Impatiens decurva Ruchis. & S.B. Janssens, 2018

Ruchisansakun, Saroj, Suksathan, Piyakaset, Niet, Timotheüs Van Der, Smets, Erik F. & Janssens, Steven B., 2018, Three new species of Impatiens (Balsaminaceae) from Myanmar, Phytotaxa 338 (1), pp. 63-74 : 64-67

1. Impatiens decurva Ruchis. & S.B. Janssens , sp. nov. ( Figs. 1 , 2 )

Impatiens decurva Ruchis. & S.B. Janssens is similar to I. pendula B. Heyne ex Wight & Arn. (1834: 136) but differs in having congested leaves towards the stem apex, a pilose midrib on the dorsal petal, pink lateral united petals with a white base, an unequally bilobed apex of the lower lateral united petals, and a strongly decurved pedicel in fruiting stage.

Type:— MYANMAR. Shan State: Kalaw, 20°39’21.92”N 96°34’55.89”E, 27 Sep. 2015, S. Ruchisansakun & Makino BG Exped.734 (holotype L!, isotypes L!, RAF!, RANG!).

Lithophytic annual herb 6–30 cm tall. Stem erect, 1–3 mm in diam., cylindrical, simple to moderately branched, red, glabrous. Leaves spirally arranged, mostly crowded towards the apex of stem. Petiole absent or to 7 mm long, ca. 1 mm in diam., pale green to green to pink, glabrous. Lamina 10–40 × 5–15 mm, ovate to elliptic to obovate, apex acute to acuminate, base cuneate to attenuate, margin shallowly serrate, adaxial surface green, margin pilose, abaxial surface pale green and glabrous, with 3–5 long red hairs along the margin near the base; lateral veins 3–5 pairs, adaxially pilose. Flowers solitary, axillary, erect, 10–11 × 8–10 mm, 6–8 mm deep, pale pink and white, with two yellow spots at center. Bracts <1 × <1 mm, linear to narrowly lanceolate, the apex acute and mucronate, base cuneate, green with red apex, persistent. Pedicel 10–15 mm long, <1 mm in diam., pale green, pilose. Lateral sepals 2, <1 × 1 mm, free, ovate to lanceolate, apex acute, base obtuse, pale green with red tip, glabrous. Lower sepal 4–5 × 2.5–3 mm, ca. 2 mm deep, navicular, apex acuminate and mucronate, white with red tip, pilose outside, spurless. Dorsal petal 4–5 × 5–5.5 mm, broadly ovate, cucullate, apex round and slightly mucronate, base truncate to shallowly cordate, white, mostly glabrous but pilose on midrib, abaxial midvein simple or with narrow crest, ca. 1 mm wide, green. Lateral united petals 7–9 mm long, free: upper petals 4.5–5 × 2.5–3 mm, ovate, apex round, base cuneate, white to pale pink; the lower petals 6–7 × 3.5–4 mm, free, elliptic to obovate, apex unequally bilobed, pink, each with a yellow spot at base. Stamens 5: filaments ca. 2.5 mm long, white to pale pink; anthers pale pink. Ovary 2 mm long, <1 mm in diam., 5-carpellate, green, pilose. Fruits 8–10 mm long, 2.5–3 mm in diam., short fusiform, 5–lobed, green, pilose; pedicel strongly decurved in middle during fruiting stage. Seeds ca. 2 mm long, ovoid, 9–11 per fruit, brown.

Phenology:— Flowering from September to October; fruiting in October.

Distribution:— Endemic to Myanmar (Shan State).

Ecology:— Growing in limestone soils on a mountain summit in open, fragmented evergreen forest, 1500–1600 m elevation.

Proposed IUCN conservation assessment:— Critically Endangered B1ab (i, ii, iii) + 2ab (i, ii, iii). This species is only known from the type locality. The extent of occurrence is estimated as <5 km, where it occurs as a small population ( IUCN 2012).

Etymology:— The specific epithet refers to the decurved pedicel at the early fruiting stage.

Note:— Impatiens decurva Ruchis. & S.B. Janssens is the only spurless species in Myanmar with solitary flowers and spirally arranged leaves. Although it morphologically resembles I. pendula , I. decurva possesses a distinct characters: the leaves clustered towards the stem apex, a pilose midrib on the dorsal petal, the apex of the lower lateral united petals unequally bilobed, and a pedicel that is strongly decurved from the middle in the early fruiting stage. Impatiens decurva resembles I. muscicola Craib (1926: 162) in morphology. Impatiens muscicola is a species endemic to northern Thailand, and differs from I. decurva in having lower lateral petals with an unequally bilobed apex and a pedicel that is strongly decurved in the middle during the fruiting stage.

Phylogenetic analysis: —Bayesian phylogenetic analyses of the combined ITS and atpB-rbcL dataset confirm its position within subgen. Impatiens sect. Uniflorae Hook. f. & Thomson (1860: 113) ( Yu et al. 2015) ( Fig. 3 ). Despite the morphological similarity with I. muscicola Craib , I. decurva appears more closely related to the Myanmar species I. florulenta Hook. f. (1905: 25 & 32), a long-spurred species distributed in the same area.

Pollination ecology: —We predict that I. decurva relies on autonomous self-pollination. This prediction is based on the strong similarity in floral features, including flower size, between I. decurva and I. muscicola . For the latter species autonomous self-pollination was experimentally confirmed ( Ruchisansakun et al. 2016).

Figures

FIGURE 1. Impatiens decurva. A. Habit; B. Front view of flower; C. Lateral view of flower; D. Fruit; E. Pedicel and ovary; F. Lateral sepals; G. Lower sepal; H. Dorsal petal; I. Lateral united petals. Drawn by Saroj Ruchisansakun.

FIGURE 2. Impatiens decurva. A. Lateral view of flower; B. Front view of flower; C. Habit in situ. Photographs by Saroj Ruchisansakun.

FIGURE 3. Majority-rule consensus tree from Bayesian phylogenetic analyses of a combined dataset of nuclear ITS and plastid atpBrbcL DNA sequences. Impatiens decurva and I. oblongata are highlighted in bold font. Bayesian posterior probabilities are indicated at each node.