Introduction

The genus Rhabdotosperma Hartl belongs to the family Scrophulariaceae in the tribe Scrophularieae (Oxelman et al. 2005). It was separated as a new genus from Verbascum by Hartl (1977), (formerly recognised as Celsia L. and Verbascum L. sect. Aulacospermae by Murbeck 1925, 1933) on account of seed morphology, a distinction followed by Lobin & Porembski (1994) and Fischer (2004). Rhabdotosperma species are distinguished from their closely related Verbascum sister species by having longitudinally furrowed seeds (aulacospermous), a dilated to disciform stigma, and the absence of accessory flowers (Hartl 1959, 1977; Fischer 2004). The genus comprises seven species (Hartl 1977; Lobin & Porembski 1994; Fischer 2004; Christenhusz et al. 2017).

In tropical Africa, six species of Rhabdotosperma were listed by Hartl (1977), namely, R. densifolium (Hook.f.) Hartl, R. brevipedicellatum (Engl.) Hartl, R. ledermannii (Schltr. ex Murb.) Hartl, R. scrophulariifolium (Hochst. ex A.Rich.) Hartl, R. keniense (Murb.) Hartl, and R. schimperi (Skan) Hartl. In the Flora of Tropical East Africa, Ghazanfar et al. (2008) treated the last two species as synonyms of R. brevipedicellatum and R. scrophulariifolium, respectively. Fischer (2006), however, treated R. keniense and R. schimperi as distinct taxa. Rhabdotosperma bottae (Deflers) Hartl is the only species reported from Yemen (Hartl 1977; Huber-Morath 1984; Wood 1997) and Saudi Arabia (Collenette 1985, 1998, 1999; Chaudhary 2001) in the Arabian Peninsula.

As part of an ongoing taxonomic revision of Verbascum and Rhabdotosperma in the Arabian Peninsula, some interesting specimens of Rhabdotosperma were examined that did not match any known species in the area or the surrounding regions. A comprehensive investigation was carried out to ascertain whether these specimens warranted taxonomic distinction.

Materials and Methods

Morphological and ecological data were gathered from relevant literature, herbarium specimens, and the first author’s own observations during fieldwork in Saudi Arabia. Herbarium specimens were examined from BM, E, K, KSU, RIY, and the JSTOR Global Plants platform (https://plants.jstor.org/). The new species was assessed following the IUCN Red List Categories and Criteria (IUCN 2012), and the Geospatial Conservation Assessment Tool (GeoCAT) was used to analyse and estimate the extent of occurrence (EOO) and area of occupancy (AOO) with a grid size of 2 km2 (Bachman et al. 2011). The distribution map was made with QGIS software version 3.22 (2022).

Taxonomic Treatment

Rhabdotosperma saudiarabicum A.Alzahrani, sp. nov. Type: Saudi Arabia, Abha, Jabal Al-Soudah, Al-Soudah, 25 km NW of Abha, 2800 m, 22 Feb. 1982, I. S. Collenette 3316 (holotype K!).

http://www.ipni.org/urn:lsid:ipni.org:names:77306699-1

Biennial herb, dark green, simple or rarely branched, woody at the base, up to 40 cm tall. Indumentum dense glandular hairs. Stems erect, terete to angular. Basal leaves alternate, oblong-ovate, 8 – 10 × 3 – 4 cm, apex obtuse, base cordate, margins crenate, lamina dark green with sparse simple hairs on the veins below; petiole 4.5 – 6.5 cm, winged with 1 – 2 small lateral lobes. Cauline leaves oblong to oblong-ovate, 3.5 – 6.7 × 1.5 – 2.6 cm, apex obtuse, base cordate or semi-amplexicaul; sessile or petiole 0.8 – 2 cm. Inflorescence racemose; flowers single in the axil of bracts. Upper bracts lanceolate-triangular, 4 – 5 mm, acute. Lower bracts cordate-triangular, 10 – 40 mm, acute. Pedicel covered with dense glandular hairs up to 6 mm long. Calyx 4 – 5 mm, lobes oblong, acute or mucronate, dense glandular hairs. Corolla 15 – 20 mm across, yellow with red streaks on the upper side and around the throat, without pellucid glands, tubeless, glabrous inside, sparse glandular hairs outside. Stamens 4, 3 – 5 mm long. Filaments red with yellowish-red hairs, two anterior glabrous near the apex, two posteriors with hairs up to the anthers. Anthers two anterior inserted obliquely on filaments, two posteriors with reniform anthers. Ovary pyriform-ovoid, sparse glandular hairs. Style up to 4 mm long, filiform, green or red. Stigma disciform. Capsule 6 – 8 × 4 – 5 mm, pyriform-ovoid, sparse glandular hairy. Seeds 0.6 – 0.7 × 0.4 – 0.5 mm, brownish, oblong-cylindrical, aulacospermous. Figs 1, 2 & 3.

Fig. 1
figure 1

Rhabdotosperma saudiarabicum. A, B habit, from Al-Soudah, Asir mountains, Abha, Saudi Arabia. photos: s. collenette.

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Fig. 2
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Rhabdotosperma saudiarabicum. A leaf and petiole with two small lateral lobes (white arrow); B calyx and upper bracts (white arrow); C stems with glandular hairs (white arrow); D, E filaments with two anterior anthers inserted obliquely (white arrows); F capsules; G stigma disciform (white arrow); H seeds aulacospermous. A – E from I. S. Collenette 3316; F – H from L. Boulos & A. S. Ads 14165. photos: ali alzahrani.

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Fig. 3
figure 3

Rhabdotosperma saudiarabicum. A habit; B leaf and petiole with two small lateral lobes; C flower (showing two anterior anthers inserted obliquely); D calyx and upper bract (densely covered in glandular hairs); E capsule; F seed. A – D from I. S. Collenette 3316; E – F from L. Boulos & A. S. Ads 14165. drawn by s. almalki.

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recognition. Rhabdotosperma saudiarabicum is morphologically similar to R. bottae and R. scrophulariifolium, sharing with these two species similarly winged petioles, oblong calyx lobes, racemose inflorescences, four stamens, two anterior filaments that are glabrous near the top, pyriform-ovoid capsules, and oblong-cylindrical seeds. However, the new species can be distinguished from R. bottae by its alternate leaves (vs rosette), oblong-ovate leaves (vs oblong to oblong-lanceolate), crenate margins (vs crenate-denticulate), it has sparse, simple hairs on leaf veins below (vs dense, simple hairs), 5 – 6 mm long pedicels (vs 10 – 20 mm), two anterior filaments 3 – 5 mm long (vs 7 – 8 mm), two anterior anthers inserted obliquely on filaments (vs inserted decurrent longitudinally), style length of 3 – 4 mm (vs 8 – 15 mm), and capsules covered with sparse glandular hairs (vs glabrous). Furthermore, R. saudiarabicum differs from R. scrophulariifolium in having dense glandular indumentum (vs glandular with sparse, simple and forked hairs), oblong-ovate leaves (vs oblong), crenate margins (vs toothed-crenate), sparse, simple hairs on leaf veins below (vs dense, simple hairs on the veins on both sides), 5 – 6 mm long pedicels (vs 6 – 30 mm), calyx 4 – 5 mm long (vs 6 – 9 mm), and capsules covered with sparse glandular hairs (vs glabrous). Morphological comparison of these three species is given in Table 1.

Table 1. Morphological comparison of Rhabdotosperma saudiarabicum and similar species.
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distribution. Al-Soudah, Asir Mountains, Abha, southwestern Saudi Arabia (Map 1).

Map 1
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Distribution map of Rhabdotosperma saudiarabicum in Saudi Arabia.

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specimens examined. saudi arabia. Abha: Jabal Al-Soudah, Al-Soudah, 25 km NW of Abha, 2800 m, 22 Feb. 1982, I. S. Collenette 3316 (K!, holotype); Al-Soudah, 3048 m, 1 July 1946, D. Vesey-FitzGerald 16082/4 (BM!); Al Mahmoud, 35 km N of Abha, 10 km below Jabal Al-Soudah, 21 May 1980, L. Boulos & A. S. Ads 14165 (K!); Waterfall beauty spot 10 km NW of Abha, Jabal Al-Soudah Road, 2530 m, 8 May 1985, I. S. Collenette 5368 (E!); Al-Soudah, 11 Aug. 1952, J. D. Tothill 116 (BM!).

habitat. The species is known from evergreen woodlands in the Asir mountains from Saudi Arabia at altitudes ranging from 2500 – 3000 m, where it grows in granite crevices and near streams. Associated plants observed within the locality include Juniperus procera Hochst. ex Endl., Vachellia origena (Hunde) Kyal. & Boatwr., Hypericum revolutum Vahl, Dodonaea viscosa subsp. angustifolia (L.f.) J.G.West, Erica arborea L., Rosa abyssinica R.Br. ex Lindl., Clutia lanceolata Forssk., Euryops arabicus Steud. ex Jaub. & Spach, Nepeta deflersiana Schweinf. ex Hedge, Lavandula dentata L., Mentha longifolia var. schimperi (Briq.) Briq., Plantago lanceolata L. and Veronica anagallis-aquatica L.

conservation status. Rhabdotosperma saudiarabicum is endemic to Al-Soudah in the Asir Mountains of Saudi Arabia; three localities of five specimens are known from this area. The species was last recorded in 1985 by Collenette. However, during fieldwork in Al-Soudah in 2019, 2020, and 2021, the first author failed to find it, indicating that it might be either very rare or extinct from the type locality. The locality is under threat from suburban and agricultural expansion, colonisation by invasive species such as Opuntia ficus-indica (L.) Mill., Nicotiana glauca Graham, Tagetes minuta L., Argemone ochroleuca Sweet, and Verbesina encelioides (Cav.) Benth. & Hook.f. ex A.Gray, and recently, as a tourism destination and recreation area. Due to the close proximity of the localities where specimens of R. saudiarabicum were recorded (Map 1), and to the fact that they face the same threats, we treated them as a single location based on the IUCN’s definition of “location”. Additionally, we estimated that the number of mature individuals may be less than 50, its extent of occurrence is 11.566 km2, and the area of occupancy is 12 km2. Therefore, we assessed the species as Critically Endangered (CR), meeting B1ab (iii)+D.

phenology. Rhabdotosperma saudiarabicum was collected with flowers from February to August and in fruit from May to July.

etymology. The specific epithet is derived from Saudi Arabia, where this species is endemic.

notes. Specimens of this newly described species were included in Verbascum melhanense (Murb.) Hub.-Mor. or R. bottae in all published floras of Saudi Arabia (Collenette 1985, 1998, 1999; Chaudhary 2001), leading to its misidentification over the last 75 years. Seed morphology is the most distinctive and stable character separating Verbascum and Rhabdotosperma (Murbeck 1925, 1933; Huber-Morath 1978; Hartl 1977). Hence, V. melhanense is not included in this morphological comparison of similar species presented in Table 1 because of the transversally elongated seeds (bothrospermous, Hartl 1959) that characterise Verbascum.