Abstract: The pollen morphology of 30 species belonging to 24 genera representing 6 tribes of Asteraceae have been investigated with the aid of the light microscope. The results confirm pollen grains of this family exhibit considerable variation in their morphological characteristics as size, aperture type and pattern of exin sculpturing. Pollen grains of all taxa examined are single and can be divided into 2 groups: I-Simple trizonocolporate with echinate or subechinate or verrucate sculpturing pattern on the exin surface which has been founded in following tribes: Cynareae, Anthemideae, Inuleae, Calenduleae and Arctotideae. This group include diffrent types of pollen exin. II- Fenestrate with echinate or echinolophate sculpturing pattern on the exine surface has been founded in Lactuceae.
INTRODUCTION
Morphological studies of pollen regardless of sexual system, diversity of distribution and knowing of special environment for preservation or growth, is important for knowing of past and present of typical growth. In addition, it is used for basic study of systematic and taxonomy of new plants, special diet provision and knowing of original honey against copy, studying of immonology and alergy (Faegri and Iversen, 1992).
Asteraceae has been variously divided into groups. Wodehouse (1935) collected and presented investigating information of 1500 title that is published by him. Erdtman (1960, 1969 and 1971) has also studied about it.
The first survey on the pollen morphology and structure of Anthemideae were made by Wodehouse (1926, 1928). He commented that the species of Anthemis, Leucanthemum and Chrysanthemum which are insect pollinated, have grains with well developed spines and a thick exine while species of Artemisia which are wind pollinated have grains with vestigial or absent spines and a much less thick exine.
Wodehouse (1935) characterized the basic exomorphology of Lactuceae pollen and described 2 basic pollen type (echinate and echinolophate). Tomb (1975) and Blackmore (1982, 1984) studied pollen grains of Lactuceae.
The evolution of pollen morphological characters for the systematics of the Cynareae has gained through the pioneer studies on the Centaurea by Pehlivan (1995). And through those on the Echinops by Blackmore (1990).
Stix (1960) characterized the basic morphology of Asreraceae pollen in central Europe and discribed 12 basic pollen types.
It has very scientific usage and necessity to studying and knowing of pollen grains in flora of Iran.
The present study aimed to describe the pollen morphology of the selected Iranian taxa of Asteraceae to find out the inter-generic and specific relationships and to solve some taxonomic problems.
MATERIALS AND METHODS
For the purpose of studying pollen grains in Asteraceae, specimens were collected in their flowering period. Pollen grains have been taken from anther and were acetolysed according to Erdtman (1960) method. Thirty samples of each species have been examined. All pollen samples were prepared for light microscope and their size measured by micrometer. Total information shows in Table 1-3 and Fig. 1-3.
RESULTS AND DISCUSSION
The pollen morphology of 30 species belonging to 24 genera representing 6 tribes of Asteraceae have been investigated with the aid of the light microscope (Table 1, 2 and Fig. 1-3).
The results confirm pollen grains of this family exhibit considerable variation in their morphological characters as size, aperture type and pattern of exin sculpturing. Pollen grains all of examined taxa are single and distinguished to two groups:
| Table 1: | Taxa that used in pollen studies |
| Table 2: | Specimens and collecting data |
| Table 3: | Pollen characters of Asteraceae taxa examined pollen shape (S), measurment (μm) of the polar (P), equatoral (E) axes and shape index (P/E) |
| • | Trizonocolporate with echinate or subechinate or verrucate sculpturing pattern on the exin surface which has been founded in following tribes: Cynareae, Anthemideae, Inuleae, Calenduleae and Arctotideae. This group includes different types of pollen. |
| • | Fenestrate with echinate or echinolophate sculpturing pattern on the exine surface has been founded in Lactuceae. |
Pollen grains of Lactuceae have systematical and phylogenetical value because of special pattern of exin sculptusing. Echinate is more primitive than echinolophate (Woodehouse, 1935; Blackmore, 1984). Pollen grain in Anthemideae has coarse columellae visible under the structured tectum and echinae (Heywood and Humphries, 1975).
It shows comparisons of pollen grain in different taxa of Asteraceae that Cynareae has diversity. You can see 5 different types of pollen (Echinops type, Carduus type, Cirsium type, Centaurea type and Serratula type). These investigations show that most of the pollen grains of cynareae are echinate or subechinate but seldom they are verrucate. They are in circular or elleptical shapes and existence of endocolpus in equatorial pole is special character.
Pollens in Echinops type (includes: Echinops) are very big. Grain usually are >90 μm in polar axes with pattern of subechinate sculpturing on the exin surface and thick columellae. Serratula type (includes: Arctium and Zoegea) has circular shape with subechinate pattern and Columellae visible underneath the tectum (tectum and echinae are traversed by rods which are much finer and more densly packed than the columellae.
| Fig. 1: | Light microscopic photographs of the Asteraceae pollen grains |
| Fig. 2: | Light microscopic photograph of the Asteraceae pollen grains |
| Fig. 3: | Light microscopic photographs of the Asteraceae pollen grains |
Cousinia type has verrucate pattern. Centaurea type (includes: Cousinia, Centaurea jurinea and Outreya has pollen grains <90 μm with subechinate or verrucate pattern. Carduus type (includes: Carduus and Carthamus) has echinate pattern and no endocolpus around equator of grain and no Columellae visible underneath the tectum (only the fine, densly packed rods of tectum are visible. In Circium type (includes: Circium) columella slanding slightly underneath the tectum at the point where the echinae are situated, thus giving a star pattern to each echina. Colpi fairly short.
With paying attention to it, that some genera of Cynareae (e.g., Cousinia eriobasis Centaurea depressa, Centaurea bruguieran, Centaurea depressa, Outreya and Centaurea jurinea) have verrucate pattern so, they are considered primitive (Moor et al., 1991). During the evolutional improvement verrucate pattern change to echinate pattern. Also polar view change from triangular- circular to circular shape. So Carduus is considered as evolutional turning point in this tribe. In Lactuceae genera with evolutional improvement echinate pattern change to echinolophate. Echinolophate can be seen in most of taxa in this tribe and in a few other tribe (e.g., Vernonieae) of this family but it has not been seen in other families and it shows that Asteraceae is very advanced.
According to shape index (P/E) there are 4 shapes such as: spherical (1.00), Prolatespherical (1.01-1.13), subprolate (1.14-132) and prolate (1.33-2.00) in Asteraceae (Table 3).
ACKNOWLEDGMENT
We would like to grateful to Mrs. Khatereh Emadzade for her co-operation.