Jour. Korean For. Soc. Vol. 99, No. 5, pp. 663~672 (2010) JOURNAL OF KOREAN FOREST SOCIETY Pollen Morphology of the Genus Rhododendron (Ericaceae) in Korea Joonmoh Park1 and Unsook Song2* 1Jeollabuk-Do Forest Environment Research Inst., Jinan 567-883, Korea 2Inst. of Agricultural Science & Technology, Chonbuk Nat. University, Jeonju, 561-756, Korea Abstract : The pollen morphology of eleven species and three forms of the genus Rhododendron (Ericaceae) in Korea was examined using light, scanning electron and transmission electron microscopy. The pollen grains are grouped in permanent tetrahedral tetrads; viscin threads are present on the tetrads. The hexacolporate tetrads are of tricolporate monads whereby the apertures form in pairs at six points in the tetrad. The exine sculpture pattern is rugulate, scabrate or verrucate on mesocolpium but psilate, rugulate or microscabrate around the aperture on apocolpium. The exine of Korean Rhododendron pollen consists of tectum, columella, foot layer and endexine. The surface of viscin threads is more or less smooth. The threads are sometimes tangled together and occasionally divided into strands. Six types are recognized based on the pollen morphology. The types are: (1) Micranthum-type (rugulate mesocolpium and psilate apocolpium), (2) Tomentosum-type (rugulate mesocolpium and apocolpium), (3) Aureum-type (scabrate mesocolpium and rugulate apocolpium), (4) Brachycarpum-type (scabrate mesocolpium and rod shaped-microscabrate apocolpium), (5) Schlippenbachii- type (scabrate mesocolpium and round-microscabrate apocolpium) and (6) Weyrichii-type (verrucate mesocolpium). Key words : pollen type, Rhododendron, tetrahedral tetrad, viscin thread Introduction Lee, 1988) tried to solve those problems, there has been no taxonomic progress because of almost the same char- The Rhododendron L., one of the largest genera in the acters and methods used in the studies. Ericaceae, belongs to the Rhododendroideae (Rehder, The pollen of Rhododendron is a tetrahedral tetrad com- 1940; Leach, 1961; Cronquist, 1981). It consists of about bined from four monads while pollen grains of other plants 1,200 taxa, and widespread in temperate, cool, subtrop- are commonly monads. The size of tetrads and exine strat- ical regions and in tropical mountains except for Africa ification vary according to authors (Wodehouse, 1935; Erdt- and South America regions (Leach, 1961; Bailey and man, 1952; 1969; Yang, 1952; Waha, 1982; Praglowski and Bailey, 1978; Galle, 1995). There are 22 native Rhodo- Grafstrom, 1985). The genus has been palynologically attrac- dendron taxa in Korea, and their distribution ranges tive because of viscin threads (Bowers, 1930; Ikuse, 1956; from very low areas along seacoasts to ca. 2,000 m high Oldfield, 1959; Ueno, 1978; Foss, 1988; Keri and Zetter, mountains (Lee, 1989). 1992; Crepet, 1996; Abraham-Peskir et al., 1997). Since R. schlippenbachii Maxim. was firstly documented Pollen morphology of Korean Rhododendron has been in 1870 as a Korean Rhododendron species (Nakai, 1919), reported (Lee, 1979; Jang, 1986; Choo, 1987) briefly there have been a number of fragmentary systematic using light microscopy and scanning electron micros- studies on the genus in Korea. Nakai (1919) reported 12 copy. There have not been any transmission electron species and three varieties of the Korean Rhododendron. microscopic studies on Korean Rhododendron, yet. However, some scientific names and morphological Therefore, this study aimed to examine eleven species characters proposed by Nakai were turned out not to be and three forms of Korean Rhododendron using light, correct. The number range of Rhododendron taxa has scanning electron and transmission electron microscopy, been 17 to 23 by different scientists (Nakai, 1952; Chung, and further contribute to the systematics of Korean 1957; Lee, 1989; Lee, 1996a; Lee, 1996b). Though recent Rhododendron. systematic studies (Choo, 1987; Hwang, 1987; 1999; Materials and Methods *Corresponding author E-mail: [email protected] Pollen material from 11 species, 3 forms of Korean 663 664 Jour. Korean For. Soc. Vol. 99, No. 5 (2010) microtome (Rechert-Jung, Type 701704). The exine struc- ture and viscin thread were examined by a JEM-1200EXII at 1,500-10,000X (2,000 accelerating voltage). Terminology Terminology follows Punt et al. (2007). Results 1. Pollen morphology of Korean Rhododendron taxa The pollen grains of the genus are compound grains. Four monads are combined into a tetrahedral tetrad hav- ing viscin threads. Each pollen grain of the tetrad is iso- polar and radially symmetrical, either circular (Figure 2c) or Figure 1. Schematic representation of tetrad tetrahedral somewhat rounded triangular (Figure 3c) in polar view, grains of Rhododendron indicating the positions of pollen and oblate to oblate spheroidal in equatorial view. The characters measured (TD=tetrad diameter, DT=diameter tetrads are rhomboidal. The hexacolporate tetrads are of of top grain, AD=apocolpium diameter, PL=polar axis length, EW=equatorial width, CL=colpus length and CW=colpus tricolporate monads whereby the apertures form in pairs width). at six points in the tetrad. There are 12 lalongate endoapertures in the tetrad. The mesocolpium is sca- brate, rugulate or verrucate while the aperture area is Rhododendron was collected throughout the country microscabrate, rugulate or psilate (Figures 2-4). from March 1998 to July 1999. Pollen of five species; R. The exine of the Korean Rhododendron pollen con- aureum, R. confertissimum, R. dauricum, R. tomentosum sists of tectum, columella, foot layer and endexine. The and R. redowskianum in North Korea was extracted from exine thickness shows a tendency to get thicker toward the specimens in Chonbuk Nat. University and Seoul apertures. The tectum is thicker than the columella. And Nat. University herbaria (Specimens Investigated). The the foot layer is thicker than the endexine. The tectum is pollen specimens are preserved in the herbarium of the either eutectate or tectate-perforate according to taxa. The Department of Forest Resources, Chonbuk Nat. Univer- columella is of either rod shape (Figure 2f) or granule sity, Korea. (Figure 3d). The foot layer is the thickest while endexine For LM, the pollen grains were acetolysed and mounted is the thinnest with some irregular grooves (Figure 2b). in glycerol jelly (Erdtman, 1952; Song and Kim, 1999). The viscin threads are almost smooth. Most of them An Olympus B201 was used to measure seven charac- are long and drooping or tangled. Sometimes, the threads ters; tetrad diameter (TD), diameter of top grain (DT), are branched into two strands and some are very short. apocolpium diameter (AD), monad polar axis length (PL), monad equatorial width (EW), colpus length (CL) 2. Pollen grain size and colpus width (CW) at 400X (Figure 1). Fifty tetrads The diameter range of tetrads is 30.0 to 70.0 µm which per taxon were investigated. are medium to big pollen grains according to Erdtman For SEM, acetolysed pollen grains were dehydrated in (1952). Rhododendron tomentosum is the smallest with ethanol series and transferred onto aluminum stubs (Har- the mean size of 32.0 µm while R. schlippenbachii f. ley and Daly, 1995; Harley et al., 2005). The pollen sus- albiflorum is the biggest (60.9 µm). The pollen size of pensions on the stubs were covered with an inverted monads and tetrads varies according to taxa (Table 1). glass beaker and left to evaporate at room temperature. The pollen grains were coated with ca. 90 nm of plat- 3. Aperture inum (Cressington Sputter Coater Q108). The examina- The monads are isopolar. The hexacolporate tetrads tion of pollen grains was conducted with a JEOL JSM- are of tricolporate monads whereby the apertures form in 5600 LV at 650-10,000X (2,000 accelerating voltage). pairs at six points in the tetrad. There are 12 lalongate For TEM, pollen was fixed in 2% of glutaraldehyde endoapertures. Rhododendron tomentosum has the short- solution for 90 minutes and in 1% of OsO4 solution for est aperture length (14.1 µm) while R. weyrichii has the another 90 minutes, and dehydrated using propylene oxide. longest (21.5 µm) one. For the aperture diameter, R. tomen- Then the pollen was embedded in epoxy resin for two tosum has the shortest (2.30 µm) one while R. yedoense hours and the ultra-thin sectioning was done by an Ultra- f. poukhanense has the longest (3.40 µm) one (Table 1). Pollen morphology of the genus Rhododendron (Ericaceae) in Korea 665 Figure 2. LM, SEM and TEM images of pollen types I (a-b) and II (c-f). a-b. R. micranthum. (a) a tetrad showing rugulate mesocolpium but psilate apocolpium and aperture areas (SEM), (b) a part of exine on mesocolpium showing tectum (arrow, te), columellae (arrow, co), foot layer (arrow, fl) and endexine (arrow, en) (TEM). c, e, f. R. redowskianum. (c) a high focused tetrahedral tetrad (LM), (e) a rugulate tetrad (SEM), and (f) a part of exine showing tectum (te), columellae (co), foot layer (fl) and very thin endexine (en) (TEM). d. R. tomentosum of detail of apertural region (SEM). 4. Equatorial and polar view rounded triangular in polar view. The every pollen grain in equatorial view is oblate to oblate spheroidal (P/E=0.68-0.95) but the mean P/E of 5. Exine pattern each taxon shows that the pollen is suboblate. The tet- The exine sculpture pattern is rugulate, scabrate or verru- rads are rounded lozenge shaped in equatorial view. They cate on mesocolpium but psilate, rugulate or microsca- are radially symmetrical and either circular or somewhat brate around the aperture and on apocolpium. Rhododendron 666 Jour. Korean For. Soc. Vol. 99, No. 5 (2010) Table 1. Pollen morphological data of the genus Rhododendron in Korea by LM. TD DT AD PL EW Aperture Taxon P/E (µm) (µm) (µm) (µm) (µm) CL(µm) CW(µm) Rhododendron aureum 53.0±2.9 35.2±2.6 20.1±1.3 31.3±2.4 36.5±1.4 0.86±0.04 17.1±1.8 2.80±0.26 R.