Pollen Morphology of the Genus Rhododendron (Ericaceae) in Korea

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663

F

OREST

S

OCIETY

Pollen Morphology of the Genus Rhododendron (Ericaceae) in Korea Joonmoh Park

¹

and Unsook Song

²^*

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

The

Rhododendron

L., one of the largest genera in the Ericaceae, belongs to the Rhododendroideae (Rehder, 1940; Leach, 1961; Cronquist, 1981). It consists of about 1,200 taxa, and widespread in temperate, cool, subtrop- ical regions and in tropical mountains except for Africa and South America regions (Leach, 1961; Bailey and Bailey, 1978; Galle, 1995). There are 22 native

^Rhodo- dendron

taxa in Korea, and their distribution ranges from very low areas along seacoasts to

ca

. 2,000 m high mountains (Lee, 1989).

Since

R. schlippenbachii

Maxim. was firstly documented in 1870 as a Korean

Rhododendron

species (Nakai, 1919), there have been a number of fragmentary systematic studies on the genus in Korea. Nakai (1919) reported 12 species and three varieties of the Korean

Rhododendron

. However, some scientific names and morphological characters proposed by Nakai were turned out not to be correct. The number range of

Rhododendron

taxa has been 17 to 23 by different scientists (Nakai, 1952; Chung, 1957; Lee, 1989; Lee, 1996a; Lee, 1996b). Though recent systematic studies (Choo, 1987; Hwang, 1987; 1999;

Lee, 1988) tried to solve those problems, there has been no taxonomic progress because of almost the same char- acters and methods used in the studies.

The pollen of

Rhododendron

is a tetrahedral tetrad com- bined from four monads while pollen grains of other plants are commonly monads. The size of tetrads and exine strat- ification vary according to authors (Wodehouse, 1935; Erdt- man, 1952; 1969; Yang, 1952; Waha, 1982; Praglowski and Grafstrom, 1985). The genus has been palynologically attrac- tive because of viscin threads (Bowers, 1930; Ikuse, 1956;

Oldfield, 1959; Ueno, 1978; Foss, 1988; Keri and Zetter, 1992; Crepet, 1996; Abraham-Peskir

et al

., 1997).

Pollen morphology of Korean

Rhododendron

has been reported (Lee, 1979; Jang, 1986; Choo, 1987) briefly using light microscopy and scanning electron micros- copy. There have not been any transmission electron microscopic studies on Korean

Rhododendron

, yet.

Therefore, this study aimed to examine eleven species and three forms of Korean

Rhododendron

using light, scanning electron and transmission electron microscopy, and further contribute to the systematics of Korean

Rhododendron

.

Materials and Methods

Pollen material from 11 species, 3 forms of Korean

*Corresponding author

E-mail: [email protected]

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664 Jour. Korean For. Soc. Vol. 99, No. 5 (2010)

Rhododendron

was collected throughout the country from March 1998 to July 1999. Pollen of five species;

R.

aureum, R. confertissimum, R. dauricum, R. tomentosum

and

R. redowskianum

in North Korea was extracted from the specimens in Chonbuk Nat. University and Seoul Nat. University herbaria (Specimens Investigated). The pollen specimens are preserved in the herbarium of the Department of Forest Resources, Chonbuk Nat. Univer- sity, Korea.

For LM, the pollen grains were acetolysed and mounted in glycerol jelly (Erdtman, 1952; Song and Kim, 1999).

An Olympus B201 was used to measure seven charac- ters; tetrad diameter (TD), diameter of top grain (DT), apocolpium diameter (AD), monad polar axis length (PL), monad equatorial width (EW), colpus length (CL) and colpus width (CW) at 400X (Figure 1). Fifty tetrads per taxon were investigated.

For SEM, acetolysed pollen grains were dehydrated in ethanol series and transferred onto aluminum stubs (Har- ley and Daly, 1995; Harley

et al

., 2005). The pollen sus- pensions on the stubs were covered with an inverted glass beaker and left to evaporate at room temperature.

The pollen grains were coated with

ca

. 90 nm of plat- inum (Cressington Sputter Coater Q108). The examina- tion of pollen grains was conducted with a JEOL JSM- 5600 LV at 650-10,000X (2,000 accelerating voltage).

For TEM, pollen was fixed in 2% of glutaraldehyde solution for 90 minutes and in 1% of OsO

4

solution for another 90 minutes, and dehydrated using propylene oxide.

Then the pollen was embedded in epoxy resin for two hours and the ultra-thin sectioning was done by an Ultra-

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 somewhat rounded triangular (Figure 3c) in polar view, and oblate to oblate spheroidal in equatorial view. The tetrads are rhomboidal. The hexacolporate tetrads are of tricolporate monads whereby the apertures form in pairs 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 microscabrate, rugulate or psilate (Figures 2-4).

The exine of the Korean Rhododendron pollen con- sists of tectum, columella, foot layer and endexine. The exine thickness shows a tendency to get thicker toward apertures. The tectum is thicker than the columella. And the foot layer is thicker than the endexine. The tectum is either eutectate or tectate-perforate according to taxa. The columella is of either rod shape (Figure 2f) or granule (Figure 3d). The foot layer is the thickest while endexine is the thinnest with some irregular grooves (Figure 2b).

The viscin threads are almost smooth. Most of them are long and drooping or tangled. Sometimes, the threads are branched into two strands and some are very short.

2. Pollen grain size

The diameter range of tetrads is 30.0 to 70.0

^µ

m which are medium to big pollen grains according to Erdtman (1952).

Rhododendron tomentosum

is the smallest with the mean size of 32.0

^µ

m while

R. schlippenbachii

f.

albiflorum

is the biggest (60.9

^µ

m). The pollen size of monads and tetrads varies according to taxa (Table 1).

3. Aperture

The monads are isopolar. The hexacolporate tetrads are of tricolporate monads whereby the apertures form in pairs at six points in the tetrad. There are 12 lalongate endoapertures.

has the short- est aperture length (14.1

^µ

m) while

R. weyrichii

has the longest (21.5

^µ

m) one. For the aperture diameter,

R. tomentosum

has the shortest (2.30

^µ

m) one while

R. yedoense

f.

poukhanense

has the longest (3.40

^µ

m) one (Table 1).

Figure 1. Schematic representation of tetrad tetrahedral

grains of

Rhododendron

indicating the positions of pollen

characters measured (TD=tetrad diameter, DT=diameter

of top grain, AD=apocolpium diameter, PL=polar axis length,

EW=equatorial width, CL=colpus length and CW=colpus

width).

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4. Equatorial and 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 each taxon shows that the pollen is suboblate. The tet- rads are rounded lozenge shaped in equatorial view. They are radially symmetrical and either circular or somewhat

rounded triangular in polar view.

5. Exine pattern

The exine sculpture pattern is rugulate, scabrate or verru- cate on mesocolpium but psilate, rugulate or microsca- brate around the aperture and on apocolpium.

Rhododendron

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).

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micranthum

is characterized with its psilate pattern in aperture margins and on apocolpium (Figure 2a). The rest 13 taxa are rugulate or microscabrate around the aperture and on apocolpium.

Rhododendron weyrichii

is verrucate having wart-like sexine elements of wider than 1

^µ

m on mesocolpium while the others are rugulate or microscabrate on mesocolpium (Figure 4f).

Rhododen- dron brachycarpum

is characterized by the rod shaped elements of the microscabrate apocolpium and aperture (Figure 3e). The rest eight taxa with microscabrate apo- colpium and aperture have round shaped elements instead. These various exine patterns are major features to be used in the key to six Korean

Rhododendron

pol- len types (Figures 2-4, Table 2).

6. Exine stratification

The exine of the Korean

Rhododendron

pollen con- sists of four layers: the tectum, the columella, the foot layer and the endexine. The exine thickness shows a ten- dency to get thicker toward apertures. Exine thickness of the genus is 0.97 to 2.50

^µ

m. The exine thickness of

R.

confertissimum

is the thinnest being 1.19

^µ

m.

Rhodo- dendron weyrichii

has the thickest exine thickness with the range of 2.00 to 2.50 (mean 2.22)

^µ

m while the rest 13 taxa are in 0.97 to 1.92

^µ

m (Table 2).

The tectum of

R. micranthum

(Figure 2b),

R. tomentosum

, and

R. redowskianum

(Figure 2f) is nearly eutec- tate while that of the rest 11 taxa is close to tectate- perforate (Figure 2b) or semitectate. The columella is of either rod shape or granule. The foot layer is the thickest while endexine is the thinnest with some irregular grooves.

7. Viscin thread

The genus has viscin threads on the pollen surface.

The diameter of the threads is 0.15 to 0.62 .

Rhododen- dron micranthum

has the narrowest thread diameter (0.22

^µ

m) but

R. mucronulatum

f.

ciliatum

has the wid- est one (0.51

^µ

m). The thread surface is nearly smooth.

Long pendent threads are sometimes tangled together and occasionally divided into two strands (Figure 2e).

Relatively short threads are observed as well (Figures 2a, 2e, 3a, 3e, and 4d).

8. Key to the Rhododendron pollen types in Korea

Six pollen types of 11 species and 3 forms of Korean

Rhododendron

are recognized based on differences in exine sculpture pattern and exine thickness.

1. Rugulate on mesocolpium

2. Psilate on apocolpium and around the aperture

···Micranthum

-type (I) 2. Rugulate on apocolpium and around the aperture

···

Tomentosum

-type (II) 1. Scabrate or verrucate on mesocolpium

3. Scabrate mesocolpium with exine thickness of thinner than 1.92

^µ

m

4. Rugulate on apocolpium and around the aper- ture

··· Aureum

-type (III) 4. Microscabrate on apocolpium and around the

aperture

5. Microscabrate pattern with rod shaped ele- ments

···Brachycarpum

-type (IV) Table 1. Pollen morphological data of the genus

Rhododendron

in Korea by LM.

Taxon TD

(

^µ

m) DT

(

^µ

m) AD

(

^µ

m) PL

(

^µ

m) EW

(

^µ

m) P/E Aperture

CL(

µ

m) CW(

µ

m)

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

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5. Microscabrate pattern with round shaped ele- ments

···Schlippenbachii

-type (V) 3. Verrucate mesocolpium with exine thickness of

thicker than 2.00

^µ

m

··· Weyrichii

-type (VI)

1) Micranthum-type (Pollen type I)

(Figures 2a-2b, Table 2). The tetrad is 30.0 to 35.0 (mean 32.2)

^µ

m. Each pollen grain of the tetrad is cir- cular or rounded triangular in polar view, suboblate (P/

E=0.71-(0.80)-0.91) in equatorial view. Exine is rugulate

on mesocolpium and psilate on apocolpium and around

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).

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6 6 8 J o u r . K o r e a n F o r . S o c . V o l . 9 9 , N o . 5 ( 2 0 1 0 )

Ta ble 2 . P oll en m orp ho logi ca l d ata of th e g en us

Rhododendron

in Ko rea by SE M an d T EM . Ta xo n Sc ulpt ure p att ern SE T(

µ

m) NET(

µ

m) EX T (

µ

m)

③

+

⑥

VT D (

µ

m) Po llen type

Me soco lpiu m Ap oc olpi um & ap ertu re reg ion TET

①

COT

②①

+

②

=

③

FO T

④

ENT

⑤④

+

⑤

=

⑥ Rhododendron aureum

scab rat e rugu lat e 0.34 ±0 .04 0.15± 0.04 0.4 9± 0.0 6 0.6 1± 0.05 0.2 2± 0.0 7 0.8 3± 0.08 1.3 2± 0.0 8 0.5 1± 0.0 4 IV

R. brachycarpum

scab rat e mi crosca brat e (ro d-s ha pe d) 0.4 6±0 .11 0.2 4± 0.0 7 0.71 ±0 .09 0.6 1± 0.0 9 0.16 ±0 .06 0.7 7± 0.0 8 1.47 ±0. 13 0.2 8±0 .03 III

R. confertissimum

r ug ula te rugu lat e 0.49 ±0 .06 0.26± 0.04 0.7 6± 0.0 7 0.3 7± 0.03 0.0 6± 0.0 2 0.4 3± 0.03 1.1 9± 0.0 6 0.4 9± 0.0 4 II

R. dauricum

scab rat e mi crosca brat e (ro un de d) 0.4 0±0 .05 0.3 2± 0.0 4 0.72 ±0 .05 0.4 4± 0.0 2 0.06 ±0 .02 0.5 1± 0.0 2 1.22 ±0. 06 0.3 2±0 .03 V

R. micranthum

ru gu lat e psi lat e 0.3 8±0 .09 0.1 5± 0.0 5 0.53 ±0 .13 0.5 7± 0.0 6 0.15 ±0 .06 0.7 2± 0.0 9 1.25 ±0. 16 0.2 2±0 .03 I

R. mucronulatum

scab rat e mi crosca brat e (ro un de d) 0.4 4±0 .06 0.2 1± 0.0 3 0.65 ±0 .08 0.4 8± 0.0 8 0.12 ±0 .04 0.6 0± 0.0 6 1.25 ±0. 13 0.2 8±0 .05 V

R. m

. f.

ciliatum

scab rat e mi crosca brat e (ro un de d) 0.3 8±0 .08 0.2 3± 0.0 7 0.60 ±0 .12 0.4 4± 0.0 7 0.16 ±0 .08 0.5 9± 0.0 8 1.20 ±0. 17 0.5 1±0 .06 V

R. redowskianum

ru gu lat e rug ula te 0.5 6±0 .05 0.2 4± 0.0 4 0.80 ±0 .06 0.6 2± 0.1 1 0.16 ±0 .05 0.7 8± 0.0 5 1.58 ±0. 09 0.6 7±0 .13 II

R. schlippenbachii

scab rat e mi crosca brat e (ro un de d) 0.4 9±0 .09 0.2 2± 0.0 5 0.71 ±0 .09 0.6 5± 0.0 3 0.18 ±0 .05 0.8 3± 0.1 2 1.54 ±0. 20 0.3 5±0 .05 V

R. s.

f.

albiflorum

scab rat e mi crosca brat e (ro un de d) 0.5 7±0 .14 0.1 5± 0.0 4 0.73 ±0 .14 0.5 7± 0.0 6 0.19 ±0 .07 0.7 6± 0.1 0 1.49 ±0. 23 0.3 8±0 .04 V

R. tomentosum

r ug ula te rugu lat e 0.45 ±0 .05 0.23± 0.03 0.6 8± 0.0 7 0.5 4± 0.08 0.1 4± 0.0 7 0.6 8± 0.08 1.3 7± 0.1 3 0.2 2± 0.0 4 II

R. tschonoskii

scab rat e mi crosca brat e (ro un de d) 0.4 2±0 .05 0.2 2± 0.0 5 0.64 ±0 .06 0.4 6± 0.0 8 0.16 ±0 .06 0.6 2± 0.0 9 1.26 ±0. 14 0.3 0±0 .03 V

R. yedoense

f.

poukhanense

scab rat e mi crosca brat e (ro un de d) 0.6 3±0 .07 0.1 6± 0.0 3 0.79 ±0 .07 0.6 8± 0.1 0 0.19 ±0 .04 0.8 7± 0.0 9 1.66 ±0. 12 0.3 6±0 .03 V

R. weyrichii

v err uca te mi crosca brat e (ro un de d) 0.6 8±0 .08 0.3 2± 0.0 8 1.00 ±0 .13 0.8 4± 0.0 6 0.38 ±0 .05 1.2 2± 0.0 7 2.22 ±0. 18 0.2 8±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

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the aperture. Tectum is nearly eutectate and exine thickness is 1.02 to 1.44 (mean 1.25)

^µ

m. Endexine is thicker than ectexine. The viscin threads, with a diam- eter range of 0.15 to 0.27 (mean 0.22)

^µ

m, are smooth and either drooping or tangled. Taxon included:

R.

micranthum.

2) Tomentosum-type (Pollen type II)

(Figures 2c-2f, Table 2). The tetrad is 30.0 to 50.5 (mean 41.2)

^µ

m. Each pollen grain of the tetrad is cir- cular or rounded triangular in polar view, suboblate (P/

E=0.68-(0.80)-0.92) in equatorial view. Exine is rugulate on mesocolpium, apocolpium and around the aperture.

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).

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Tectum of

R. tomentosum

and

R. redowskianum

is nearly eutectate when

R. confertissimum

is tectate-perforate.

Exine thickness is 1.08 to 1.76 (mean 1.38)

^µ

m. Ectex- ine of

R. confertissimum

and

R. redowskianum

is thicker than endexine. The thickness of ectexine and endexine of

R. tomentosum

is similar. The viscin threads, with a diameter range of 0.15 to 0.91 (mean 0.46)

^µ

m, are smooth and either drooping or tangled. They sometimes get divided into two strands (Figure 2e) and can be relatively short.

Taxa included:

R. confertissimum, R. tomentosum

and

R.

redowskianum

.

3) Aureum-type (Pollen type III)

(Figures 3a-3b, Table 2). The tetrad is 50.0 to 60.0 (mean 53.0)

^µ

m. Each pollen grain of the tetrad is cir- cular or rounded triangular in polar view, suboblate (P/

E=0.79-(0.86)-0.95) in equatorial view. Exine is scabrate on mesocolpium and regulate on apocolpium and around the aperture. Tectum is either tectate-perforate or semi- tectate, and exine thickness is 1.20 to 1.43 (mean 1.32)

µ

m. Endexine is thicker than ectexine. The viscin threads, with a diameter range of 0.45 to 0.59 (mean 0.51)

^µ

m, are smooth and either drooping or tangled. Taxon included:

R. aureum

.

4) Brachycarpum-type (Pollen type IV)

(Figures 3c-3e, Table 2). The tetrad is 45.5 to 55.0 (mean 51.0)

^µ

m. Each pollen grain of the tetrad is cir- cular or rounded triangular in polar view, suboblate (P/

E=0.74-(0.81)-0.91) in equatorial view. Exine is scabrate on mesocolpium and microscabrate with rod shaped ele- ments on apocolpium and around the aperture. Tectum is either tectate-perforate or semitectate and exine thick- ness is 1.28 to 1.76 (mean 1.47)

^µ

m. Endexine is thicker than ectexine. The viscin threads, with a diameter range of 0.24 to 0.36 (mean 0.28)

^µ

m, are smooth and either drooping or tangled. Taxon included:

R. brachycarpum

.

5) Schlippenbachii-type (Pollen-type V)

(Figures 4a-4d, Table 2). The tetrad is 39.5 to 70.0 (mean 51.1)

^µ

m. Each pollen grain of the tetrad is cir- cular or rounded triangular in polar view, suboblate (P/

E=0.69-(0.79)-0.93) in equatorial view. Exine is scabrate on mesocolpium, and microscabrate with rounded ele- ments on apocolpium and around the aperture. Tectum is either tectate-perforate or semitectate. Exine thickness is 0.97 to 1.92 (mean 1.40)

^µ

m. Ectexine of

R. dauricum, R. mucronulatum, R. mucronulatum

f.

ciliatum

and

R.

tschonoskii

is thicker than endexine. However, endexine of

R. schlippenchii, R. schlippenchii

f.

albiflorum

and

R.

yedoense

f

. poukhanense

is thicker than ectexine. The thickness of ectexine and endexine of

R. hypoleucum

is similar. The viscin threads, with a diameter range of 0.20

to 0.62 (mean 0.36)

^µ

m, are smooth and either drooping or tangled. They sometimes get divided into two strands and can be relatively very short. Taxa included:

R. dauricum, R. mucronulatum, R. mucronulatum

f.

ciliatum

,

R.

schlippenchii

,

R. schlippenchii

f.

albiflorum

,

R. tschonoskii

and

R. yedoense

f

. poukhanense

.

6) Weyrichii-type (Pollen type VI)

(Figures 4e-4f, Table 2). The tetrad is 51.0 to 60.0 (mean 54.2)

^µ

m. Each pollen grain of the tetrad is cir- cular or rounded triangular in polar view, suboblate (P/

E=0.73-(0.80)-0.94) in equatorial view. Exine is verru- cate on mesocolpium and microscabrate with rounded elements on apocolpium and on the aperture margins.

Tectum is either tectate-perforate or semitectate and exine thickness is 2.00 to 2.50 (mean 2.22)

^µ

m which is significantly thicker than those of the rest 13 taxa (0.97- 1.92

^µ

m). Endexine is significantly thicker than ectex- ine. The viscin threads, with a diameter range of 0.23 to 0.32 (mean 0.28)

^µ

m, are smooth and either drooping or tangled (Figure 4d). Taxon included:

R. weyrichii

.

Discussion

The exine sculpture pattern of

Rhododendron

pollen was reported to be microreticulate connected with micros- cabrae (Lee, 1979; Jang, 1986). In the present study, it is rather various according to pollen types; scabrate, rugu- late or verrucate on mesocolpium, psilate, rugulate or microscabrate on apocolpium and the aperture margins.

The difference among past studies might result from the fact that those early studies were based on LM results, and did not include SEM observations.

, formerly included in the genus

Ledum

(Ericaceae) has recently been transferred to

Rhododendron

as a result of further systematic studies (Chamberlain

et al

., 1996). Another species,

R. redowskianum

, formerly included in

Therorhodion

(Kron and Judd, 1990) was transferred to

Rhododendron

by Kron (1997). Notably the pollen of both species as well as

R.

confertissimum

, have a similar rugulate exine pattern (

Tomentosum

pollen type). This lends some support to the systematic opinion of Chamberlain

et al

. (1996) regarding the position of

R. tomentosum

and

R. confertissimum

in section

Rhododendron

(subgenus

Rhododen- dron

). However,

R. redowskianum

is in a different subgenus,

Therorhodion

.

Erdtman (1952) pointed out that the tetrad pollen mor-

phology of Ericaceae is similar to that of Empetraceae,

Epacridaceae and Pyrolaceae in the order Ericales. The

exine layers of the present study; ectexine and the

endexine, are similar to the exine stratification of Empe-

traceae (Kim

et al

., 1988). Empetraceae, along with

(9)

Epacridaceae and Pyrolaceae, is now included in the Eri- caceae, following DNA studies (Angiosperm Phylogeny Group, 1998; 2003).

Rhododendron micranthum

was so characterized with its pollen grain size, exine pattern and structure from the rest taxa in the genus that it could make its own pollen type (

Micranthum

-type) by itself. Even though Cham- berlain

et al

. (1996) put

R. micranthum

along with

R.

mucronulatum

in the same subgenus

Rhododendron

, there was no significant pollen morphological similarity between these two species. However, the present pollen morphological results are in agreement with Hwang (1999)’s study. He reviewed 20

Rhododendron

taxa based on 51 morphological characters and also reported that

R. micranthum

was quite independent.

Conclusion

The various exine pattern being scabrate, rugulate or ver- rucate in mesocolpium area, and psilate, rugulate or micros- cabrate in apocolpium and aperture areas, was the major character to create a key to six pollen types in Korean

Rhododendron

. The Korean

Rhododendron

exine consists of four layers; tectum, columella, foot layer and endexine. The exine thickness of

R. weyrichii

was thicker than those of the rest taxa.

Rhododendron micranthum

was distinguished from the others in the pollen grain size and exine pattern.

Acknowledgements

Lots of thanks are due to Dr. Madeline Harley at Royal Botanic Gardens, Kew, UK for her helpful com- ments, and Prof. Lee, Wang-Hyu at College of Agricul- ture & Life Sciences and Mr. Kim, Wan-Cheol at the EM Lab., College of Medicine, Chonbuk Univ., Korea for their SEM & TEM technical assistance.

Specimens Investigated

Rhododendron aureum

Georgi. Democratic People's Republic of KOREA: Baekdusan, Hambuk. CNU-98-100.

R. brachycarpum

D. Don. ex G. Don. Republic of KOREA: Sadong, Ulleungdo, Gyeonbuk. JMP RB060699.

Republic of KOREA: Socheongbong, Seoraksan, Gang- won. JMP RB120699. Republic of KOREA: Jungcheon- gbong, Seoraksan, Gangwon. JMP RB140699.

R. confertissimum

Nakai. Democratic People's Repub- lic of KOREA: Bujeongowon, Hamnam. SNU-1937-84.

R. dauricum

L.. Democratic People's Republic of KOREA:

Agr. Division Research Institute, Hamnam. SNU-1927-79.

R. micranthum

Turcz.. Republic of KOREA: Daeseong- san, Danyang, Chungbuk. JMP RMI040698.

R. mucronulatum

Turcz.. Republic of KOREA: Gyebuk,

Jangsu, Jeonbuk. JMP RMU040498. Republic of KOREA:

Tookdong valley, Deogyusan, Jeonbuk. JMP RMU100499.

R. mucronulatum

f.

ciliatum

(Nakai) Kitag.. Republic of KOREA: Nogodan, Jirisan, Jeonnam. JMP RMC040699.

Republic of KOREA: Banyabong, Jirisan, Jeonbuk. JMP RMC050699. Republic of KOREA: Hangyeryeong, Seorak- san, Gangwon. JMP RMC080699. Republic of KOREA:

Jungheongbong, Seoraksan, Gangwon. JMP RMC090699.

Republic of KOREA: Namdeogyusan, Gyeongnam. JMP RMC130699.

R. redowskianum

Maxim.. Democratic People's Republic of KOREA: Baekdusan, Hambuk. JMP RR230898.

R. schlippenbachii

Maxim.. Republic of KOREA: Hyang- jeokbong, Deogyusan, Jeonbuk. JMP RS050699. Republic of KOREA: Haeinsa, Gayasan, Gyeongnam. JMP RS200599. Republic of KOREA: Hangyeryeong, Seorak- san, Gangwon. JMP RS080699. Republic of KOREA:

Seongsamjae, Jirisan, Jeonnam. JMP RS110699. Republic of KOREA: Banyabong, Jirisan, Jeonbuk. JMP RS120699.

R. schlippenbachii

f.

albiflorum

Y. N. Lee. Republic of KOREA: Daecheongbong, Mt. Seorak, Gangwon. JMP RSA080699.

R. tomentosum

Harmaja. Democratic People's Repub- lic of KOREA: Hamnam. SNU-1925-125.

R. tschonoskii

Maxim.. Republic of KOREA: Namsan- jeilbong, Gayasan, Gyeongnam. JMP RT150698. Republic of KOREA: Namdeogyusan, Gyeongnam. JMP RT120699.

Republic of KOREA: Sangwangbong, Gayasan, Gyeo- ngnam. JMP RT200699. Republic of KOREA: Namsan- jeilbong, Gayasan, Gyeongnam. JMP RT210699. Republic of KOREA: Banyabong, Jirisan, Jeonbuk. JMP RT040799.

R. yedoense

f.

poukhanense

(H. Lév.) M. Sugim. ex T.

Yamaz.. Republic of KOREA: Gyebuk, Jangsu, Jeonbuk.

JMP RY250498. Republic of KOREA: Seongsamjae, Jirisan, Jeonnam. JMP RY300499. Republic of KOREA:

Deogyusan, Jeonbuk. JMP RY040599. Republic of KOREA:

Jirisan, Jeonnam. JMP RY070599.

R. weyrichii

Maxim.. Republic of KOREA: Halla Arbo- retum, Jeju. JMP RW110599.

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(Received January 20, 2010; Accepted May 13, 2010)