Taxonomical notes on plants of southern Japan II. Variation in the Rhododendron obtusum group in the Kirishima Mis. southern Kytushu.

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  • 西南日本の植物雑記II. : 霧島山系におけるミヤマキリシマ, キリシマツツジ, ヤマツツジ諸集団の形質変異
  • 西南日本の植物雑記 2.霧島山系におけるミヤマキリシマ,キリシマツツジ,ヤマツツジ諸集団の形質変異
  • セイナン ニホン ノ ショクブツ ザッキ 2.キリシマ サンケイ ニ オケル

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Morphological variation of natural populations of the Rhododendron obtusum group (R. obtusum, R. kaempferi and R. kiusianum) were studied at 19 sites in the Kirishima Mts., southern Kyushu. In this part of southern Japan R. kaempferi (southern type) grows wide on rocky places and along forest edges on lower mountain slopes up to 800m alt. Rhododendron kiusianum occurs higher on open slopes from ca. 1000m alt. to the summits of volcanic mountains and usually forms large populations. In the zone between these two species polymorphic R. obtusum occurs in open forests, around forest edges and on roadsides at 800-1000m alt. Many morphological characteristics of this species are intermediate between R. kaempferi and R. kiusianum, and flower color varies immensely. Undoubtedly, this species originated through natural hybridization of the two species. Rhododendron kaempferi and R. kiusianum are crossed easily and freely and produce viable F_1,F_2 and further generations in cultivation. In the field, however, these two species have different pollinator syndromes. The main visitors for R. kaempferi are Lepidoptera (Byasa alcinous and Papilio spp.) and the honey bee Apis mellifera. The large red flowers are sometimes also visited by Diptera and small bees. In contrast, the small purplish flowers of R. kiusianum are visited by various species of Diptera and small bees. Large butterflies usually did not visit. Different pollinator assemblages and habitat segregation may isolate these two species. Rhododendron obtusum is visited by all the common pollinators of R. kiusianum and R. kaempferi, e.g., Lepidoptera (Byasa alcinous and Pieris melete), Diptera (Eristalis tenax and Phytomia zonata), and Hymenoptera (Bombus ardens and Apis mellifera). Through this pollinator assemblage, R. obtusum seems to be an effective route of gene introgression between R. kaempferi and R. kiusianum.

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