Solanum plastisexum Martine & McDonnell, 2019

McDonnell, Angela J., Wetreich, Heather B., Cantley, Jason T., Jobson, Peter & Martine, Christopher T., 2019, Solanum plastisexum, an enigmatic new bush tomato from the Australian Monsoon Tropics exhibiting breeding system fluidity, PhytoKeys 124, pp. 39-55 : 39

Solanum plastisexum Martine & McDonnell sp. nov. Figs 1 , 2D , 5

Diagnosis.

Like Solanum eburneum , Solanum watneyi and Solanum succosum , but differing by having elliptic, unlobed (or rarely very shallowly lobed) leaves, small apical leaves, long calyx lobes on the staminate flowers and fully erect staminate inflorescence branches.

Type.

AUSTRALIA. Northern Territory: ~42 km E of Top Springs, on and around the Buchanan Highway, 16°42.274'S, 132°07.446'E, elev. 286 m, 23 May 2016 (fl, fr), C.T. Martine 4258, J.T. Cantley, L.M. Lacey, & P.C. Jobson (holotype: DNA; isotypes to be distributed to BM, BUPL, MEL, NY, PERTH)

Description.

Erect perennial herb 50-80 cm tall. Stems slender, woody at base, upright even when weighted by fruits; single stemmed, with some lateral branching on mature stems. Foliage and stems grey to grey-green, becoming slightly more yellow-green with age; indumentum of stems, leaves and inflorescences composed of stellate trichomes with the stalk, these short, appressed and very dense throughout (of Type 1 sensu Bean 2004 and Seithe 1979); stalk 0.05-0.1 mm long, with 4-6 rays 0.2-0.4 mm long, the midpoint elongate, to 0.4 mm long. Prickles scattered throughout, tan, straight, slightly widened at base, fine, 1-2 mm long. Sympodial units difoliate, the leaves solitary or geminate. Mature leaves 3-12 cm × 0.7-2.4 cm, lanceolate (elliptic), with 3-7 pairs of primary veins, with few prickles along base of abaxial midvein; both sides closely and very densely stellate-pubescent; base tapering; margins entire, occasionally sparsely shallowly lobed; apex acute; petiole 0.3-1.8 cm long with 0-4 prickles along base of adaxial midvein. Inflorescence a supra-axillary cyme 1-15 cm long, complete inflorescence consisting of a basal hermaphrodite flower and a distal group of 2-many staminate flowers with 2-3 staminate flowers typically open at the same time; peduncle typically 1.0-10.0 mm long. Flowers 5-merous, heterostylous and the plants andromonoecious. Hermaphrodite flower ca. 1.5-3 cm below the oldest staminate flower, opening first; pedicel ca. 1 cm long at anthesis, elongating in fruit, armed with 20-50 prickles, each 1-3 mm long; calyx lobes 16-18 mm long, fused for first 2-3 mm, some occasionally fused along most of their length with sepals arranged 2+2+1, armed with 40 -100 long, straight prickles and dense stellate trichomes; corolla 3.1-4.0 cm in diameter, lavender to medium purple, rotate, glabrous; stamens equal; filaments ca. 1.0 mm long; anthers 5 mm long, oblong to lanceolate, poricidal, in a tight anther cone; ovary glabrous, ca. 1.5 mm diameter at anthesis; style (including capitate stigma) dimorphic, 2.5-5 mm or 7-10.5 mm long, curved. Staminate flowers with pedicels 4-7 mm long, unarmed; calyx lobes 16-17 mm long, fused at the base, occasionally 2+2+1 as in hermaphroditic flowers, prickles absent; corolla 1.7-3.5 cm in diameter, lavender to medium purple, rotate; stamens of same proportions as in hermaphrodite flowers; ovary, style and stigma vestigial and diminutive; rachis bearing staminate flowers often deciduous in fruit. Fruit a globose berry 1.4-2.7 cm long, 1.7-2.8 cm in diameter, light green with darker green striations when young, maturing to creamy yellow; flesh firm; locules 2, with little liquid; fruit wall ca. 3-4 mm thick; fruits retained on plant after maturation; fruiting pedicels 2.4-4.1 cm long; fruiting calyx covering 1/4 to 1/3 of developed fruit, the lobes narrowly deltoid, long-acuminate, tips acute, turning brown and weakly reflexing at maturity, very densely short stellate-pubescent and armed with sharp spines 2-5 mm long, these single or paired along the calyx sutures. Seeds up to ~150 per fruit, 3.0-4.5 mm long, dark brown to black, flat, reniform, the surfaces finely reticulate.

Distribution and ecology.

Solanum plastisexum is currently known from a restricted range on and around the Buchanan Highway in the sub-arid, monsoon-influenced zone of the northern region of the Australian Northern Territory (Figs 4 , 6 ). The species is locally abundant in two to three sites along and off of the road in Corymbia dichromophloia (F.Muell.) K.D.Hill & L.A.S.Johnson and Petalostigma pubescens Domin savanna woodland alongside Aristida L., Heteropogon contortus (L.) P.Beauv. ex Roem. & Schult. and other herbaceous and shrubby associates. The distribution of plants along annually-graded roadsides suggests that the species is, like many of its congeners, disturbance-adapted.

Nothing is known about the biotic interactions local fauna have with this species, although the floral morphology suggests the typical Solanum buzz pollination syndrome ( Anderson and Symon 1988). Plants encountered in 2018 bore many mature fruits not taken nor eaten by frugivores, but the exposed, fleshy berries may indicate biotic seed dispersal via ingestion ( Martine et al. 2019).

Phenology.

The handful of collections that have been made of S. plastisexum that include flowers are all from the end of the wet season through the early months of the dry season, from January to June. Mature fruiting specimens have been collected in June.

Etymology.

The name is based on the Latin “plastus” ( “deceptive,” but derived from the Greek “plastikos/plasticos/plasticus” for "able to be molded, changeable") and the Latin “sexus” for sex. We suggest the use of Dungowan Bush Tomato for the common name of this species, which refers to the cattle station on which the majority of the collections have been made.

Preliminary assessment of conservation status.

Solanum plastisexum is known from only two to three extant populations, each consisting of a few dozen individuals (with some likelihood of clonality) and two historical (pre-2000) collections (Fig. 1D ). The currently-known distribution of the species is not under conservation protection, but one of the populations appears to have been stable since at least the 1970s. When evaluated using the IUCN Red List Categories and Criteria for extinction risk ( IUCN 2012), S. plastisexum falls into the Vulnerable (VU) category under Criterion B (B1ab(iii)+2ab(iii)). The VU designation is the lowest of three threatened categories, but indicates the taxon still faces a high risk of extinction in the wild. It has an Area of Occupancy that is likely much less than <20 km2 and an Extent of Occurrence <5 km2, as calculated using the geocat.kew.org online tool. There are fewer than 10 known locations that are possibly fragmented and a decline in overall habitat quality is likely, given population localities along a road.

Specimens examined.

AUSTRALIA. Northern Territory: 92 km W. of Dunmarra, 16°44'S, 132°10'E, 20 June 1974 (fr), P.K. Latz 5482 (DNA [DNA A0041776]); Dungowan Station, 16°44'S, 132°17'E, 10 January 1978 (fl), T. S. Henshall 1914 (DNA [DNA A0054101]); Buchanan Highway, 46.1 km E. of Top Springs, 16°43.140'S, 132°09.511'E, 22 May 2016 (fl), C.T. Martine, J.T. Cantley, L.M. Lacey & P. Jobson 4260 (DNA, BUPL); Buchanan Highway, 42.1 km E. of Top Springs, 16°42.274'S, 132°07.446'E, 30 May 2018 (fl, fr), C.T. Martine, A.J. McDonnell, J.T. Cantley, & P. Jobson 4743 (NT, DNA, BUPL); Buchanan Highway, 46.1 km E. of Top Springs, 16°43.140'S, 132°09.511'E, 30 May 2018 (fl, fr), C.T. Martine, A.J. McDonnell, J.T. Cantley, & P. Jobson 4745 (DNA, BUPL).

Figures

Figure 1. Morphology and the earliest-known herbarium specimen of Solanum plastisexum. A Flowering stem with a single staminate flower in 2016 B Mature fruit C Erect inflorescences bearing staminate flowers in 2018 and D Specimen collected by P. Latz in 1974, held at DNA and annotated by D. Symon with an annotation indicating his confusion about the reproductive morphology of the specimen (male rachis visible above fruit on far left).

Figure 2. Closely related species of andromonoecious bush tomatoes included in this study. A Solanum jobsonii B S. watneyi C S. succosum D S. plastisexum E S. diversiflorum and F S. eburneum. Colours associated with each taxon also used in Figs 3, 4.

Figure 4. Map showing geographic distribution of all taxa compared in this study. red points = S. jobsonii, blue points = S. diversiflorum, black points = S. eburneum, pink points = S. watneyi, yellow points = S. succosum and purple asterisk = S. plastisexum. All points are based on specimens databased in the Australasian Virtual Herbarium (https: // avh. chah. org. au /) and specimens held at BUPL.

Figure 5. Scan of holotype of Solanum plastisexum, held at DNA.

Figure 6. Typical habitat of Solanum plastisexum, taken from the Buchanan Highway.