Jour. Korean For. Soc. Vol. 99, No. 5, pp. 663~672 (2010) JOURNAL OF KOREAN FOREST SOCIETY

Pollen Morphology of the Genus () 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 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. brachycarpum 51.0±2.9 32.4±1.5 18.4±2.4 27.9±1.6 34.4±1.9 0.81±0.05 18.4±1.6 2.90±0.39 R. confertissimum 44.6±3.2 30.6±1.4 16.6±1.8 25.2±2.6 31.2±2.1 0.81±0.07 15.4±1.2 2.65±0.34 R. dauricum 45.7±1.9 30.6±0.9 16.2±1.1 25.3±2.0 33.0±2.9 0.77±0.03 16.5±2.4 2.95±0.28 R. micranthum 32.2±1.3 22.4±1.2 9.8±0.9 18.0±1.2 22.4±0.9 0.80±0.06 14.3±1.5 2.70±0.42 R. mucronulatum 49.0±2.3 32.4±3.3 17.9±1.8 27.3±1.4 33.9±1.5 0.81±0.04 18.4±2.3 3.00±0.33 R. m. f. ciliatum 52.3±6.5 32.1±2.0 17.7±1.2 28.2±2.7 36.0±4.1 0.79±0.07 17.5±2.0 2.80±0.54 R. redowskianum 46.9±1.9 31.4±2.8 15.9±1.2 27.5±1.6 34.0±2.0 0.81±0.04 14.7±1.9 2.50±0.41 R. schlippenbachii 59.7±5.5 37.8±3.0 19.3±1.9 34.2±3.1 40.8±3.3 0.84±0.06 21.3±2.5 2.80±0.67 R. s. f. albiflorum 60.9±4.3 40.4±2.6 21.6±1.1 35.3±2.7 44.0±3.3 0.80±0.02 21.3±1.7 3.15±0.88 R. tomentosum 32.0±2.2 21.7±1.2 11.2±1.1 18.3±1.9 23.2±2.2 0.79±0.06 14.1±2.4 2.30±0.48 R. tschonoskii 41.9±2.4 28.8±1.2 14.4±0.9 22.6±1.3 28.8±1.4 0.79±0.05 16.0±1.2 2.90±0.32 R. yedoense f. poukhanense 51.1±1.9 34.5±2.5 18.5±1.1 26.1±1.8 34.2±1.4 0.76±0.05 20.6±2.6 3.40±0.99 R. weyrichii 54.2±2.7 36.7±2.0 19.3±1.6 29.3±1.9 36.6±1.3 0.80±0.07 21.5±3.3 2.55±0.37 TD=Tetrad diameter, DT=Diameter of top grain, AD=Apocolpium diameter, PL=Polar axis length, EW=Equatorial width, P/E=PL/ EW, CL=Colpus length and CW=Colpus width micranthum is characterized with its psilate pattern in 7. Viscin thread aperture margins and on apocolpium (Figure 2a). The The genus has viscin threads on the pollen surface. rest 13 taxa are rugulate or microscabrate around the The diameter of the threads is 0.15 to 0.62 . Rhododen- aperture and on apocolpium. Rhododendron weyrichii is dron micranthum has the narrowest thread diameter verrucate having wart-like sexine elements of wider than (0.22 µm) but R. mucronulatum f. ciliatum has the wid- 1 µm on mesocolpium while the others are rugulate or est one (0.51 µm). The thread surface is nearly smooth. microscabrate on mesocolpium (Figure 4f). Rhododen- Long pendent threads are sometimes tangled together dron brachycarpum is characterized by the rod shaped and occasionally divided into two strands (Figure 2e). elements of the microscabrate apocolpium and aperture Relatively short threads are observed as well (Figures (Figure 3e). The rest eight taxa with microscabrate apo- 2a, 2e, 3a, 3e, and 4d). colpium and aperture have round shaped elements instead. These various exine patterns are major features 8. Key to the Rhododendron pollen types in Korea to be used in the key to six Korean Rhododendron pol- Six pollen types of 11 species and 3 forms of Korean len types (Figures 2-4, Table 2). Rhododendron are recognized based on differences in exine sculpture pattern and exine thickness. 6. Exine stratification The exine of the Korean Rhododendron pollen con- 1. Rugulate on mesocolpium sists of four layers: the tectum, the columella, the foot 2. Psilate on apocolpium and around the aperture layer and the endexine. The exine thickness shows a ten- ················································Micranthum-type (I) dency to get thicker toward apertures. Exine thickness of 2. Rugulate on apocolpium and around the aperture the genus is 0.97 to 2.50 µm. The exine thickness of R. ············································ Tomentosum-type (II) confertissimum is the thinnest being 1.19 µm. Rhodo- 1. Scabrate or verrucate on mesocolpium dendron weyrichii has the thickest exine thickness with 3. Scabrate mesocolpium with exine thickness of the range of 2.00 to 2.50 (mean 2.22) µm while the rest thinner than 1.92 µm 13 taxa are in 0.97 to 1.92 µm (Table 2). 4. Rugulate on apocolpium and around the aper- The tectum of R. micranthum (Figure 2b), R. tomen- ture tosum, and R. redowskianum (Figure 2f) is nearly eutec- ················································ Aureum-type (III) tate while that of the rest 11 taxa is close to tectate- 4. Microscabrate on apocolpium and around the perforate (Figure 2b) or semitectate. The columella is of aperture either rod shape or granule. The foot layer is the thickest 5. Microscabrate pattern with rod shaped ele- while endexine is the thinnest with some irregular ments grooves. ·································Brachycarpum-type (IV) Pollen morphology of the genus Rhododendron (Ericaceae) in Korea 667

Figure 3. LM, SEM and TEM images of pollen types III (a-b) and IV (c-e). a-b. R. aureum. (a) a part of scabrate mesocolpium (SEM), (b) showing tectate tectum (arrow, te), columellae (arrow, co), foot layer (arrow, fl) and endexine (arrow, en) (TEM). c-e. R. brachycarpum. (c) a low focused tetrad showing a polar view tetrad of lalongate apertures (arrows) (LM), (d) a part of the exine on mesocolpium showing tectum (te), columellae (co), foot layer (fl) and endexine (en) (TEM), and (e) rod-shaped microscabrate sculpturing on apocolpium with viscin threads (SEM).

5. Microscabrate pattern with round shaped ele- 1) Micranthum-type (Pollen type I) ments (Figures 2a-2b, Table 2). The tetrad is 30.0 to 35.0 ································Schlippenbachii-type (V) (mean 32.2) µm. Each pollen grain of the tetrad is cir- 3. Verrucate mesocolpium with exine thickness of cular or rounded triangular in polar view, suboblate (P/ thicker than 2.00 µm E=0.71-(0.80)-0.91) in equatorial view. Exine is rugulate ················································· Weyrichii-type (VI) on mesocolpium and psilate on apocolpium and around 6 86

Table 2. Pollen morphological data of the genus Rhododendron in Korea by SEM and TEM. Sculpture pattern SET(µm) NET(µm) EXT VTD Pollen Taxon Apocolpium & (µm) (µm) type Mesocolpium aperture region TET① COT②①+②=③ FOT④ ENT⑤④+⑤=⑥ ③+⑥ Rhododendron aureum scabrate rugulate 0.34±0.04 0.15±0.04 0.49±0.06 0.61±0.05 0.22±0.07 0.83±0.08 1.32±0.08 0.51±0.04 IV

R. brachycarpum scabrate microscabrate (rod-shaped) 0.46±0.11 0.24±0.07 0.71±0.09 0.61±0.09 0.16±0.06 0.77±0.08 1.47±0.13 0.28±0.03 III J uo R. confertissimum rugulate rugulate 0.49±0.06 0.26±0.04 0.76±0.07 0.37±0.03 0.06±0.02 0.43±0.03 1.19±0.06 0.49±0.04 II r K . o R. dauricum scabrate microscabrate (rounded) 0.40±0.05 0.32±0.04 0.72±0.05 0.44±0.02 0.06±0.02 0.51±0.02 1.22±0.06 0.32±0.03 V er na R. micranthum rugulate psilate 0.38±0.09 0.15±0.05 0.53±0.13 0.57±0.06 0.15±0.06 0.72±0.09 1.25±0.16 0.22±0.03 I F ro . R. mucronulatum scabrate microscabrate (rounded) 0.44±0.06 0.21±0.03 0.65±0.08 0.48±0.08 0.12±0.04 0.60±0.06 1.25±0.13 0.28±0.05 V S co V . R. m. f. ciliatum scabrate microscabrate (rounded) 0.38±0.08 0.23±0.07 0.60±0.12 0.44±0.07 0.16±0.08 0.59±0.08 1.20±0.17 0.51±0.06 V o .l R. redowskianum rugulate rugulate 0.56±0.05 0.24±0.04 0.80±0.06 0.62±0.11 0.16±0.05 0.78±0.05 1.58±0.09 0.67±0.13 II 9 N , R. schlippenbachii scabrate microscabrate (rounded) 0.49±0.09 0.22±0.05 0.71±0.09 0.65±0.03 0.18±0.05 0.83±0.12 1.54±0.20 0.35±0.05 V o 5 . ( R. s. f. albiflorum scabrate microscabrate (rounded) 0.57±0.14 0.15±0.04 0.73±0.14 0.57±0.06 0.19±0.07 0.76±0.10 1.49±0.23 0.38±0.04 V 02 01 R. tomentosum rugulate rugulate 0.45±0.05 0.23±0.03 0.68±0.07 0.54±0.08 0.14±0.07 0.68±0.08 1.37±0.13 0.22±0.04 II ) R. tschonoskii scabrate microscabrate (rounded) 0.42±0.05 0.22±0.05 0.64±0.06 0.46±0.08 0.16±0.06 0.62±0.09 1.26±0.14 0.30±0.03 V R. yedoense f. poukhanense scabrate microscabrate (rounded) 0.63±0.07 0.16±0.03 0.79±0.07 0.68±0.10 0.19±0.04 0.87±0.09 1.66±0.12 0.36±0.03 V R. weyrichii verrucate microscabrate (rounded) 0.68±0.08 0.32±0.08 1.00±0.13 0.84±0.06 0.38±0.05 1.22±0.07 2.22±0.18 0.28±0.02 VI TET=Tectum thickness, COT=Colummella thickness, SET=Sexine thickness, FOT=Foot layer thickness, ENT=Endexineness, NET=Nexine thickness, EXT=Exine thickness, VTD=Viscin thread diameter, Pollen type I=Type Micranthum, II=Type Hypoleucum, III=Type Aureum, IV=Type Brachycarpum, V=Type Schlippenbachii and VI=Type Weyrichii Pollen morphology of the genus Rhododendron (Ericaceae) in Korea 669

Figure 4. LM and SEM images of pollen types V (a-d) and VI (e-f). a, c. Rhododendron tschonoskii. LM. (a) a high focused tetrad, (c) a low focused tetrad showing tricolporate apertures (arrows) (LM) b. R. yedoense f. poukanense of microscabrate mesocolpium with round shaped elements (SEM). d. R. schlippenbachii f. albiflorum of 4 tetrads with tangled viscin threads (SEM). e-f. R. weyrichii. (e) a high focused tetrad (LM), (f) a part of verrucate mesocolpium but rounded microscabrate around the aperture (SEM). the aperture. Tectum is nearly eutectate and exine 2) Tomentosum-type (Pollen type II) thickness is 1.02 to 1.44 (mean 1.25) µm. Endexine is (Figures 2c-2f, Table 2). The tetrad is 30.0 to 50.5 thicker than ectexine. The viscin threads, with a diam- (mean 41.2) µm. Each pollen grain of the tetrad is cir- eter range of 0.15 to 0.27 (mean 0.22) µm, are smooth cular or rounded triangular in polar view, suboblate (P/ and either drooping or tangled. Taxon included: R. E=0.68-(0.80)-0.92) in equatorial view. Exine is rugulate micranthum. on mesocolpium, apocolpium and around the aperture. 670 Jour. Korean For. Soc. Vol. 99, No. 5 (2010)

Tectum of R. tomentosum and R. redowskianum is nearly to 0.62 (mean 0.36) µm, are smooth and either drooping eutectate when R. confertissimum is tectate-perforate. or tangled. They sometimes get divided into two strands Exine thickness is 1.08 to 1.76 (mean 1.38) µm. Ectex- and can be relatively very short. Taxa included: R. dau- ine of R. confertissimum and R. redowskianum is thicker ricum, R. mucronulatum, R. mucronulatum f. ciliatum, R. than endexine. The thickness of ectexine and endexine schlippenchii, R. schlippenchii f. albiflorum, R. tschonoskii of R. tomentosum is similar. The viscin threads, with a and R. yedoense f. poukhanense. diameter range of 0.15 to 0.91 (mean 0.46) µm, are smooth and either drooping or tangled. They sometimes get divided 6) Weyrichii-type (Pollen type VI) into two strands (Figure 2e) and can be relatively short. (Figures 4e-4f, Table 2). The tetrad is 51.0 to 60.0 Taxa included: R. confertissimum, R. tomentosum and R. (mean 54.2) µm. Each pollen grain of the tetrad is cir- redowskianum. cular or rounded triangular in polar view, suboblate (P/ E=0.73-(0.80)-0.94) in equatorial view. Exine is verru- 3) Aureum-type (Pollen type III) cate on mesocolpium and microscabrate with rounded (Figures 3a-3b, Table 2). The tetrad is 50.0 to 60.0 elements on apocolpium and on the aperture margins. (mean 53.0) µm. Each pollen grain of the tetrad is cir- Tectum is either tectate-perforate or semitectate and cular or rounded triangular in polar view, suboblate (P/ exine thickness is 2.00 to 2.50 (mean 2.22) µm which is E=0.79-(0.86)-0.95) in equatorial view. Exine is scabrate significantly thicker than those of the rest 13 taxa (0.97- on mesocolpium and regulate on apocolpium and around 1.92 µm). Endexine is significantly thicker than ectex- the aperture. Tectum is either tectate-perforate or semi- ine. The viscin threads, with a diameter range of 0.23 to tectate, and exine thickness is 1.20 to 1.43 (mean 1.32) 0.32 (mean 0.28) µm, are smooth and either drooping or µm. Endexine is thicker than ectexine. The viscin threads, tangled (Figure 4d). Taxon included: R. weyrichii. with a diameter range of 0.45 to 0.59 (mean 0.51) µm, are smooth and either drooping or tangled. Taxon included: Discussion R. aureum. The exine sculpture pattern of Rhododendron pollen 4) Brachycarpum-type (Pollen type IV) was reported to be microreticulate connected with micros- (Figures 3c-3e, Table 2). The tetrad is 45.5 to 55.0 cabrae (Lee, 1979; Jang, 1986). In the present study, it is (mean 51.0) µm. Each pollen grain of the tetrad is cir- rather various according to pollen types; scabrate, rugu- cular or rounded triangular in polar view, suboblate (P/ late or verrucate on mesocolpium, psilate, rugulate or E=0.74-(0.81)-0.91) in equatorial view. Exine is scabrate microscabrate on apocolpium and the aperture margins. on mesocolpium and microscabrate with rod shaped ele- The difference among past studies might result from the ments on apocolpium and around the aperture. Tectum is fact that those early studies were based on LM results, either tectate-perforate or semitectate and exine thick- and did not include SEM observations. ness is 1.28 to 1.76 (mean 1.47) µm. Endexine is thicker Rhododendron tomentosum, formerly included in the than ectexine. The viscin threads, with a diameter range genus Ledum (Ericaceae) has recently been transferred to of 0.24 to 0.36 (mean 0.28) µm, are smooth and either Rhododendron as a result of further systematic studies drooping or tangled. Taxon included: R. brachycarpum. (Chamberlain et al., 1996). Another species, R. redowski- anum, formerly included in Therorhodion (Kron and 5) Schlippenbachii-type (Pollen-type V) Judd, 1990) was transferred to Rhododendron by Kron (Figures 4a-4d, Table 2). The tetrad is 39.5 to 70.0 (1997). Notably the pollen of both species as well as R. (mean 51.1) µm. Each pollen grain of the tetrad is cir- confertissimum, have a similar rugulate exine pattern cular or rounded triangular in polar view, suboblate (P/ (Tomentosum pollen type). This lends some support to E=0.69-(0.79)-0.93) in equatorial view. Exine is scabrate the systematic opinion of Chamberlain et al. (1996) on mesocolpium, and microscabrate with rounded ele- regarding the position of R. tomentosum and R. confer- ments on apocolpium and around the aperture. Tectum is tissimum in section Rhododendron (subgenus Rhododen- either tectate-perforate or semitectate. Exine thickness is dron). However, R. redowskianum is in a different subgenus, 0.97 to 1.92 (mean 1.40) µm. Ectexine of R. dauricum, Therorhodion. R. mucronulatum, R. mucronulatum f. ciliatum and R. Erdtman (1952) pointed out that the tetrad pollen mor- tschonoskii is thicker than endexine. However, endexine phology of Ericaceae is similar to that of Empetraceae, of R. schlippenchii, R. schlippenchii f. albiflorum and R. Epacridaceae and Pyrolaceae in the order . The yedoense f. poukhanense is thicker than ectexine. The exine layers of the present study; ectexine and the thickness of ectexine and endexine of R. hypoleucum is endexine, are similar to the exine stratification of Empe- similar. The viscin threads, with a diameter range of 0.20 traceae (Kim et al., 1988). Empetraceae, along with Pollen morphology of the genus Rhododendron (Ericaceae) in Korea 671

Epacridaceae and Pyrolaceae, is now included in the Eri- Jangsu, Jeonbuk. JMP RMU040498. Republic of KOREA: caceae, following DNA studies (Angiosperm Phylogeny Tookdong valley, Deogyusan, Jeonbuk. JMP RMU100499. Group, 1998; 2003). R. mucronulatum f. ciliatum (Nakai) Kitag.. Republic of Rhododendron micranthum was so characterized with KOREA: Nogodan, Jirisan, Jeonnam. JMP RMC040699. its pollen grain size, exine pattern and structure from the Republic of KOREA: Banyabong, Jirisan, Jeonbuk. JMP rest taxa in the genus that it could make its own pollen RMC050699. Republic of KOREA: Hangyeryeong, Seorak- type (Micranthum-type) by itself. Even though Cham- san, Gangwon. JMP RMC080699. Republic of KOREA: berlain et al. (1996) put R. micranthum along with R. Jungheongbong, Seoraksan, Gangwon. JMP RMC090699. mucronulatum in the same subgenus Rhododendron, Republic of KOREA: Namdeogyusan, Gyeongnam. JMP there was no significant pollen morphological similarity RMC130699. between these two species. However, the present pollen R. redowskianum Maxim.. Democratic People's Republic morphological results are in agreement with Hwang of KOREA: Baekdusan, Hambuk. JMP RR230898. (1999)’s study. He reviewed 20 Rhododendron taxa R. schlippenbachii Maxim.. Republic of KOREA: Hyang- based on 51 morphological characters and also reported jeokbong, Deogyusan, Jeonbuk. JMP RS050699. Republic that R. micranthum was quite independent. of KOREA: Haeinsa, Gayasan, Gyeongnam. JMP RS200599. Republic of KOREA: Hangyeryeong, Seorak- Conclusion san, Gangwon. JMP RS080699. Republic of KOREA: Seongsamjae, Jirisan, Jeonnam. JMP RS110699. Republic The various exine pattern being scabrate, rugulate or ver- of KOREA: Banyabong, Jirisan, Jeonbuk. JMP RS120699. rucate in mesocolpium area, and psilate, rugulate or micros- R. schlippenbachii f. albiflorum Y. N. Lee. Republic of cabrate in apocolpium and aperture areas, was the major KOREA: Daecheongbong, Mt. Seorak, Gangwon. JMP character to create a key to six pollen types in Korean RSA080699. Rhododendron. The Korean Rhododendron exine consists of R. tomentosum Harmaja. Democratic People's Repub- four layers; tectum, columella, foot layer and endexine. The lic of KOREA: Hamnam. SNU-1925-125. exine thickness of R. weyrichii was thicker than those of the R. tschonoskii Maxim.. Republic of KOREA: Namsan- rest taxa. Rhododendron micranthum was distinguished jeilbong, Gayasan, Gyeongnam. JMP RT150698. Republic from the others in the pollen grain size and exine pattern. of KOREA: Namdeogyusan, Gyeongnam. JMP RT120699. Republic of KOREA: Sangwangbong, Gayasan, Gyeo- Acknowledgements ngnam. JMP RT200699. Republic of KOREA: Namsan- jeilbong, Gayasan, Gyeongnam. JMP RT210699. Republic Lots of thanks are due to Dr. Madeline Harley at of KOREA: Banyabong, Jirisan, Jeonbuk. JMP RT040799. Royal Botanic Gardens, Kew, UK for her helpful com- R. yedoense f. poukhanense (H. Lév.) M. Sugim. ex T. ments, and Prof. Lee, Wang-Hyu at College of Agricul- Yamaz.. Republic of KOREA: Gyebuk, Jangsu, Jeonbuk. ture & Life Sciences and Mr. Kim, Wan-Cheol at the JMP RY250498. Republic of KOREA: Seongsamjae, EM Lab., College of Medicine, Chonbuk Univ., Korea Jirisan, Jeonnam. JMP RY300499. Republic of KOREA: for their SEM & TEM technical assistance. Deogyusan, Jeonbuk. JMP RY040599. Republic of KOREA: Jirisan, Jeonnam. JMP RY070599. Specimens Investigated R. weyrichii Maxim.. Republic of KOREA: Halla Arbo- retum, Jeju. JMP RW110599. Rhododendron aureum Georgi. Democratic People's Republic of KOREA: Baekdusan, Hambuk. CNU-98-100. Literature Cited R. brachycarpum D. Don. ex G. Don. Republic of KOREA: Sadong, Ulleungdo, Gyeonbuk. JMP RB060699. 1. Abraham-Peskir, J.V., Searle, R. and Medenwaldt, R. 1997. Republic of KOREA: Socheongbong, Seoraksan, Gang- A new technique of examining pollen-connecting threads. won. JMP RB120699. Republic of KOREA: Jungcheon- Grana 36: 343-346. gbong, Seoraksan, Gangwon. JMP RB140699. 2. The Angiosperm Phylogeny Group. 1998. An ordinal R. confertissimum Nakai. Democratic People's Repub- classification for the families of flowering plants. Annals of lic of KOREA: Bujeongowon, Hamnam. SNU-1937-84. the Missouri Botanical Garden 85: 531-553. R. dauricum L.. Democratic People's Republic of KOREA: 3. The Angiosperm Phylogeny Group. 2003. An update of Agr. Division Research Institute, Hamnam. SNU-1927-79. the Angiosperm Phylogeny Group classification for the R. micranthum Turcz.. Republic of KOREA: Daeseong- orders and families of flowering plants: APG II. Botan- san, Danyang, Chungbuk. JMP RMI040698. ical Journal of the Linnean Society 141: 399-436. R. mucronulatum Turcz.. Republic of KOREA: Gyebuk, 4. Bailey, L.H. and Bailey, E.Z. 1978. Hortus Third. Mac- 672 Jour. Korean For. Soc. Vol. 99, No. 5 (2010)

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