Bot. Bull. Acad. Sin. (2005) 46: 75-81

LIAO et al. Cuscuta pollen morphology in Taiwan

Pollen morphology of Cuscuta (Convolvulaceae) in Taiwan

Gwo-Ing LIAO1,*, Ming-Yih CHEN2, and Chang-Sheng KUOH1

1Department of Life Sciences, National Cheng-Kung University, Tainan 701, Taiwan

2Department of Life Sciences, National Chung-Hsing University, Taichung 402, Taiwan

(Received March 8, 2004; Accepted August 31, 2004)

Abstract. The pollen morphology of five taxa of Cuscuta, C. australis, C. campestris, C. chinensis, C. japonica var. formosana and C. japonica var. japonica (Convolvulaceae) in Taiwan was investigated by using LM, SEM and TEM. The pollen of Cuscuta is three to six colpate, zonocolpate or pantocolpate colpus with granules, tectum with scabrate processes, ektexine reticulate or finely reticulate, and endexine thinner than ektexine. Two distinct pollen types of Cuscuta spp. in Taiwan are recognized. Type 1 is small, and has colpus with granules, and ektexine finely reticulate, including C. australis, C. campestris and C. chinensis. Type 2 is medium in size, and has colpus with granules, scabrate processes on surface of granule and ektexine reticulate, including C. japonica var. formosana and C. japonica var. japonica. Pollen types correspond with Yuncker's subdivision of the genus Cuscuta, type 1 for subgenus Grammica and type 2 for subgenus Monogyna. Meanwhile, pollen characteristics are also helpful in identifying species.

Keywords: Cuscuta; Dodder; Pollen; Taiwan.

Introduction

Dodders (genus Cuscuta L.) are parasitic flowering plants of the Convolvulaceae family (Yuncker, 1932; Kuijt, 1969; Parker and Riches, 1993; Liao et al., 2000). They are, however, sometimes assigned to the Cuscutaceae family (Hadac and Chrtek, 1970; Chrtek and Osbornova, 1991). This genus is globally distributed, with most species in the tropics and subtropics, and some in the temperate regions (Beliz, 1986). The number of species varies between 100 and 200 (Yuncker, 1932; Beliz, 1986; Parker and Riches, 1993; Fang et al., 1995; Staples and Yang 1998). Liao et al. (2000) considered three species and two varieties of Cuscuta in Taiwan, including C. australis, C. campestris, C. chinensis, C. japonica var. formosana and C. japonica var. japonica.

The identification of Cuscuta depends mostly on the detailed characteristics of the flowers, including the shapes of perianth segments, shapes of stigma, number of styles, infra-staminal scales attached to the filaments, as well as the dehiscence of the capsule. The characteristics of the scales and fruits are often difficult to determine, even in fresh materials. Furthermore, even these characteristics are difficult to observe in a dried specimen (Parker and Riches, 1993). Cuscuta campestris, a North American species recently introduced in several countries of the Old World, has often been misidentified (Parker and Riches, 1993). In Taiwan, it has been misidentified as C. australis or C. chinensis (Liao et al., 2000).

Information concerning pollen morphology can be used in the analysis of fossil pollen (Moore et al., 1991; Martin, 2001), airborne pollen (Nilsson et al., 1977; Lewis and Vinay, 1983), and honey pollen (Crompton and Wojtas, 1993). It is also used as taxonomical characteristics (Hsiao and Kuoh, 1995; Perveen and Qaiser, 1998). This study describes several pollen morphological characteristics of Cuscuta in Taiwan. The aim is to provide more information on the fine structural characteristics, and a Key is prepared for the identification of Taiwan taxa of Cuscuta.

Materials and Methods

Eleven pollen samples representing five taxa of Cuscuta in Taiwan were examined by light microscopy (LM), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). All pollen samples were freshly collected, except that of Cuscuta australia, which was obtained from the herbarium of National Pingtung University of Science and Technology, Taiwan (PPI) (Table 1).

Flowers were fixed using F. A. A. for 12-24 h and then stocked in 70% alcohol. For LM and TEM, anthers were dissected, gradually dehydrated in an acetone series, slowly infiltrated/embedded in Spurr epoxy resin, and then thick- and thin-sectioned on an ultramicrotome using a diamond knife. Thick sections (500 nm) were stained with toluidine blue, and examined on a Zeiss compound microscope. Ultrathin sections (90 nm) were stained with 4% aqueous uranyl acetate and 0.4% lead citrate, and examined/imaged with a Zeiss 10C TEM at 80 kV. For SEM, anthers were critical point dried. Pollen grains were then removed from the anthers, mounted onto aluminum stubs, coated with gold palladium, and then examined/imaged using a Hitachi H2500 SEM at 15 kV.

*Corresponding author. Tel: +886-6-2757575 ext 65540; Fax: +886-6-2742583; E-mail: liaomi@mail.ncku.edu.tw


Botanical Bulletin of Academia Sinica, Vol. 46, 2005

The size of the pollen was measured by polar axis and equatorial axis when observed from the equatorial view, and 30 pollen grains were measured per sample. Studied pollen was characterized in size as small (10-25 m) or medium (25-50 m) according to Erdtman (1952). Pollen terminology followed Punt et al. (1994).

Results

Description of Each Taxa

Cuscuta australis R. Br. (Figure 1, Table 2)

Pollen grains 3-colpate, zonocolpate, oblate spheroidal to prolate (P/E= 0.93-1.38) in equatorial view, small in size (15.5-22.7 15.1-20.7 m); circular in polar view, 15.1-20.7 m in diameter; colpi 12.4-18.16 2.2-3 m, surface with granules; exine 1.3 m thick; tectum with scabrate processes; ektexine finely reticulate; endexine thinner than ektexine.

Cuscuta campestris Yuncker (Figure 2, Table 2)

Pollen grains 6-colpate, pantocolpate, suboblate to prolate spheroidal (P/E= 0.76-1.13) in equatorial view, small in size (17.9-22.7 18.3-24.7 m); circular in polar view, 18.3-24.7 m in diameter; colpi 10.7-15.3 2.7-6.7 m, surface with granules; exine 1.8 m thick; tectum with scabrate processes; ektexine finely reticulate; endexine thinner than ektexine.

Cuscuta chinensis Lam. (Figure 3, Table 2)

Pollen grains 3-colpate, zonocolpate, oblate to subprolate (P/E= 0.74-1.28) in equatorial view, small in size (16.7-23.5 17.1-26.7 m); circular in polar view, 17.1-26.7 m in diameter; colpi as long as polar axes, 16.7-23.5 5-6.7 m, surface with granules; exine 1.9 m thick; tectum with scabrate processes; ektexine finely reticulate; endexine thinner than ektexine.


LIAO et al. Cuscuta pollen morphology in Taiwan

Cuscuta japonica Choisy var. formosana (Hay.) Yuncker (Figure 4, Table 2)

Pollen grains 4- (rarely 5- or 3-) colpate, zonocolpate, oblate to subprolate (P/E= 0.74-1.18) in equatorial view, medium in size (22.3-32.6 22.3-34.6 m); inter-angular in polar view, 22.3-34.6 m in diameter; colpi 12.5-17.3 4.8 m, surface with granules, scabrate processes on surface of granule; exine 4.0 m thick; tectum with scabrate processes; ektexine reticulate; endexine thinner than ektexine.

Notes. Cuscuta japonica var. formosana in Puli, Nantou County, produces a mixture pollen including normally zonocolpate and variously large pantocolpate grains.

Cuscuta japonica Choisy var. japonica (Figure 5, Table 2)

Pollen grains 4- (or 5-, rarely 3-) colpate, zonocolpate, suboblate to prolate spheroidal (P/E= 0.82-1.07) in equatorial view, medium in size (24.7-34.6 27.0-37.8 m); inter-angular in polar view, 27.0-37.8 m in diameter; colpi 15.3-20 3.3-5.3 m, surface with granules, scabrate processes on surface of granule; exine 3.9 m thick; tectum with scabrate processes; ektexine reticulate; endexine thinner than ektexine.

Key to the Taxa of Cuscuta Pollen in Taiwan

1. Grains small, 10-25 mm; ektexine finely reticulate 2

2. Grain pantocolpate C. campestris

Figure 1. Micrographs of Cuscuta australis pollen. A-D: SEM; E-G: TEM. A, equatorial view; B, polar view; C, colpus membrane with granules; D, surface pattern at the mesocolpium; E, transverse section through equatorial plane at mesocolpium showing prominent columellae (C), arrow indicates scabrate process on tectum (T); F, section showing the thickening of the endexine (E) and the absence of the tectum at apertural region; G, oblique section through plane between apocolpium and mesocolpium, only one colpus (double arrow) is visible. Note that some areas show slight shrinkage when mounted on the large slide (arrow). Bars: A, B = 5 m; C, G = 2 m; D-F = 1 m.


Botanical Bulletin of Academia Sinica, Vol. 46, 2005

Sengupta pointed out that the pollen grains of 19 species are tricolpate and scrobiculate, the pollen grains of C. monogyna are tricolpate and reticulate, and the pollen grains of C. reflexa are penta-hexa-colpate and reticulate. Meanwhile he also postulated that Cuscuta pollen evolved from tricolpate to penta-hexa-colpate with a diagram. The five taxa of Cuscuta we studied were not examined in Sengupta's work. The pollen of Cuscuta in Taiwan is three to six colpate, zonocolpate or pantocolpate, colpus with granules, tectum with scabrate processes, ektexine reticulate or finely reticulate, and endexine thinner than ektexine, hence we distinguished two main pollen types of Cuscuta spp. in Taiwan, based on the pollen size and fine structure of pollen wall mainly using electron microscopy. Type 1

2. Grain zonocolpate C. australis, C. chinensis

1. Grains medium, 25-50 m; ektexine reticulate

C. japonica var. formosana, C. japonica var. japonica

Discussion

Sengupta has an important work on the pollen morphology of Convolvulaceae (Sengupta, 1972). His work distinguished four main pollen types in Convolvulaceae, based on the number and distribution of apertures mainly using light microscopy. The pollen types are tricolpate, penta-hexa-colpate, dodecacolpate, and pantoporate, respectively. When dealing with the genus Cuscuta,

Figure 2. Micrographs of Cuscuta campestris pollen. A-E: SEM; F-H: TEM. A, equatorial view; B, polar view; C, subequatorial view; D, colpus membrane with granules; E, surface pattern at the mesocolpium; F, transverse section through equatorial plane at mesocolpium showing prominent columellae (C), arrow indicates scabrate process on tectum (T), note the columellae is longer than that in Figure 1- E; G, section showing the thickening of the endexine (E) and the absence of the tectum at the apertural region; H, transvers section of pollen grain at the subpolar region, three colpi are visible. Note that some areas show slight shrinkage when mounted on the large slide (arrow). Bars: A-C = 5 m; D, H = 2 m; E-G = 1 m.


LIAO et al. Cuscuta pollen morphology in Taiwan

is small, and has a colpus with granules, and ektexine finely reticulate, including C. australis, C. campestris and C. chinensis. Type 2 is medium in size, and has a colpus with granules, scabrate processes on surface of granule and ektexine reticulate, including C. japonica var. formosana and C. japonica var. japonica.

Yuncker (1932) divided the genus Cuscuta into three subgenera, subgenus Grammica, subgenus Cuscuta, and subgenus Monogyna, respectively, with these based on

the arrangement of the styles and the shapes of the stigmas. In this preliminary study, we find that the pollen types we proposed correspond with Yuncker's subdivision of the genus Cuscuta, type 1 for subgenus Grammica and type 2 for subgenus Monogyna. The relation between the fine structure of the pollen wall and the subdivision of the genus Cuscuta are worth further study with more taxa.

Huang (1972) studied the pollen morphology of two species of Cuscuta in Taiwan using light microscopy. He de

Figure 3. Micrographs of Cuscuta chinensis pollen. A-D: SEM; E-G: TEM. A, equatorial view; B, polar view; C, colpus membrane with granules; D, surface pattern at the mesocolpium; E, transverse section through equatorial plane at mesocolpium showing prominent columellae (C), arrow indicates scabrate process on tectum (T), note the columellae is longer than that in Figure 1- E; F, section showing the thickening of the endexine (E) and the absence of the tectum at the apertural region (arrow); G, transverse section of pollen grain at the equatorial region, three colpi are visible. Note that some areas show slight shrinkage when mounted on the large slide (arrow). Bars: A, B = 5 m; C, G = 2 m; D-F = 1 m.


Botanical Bulletin of Academia Sinica, Vol. 46, 2005

ture of C. japonica var. formosana has ektexine reticulate. The difference between these results is probably due to the use of different microscopies.

Cuscuta campestris is a dodder common worldwide (Parker and Riches, 1993). It was firstly reported in Taiwan by Liao et al. in 2000. In Taiwan, the first collected Cuscuta campestris was misidentified as C. chinensis in 1964. Then, another specimen was misidentified as C. australis in 1971.

scribed the pollen of C. chinensis as tectum psilate, and C. japonica var. formosana as sexine rugulate. Wang et al. (1995) investigated the pollen morphology of C. chinensis from China using light microscopy, and showed that the nexine was as thick as the sexine. However, the observations in our work showed that the tectum of the pollen of C. chinensis has scabrate processes, and endexine is thinner than ektexine. Moreover, the pollen sculp

Figure 4. Micrographs of Cuscuta japonica var. formosana pollen. A-F, I & J: SEM; G & H: TEM. A, equatorial view; B-D, polar view; E, colpus membrane with granules, note the scabrate processes (arrows) on surface of granule; F, surface pattern at the mesocolpium; G, transverse section through equatorial plane at mesocolpium showing prominent columellae (C), endexine (E) and intine (I), arrow indicates scabrate on tectum (T); H, section showing the thickening of the endexine (E) and the absence of the tectum at the apertural region; I-J, showing variously large pantocolpate grains and normally zonocolpate grains. Bars: A-D = 5 m; E, F = 2 m; G, H = 1 m; I = 20 m; J = 10 m.


LIAO et al. Cuscuta pollen morphology in Taiwan

Figure 5. Micrographs of Cuscuta japonica var. japonica pollen. A-F: SEM; G & H: TEM. A, equatorial view; B-D, polar view; E, colpus membrane with granules, note the scabrate processes on surface of granule; F, surface pattern at the mesocolpium; G, transverse section through equatorial plane at mesocolpium showing prominent columellae (C), arrow indicates scabrate on tectum (T). Note both endexine (double arrow) and intine (arrowhead) are thinner than those in Figure 4- G; H, section showing the thickening of the endexine (E) and the absence of the tectum at the apertural region. Scale bars: A-D = 5 m; E, F = 2 m; G, H = 1 m.

Thereafter, almost all collected species were misidentified as C. australis. These misidentifications followed from similarities in gross morphology. Actually, the characteristics of pollen of C. campestris can be easily distinguished from those of the pollen of other species in Cuscuta, since the pollen of C. campestris is pantocolpate and the other pollen are zonocolpate.

Although the gross morphology and characteristics of the pollen of Cuscuta japonica var. formosana and C. japonica var. japonica are very similar, the tongue-shaped stigma lobes and shorter corolla, and thicker endexine and intine of C. japonica var. formosana distinguish it from C. japonica var. japonica.

Moore et al. (1991) stated that the pantocolpate condition of some species, including Cuscuta, that are normally trizonocolpate, is thought to be associated with meiotic irregularities or increasing ploidy levels, and sometimes

with hybridization. In this work, Cuscuta japonica var. formosana in Puli, Nantou County, also produces a mixture of pollen, including normally zonocolpate grains and variously large pantocolpate grains. Although the cause of the dimorphism of C. japonica var. formosana is not yet clear, it is not a problem when using pollen characteristics in the key for identification of Cuscuta spp. in Taiwan.

Acknowledgments. Dr. S. Z. Yang, the Curator of the PPI, is appreciated for loaning the specimens of Cuscuta australis. Mr. M. H. Chuang is also appreciated for his assistance in TEM preparation and photography.

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Botanical Bulletin of Academia Sinica, Vol. 46, 2005

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