Hydrangea acuminata subsp. yakushimensis Yahara & Tagane, 2022

Hirota, Shun K., Yahara, Tetsukazu, Fuse, Kengo, Sato, Hiroyuki, Tagane, Shuichiro, Fujii, Shinji, Minamitani, Tadashi & Suyama, Yoshihisa, 2022, Molecular phylogeny and taxonomy of the Hydrangea serrata complex (Hydrangeaceae) in western Japan, including a new subspecies of H. acuminata from Yakushima, PhytoKeys 188, pp. 49-71 : 49

Hydrangea acuminata subsp. yakushimensis Yahara & Tagane subsp. nov.

Figures 4 , 5

Diagnosis.

Hydrangea acuminata subsp. yakushimensis is different from Hydrangea acuminata subsp. acuminata in that it has smaller capsules, 2.2-2.7 mm long with calyx tube 1.2-1.4 mm long and projected apical part including persistent style 1.0-1.3 mm (vs. capsules 3.2-5.1 mm long with calyx tube 1.6-3.4 mm and projected apical part including persistent style 1.5-2.0 mm), a larger infructescence attaining to 7 × 12 cm (vs. attaining to 6.3 × 8.7 cm), leaves glabrous adaxially except veins (vs. hairy) and glabrous or only slightly hairy abaxially except for tufted hairs at axils of lateral veins (vs. hairy overall on abaxial surface).

Type.

Japan. Kagoshima Pref.: Yakushima Migitani , on cliff along stream, 30.34255555°N, 130.48100000°E, 1520 m elevation, 9 September 2020, with fruits, K. Fuse JPN1799 (holotype: KYO!) .

Description.

Shrubs 1-1.5 m tall. First year’s twigs green when fresh, with dark purple lenticels, glabrous, terete. Old twigs pale brown; bark not peeled off. Leaves opposite; petioles purplish green, 1.7-3 cm long, glabrous; leaf blade adaxially green, abaxially light green when fresh, pale green when dried, elliptic, 9-12 × 4.6-6.4 cm, papery, adaxially glabrous except veins which are covered with minute hairs, abaxially glabrous or only sparsely hairy except for tufted hairs at axils of lateral veins, secondary veins 6-9 on each side of midvein, adaxially slightly sunken, abaxially slightly elevated, base broadly cuneate, apex long acuminate, margin serrate, teeth 2-3 mm high, 13-31 along each side of the margin. Inflorescences corymbose cymes, 2-7 cm long, 4-12 cm in diam., densely pubescent, apex flat to slightly arcuate, 3-5-branched; the longest internode of each branch 1.5-2.5 cm long, densely pubescent; infructescence attaining to 7 × 12 cm. Marginal showy flowers light blue, on pedicel 1-2 cm long; sepals 3 to 5, rhomboid-elliptic, 0.8-1.4 × 0.5-1.1 cm, glabrous, apex obtuse, base rounded to cuneate, margin entire. Fertile flowers protandrous, light blue. Male-stage flowers on pedicel 1-1.8 mm long; calyx tube funnel-shaped, ca. 1 mm long, 0.8 mm in diam., lobes 5, triangular, 0.5 × 0.4 mm, apex acute; petals 5, light blue, elliptic, 2-2.2 × 1 mm, glabrous, apex acute; stamens 10, light blue, subequal, filaments 1.5-3 mm long, glabrous, anthers white, globular, 0.6 mm in diam.; ovary nearly 1/2 superior, style 3, connate at base, slightly spreading, dark blue, ca. 0.7 mm long, stigma flat. In female-stage flowers, petals and stamens fallen off; ovary nearly 1/2 superior; calyx tube light blue, ca. 1 mm long; style darker blue, spreading, ca. 1 mm long; capsules 2.2-2.7 mm long; calyx tube subglobose, 1.2-1.4 mm long, 1.5-2 mm in diam., projected apical part including persistent styles 1.5 mm long. Seeds light brown, elliptic, 0.8 × 0.5 mm, not winged.

Japanese name.

Yakushima-ruri-ajisai.

Phenology.

Flowers were collected in July and August, and fruits were collected in September.

Distribution and habitat.

Yakushima (Yaku Island), Japan (endemic). The distribution of H. acuminata subsp. yakushimensis is restricted to cliffs along streams at Yakushima. It mainly grows in the mountain-top area from 1520 to 1750 m, but one population occurs at an elevation of 575 m, along the Miyanoura River.

Etymology.

The specific epithet is derived from the type locality, Yakushima.

IUCN Conservation status.

Endangered (EN) based on criterion D; the population size is above 50, but less than 250.

Additional specimens examined.

Japan. Kagoshima Pref., Yakushima: Mt. Nagata , on cliff, 30.343799°N, 130.492056°E, 1750 m elevation, 2 August 2005, with flowers, T. Yahara, S. Tagane, K. Fuse & T. Saito 0791 (FU!); Kamisamano-kubo, on cliff, 30.343799°N, 130.492056°E, 1750 m elevation, 2 August 2005, with flowers, T. Yahara, S. Tagane, K. Fuse & T. Saito 0792 (FU!); ditto, with flowers, T. Yahara, S. Tagane, K. Fuse, T. Saito 0793 (FU!); Nemachino-kubo, on cliff, 30.345465°N, 130.49468230°E, 1740 m elevation, 12 July 2006, sterile, S. Tagane & K. Fuse 1065 (FU!); Migitani, on cliff along stream, 30.34255555°N, 130.48100000°E, 1520 m elevation, 13 July 2006, with flowers, S. Tagane & K. Fuse 1103, 1104, 1105 (FU!); Sensuikyo, 30.372031°N, 130.504266°E, 575 m elevation, 31 August 2020, sterile, K. Fuse JPN1708 (FU!) .

Figures

Figure 4. Molecular phylogenetic tree reconstructed using cpDNA sequences. Bootstrap values are shown on the nodes. Nodes supported by less than 60 % bootstrap values are not shown.

Figure 5. Hydrangea acuminata subsp. yakushimensis Yahara & Tagane A a tree growing on cliff along stream B a fruiting twig of the specimen JPN 1799 (holotype) C lower leaf surface of the specimen JPN 1799. Scale bars: 20 cm (A); 10 cm (B); 2 cm (C).