﻿Taxonomic notes on the genus Itea (Iteaceae)

﻿Abstract The genus Itea (Iteaceae) is recognised as a genus with about 21 extant species of shrubs and trees. Within the genus, most species have oblong to elliptical leaves. The lanceolate and obolanceolate leaves are only found in three species, vix Iteaamoena Chun, Iteariparia Collett & Hemsl. and Iteatenuinervia S. Y. Liu. The results of our examination of literature, specimens and living plants in the wild have shown that Iteatenuinervia is conspecific with Iteariparia and is here reduced to a synonym of the latter species. The morphological description, colour pictures, voucher specimens, distribution map of Iteariparia and its related Iteaamoena, together with the morphological comparisons between the two species, are provided in this study.


Introduction
Itea L. (Iteaceae) was first described based on the species I. virginica L. from eastern North America (Linnaeus 1753) and later it was reported in Asia, for example, I. macrophylla Wall.from tropical Asia (Wallich 1824), I. nutans Royle from western Himalaya (Royle 1835) and I. japonica Oliv.from Japan (Oliver 1865).Itea was previously divided into two sections based on its deciduous character, i.e. sect.Deciduae Engl.(= sect.Itea) and sect.Sempervirentes Engl.(Engler 1891).The former section comprises only two species with deciduous leaves, I. virginica and I. japonica, and the latter includes the other species with evergreen leaves.Recently, Choristylis rhamnoides Harv.(Harvey 1842) from southeastern Africa was transferred to Itea as the third section, i.e. sect.Choristylis (Harv.)Jordaan (Kubitzki 2007;Jordaan 2012).Thus, Itea was considered as an unusual genus with an East Asian-eastern North American-southeastern African disjunction pattern (Kubitzki 2007;Tian et al. 2021).The genus comprises about 21 extant species of shrubs and trees, including one in Africa, one in North America and 19 species in temperate, subtropical and tropical Asia (Tian et al. 2021;POWO 2023).
The leaf shapes of the genus Itea are usually oblong to elliptical, while the lanceolate and oblanceolate leaves only appear in three species, including I. riparia Collett & Hemsl. (Collett and Hemsley 1890), I. amoena Chun (Chun 1934) and I. tenuinervia S.Y. Liu (Liu 2001;Tian et al. 2021).Itea riparia was PhytoKeys 239: 59-72 (2024), DOI: 10.3897/phytokeys.239.117851 Zhu-Qiu Song & Bu-Yun Zhang: Taxonomic notes on the genus Itea (Iteaceae) first published based on specimens collected on river-banks from Shan Hills, southern Shan States, Myanmar and it was characterised by narrow leaves, straight branches and erect racemes (Collett and Hemsley 1890).Gagnepain (1916) originally described I. thorelii based two gatherings from Vietnam and Laos and considered that it is very similar to I. riparia, but is different from the latter in the leaf width, the shape of anthers and the length of inflorescence, flower, stamen and ovary.Craib (1931) observed that some specimens of I. riparia from Thailand match I. thorelii in some respects.In China, I. riparia was recorded in Yunnan and I. thorelii was reported in Yunnan and Guangxi (Wu 1977;Fang and Qin 1985;Jin 1995), but the two species were merged in the Flora of China due to absence of obvious different characters (Jin and Ohba 2001).The earlier name I. riparia was adopted as its accepted name (Jin and Ohba 2001;Esser 2005;Chen and Liu 2015).The species grows on river-banks or near streams.Chun (1934) described another Itea species with narrow leaves, Itea amoena, based on specimens from southern Guangxi, China and noted that it is well characterised by narrow, long-acuminate leaves and by long lanceolate calyx lobes.The species was previously identified as "Itea riparia var.auminata Hu var.nov.", based on Ren-Chang Ching 8059 (Chun 1934) and recognised as "Itea lanceolata Merr.n. sp." based on Kuan-Kwang Tsoong 1868 (Jin 1995).However, the latter two names have never been validly published.This species grows near streams, just like I. riparia (Fig. 5).Besides the habitat, I. amoena and I. riparia are also similar in the habit of evergreen shrubs, the lanceolate leaves and terminal inflorescences.Liu (2001) described Itea tenuinervia on the basis of a flowering collection found near streams in Shuolong Town, Daxin County, southwestern Guangxi, i.e.Shou-Yang Liu 2893 (Fig. 1A) and it was morphologically compared with I. riparia and I. thorelii in the protologue.However, our examination of literature and specimens showed that there are no obvious differences between I. tenuinervia and I. riparia (including I. thorelii).Thus, this study treats Itea tenuinervia as a new synonym of I. riparia and discusses the morphological differences between I. riparia and I. amoena.

Itea amoena
Distribution.The species is only found in Guangxi, southern China (Fig. 6).It usually occurs near streams under evergreen forests, with elevation range from 90 to 800 m.Specimens examined.China.Taxonomic notes.Itea amoena can be readily distinguished from I. riparia in its long narrower leaves (length: width = 4.1-9.5 and ratio mean = 6.5 vs. 2.6-5.2 and 3.6), raised lateral and reticulate veins in the adaxial leaf surface (vs.impressed veins), floral petals reflexed at anthesis (vs.floral petals erect at anthesis), floral petals deciduous at fruiting (vs.becoming green and thickened at fruiting), lanceolate calyx lobes (vs.triangular calyx lobes) and fruits with two fused carpels (vs.two obviously divergent carpels).In distribution, I. amoena is restricted to southern Guangxi and it does not overlap with I. riparia (Fig. 6).

Figure 3 .
Figure 3. Itea riparia from Daxin County, Guangxi, China A habitat B leaves in adaxial and abaxial views C fruiting branchlets D infructescence E fruits, showing two divergent carpels per fruit and becoming green and thickened petals and persistent sepals and petals.Voucher specimens: Zhu-Qiu Song et al.JZ20230549 (IBSC).Scale bar: 2 cm (B).

Figure 5 .
Figure 5. Itea amoena from Shangsi County, Guangxi, China A habitat B leaves in adaxial and abaxial views C flowering branchlet D branchlet E part of inflorescences, showing usually 2 or 3 flowers per node F part of inflorescences, showing floral petals reflexed at anthesis G flowers with the different parts separated H flowers at the end of flowering, showing two fused carpels per fruit and deciduous petals I mature fruits.Voucher specimens: Zhu-Qiu Song et al.JZ20230531, JZ20230537 (IBSC).Scale bars: 2 cm (B); 4 cm (C); 5 mm (G).
).It usually occurs near streams under forests, with elevation range from 200 to 1400 m.