Astragalus tijuanensis A. E. Estrada, Rebman, C. González & Villarreal, 2022

Castillón, Eduardo Estrada, Rebman, Jon P., González, Carlos & Quintanilla, José Ángel Villarreal, 2022, Astragalus tijuanensis and Astragalus brauntonii var. lativexillum, a new species and a new variety in the genus Astragalus (Fabaceae) from extreme northwestern Baja California, Mexico, Phytotaxa 577 (1), pp. 1-13 : 2-3

Astragalus tijuanensis A. E. Estrada, Rebman, C. González & Villarreal , sp. nov. ( Fig. 1 and Fig. 2 )

Morphologically similar to Astragalus palmeri A. Gray , but differing in type and stipule size, number of flowers per raceme, petal colors, curvature and size of the banner, number of ovules, and diameter of the pod and the absence of pubescence.

Type:— MEXICO. Baja California (Municipio Tijuana ): along Boulevard 2000 between Rosarito and Tijuana, in the vicinity of the south side of the metropolitan area of the city of Tijuana , 32° 20’11.04” N, 116°57’44.593” W, alt. 171 m, 5 June 2022, C. González 104 (Holotype SD!; isotype MEXU!) .

Herbaceous, caulescent, perennial with deep single buried root, without superficial crown of a woody taproot, 13 × 0.7 cm long. Stems three to several, up to 30 cm long, 3–7.5 mm diameter, erect, arising from the base, the lower part of the stems, 7–10 cm, gray color, with stipules and lower leaves persistent, the leaves dried throughout, without leaflets, the upper part of the stem green, striate, minutely and densely strigulose, the trichomes 0.1–0.3 mm long, appressed, white. Stipules free or semi–amplexicaul, clasping ¼ to ½ of the stem´s circumference, 2–3 × 3–4 mm, persistent, almost always wider than longer, triangular to triangular–ovate, white to light brown, sparsely strigulose or glabrate, with only few trichomes on the edges. Leaves odd–pinnate, 4.5–8.5 cm long, petiole (3–)11–12(–17) mm long, leaflets (17)21–29(33) per leaf, opposite, rarely some of them alternate, (2–)8–10 × 0.7–1 mm, linear to linear–oblong, petiolule 0.2 mm long, separated from each other by 3–4 mm, glabrate and light–green adaxially, and dark–green and sparsely strigulose abaxially as stem pubescence, flattened or slightly concave, acute at both ends. Inflorescences in axillary open racemes, 4.5–5 cm long, peduncles erect or incurved–ascending, when young (in flower stage), 2–3 cm long, elongating with age (5–)7–11(–14) cm long (in fruit), exceeding the length of the leaves, sparsely strigulose, 0.1–0.2 mm long, the trichomes white; floriferous axis 1.5–2 cm long, elongating 2.2–5 cm long in fruit, with 13–19 flowers per raceme. Bracts 0.6–1.2 × 0.7–1 mm long, ovate to triangular, white to green, opaque to somewhat translucent, sometimes with slight purple tones in the upper half, subglabrate to sparsely strigulose, sometimes the midvein is evident, pedicel ascending or spreading, arched outward, 1–1.3 mm long, slightly thickened in fruit, caducous; bracteoles absent. Flowers 12–13 mm long, calyx (3.5–)3.9–4(–4.2) × 2.5–2.7 mm, campanulate to wide–campanulate, the tube 3.2–3.3 mm long, inequilaterous or slightly oblique sparsely strigulose, the trichomes white and purple mixed, occasionally subglabrous with few trichomes, the teeth 0.7–0.9 mm long, triangular to triangular–subulate, separated by obtuse or rounded sinuses; petals white with purple tones, the banner sessile, 12.5–13 × 6.3–7 mm, obovate, elliptic obovate to obovate–rhombic, gradually attenuated at the base, the notch 0.4–0.5 mm deep, folded at the middle, its margins then folded backward, slightly bent backward from the keel, recurved through 25°–30° from the vertical; wings 11.9–12.2 × 2–2.3 mm, the claw 5–5.2 mm long, the blade 6.8–7 mm long, straight, narrowly obliquely oblong–obovate, rounded to truncate at apex, basally auriculate; keel 9.4–9.8 × 2.5–2.8 mm, the claw 4.8–5 mm long, the blade 4.6–5 mm long, half obovate, incurved, apex rounded, auriculate; Stamens 10–merous, diadelphous, 9 fused by their filaments into a white striped sheath 5–5.2 mm long, the free part 4–4.3 mm long, enfolding the ovary, the vexilar stamen 8.6–9.3 mm long, standing free; anthers yellow, round, 0.9–1 × 0.2 mm. Ovary unilocular, 32–38 ovulate, sessile, linear–oblong, green, glabrous, 4.2–4.8 × 0.8 mm, the style linear, glabrous, style 4–4.5 mm long, glabrate, incurved, stigma terminal, minute, glabrous. Pod spreading or slightly ascending, sessile, (17–)19–20(–24) × 6–8 mm, incurved, semi–lunate in profile, at first subcylindrical to elliptic, sub–fleshy, acute in both ends, and at maturity strongly compressed laterally or low convex on the lateral faces, somewhat plump (not inflated like a bladder), ventral suture concave, keeled, dorsal suture almost straight to low convex, base acute, apiculate and rigid at the apex, the beak 0.7–1 mm long, with the sub–persistent style, the valves coriaceous, stramineous, brown to light–brown, glabrous, noticeably transversely reticulate and wrinkled, unilocular, septum absent; dehiscence apical; seeds 1.9–2.2 mm long, mitten–shaped, black, opaque.

Distribution and habitat:— As far as we know, Astragalus tijuanensis is only known from the type locality on the south side of the metropolitan area of Tijuana, Baja California to the east of Rosarito ( Fig. 3 ). This species was found in a recently burned, disturbed, chaparral area on open gravelly or clayish hillsides, low slopes or flats, on clayish substrate associated with Malosma laurina Engl. (1883: 339) , Xylococcus bicolor Nutt. (1842: 259) , Malacothamnus fasciculatus Greene (1906: 208) , Adenostoma fasciculatum Hook. & Arn. (1832: 139) , Deinandra fasciculata Greene (1897: 424) , Deinandra conjugens (D.D. Keck) B.G. Baldwin (1999: 468) , Heteromeles arbutifolia (Lindl.) M. Roem. (1847: 105) , Nolina interrata Gentry (1946: 181) , Gutierrezia californica (DC.) Torr. & A.Gray (1842: 193) , Eriogonum fasciculatum var. foliolosum S. Stokes (1910: 351) , Ceanothus verrucosus Nutt (1838: 267- 268) , Adolphia californica S. Watson (1876: 126) , and Nicotiana glauca Graham (1828: 2837) .

Etymology:— The name of this new species refers to the type locality in the municipality of Tijuana, and to date, it is the only place known where this species grows in a small population.

Phenology:— According to the presence of reproductive structures on the type specimen, flowering likely started in early May and was continuing in early June while fruiting probably started in early June and will likely continue until the end of July or early August.

Conservation status:— This new species was discovered while surveying and documenting populations of a cross-border rare plant species called Deinandra conjugens as part of a binational collaborative project. This new taxon was only encountered in a single population at the type locality. Even though we documented other regional populations of Deinandra conjugens in the area, this new Astragalus was not found elsewhere. The only known population is found within the Tijuana metropolitan area, which is being strongly affected by human activity (personal observation). According to GeoCAT, the extent of its distribution and the number of occurrences, as recommended by the IUCN Red List guidelines ( IUCN 2017), Astragalus tijuanensis should be placed in the Critically Endangered (CR) category, criteria B2ab(iii). Under the current conditions of impact due to fires, the presence of livestock, and the development of human settlements in the area of distribution of this new species, it faces a high risk of extinction.

Figures

FIGURE 1. Astragalus tijuanensis, A) Whole plant showing leaves, inflorescences, flowers and fruits. B) Stipules. C) Bracts. D) inflorescence enlarged to see detail of the flowers. E) Calyx. F) Banner: front view (left) and side view (right). G) Wing. H) Keel. I) Stamens. J) Ovary. K) Fruit. L) Seed.

FIGURE 2. Astragalus tijuanensis, part of the plant showing its leaves, inflorescences, flowers, and fruits.

FIGURE 3. Map showing the distribution of Astragalus tijuanensis (white star) and A. brauntonii var. lativexillum (black dot) in Tijuana, Baja California, Mexico.