Rhododendron Xanneliesae, which was received by the Arnold Arboretum in 1921 from Hunnewell Pinetum documented to be a Rhododendron arborescens / Rhododendron calendualceum interaction, has been determined to be a tetraploid using flow cytometry by
Dr. João Loureiro, Dr. Silvia Castro, José Cerca, and Mariana Castro
Plant Ecology and Evolution Group,
Centre for Functional Ecology,
Department of Life Sciences,
Faculty of Science and Technology,
University of Coimbra, Portugal.
Rhododendron Xanneliesae is very seed fertile.
Rhododendron calendulaceum is a tetraploid species. Rhododendron arborescens is a diploid species.
Diploid / tetraploid interaction in deciduous azaleas normally results in triploid offspring.
"A monograph of azaleas: Rhododendron subgenus Anthodendron" By Ernest Henry Wilson, Alfred Rehder
"This hybrid originated accidentally at the Arnold Arboretum and was raised probably in 1896 from seed of R. calendulaceum or R. arborescens collected in the Arboretum. It is exactly intermediate between R. calendulaceum and R. arborescens and I have little doubt that it is a hybrid between these two species. The first flowers of R. arborescens are usually just beginning to open about the middle of June when late blooming forms of R. calendulaceum bear the last flowers; and with dichogamous plants this is just a favorable condition for cross-fertilization. From R. calendulaceum it differs chiefly in the glaucous and glabrous under side of the leaves, only the midrib being furnished with strigose hairs and slightly pubescent toward the base, in the very sparingly hairy branchlets and in the longer corollatube of the pinkish white fragrant flowers marked with a large yellow blotch and in the style puberulous only near the base. From R. arborescens it differs in the slightly pilose and slightly puberulous branchlets, glabrescent toward the base, in the pubescent and strigose midrib of the under side of the leaves, in the large yellow blotch on the upper lobe of the pinkish white flowers, in the shorter ovate calyx-lobes and in the style being puberulous near the base.
It is a shrub of vigorous habit, very handsome in flower with its large fragrant, pinkish white flowers marked with a conspicuous yellow blotch; the pinkish corollatube is rather densely furnished with short glandular hairs and the style is purple toward the apex. The leaves are elliptic or broadly elliptic and somewhat bluish green above."
Summary of Ploidy Levels
Diploid Rhododendron arborsecens,
Tetraploid Rhododendron Xanneliesae,
Tetraploid Rhododendron calendulaceum
Source: Arnold Arboretum Accession Number: 10767
John and Sally Perkins
