Bruguiera × rhynchopetala (W.C.Ko) N.C.Duke & X.J.Ge, 2006

Duke, N. C. & Ge, X. - J., 2011, Bruguiera (Rhizophoraceae) in the Indo-West Pacific: a morphometric assessment of hybridization within single-flowered taxa, Blumea 56 (1), pp. 36-48 : 45-46

3. Bruguiera × rhynchopetala (W.C.Ko) N.C.Duke & X.J.Ge , comb. & stat. nov. — Fig. 4 ; Map 1 , 2

Bruguiera sexangula var. rhynchopetala W.C.Ko,Acta Phyotax.Sin. 16 (1978) 110.— Type: Yue-74, 03190 (IBSC), China, Hainan. Qiongshan, Yanfeng , mangrove, 9 Sept. 1974.

Bruguiera × rhynchopetala N.C.Duke (2006) 122–123, nom. nud.

Natural hybrid of Bruguiera gymnorhiza (L.) Savigny × B. sexangula (Lour.) Poir. Differs from B. sexangula but similar to B. gymnorhiza in presence of 2 –3 bristles, much greater than 1 mm long, distinctly exceeding apex of petal lobes. Differs from B. gymnorhiza but similar to B. sexangula in surface of calyx often with distinct ribbing on tube corresponding to lobe junctures on closed mature buds, spine between petal lobes always shorter than lobes.

Intermediate characters between B. gymnorhiza and B. sexangula include: bristle length on apex of petal lobe, c. 1–2 mm long (bristles of B. sexangula 0 –1 mm; 2 – 4 mm for B. gymnorhiza ); lenticels (notable for China) between 1–2 cm ( B. sexangula c. 1 cm; B. gymnorhiza c. 2 cm); and bark (notable for China) with both horizontal and vertical fissures ( B. sexangula has vertical; B. gymnorhiza has predominately horizontal) ( Table 3).

Etymology. ‘rhyncho-petala’ means ‘beaked petals’ (in Greek),and refers to the bristles at the tips of petal lobes of this hybrid species.

Tree or shrub to 15–35 m, evergreen, columnar or multi-stemmed, branching mostly sympodial, stem base with flat, fin-like buttresses to 0.5 m high. Exposed root stilts occasionally on lower stem, pneumatophores knee-like, to 25 cm. Bark grey to pale-brown, with horizontal and vertical fissures, with a few large corky lenticels of 1– 2 cm diam, especially on buttresses. Foliage comprised of compact rosettes of paired leaves, clustered at 4– 8(–10) leaf scar nodes down from apical shoot, terminal, spicate, prominent, red-green, 3 –7(–8) cm long. Interpetiolar stipules paired, lanceolate, green to yellowish, occasional pinkish tinge, enclose terminal bud to 8 cm long. Leaves opposite, simple, blade elliptic to elliptic-oblong, smooth, glossy green, (6–)7–15(–21) cm long, 3 –6(–8) cm wide, (3–)4–8(–9) cm shape length, length/width ratio 2.1–2.6, length/shape ratio 1.9– 2.1, margin entire, apex acute, base cuneate; petiole green, to 2.4– 4.4 cm long. Inflorescence axillary, 1-flowered, buds generally nodding, maturing within leafy rosette; peduncle green, 3.9–19(– 23) mm long, 1.9– 2.7(– 3.2) mm wide; mature buds present at 1–2 internodal segments below apical shoot; mature hypocotyls present at 4 – 6 internodal segments below apical shoot. Mature flower buds green with rosy blush to all green, 29.4– 41 mm long, 4.3 – 6.1 mm wide around calyx tube, 7.6 –10 mm wide at sepal lobes, distil tip acute; calyx tube turbinate, smooth to ribbed, with 9 –12(–13) lobes, slender pointed, longer than tube, 17.2– 26 mm long, margins on closed bud smooth, to slightly indented or slightly raised; petals 9 –12, creamy white, turning orange brown at anthesis, 14–18 mm long, 1.9 – 2.8 mm closed width, bilateral folded, 3.5– 5.6 mm open width, bilobed; lobes 3.2– 6.2 mm long, densely fringed with hairs along outer margins, apices rounded with 1–3 bristles near top, 1.2– 2.3 mm long, sinus between lobes with hair-like spine, 2.5 – 5 mm long, not exceeding lobes, spine/petal lobe length ratio 0.5 – 0.9; stamens 18 –24, creamy white turning orange brown at anthesis, 12.2 –18 mm long, 0.2– 0.5 mm wide, compressed pair within closed petal, dehiscing precociously when triggered, anthers linear, creamy pale yellow turning brown at anthesis, 5.6 –7.3 mm long, ± 0.7 mm wide; style filiform, smooth, pale green, 17–25 mm long, to 0.9 –1.5 mm wide, stigma minutely 3(–4)-lobed at tip, mounted centrally within calyx bowl 2.7–4.4 mm wide, 3.4–7.3 mm deep. Mature fruit cryptic within slightly enlarged calyx tube, turbinate, smooth to ribbed, 41–44 mm long, 14–19 mm wide, lobes slightly reflexed, if at all, 18– 24 mm long, 20– 35 mm spread width; germination viviparous, hypocotyl emergent from calyx with maturation. Mature hypocotyl cigar-shaped, straight, green, 9 –15 cm long, 13–19 mm at widest point, 39–75 mm shape length, 4 – 8 mm width at plumule end, length/width ratio 5.8 – 8.6, length/shape ratio 1.6 – 2.6, some longitudinal ribbing, distil end blunt, plumule 2 – 3.7 mm long, buoyant dispersal agent.

Phenology — In Australia, flowering peaks in August and September, and maturation of propagules occurs in September to December. In China, flowering was observed in February to June,and generally differing by around six months in the northern hemisphere. The Bruguiera hybrid differs from Rhizophora hybrids as it commonly produces mature-sized and well-formed hypocotyls. By contrast, Rhizophora hybrid individuals rarely produce mature hypocotyls ( Duke 2010).

Ecology and local influences — Common in the mid intertidal zone of intermediate-upstream estuarine position. Notable occurrences within the estuarine range of B. sexangula , and overlapping with B. gymnorhiza . The estuarine range therefore is predominately upstream, favouring more freshwater-influenced estuaries. Usually located in mid to higher intertidal portions of mangrove stands.

Distribution — Distribution in China is restricted to north-eastern coast of Hainan Island, but records lacking elsewhere in China. In Australia (Duke 2006), the hybrid occurs in east coast estuaries of Queensland from Jacky Creek (10°54' S, 142°32' E) in the north, to the Herbert River (18°32' S, 146°19' E) in the southeast. The hybrid may occur only where parental forms co-exist.

Specimens examined. AUSTRALIA, Queensland, Daintree River, Norman C. Duke, D001, D020 ( BRIU) . – CHINA, Hainan, Qiongshan, Yanfeng , 26 Sept. 1995, Wang Rui-Jiang 035 ( IBSC) ; Qiongshan, Yanfeng, Shanwei , 3 Oct. 1983, Ko Wan-Cheung 497 ( IBSC) ; Wenchang, Bianhai, Sept. 1979, Ko Wan-Cheung 7 ( IBSC) ; Wenchang, Shagang , 4 Aug. 1957, Li Dong- Sheng & Zhang Jing-Qing 80292 ( IBSC) ; Wenchang, Qinglangang , March, 1956, CAS Hainan Vegetation Investigation Team 1024 ( IBSC) ; Wenchang, Qinglangang , 16 April 1956, 01055 ( IBSC) .

Note — Bruguiera × rhynchopetala is the hybrid of B. gymnorhiza and B. sexangula . Discrimination of taxa is based on calyx ribbing, bristle number and petal lobe length. The hybrid entity is distinguished by petals having 1–2 relatively short bristles at lobe tips. By comparison, B. sexangula has no bristles, or at least only very short ones less than 1.25 mm long, not or barely exceeding the apex of the petal lobe. The hybrid is more comparable with B. gymnorhiza which has petal lobes with 2 –3, rarely 4, bristles of 2 – 4 mm long, and distinctly exceeding the apex. In field surveys in China, putative hybrids were identified using intermediate characteristics of bark fissuring and lenticels. In Australia, bark characteristics were not as useful, so initial field identification was based on tree shape being more erect, columnar, with plank-like buttresses.

Figures

Map 1 Global distributions of the three large, single-flowered Bruguiera species, B. exaristata, B. gymnorhiza and B. sexangula. Note that B. sexangula was introduced to the Hawaiian islands (Allen et al. 2000).

Map 2 Conjectured distribution of Bruguiera × rhynchopetala, based on the area of overlap for B. sexangula and B. gymnorhiza (see Fig. 1). Collection sites in China and Australia are indicated by triangle symbols.

Fig. 4 Bruguiera × rhynchopetala. a. Habit, Escape River, Queensland,Australia; b. trunk and buttresses,Hainan, China; c. foliage; d. leaves; e. leafy rosette with reproductive parts; f. closed mature flower bud; g. open mature flower bud showing petals; h. immature fruit stage; i. open petal; j. diagram of open petal; k. mature hypocotyl. — Scale bars: a–j = 10 mm.