Rhododendron is a genus ofshrubs and small to (rarely) largetrees, the smallest species growing to 10–100 cm (4–40 in) tall, and the largest,R. protistum var.giganteum, reported to 30 m (100 ft) tall.[10][11] Theleaves are spirally arranged; leaf size can range from 1–2 cm (0.4–0.8 in) to over 50 cm (20 in), exceptionally 100 cm (40 in) inR. sinogrande. They may be eitherevergreen ordeciduous. In some species, the undersides of the leaves are covered with scales (lepidote) or hairs (indumentum). Some of the best known species are noted for their many clusters of large flowers. A recently discovered species in New Guinea has flowers up to six inches (fifteen centimeters) in width,[12] the largest in the whole genus. The accompanying photograph shows it as having sevenpetals. There are alpine species with small flowers and small leaves, and tropical species such assectionVireya that often grow asepiphytes. Species in this genus may be part of the heath complex inoak-heath forests in eastern North America.[13][14]
They have frequently been divided based on the presence or absence of scales on the abaxial (lower) leaf surface (lepidote or elepidote). These scales, unique to subgenusRhododendron, are modified hairs consisting of a polygonal scale attached by a stalk.[3]
Rhododendron is the largest genus in the familyEricaceae, with over 1,000 species,[16][17] (though estimates vary from 850 to 1,200)[18][19] and ismorphologically diverse. Consequently, thetaxonomy has been historically complex.[15]
Linnaeus' six species ofAzalea wereAzalea indica,A. pontica,A. lutea,A. viscosa,A. lapponica andA. procumbens (nowKalmia procumbens), which he distinguished fromRhododendron by having fivestamens, as opposed to ten. As new species of what are now consideredRhododendron were discovered, they were assigned to separate genera if they seemed to differ significantly from the type species. For instanceRhodora (Linnaeus 1763) forRhododendron canadense,Vireya (Blume 1826)[24] andHymenanthes (Blume 1826) forRhododendron metternichii, nowR. degronianum. Meanwhile, other botanists such asSalisbury (1796)[25] and Tate (1831)[26] began to question the distinction betweenAzalea andRhododendron, and finally in 1836,Azalea was incorporated intoRhododendron[27] and the genus divided into eight sections. Of theseTsutsutsi (Tsutsusi),Pentanthera,Pogonanthum,Ponticum andRhodora are still used, the other sections beingLepipherum,Booram, andChamaecistus. This structure largely survived till recently (2004), following which the development of molecular phylogeny led to majorre-examinations of traditional morphological classifications,[22][23] although other authors such as Candolle, who described six sections,[28] used slightly different numeration.
Soon, as more species became available in the nineteenth century so did a better understanding of the characteristics necessary for the major divisions. Chief amongst these wereMaximovicz'sRhododendreae Asiae Orientali[29] andPlanchon. Maximovicz used flower bud position and its relationship with leaf buds to create eight "Sections".[30]Bentham and Hooker used a similar scheme, but called the divisions "Series".[31] It was not until 1893 thatKoehne appreciated the significance of scaling and hence the separation of lepidote and elepidote species. The large number of species that were available by the early twentieth century prompted a new approach whenBalfour introduced the concept of grouping species intoseries.The Species of Rhododendron[32] referred to this series concept as the Balfourian system. That system continued up to modern times in Davidian's four volumeThe Rhododendron Species.[33]
The next major attempt at classification was bySleumer who from 1934 began incorporating the Balfourian series into the older hierarchical structure of subgenera and sections, according to theInternational Code of Botanical Nomenclature, culminating in 1949 with his "Ein System der GattungRhododendron"[34] and subsequent refinements.[35][36] Most of the Balfourian series are represented by Sleumer as subsections, though some appear as sections or even subgenera. Sleumer based his system on the relationship of the flower buds to the leaf buds, habitat, flower structure, and whether the leaves were lepidote or non-lepidote. While Sleumer's work was widely accepted, many in the United States and the United Kingdom continued to use the simpler Balfourian system of the Edinburgh group.
Sleumer's system underwent many revisions by others, predominantly the Edinburgh group in their continuingRoyal Botanic Garden Edinburgh notes. Cullen of the Edinburgh group, placing more emphasis on the lepidote characteristics of the leaves, united all of the lepidote species into subgenusRhododendron, including four of Sleumer's subgenera (Rhododendron,Pseudoazalea,Pseudorhodorastrum,Rhodorastrum).[36][37] In 1986 Philipson &Philipson raised two sections of subgenusAleastrum (Mumeazalea,Candidastrum) to subgenera, while reducing genusTherorhodion to a subgenus ofRhododendron.[38] In 1987 Spethmann, addingphytochemical features proposed a system with fifteen subgenera grouped into three 'chorus' subgenera.[39]
A number of closely related genera had been included together withRhododendron in a former tribe, Rhodoreae. These have been progressively incorporated intoRhododendron.[40] Chamberlain and Rae moved themonotypic sectionTsusiopsis together with the monotypic genusTsusiophyllum into sectionTsutsusi,[41] while Kron & Judd reduced genusLedum to a subsection of sectionRhododendron.[42] Then Judd & Kron moved two species (R.schlippenbachii andR.quinquefolium) from sectionBrachybachii, subgenusTsutsusi and two from sectionRhodora, subgenusPentanthera (R. albrechtii,R. pentaphyllum) into sectionSciadorhodion, subgenusPentanthera.[43] Finally Chamberlain brought the various systems together in 1996, with 1,025 species divided into eight subgenera. Goetsch (2005) provides a comparison of the Sleumer and Chamberlain schemata (Table 1).[3][15][23][44][45]
The era of molecular analysis rather than descriptive features can be dated to the work of Kurashige (1988) and Kron (1997) who usedmatKsequencing. Later Gaoet al. (2002) usedITS sequences[46] to determine acladistic analysis. They confirmed that the genusRhododendron wasmonophyletic, with subgenusTherorhodion in thebasal position, consistent with the matK studies. Following publication of the studies of Goetschet al. (2005) withRPB2,[3] there began an ongoing realignment of species and groups within the genus, based on evolutionary relationships. Their work was more supportive of Sleumer's original system than the later modifications introduced by Chamberlainet al..[3][47]
The major finding of Goetsch and colleagues was that all species examined (exceptR. camtschaticum, subgenusTherorhodion) formed three majorclades which they labelledA,B, andC, with the subgeneraRhododendron andHymenanthes as monophyletic groups nested within cladesA andB, respectively. By contrast subgeneraAzaleastrum andPentanthera werepolyphyletic, whileR. camtschaticum appeared as asister to all other rhododendrons. The small polyphyletic subgeneraPentanthera andAzaleastrum were divided between two clades. The four sections ofPentanthera between cladesB andC, with two each, whileAzaleastrum had one section in each ofA andC.
Thus subgeneraAzaleastrum andPentanthera needed to be disassembled, andRhododendron,Hymenanthes andTsutsusi correspondingly expanded. In addition to the two separate genera included underRhododendron by Chamberlain (Ledum,Tsusiophyllum), Goetschet al.. addedMenziesia (cladeC). Despite a degree ofparaphyly, the subgenusRhododendron was otherwise untouched with regard to its three sections but four other subgenera were eliminated and one new subgenus created, leaving a total of five subgenera in all, from eight in Chamberlain's scheme. The discontinued subgenera arePentanthera,Tsutsusi,Candidastrum andMumeazalea, while a new subgenus was created by elevating subgenusAzaleastrum sectionChoniastrum to subgenus rank.
SubgenusPentanthera (deciduous azaleas) with its four sections was dismembered by eliminating two sections and redistributing the other two between the existing subgenera in cladesB (Hymenanthes) andC (Azaleastrum), although the name was retained in sectionPentanthera (14 species) which was moved to subgenusHymenanthes. Of the remaining three sections, monotypicViscidula was discontinued by movingR.nipponicum toTsutsusi (C), whileRhodora (2 species) was itself polyphyletic and was broken up by movingR.canadense to sectionPentanthera (B) andR.vaseyi to sectionSciadorhodion, which then became a new section of subgenusAzaleastrum (C).
SubgenusTsutsusi (C) was reduced to section status retaining the name, and included in subgenusAzaleastrum. Of the three minor subgenera, all inC, two were discontinued. The single species ofmonotypic subgenusCandidastrum (R.albiflorum) was moved to subgenusAzaleastrum, sectionSciadorhodion. Similarly the single species in monotypic subgenusMumeazalea (R.semibarbatum) was placed in the new sectionTsutsusi, subgenusAzaleastrum. GenusMenziesa (9 species) was also added to sectionSciadorhodion. The remaining small subgenusTherorhodion with its two species was left intact. Thus two subgenera,Hymenanthes andAzaleastrum were expanded at the expense of four subgenera that were eliminated, althoughAzaleastrum lost one section (Choniastrum) as a new subgenus, since it was a distinct subclade in A. In all,Hymenanthes increased from one to two sections, whileAzaleastrum, by losing one section and gaining two increased from two to three sections.[3] (See schemata underSubgenera.)[47]
Taxonomic changes within genusRhododendron
Chamberlain (1996)
Goetsch (2005)
Genus
Subgenus
Section
Species
Subgenus
Section
Menziesa
9 species
Azaleastrum C
Sciadorhodion
Rhododendron
Candidastrum
R. albiflorum
Pentanthera
Sciadorhodion
4 species
Rhodora
R. vaseyi
R. canadense
Hymenanthes B
Pentanthera
Pentanthera
14 species
Viscidula
R. nipponicum
Azaleastrum C
Tsutsusi
Tsutsusi
Brachycalyx
15 species
Tsutsusi
65 species
Mumeazalea
R. semibarbatum
Azaleastrum
Choniastrum
11 species
Choniastrum A
Subsequent research has supported the revision by Goetsch, although has largely concentrated on further defining the phylogeny within the subdivisions.[48] In 2011 the two species ofDiplarche were also added toRhododendron,incertae sedis.[49]
In the later traditional classification, attributed to Chamberlain (1996), and as used byhorticulturalists and theAmerican Rhododendron Society,[50]Rhododendron has eight subgenera based onmorphology, namely the presence of scales (lepidote),deciduousness of leaves, and the floral and vegetative branching patterns, after Sleumer (1980).[3][15][36] These consist of four large and four small subgenera. The first two subgenera (Rhododendron andHymenanthes) represent the species commonly considered as 'Rhododendrons'. The next two smaller subgenera (Pentanthera andTsutsusi) represent the 'Azaleas'. The remaining four subgenera contain very few species.[51] The largest of these is subgenusRhododendron, containing nearly half of all known species and all of the lepidote species.
SubgenusRhododendronL.: Small leaf orlepidotes (scales on the underside of the leaves). 3 sections, 462 species,type species:R. ferrugineum.
For a comparison of the Sleumer and Chamberlain systems, see Goetschet al. (2005) Table 1.[3]
This division was based on a number of what were thought to be key morphological characteristics. These included the position of the inflorescence buds (terminal or lateral), whether lepidote or elepidote, deciduousness of leaves, and whether new foliage was derived fromaxils from previous year's shoots or the lowest scaly leaves.
DeciduousRhododendron luteum in fall colorEvergreen azaleaRhododendron kaempferiEvergreen azalea cultivar leaf color before sheddingRhododendron afterfreezing rain
Following thecladistic analysis of Goetschet al. (2005)[3] this scheme was simplified, based on the discovery of three majorclades (A, B, C) as follows.
Clade A
SubgenusRhododendronL.: Small leaf orlepidotes (scales on the underside of the leaves). 3 sections, about 400 species,type species:R.ferrugineum.
The larger subgenera are further subdivided into sections and subsections[51] Some subgenera contain only a single section, and some sections only a single subsection. Shown here is the traditional classification, with species number after Chamberlain (1996), but this scheme is undergoing constant revision. Revisions by Goetschet al. (2005)[3] and by Cravenet al. (2008)[48] shown in (parenthetical italics). Older ranks such as Series (groups of species) are no longer used but may be found in the literature, but the American Rhododendron Society still uses a similar device, called Alliances[50]
SubgenusRhododendron L. (3 sections, 462 species: increased to five sections in 2008)
(Discovereya (Sleumer) Argent, raised from Vireya)
Pogonathum Aitch. & Hemsl. (13 species; Himalaya and adjacent mountains)
(Pseudovireya (C.B.Clarke) Argent, raised from Vireya)
Rhododendron L. (149 species in 25 subsections; temperate to subarctic Northern Hemisphere)
Vireya (Blume) Copel.f. (300 species in 2 subsections; tropical southeast Asia, Australasia. At one time considered separate subgenus[52])
SubgenusHymenanthes (Blume) K.Koch (1 section, 224 species) (Increased to two sections)
PonticumG. Don (24 subsections)
(Pentanthera(G. Don) Pojarkova (2 subsections – new section, moved from subgenusPentanthera)
Species of the genusRhododendron are widely distributed between latitudes 80°N and 20°S and arenative to areas from North America toEurope,Russia, andAsia, and fromGreenland toQueensland,Australia and theSolomon Islands.[15] Thecentres of diversity are in theHimalayas andMaritime Southeast Asia,[46] with the greatest species diversity in the Sino-Himalayan region, Southwest China and northernBurma, from India –Himachal Pradesh,Uttarakhand,Sikkim andNagaland toNepal, northwesternYunnan and westernSichuan and southeasternTibet. Other significant areas of diversity are in the mountains ofKorea,Japan andTaiwan. More than 90% ofRhododendronsensu Chamberlain belong to the Asian subgeneraRhododendron,Hymenanthes and sectionTsutsusi. Of the first two of these, the species are predominantly found in the area of the Himalayas and Southwest China (Sino-Himalayan Region).[3]
The 300tropical species within theVireya section of subgenusRhododendron occupy theMaritime Southeast Asia from their presumed Southeast Asian origin to Northern Australia, with 55 known species inBorneo and 164 inNew Guinea. The species in New Guinea are native tosubalpine moistgrasslands at around 3,000 metres above sea level in the Central Highlands.[52] SubgeneraRhododendron andHymenanthes, together with sectionPentanthera of subgenusPentanthera are also represented to a lesser degree in the Mountainous areas of North America andWestern Eurasia. SubgenusTsutsusi is found in the maritime regions ofEast Asia (Japan, Korea, Taiwan,East China), but not in North America or Eurasia.[3][30]
Rhododendron ponticum has become invasive inIreland[59] and theUnited Kingdom.[60] It is an introduced species, spreading in woodland areas and replacing the natural understory.R. ponticum is difficult to eradicate, as its roots can make new shoots.
A number of insects either target rhododendrons or will opportunistically attack them. Rhododendron borers and variousweevils are major pests of rhododendrons, and manycaterpillars will preferentially devour them.
Rhododendron species are used as food plants by thelarvae (caterpillars) of somebutterflies andmoths; see List of Lepidoptera that feed on rhododendrons.
Major diseases includePhytophthora root rot, stem and twig fungal dieback.[61]
Rhododendron bud blast, a fungal condition that causes buds to turn brown and dry before they can open, is caused by the fungusPycnostysanus azaleae, which may be brought to the plant by the rhododendron leafhopper,Graphocephala fennahi.[62]: 562
In the UK the forerunner of the Rhododendron, Camellia and Magnolia Group (RCMG), The Rhododendron Society was founded in 1916.[63] while in Scotland species are being conserved by the Rhododendron Species Conservation Group.[64]
Both species and hybrid rhododendrons (including azaleas) are used extensively asornamental plants inlandscaping in many parts of the world, including bothtemperate andsubtemperate regions.[48] Many species and cultivars are grown commercially for thenursery trade.
Rhododendrons can be propagated by air layering or stem cuttings.[62]: 540–541 They can self-propagate by sending up shoots from the roots. Sometimes an attached branch that has drooped to the ground will root in damp mulch, and the resulting rooted plant then can be cut off the parent rhododendron. They can also be reprodcued by seed dispersal - or by horticulturalists collecting the spent flower buds and saving ad drying the seed for later germination and planting.
Rhododendron wardii var.puralbum
Rhododendrons are often valued in landscaping for their structure, size, flowers, and the fact that many of them are evergreen.[65] Azaleas are frequently used around foundations and occasionally as hedges, and many larger-leafed rhododendrons lend themselves well to more informal plantings andwoodland gardens, or as specimen plants. In some areas, larger rhododendrons can be pruned to encourage more tree-like form, with some species such asRhododendron arboreum andR. falconeri eventually growing to a height of 10–15 m (32.81–49.21 ft) or more.[65]
Rhododendrons are grown commercially in many areas for sale, and seeds were occasionally collected in the wild, a practice now rare in most areas due to the Nagoya Protocol. Larger commercial growers often ship long distances; in the United States, most of them are on the west coast (Oregon, Washington state and California). Large-scale commercial growing often selects for different characteristics than hobbyist growers might want, such as resistance to root rot when overwatered, ability to be forced into budding early, ease of rooting or other propagation, and saleability.[66]
Azaleas – group of shrubs which have smaller and thinner leaves than evergreen rhododendrons. They are generally medium-sized shrubs with smaller funnel-shaped flowers that usually have 5 stamens:
Nova Zembla Rhododendrons growing in a nursery inNew Jersey.
Like otherericaceous plants, most rhododendrons prefer acid soils with a pH of roughly 4.5–5.5; some tropical Vireyas and a few other rhododendron species grow asepiphytes and require a planting mix similar toorchids. Rhododendrons have fibrous roots and prefer well-drained soils high in organic material. In areas with poorly drained or alkaline soils, rhododendrons are often grown in raised beds using media such as composted pine bark.[77] Mulching and careful watering are important, especially before the plant is established.
A newcalcium-tolerant stock of rhododendrons (trademarked as 'Inkarho') has been exhibited at theRHSChelsea Flower Show in London (2011). Individual hybrids of rhododendrons have beengrafted on to arootstock on a single rhododendron plant that was found growing in a chalk quarry. The rootstock is able to grow in calcium-rich soil up to a pH of 7.5.[78][79]
Rhododendrons are extensively hybridized in cultivation, and natural hybrids often occur in areas where species ranges overlap. There are over 28,000cultivars of Rhododendron in the International Rhododendron Registry held by theRoyal Horticultural Society. Most have been bred for their flowers, but a few are of garden interest because of ornamental leaves and some for ornamental bark or stems. Some hybrids have fragrant flowers[80]—such as the Loderi hybrids, created by crossingRhododendron fortunei andR. griffithianum.[81] Other examples include the PJM hybrids, formed from a cross betweenRhododendron carolinianum andR. dauricum, and named after Peter J. Mezitt of Weston Nurseries, Massachusetts.[82]
Some species of rhododendron are poisonous to grazing animals because of atoxin calledgrayanotoxin in theirpollen andnectar. People have been known to become ill from eatingmad honey made bybees feeding on rhododendron andazalea flowers.Xenophon described the odd behaviour ofGreek soldiers after having consumed honey in a village surrounded byRhododendron ponticum during the march of theTen Thousand in 401 BCE.[83]Pompey's soldiers reportedly suffered lethal casualties following the consumption ofhoney made fromRhododendron deliberately left behind byPontic forces in 67 BCE during theThird Mithridatic War.[84] Later, it was recognized that honey resulting from these plants has a slightlyhallucinogenic andlaxative effect.[85] The suspect rhododendrons areRhododendron ponticum andRhododendron luteum (formerlyAzalea pontica), both found in northernAsia Minor. Eleven similar cases during the 1980s have been documented inIstanbul,Turkey.[86] Rhododendron is extremely toxic to horses, with some animals dying within a few hours of ingesting the plant, although most horses tend to avoid it if they have access to good forage. Rhododendron, including its stems, leaves and flowers, contains toxins that, if ingested by a cat's stomach, can cause seizures and even coma and death.[87]
Rhododendron species have long been used in traditional medicine.[88][89] Animal studies andin vitro research have identified possible anti-inflammatory and hepatoprotective activities which may be due to the antioxidant effects offlavonoids or otherphenolic compounds andsaponins the plant contains.[90] Xionget al. have found that the root of the plant is able to reduce the activity ofNF-κB in rats.[91]
InNepal, the flower is considered edible and enjoyed for its sour taste. The pickled flower can last for months and the flower juice is also marketed.[92]: 51 The flower, fresh or dried, is added to fish curry in the belief that it will soften the bones.[92]: 53 The juice of rhododendron flower is used to make asquash called burans (named after the flower) in the hilly regions ofUttarakhand. It is admired for its distinctive flavor and color.[93]
InUttarakhand, innorth India, the Buransh flower is deeply embedded in local culture, playing a significant role in festivals like Holi and weddings, where it is used in garlands and decorations to bless attendees. The flower is also utilized in making a healthful,antioxidant-rich juice that is popular during local festivities and summer months. Additionally, Buransh flowers are incorporated into local arts and crafts, where they are used to make colorful necklaces and jewelry, symbolizing the spiritual and physical prosperity of the community.[94]
The effects ofR. ponticum were mentioned in the 2009 filmSherlock Holmes as a proposed way to arrange afake execution.[98] It was also mentioned in the third episode of Season 2 of BBC'sSherlock, speculated to have been a part of Sherlock's fake death scheme.[citation needed]
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