Abstract
Pollen grains of 40 taxa of Rhododendron L. and its closely related genera, Therorhodion L. and Menziesia Sm., were examined by means of light microscopy and scanning electron microscopy (SEM), or SEM alone. Pollen tetrads of R. japonicum, R. schlippenbachii, R. tsusiophyllum and M. pentandra were examined also withtransmission electron microscopy. In all the genera studied, 3-colporate, oblate tosuboblate pollen grains are arranged in tetrahedral tetrads. The apocolpial pollen wall is composed of the exine - well developed tectum, columellae, foot layer and endexine, and the intine. On the contrary, the septal exine is composed of fragmentary tectum, and the two foot layers of adjacent grains are sometimes connected by columellae and endexine. Among different infrageneric taxa only the subsect. Ledum (sect. & subgen. Rhododendron) was characterized by small sized pollen tetrads, higher 2f/D value and rugulate exine sculpture. The pollen morphological characteristics overlapped each other in all other taxa. Thus palynological features showed little usefulness in the infrageneric classification of Rhododendron, however, they gave additional support to the individualgeneric status of Menziesia and Tsusiophyllum, and the sister relationship between Rhododendron and Therorhodion.DOI: http://dx.doi.org/10.3329/bjpt.v20i2.17393Bangladesh J. Plant Taxon. 20(2): 185-199, 2013
Highlights
The genus Rhododendron L., one of the largest and diverse genera of Ericaceae (Ericoideae, Rhodoreae), comprises over 1000 species (Chamberlain et al, 1996)
The pollen of Rhododendron has been studied by many researchers for taxonomic purpose as well as a part of regional flora (Huang, 1972; Vasanthy and Pocock, 1987; Fuhsiung et al, 1995; Mao et al, 2000; Terzioğlu et al, 2001; Gao et al, 2002b, c; Wang et al, 2006; Zhang et al, 2009; Miyoshi et al, 2011). All these results showed that pollen tetrads of Rhododendron were diverse in size and exine sculpture; sometime sufficient to differentiate the sections (Gao et al, 2002b; Zhang et al, 2009), but insufficient to differentiate the subgenera (Gao et al, 2002c; Wang et al, 2006)
The pollen tetrads of R. japonicum, R. schlippenbachii, R. tsusiophyllum and M. pentandra were examined with transmission electron microscopy (TEM) to study the exine stratification of respective genera
Summary
The genus Rhododendron L., one of the largest and diverse genera of Ericaceae (Ericoideae, Rhodoreae), comprises over 1000 species (Chamberlain et al, 1996). Rhododendron and closely related genera are included in the tribe Rhodoreae (Kron et al, 2002). According to Stevens (1971), tribe Rhodoreae comprises the genera Rhododendron, Therorhodion, Ledum, Tsusiophyllum and Menziesia. The genus Therorhodion was hypothesized as sister to Menziesia + Tsusiophyllum + Rhododendron (including Ledum) (Kron and Judd, 1990). Chamberlain et al (1996) recognized only 2 genera, namely Rhododendron and Menziesia. The recent classification of the Ericaceae (Kron et al, 2002) recognized four genera in this tribe, namely Diplarche, Menziesia, Rhododendron (including Ledum and Tsusiophyllum) and Therorhodion. Therorhodion is often placed in Rhododendron (Chamberlain et al, 1996; Kurashige et al, 2001; Goetsch et al, 2005); the cladistic analyses of molecular data
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