Pak. J. Weed Sci. Res. 10(1-2):87-92, 2004

Palynological Studies Of Some Weeds Of Asteraceae From Pakistan

Akbar Ali Meo and Mir Ajab Khan^[1]

Abstract

The paper is an account of pollen morphology of the some Pakistani weeds of Asteraceae by light microscopy. Pollen grains are usually isopolar, symmetrical, trionocolporate, non-lacunate and echinate in the studied species. The major palynological characters which proved useful to distinguish all the taxa of the Pakistani weeds are polar and equatorial diameter, spine length, number of spine rows between Colpi, exine width, pollen shape, P/E ratio, shape in polar and equatorial view, pollen class and aperture type. Exine thickness varies considerably among the weeds studied. This variation would prove to be a taxonomic criteria in the classification of weeds.

Key words: Weeds, pollen morphology, Asteraceae, Palynnological.

Introduction

Palynology is the science of pollen and spore morphology and has found application in plant taxonomy, plant geography, climatology, aeropalynology, criminology, allergy, stratigraphic correlation of oil bearing rocks and coal field, phar macopalynology. Pollen analysis has also been tried as a means of tracing the history of cultivated cereals. This analysis helps in qualitative analysis of drug powder and in correct identification of drug. The term palynology includes all the work dealing with both fossil and living pollens and spores.

In Pakistan, the earliest reference dates back to early sixties. Several workers like Malik et al., 1964; Bhutta, 1968; Ashraf 1973; Zahur et al., 78; Meo et al., 1988 (ab); 1989; 1999; Nasreen and Khan 1998;  Mumtaz et al., 2000; Dawar et al., 2002; Parveen 2002 and Khan, 2003 have provided a commendable quantity of basic and applied information in palynology. Moreover, at international level there has been an explosion of information published on many aspects of pollen and spores. Compositae is a eurypalynous family (Erdtman, 1952) and most of its genera posses zonocolporate pollen (Sachdeva and Malik, 1986). Wodehouse (1926) published some aspects of pollen morphology in Compositae. The pollen grains of Compositae are tricolporate, flattened or spherical and echinate with variation in size and colpus number (Wodehouse, 1935). The pollen of Astereae, Inuleae, Heliantheae, and a taxa of Anthemideae, Cynareae and Senecioneae revealed spinsose exine while those of Vernonieae and Cichorieae possess lophate or spinolose–lophate pattern (Sachdeva and Malik, 1986). Compositae is highly evolved family among angiosperms. It is generally regarded as occupying the highest position in plant kingdom due to its great preponderance and cosmopolitan range. About 206 weed species important to human being have been recorded on a world wise scale. Forty three percent of these belong to only four families; the Gramineae and Compositae alone contain no fewer than 76 weed species. The weeds of this family are wide-spread in Pakistan. Khalid (1995) taxonomically described 40 species of weeds of Compositae in Pakistan.

Materials and Methods

The florets from mature capitula were obtained from the herbarium of Quaid-e-Azam University, Islamabad (ISL). Mature, unopened buds were removed from the herbarium specimen. The grains were acetolysed for light microscopy (LM) by using the conventional procedure of Erdtman (1966) and slides were prepared by using 1% safranin mixed in glycerin jelly (commonly called Gel Safranin). Florists were treated in acetic acid for five minutes. Pollen grains were mounted in glycerin jelly and observed under Nikon Labophot microscope using oil immersion. Following measurements were undertaken (1) Polar diameter (2) Equatorial diameter (3) P/E ratio (Polar/Equatorial diameter) (4) Exine thickness (5) Spine length (6) Number of spine rows between colpi (7) Shape in polar view (8) Shape in equatorial view (9) Sculpturing (10) Aperture type.

The terminology for pollen description was adopted from Erdtman (1952), Huang (1972), Traverse (1988) and Moore et al. (1991).

Results And Discussion

Table-1 summaries the light microscopic measurements of pollen grains from the examined taxa.

Size:    The size of the pollen grain (polar–equatorial diameter excluding spines) ranged from 22.5 – 22.7mm to 31.3 – 33.3mm. There is great variation in the size of the pollen grain. Gamolepis tagetes and Syndrella nodiflora have almost similar sized pollen while rest of the species are variable. P/E ratio ranged from 0.94 to 1.30.

Symmetry and Shape: The pollen grain are radically symmetrical, isopolar. Pollen shape in equatorial view is prolate, spheroidal, prolate–spheroidal to oblate–spheroidal. Pollen shape in polar view is circular, semi-angular to semi- lobate.

Aperture: The pollen grains are trionocolporate in all the species except S. nodiflora where pollen grains are tetrazono-colporate. Aperture shape is lacunate in Artemisica indica and S. nodiflora and non-lacunate in rest of the species. Apertural membrane is echinate except Centaurea concifolia and A. indica where sculpturing is scabrate.

Spine: There is great variation in the spines in these genera. In A. indica and C. concifolia spines are absent while in rest of all the species spines are present. Spine length ranged from 2.1 to 5.7mm among the taxon. The number of spine rows between colpi ranged from 4 to 8 among the taxon.

Exine: Exine has proved a useful distinguishing character. Exine thickness ranged from 2.0 to 6.4 mm among the species.

Pollen morphological characters have proved useful to improve the classification of the taxa of the family Compositae. These characters play a key role in taxonomic debate. Tomsovic (1997) utilized pollen character as an additional information for systematic studies. He noted that species of Echinops are similar in their main characters. However, one of them is an exception E. strigosus L. distributed in Mediterranean. On the basis of all available characters he considered this species as a separate genus Psectra containing one species only.

The present study showed that maximum pollen size 31.3 mm is recorded in Launia capitata in polar view and the minimum pollen size 22.5mm is observed in A.adnata. Similarly the maximum pollen size in equatorial view is 33.3 in L. capitata while the minimum size of 22.7 mm in Anaphalis adnata.

P/E ratio plays an important role in Palynological debate. The genera like L. capitata, C. concifolia, and A. adnata showed lowest P/E ratio (0.94–0.99). Aster falconeri, Pterothica falconeri and A. indica possessed highest P/E ratios (1.11–1.30). Intermediate value of P/E ratio (1.02–1.08) was recorded in C. flexurea, G. tagetes and S. nodiflora. Exine thickness recorded highest (5.4 – 6.4mm) value in A. adnata, L. capitata, A. indica and Pterothica falconeri while the lowest value of exine thickness (2.0 – 2.2) was recorded in G. tagetes and A. falconeri. The intermediate values of exine thickness (3.2–4.6 mm) has been observed in genera like C. flexurea, C. concifolia and S. nodiflora. These results correspond with Dawar et al. (2002) who utilized exine thickness for systematic purpose in Inula (Compositae).

The character of pollen spine is of significance at specific and generic level in classification of family Compositae. Spines and spineless pollen are important in Compositae in the present study. C. concifolia and A. indica are spineless genera while all other genera have spinate pollen showing that the feature of spine is useful at specific level. These finding are in agreement with that of Keeley and Jones (1977) who reported spinate and spineless pollen in some Vernonia species and observed that both pollen and vegetative character indicate a divergence due to independent line of evolution in isolation. Wodehouse (1935) outlined the principles of morphological evolution of spine form in Compositae and suggested the reduction series from long to minute spines. The peculiar spine character represents a climax in the apertural evolution. The species with spinate pollen is a primitive character as compared to the species with spineless pollen which is considered as primitive feature due to reduction of spines within the family Compositae.

Spine length ranged from 2.1 to 5.7mm among the genera. Maximum spine length values (5.7mm) is observed in S. nodiflora and the minimum values (2.1 – 2.9mm) have been recorded in P. falconeri, A. falconeri, Crepis flexurea, L. capitata. Intermediate values (3.3 – 4.1mm) have been recorded in Gamolepis tagetes, A. adnata. In a similar fashion, the number of spine rows between colpi ranged from 4 – 8. Highest number of spine rows (7-8) have been observed in A. falconeri and lowest number (4-5) recorded in A. adnata.

The rest of the genera showed intermediate values. These results are in accordance with the findings of Clark et al. (1980), Tomb et al. (1974) and Dawar et al. (2002) who used spine length and number of spine rows between colpi in distinguishing taxonomic and pollen group in Astereae, Cichorieae and Inula in family Compositae.

Table-1. Summery of Pollen measurements, shapes and sculpturing features in some Asteraceae Weeds.

Pol.Polar Eq: Equitorial Dia.= Diameter  Tri.=Trizonocolporate Tet.= Tetrazonocolporate  

Acknowledgement

The authors are very grateful to the Department of Biological Sciences, Quaid-e-Azam University, Islamabad for the use of herbarium material.

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