This species is distinguished from Solanum dioicum W.Fitzg. (as currently delineated) and other Australian functionally dioecious Solanum species of the “Kimberley dioecious clade” by the combination of a spreading decumbent habit and the staminate inflorescence axis armed with relatively stout, spreading, straight prickles.

Australia. Northern Territory: Victoria River Valley, Judbarra National Park, off Victoria Highway (Highway 1), NW of Victoria River Roadhouse, Escarpment Walk, just off track above Garrarnawun Lookout on flat area between there and peak of outcrop, 15.61054°S, 131.11571°E, elev. 167 m, 2 June 2018 (fr), C. T. Martine, J. Cantley, A. McDonnell & P. Jobson 4748 (holotype: DNA).

Perennial spreading decumbent pale green shrub up to 30 cm tall. Main stem single, 4–12 cm tall, woody (not corky) branching 2–4 times with thickest lateral stems ca. 2–4 cm in diameter; younger stems yellow-green to tan-green in color and older woody stems eventually becoming dark tan or gray. Internodes 12–40 cm long in male plants, 30–46 cm long in functionally female plants. Stems with short, dense indumentum of porrect-stellate trichomes 0.5–1.3 mm, these mostly short stalked (occasionally on longer stalks up to 1 mm) with central midpoint ca. 0.2 mm. Prickles abundant and dense (8–10 per cm of internode), 1–8 mm long, straight, fine, widened at base, somewhat sharp. Leaves simple; blades 5–9 cm long, 1–3 cm wide, alternate, lanceolate; unarmed or with 1–3 straight prickles along adaxial midvein, soft yellow green above, slightly lighter beneath, both sides densely stellate-hairy, trichomes mostly short stalked, porrect-stellate with short central ray; apex acuminate; margins entire, sometimes ciliate; base oblique and tapering; petiole 0.5–14 mm long; Male inflorescence a scorpioid cyme 9–24 mm long with up to ca. 50 flowers (typically 1–4 flowers open at a time with previous blooms abscised); rachis densely stellate-pubescent, armed with straight prickles 5–7 mm, ca. 1 mm in diameter at the base, each subtending a flower; pedicel 3–7 mm long, sparsely armed with small prickles. Male flowers 5-merous; calyx with the tube 6–7 mm long, campanulate, armed with weak prickles ca. 2 mm long, the lobes 3–4 mm long, tipped with a linear acumen; corolla 16–27.4 mm in diameter, rotate to rotate-campanulate, pale violet; stamens equal; filaments 1–2 mm long; anthers ca. 4 mm long, tightly connivent, oblong-lanceolate to somewhat tapered, poricidal at the tips; ovary vestigial, non-functional. Female inflorescence of a solitary, morphologically cosexual flower (functionally female and producing inaperturate pollen); pedicel 7–8 mm long, sparsely armed with small prickles ca. 2 mm. Female flowers 5-merous; on; calyx with the tube campanulate, densely stellate-pubescent and armed, the prickles 5–6 mm long, straight, the lobes 5–11 mm long, unequal, long-triangular with a linear acumen, prickly; corolla 36–46 mm in diameter, rotate to rotate-campanulate, violet to pale violet; stamens equal, like those of the male flowers; filaments 1–2 mm long; anthers ca. 4 mm long, slightly spreading, poricidal at the tips; ovary ca. 5 mm in diameter at anthesis, glabrous; style ca. 5 mm long (including stigmatic surfaces), straight; stigma yellow, bifid, the lobes 1.5–2 mm long. Fruit a berry, 20–25 mm diameter, globose; immature fruit green, fleshy; mature fruit light green, drying to yellow-orange or tan, becoming leathery-reticulate and bony hard and loosely retained and partly-enclosed in calyx (75% enclosed when developing; mature, hardened fruit less than 25% enclosed), apparently detaching from calyx once hard and dry. Fruiting calyx lobes 2.1–2.8 cm long, long acuminate, tapered to a long fine tip, accrescent, slightly sticky and adherent to fruit when immature, readily separating from fruit as the berry matures, hardens, and shrinks from drying, densely armed with sharp prickles ca. 6 mm long. Seeds up to 420–586 per fruit in cultivation (two wild-collected fruits were N = 96 and N = 162), 1.1–1.5 mm in diameter, reniform dark brown to black, conspicuously and minutely reticulate.

Solanum scalarium is presently known from a single population (Fig. 3) of perhaps 50–100 individuals found within Judbarra/Gregory National Park. The species here occurs on skeletal pink soil, exposed sandstone pavement and dissected rock high above the Victoria River Valley (Fig. 4). The associated vegetation at this site is a low open woodland dominated by Corymbia terminalis (F.Muell.) K.D.Hill & L.A.S.Johnson (Myrtaceae) and Eucalyptus miniata (F.Muell.) A.Cunn. ex Schauer (Myrtaceae), with a sparse low mid story of Owenia vernicosa (F.Muell.) (Meliaceae), Calytrix exstipulata DC. (Myrtaceae), Xanthostemon paradoxus (F.Muell.) W.J.Hooker (Myrtaceae), Hibbertia spp. (Dilleniaceae), Corchorus spp. (Malvaceae), Senna oligoclada (F.Muell.) Randell (Fabaceae), Acacia sp. nov. (Fabaceae); the sparse ground layer is dominated by Cyperus cunninghamii (C.B.Clarke) C.A. Gardner (Cyperaceae) and Triodia pungens R.Br. (Poaceae). Although S. scalarium was not conspicuous on a visit by PJ and JTC in 2017, it appeared in 2018 to have sprouted vigorously from above-ground stems after fire occurring at some point in the previous 2–3 years. At the time of the type collection, plants were robust and vigorous in areas that had been burned and only represented by a few weak ramets in unburned areas dominated by Triodia pungens tussocks.

Figure 3. 

Geographic distribution of Solanum scalarium (black star) and other Australian dioecious Solanum species (source: The Australasian Virtual Herbarium).

Figure 4. 

Solanum scalarium in the field A, C, F type locality and habitat, Escarpment Walk, Judbarra National Park, Northern Territory B–D immature green fruits enclosed in prickly calyx and E prickly male rachis after male flowers have dropped off. (Photos by A.J. McDonnell.).

Pollination biology of the species is unknown, but, like other Australian congeners, the flowers are likely buzz pollinated by bees in the genera Xylocopa and Amegilla (Apidae; see Anderson and Symon 1988; Switzer et al. 2016) and likely to present high levels of pollen nutritional reward – although with slightly differential rewards available to pollen foragers from male versus functionally female flowers (Ndem-Galbert et al. 2021). A small set (N = 10) of ex situ hand pollinations conducted for this study showed that inaperturate pollen produced by functionally female flowers does not lead to fruit set when used to pollinate other females. This suggests that reproduction in S. scalarium is dependent on intersexual male-to-female outcrossing via biotic pollination like in other dioecious Solanum species.

Seed dispersal mechanism for this species is also unknown, although young fleshy fruits are mostly enclosed in a spiny calyx that gradually reflexes to some degree as fruits become dry and bony (Fig. 6D, E), suggesting that endozoochory is less likely than either ectozoochory (as a trample burr) or passive dispersal (see Symon 1979b; Martine et al. 2019). Peoples of the Walmajarri language area of the Kimberley region (west of this distribution) report that the fruits of S. dioicum (kara) are eaten by Osphranter rufus (Desmarest, 1822) (plains or red kangaroos; Doonday et al. 2013), and CTM has seen bustard birds (Ardeotis australis (Gray, 1829)) picking apart S. dioicum fruits near the northwest Kimberley coast (Martine et al. 2019). However, there is no published evidence that any extant animal acts as an effective seed dispersal agent of taxa within the “Kimberley dioecious clade” (see Martine et al. 2016b). Notably, seeds removed from green fruits (enclosed in calyces) of the type collection as well as seeds removed from older brown fruits (calyces reflexed) in cultivation were both germinable, suggesting that effective seed dispersal might happen at either stage.

Figure 5. 

Functionally male individuals of Solanum scalarium in cultivation: A leaf shape and B, C inflorescence axes of male individuals; note the distinctive spreading straight prickles that give the inflorescence axis a ladder-like appearance. (Photos by T.M. Williams.).

Figure 6. 

Functionally female individuals of Solanum scalarium in cultivation: A flower B reproductive branch C leaf shape D, E reflexing of calyx around brown, bony fruits and F overall habit. (Photos by T.M. Williams.).

Figure 7. 

A Holotype of Solanum scalarium (C.T. Martine 4748; female fruiting specimen) B paratype of S. scalarium (C.T. Martine 4796B; male specimen). (Photos by M. Sain.).

Plants encountered on 2 June 2018 were largely sterile, with several withered flowers and two mature fruits. The low fruit set in the population at the time of collection suggests that S. scalarium typically flowers earlier in the calendar year.

Latin scalare from scala, ladder or stair, and suffix aris, pertaining to; the epithet scalarium is the genitive plural of scalare, indicating ladder-like appearance of staminate inflorescence rachises, which are conspicuously and unusually armed with relatively stout, spreading, straight prickles resembling ladder steps. It is also a nod to the type locality at the Escarpment Walk, Judbarra/Gregory National Park, where stone stairs lead from the car park up to the habitat of this species. By choosing this name we acknowledge the access these steps provide to the newly-described species as well as the importance of providing broad access to nature, outdoor recreation, and scientific discovery. We suggest the use of Garrarnawun Bush Tomato for the English-language common name of the species in recognition of the Garrarnawun Lookout near where the type collection was made, a traditional meeting place of the Wardaman and Nungali-Ngaliwurru peoples whose lands overlap in this area (Parks and Wildlife Commission of the Northern Territory 2021).

While we expect that more localities for S. scalarium are likely to be found given the prevalence of similar (and less accessible) outcrops in the immediate region of the type collection, at present it is known from one protected (though frequently-visited) collection site in Judbarra/Gregory National Park (Fig. 4). Based on IUCN Red List Categories (IUCN 2012), S. scalarium should be considered Data Deficient (DD).

United States. Pennsylvania: Cultivated in Bucknell University: (Lewisburg) Burpee Research Greenhouse (staminate flowers/inflorescences, functionally female flowers, and fruits), 9 Oct 2020CT Martine & TM Williams 4796. (To be distributed to AD, BM, BUPL [Fig. 7b], DNA, NY, PERTH, US).

A comprehensive “Kimberley dioecious clade” key, including newly-recognized species, is forthcoming (Barrett and Barrett in prep). The most complete key to date can be found in Barrett (2013), which lumps the numerous variations of S. dioicum sensu lato as a single taxon. The following couplets may be inserted where S. dioicum occurs at couplet 60 in the key in Barrett (2013) and supplants the single replacement couplet 60a [previously published in Martine et al. (2016c)].

[Barrett 2013; couplet 60]