Mitutoyo vernier caliper (Mitutoyo, Kawasaki, Japan).Leaf-scale terminology follows Cowan (1950).
Closely resembles Rhododendron wilkiei Argent but differs in having leaves with laminae narrowly elliptic-oblanceolate and c.5-7 times as long as wide (vs elliptic and c.2.5 times as long as wide), venation adaxially indistinct (vs prominent) with 4-8 active pseudowhorls of leaves along the length of the stem (vs 2-3(-4) mostly terminal pseudowhorls), bracts without scales on the margin (vs with scales), yellow corollas with the tube interior red (vs red throughout), the internal surface of the corolla tube densely hairy on the proximal 3/4 (vs glabrous), and filaments hairy at the base (vs glabrous).
Etymology.The specific epithet astrophorum is derived from the Greek ἄστρον (ástron = star), and the suffix -φόρος (-phóros = bearing) in reference to the bright yellow corolla, which resembles small clusters of stars in the forest canopy.
Phenology.The species was observed on 9 April 2015 mostly in bud, and in full anthesis in June 2022.This phenology suggests a tendency to flower following the dry season, which in Palawan occurs between January and April (Worldbank.org 2022).
Proposed IUCN conservation category.Despite numerous expeditions to surrounding peaks in the VAMR, Rhododendron astrophorum has been formally only identified from its type   6).The presence of Rhododendron brookeanum H.Low ex.Lindl.on Palawan was recorded by George Argent (Pelser et al., 2011-).However, the taxon has not been further documented and the voucher Argent M270 has not been seen by the authors, thus preventing independent verification.This species has therefore been omitted from both the summary list and the revised key for Palawan Rhododendron.Despite pronounced differences in foliage size and stature, similar floral and scale macromorphologies support our hypothesis that Rhododendron astrophorum is allied with two Palawan endemic species of Rhododendron, namely R. wilkiei and R. acrophilum.Rhododendron astrophorum is unique among Palawan Rhododendron in producing brightyellow corollas with red centres, the closest colour equivalent being R. acrophilum, which produces orange corollas with yellow centres.Morphologically, the new species shares some scale and floral characteristics with both Rhododendron acrophilum and R. wilkiei but is readily distinguished in its sterile state by its distinctive, narrowly elliptic-oblanceolate leaves (Figure 2D).When in flower, Rhododendron acrophilum is distinguishable by corolla colour alone (Figure 4C).No obvious pollinators were observed during field studies of the species.Moreover, the styles have been observed to elongate towards the anthers within a matter of days post initiation of anthesis, potentially facilitating the transfer of pollen and possibly indicative of some capacity for self-pollination.However, it is not known at what point the stigma becomes receptive, nor for how long pollen is shed by the anthers.The cataloguing of new plant taxa from the VAMR has accelerated in recent years, with many taxa from a wide variety of families published (e.g.Robinson et al., 2009Robinson et al., , 2016;;Quakenbush et al., 2020;Tamayo et al., 2021), including several vireya rhododendrons (Argent, 2004).The VAMR is the largest contiguous exposed area of the Mount Beaufort Ultramafics geological terrane, a series of outcrops of Eocene origin (Okubo, 1989) that includes several mountains of > 1000 m elevation, including Brow Shoulder, Mount Shumkat, Sultan Peak, and the eponymous Mount Victoria and Anepahan.The highlands of this terrane, which arose in association with the geological events that gave rise to Mount Kinabalu in adjacent northern Borneo (Collenette, 1964;Hall, 2002), has recently seen a surge in exploration by botanists, giving rise to novel plant discoveries, including several edaphic specialists.Many of these newly described endemic species also demonstrate the close biological affinities between Palawan and Borneo, islands that have been connected in the past by land bridges (Hall, 1998;Voris, 2000;Sathiamurthy & Voris, 2006).These discoveries, of which Rhododendron astrophorum is the latest from a series we are preparing, underscore the highly endemic nature of the flora of Palawan.They also highlight the importance of further biodiversity research across the island, as well as the urgent need for conservation actions that safeguard this biologically diverse area.

Figure 3 .
Figure 3. Scanning electron micrographs of selected characters of Rhododendron astrophorum M.N.Tamayo, Y.P.Ang & A.S.Rob., sp.nov.A, Abaxial leaf surface, showing scale density; B, scales on abaxial surface, showing ray cells and lacerate margins of annular zone; C, ovary and nectary with dense indumentum of ovary scales; D, detail of nectary, with simple aciculate hairs close to ovary base (note the convex-subconical ovary scales with a less irregular margin than that of the scales in panel B); E, base of anther, showing simple subulate trichomes; F, internal surface of corolla tube, showing dense indumentum of simple hairs.Micrographs of AVAMR 85: A. Robinson.

Figure 5 .
Figure 5. A-C, Rhododendron javanicum subsp.palawanense in a bamboo thicket at a similar elevation to R. astrophorum.The Palawan subspecies has a pink corolla flushed with orange around the tube inner surface.A, Apical view of inflorescence; B and C, lateral and ventral view of corollas from left to right; D and E, Rhododendron vidalii from the vicinity of R. astrophorum, representing a new record of this species for the island of Palawan: D, apical view of flower; E, lateral view of flower.Photographs of AVAMR 102 (A-C) and AVAMR 101 (D and E): Y. P. Ang.

Figure 6 .
Figure 6.Map of Palawan, showing the distribution of Rhododendron species across the island by location.Map created by Y. P. Ang.
Vaccinium palawanense Merr.and Leptospermum sp. in the immediate vicinity.Two other species of Rhododendron sect.Schistanthe were encountered, namely, R. javanicum (Blume) Benn.subsp.palawanense Argent (Figure 5A-C) and R. vidalii Rolfe (Figure 5D,E), the latter representing a new record for Palawan.Rhododendron vidalii was formerly known to occur only in Luzon and Mindoro Islands -2 , often clustered but not overlapping, similar to leaf scales but markedly convex-subconical, annular zone with more regular margins and straighter ray cells, simple hairs absent; style 13-15 mm long, c.0.8 mm in diameter, glabrous; stigma 5-lobed, 1-1.2 mm in diameter, pale orangish red to red; fruits narrowly ellipsoid, 24-27 × c.4 mm, valves spreading, straight or occasionally twisting slightly; seeds (4-)6(-7) mm, narrowly fusiform, winged, central portion c.900 × c.25 μm.Distribution.Recorded only from the type locality.Habitat and ecology.Rhododendron astrophorum is found as an epiphyte on the larger limbs of shrubs and small trees from mid-montane ultramafic forest at c.1100-1477 m elevation at the periphery of a swamp located close to the summit of Mount Shumkat, where the vegetation was notably lower in stature and more compact than at lower elevations.Other plant species recorded in the area are Tristaniopsis oblongifolia (Merr.)Peter G.Wilson & J.T.Waterh., a single fallen flower.Should a formal determination of Rhododendron astrophorum be made for these plants, the conservation status must be reassessed.Rhododendron javanicum ssp.palawanense.Philippines.Palawan: Municipality of Narra, Victoria-Anepahan Mountain Range, Mt Shumkat, ridge on north-east to east-facing slope, in moss on bark, 1100-1200 m elevation, 11 vi 2022, AVAMR 102 (Angiosperms of Victoria-Anepahan Mountain Range) (BRIT!).