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Biocell

versión impresa ISSN 0327-9545

Biocell v.31 n.1 Mendoza ene./abr. 2007

 

Pollen morphology of Oxalis species from Buenos Aires province (Argentina)

Sonia Rosenfeldt and Beatriz G. Galati

Departamento de Biodiversidad y Biología Experimental. Facultad de Ciencias Exactas y Naturales. Universidad de Buenos Aires. Ciudad Universitaria. Pab. II. C1428EHA

Address correspondence to: Lic. Sonia Rosenfeldt. Departamento de Biodiversidad y Biología Experimental, Facultad de Ciencias Exactas y Naturales, UBA. 4º piso, Pabellón II. Ciudad Universitaria. C1428EHA Buenos Aires, ARGENTINA. E-mail: soniar@bg.fcen.uba.ar / galati@bg.fcen.uba.ar

ABSTRACT: Pollen morphology and the degree of pollen variability within nine species and two varieties of Oxalis species from Buenos Aires Province (Argentina) were studied using light microscopy  (LM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
     Oxalis pollen grain is generally 3-colpate and the shape is prolate spheroidal, oblate spheroidal or spheroidal. The exine is microreticulate. The brochi are circular to polygonal;  brochi sizes decrease near the colpi. The colpus membrane observed in unacetolysed material, is crustate or granular. The colpus length varies within a species; the largest grains are produced in stamens with long filaments and the smallest ones in stamens with short filaments.
     Four pollen types are  distinguished by means of LM, SEM and TEM. They are recognized mostly on the basis of their colpus membrane structure.
     In conclusion, the morphological similarity among Oxalis species is also reflected in their pollen morphology.

Key words: Pollen; Oxalis; Morphology; Structure.

Introduction

     The genus Oxalis L. comprises about 800 cosmopolitan species.
     Previous studies of pollen morphology in this genus are meagre and they were conducted using only light microscopy (Erdtman, 1952; Huynh, 1969 a,b) or SEM (Cerceau, 1995). Most of these reports analysed the formation of aberrant pollen. According to the apertures number and position, seven categories of grain aberrations and two succesiform series were identified by Dreyer and Van Wyk (1998) in Southern African Oxalis. Ghosh and Verma (1985) recognized three types of pollen in O. corymbosa DC. and O. dehradunebsis Raizada, and two types in O.corniculata Linn. Pollen morphology of  species of this genus from Pakistan and Taiwan (Yueyange Lake) was studied by Perveen and Qaiser (2003) and Wang and Chen (2001), respectively.
     The aim of the present study was to provide detailed pollen morphological and structural descriptions and to analyse the degree of pollen variability within Oxalis species from Buenos Aires Province (Argentina) using LM, SEM and TEM.

Materials and Methods

     This study is based on materials of nine species and two varieties of Oxalis collected from Buenos Aires Province and Buenos Aires city, Argentina. The taxa are: O. articulata Savigny (SI 27974); O. lasiopetala Zuccarini (SI 1673); O. floribunda Lehmann (SI 28745); O. paludosa Saint Hilaire (SI 1764); O. conorrhiza Jacqui (SI 2075); O. corniculata L. var. atropurpurea Planchon (CTES 43177); O. corniculata L. var. corniculata (SI 555); O. hispidula Zuccarini (SI 1345); O. perdicaria Bertero (SI 20045); O. debilis Humboldt, Bonpland et Kunth (SI 2015); O. niederleinii Knuth (SI 1042).
     For scanning electron microscopy (SEM), anthers fixed in FAA (formalin, alcohol, acetic acid) were transferred to 100º acetone and then, air dried. For exine sections, anthers were embedded in paraffin wax. Sections of 3 μm thick were transferred to xilene for ten min and air dried.
     The sputtering treatment was made with gold-palladium for 3 min. Scanning micrographs were taken with a Philips XL 30 microscope.
     For transmission electron microscopy (TEM) studies, the material was pre-fixed in 2,5% glutaraldehyde in phosphate buffer (pH 7,2) for 2 h and post-fixed in OsO at 2ºC in the same buffer for 3 h. Then, it was dehydrated in ethanol series and embedded in Spurr's resin. Fine sections were made on a Sorvall ultramicrotome, stained with uranyl acetate and lead citrate (O'Brien and Mc Cully, 1981). The sections were observed and photographed in a JEOL - JEM 1200 EX II TEM at 85.0 Kv.
     The palynological terminology used in this work is in accordance with Punt et al. (1994). Unacetolysed pollen grains from mature but not dehiscent anthers of 10 flowers were mounted in glycerine and the pollen diameter was measured using a light microscope (LM) with a calibrated ocular micrometer.

Results

General  morphology and structure

     Oxalis pollen grain is generally 3-colpate and the shape is prolate spheroidal, oblate spheroidal or spheroidal. Amb rounded or triangular. The exine is microreticulate (Figs. 1-22). The brochi are circular to polygonal. The brochi sizes decrease near the colpi (Figs. 2,4,6,8,10,12,14,16,18,20,22). The colpus membrane is granular. The colpus length varies within a species. The largest grains are produced in stamens with long filament and the smallest ones in stamens with short filament (Table 1).

TABLE 1. Characters in Oxalis species pollen

     Four pollen types are  distinguished by means of LM, SEM and TEM. They are recognised mostly on the basis of colpus membrane ornamentation :

Key to pollen types

A Colpus membrane with irregularly scattered granules...................................................................Type II
AA Colpus membrane with granules  arranged in a row on the margin.
B Granules assembled in clusters ir regularly scattered on the colpus membrane...........................Type I
BB Granules assembled in clusters aligned  on the colpus membrane center..........................................C
C Small clusters more or less aligned on the colpus membrane center ........................................Type III
CC Conspicuous clusters extensively aligned on the colpus membrane center...................................Type IV


FIGURES 1-6. Type I. SEM photographs of pollen grains of Oxalis species. Figs. 1-2. O. floribunda: Fig. 1. Polar and equatorial views, Fig. 2. Detail of polar view. Figs. 3-4. O.articulata: Fig. 3. Polar and equatorial views, Fig. 4. Detail of colpus and exine. Figs. 5-6. O. lasiopetala: Fig. 5. Polar and equatorial views, Fig. 6. Detail of colpus and exine. Scale bars: Figs. 1, 2=5μm; Fig. 3=10μm; Fig. 4= 5 μm; Fig. 5=10μm and Fig. 6=1,5μm.


FIGURES 7-14. Type II. SEM photographs of pollen grains of Oxalis species. Figs. 7-8. O. corniculata var. atropurpurea: Fig. 7. Polar and equatorial views, Fig. 8. Detail of colpus and exine. Figs. 9-10. O. corniculata var. corniculata: Fig. 9. Equatorial view, Fig. 10. Detail of colpus and exine. Figs. 11-12. O. conorrhiza: Fig. 11. Polar and equatorial views, Fig. 12. Detail of colpus and exine. Figs. 13-14. O. paludosa: Fig. 13. Polar and equatorial views, Fig. 14. Detail of colpus and exine. Scale bars: Fig. 7=20μm; Fig. 8=4μm; Fig. 9=10μm; Fig. 10=2μm ; Fig. 11= 10 μm; Fig. 12=05μm; Fig. 13=10μm and Fig. 14=4μm.


FIGURES 15-22. Types III and IV. SEM photographs of pollen grains of Oxalis species. Figs. 15-16. O. perdicaria: Fig. 15. Polar and equatorial views, Fig. 16. Detail of colpus and exine. Figs. 17-18. O. debilis: Fig. 17. Aberrant pollen grains, Fig. 18. Detail of a probable viable pollen grain. Figs. 19-20. O. hispidula: Fig. 19. Polar and equatorial views, Fig. 20. Detail of colpus and exine. Figs. 21-22. O. niederleinii: Fig. 21. Polar and equatorial views, Fig. 22. Detail of colpus and exine. Scale bars: Figs. 15, 16=10μm; Fig. 17=25μm; Fig. 18=10μm; Figs. 19, 20=4μm; Fig. 21=20μm and Fig. 22=4 μm.

Type I:

     Oblate spheroidal or spheroidal pollen grain; 19.15-33.25 x 18.62-35.91 μm. (Figs. 1-6).
     Colpus membrane granular. Small granules arranged in an irregular file on the colpus membrane margin. Granules more o less assembled in clusters irregularly scattered on the colpus membrane.
     Exine 1.1-2 mm thick. Intine thinner than exine. Foot layer very thin to absent. Columellae variable in high 0.30-1.87 mm. Endexine continuous but irregular in thickness (Figs. 23-25).
     Taxa included: O articulata Savigny, O.lasiopetala Zuccarini, O. floribunda Lehmann.


FIGURES 23-25. Type I. TEM photographs of pollen wall of Oxalis species. Section through wall showing tectum, columellae, foot layer, darkly stained endexine and intine white. Fig. 23. O. articulata. Fig. 24. O. lasiopetala. Fig. 25. SEM photograph of pollen wall section of O. articulata. c= columellae; en= endexine; in= intine; ft= foot layer; t= tectum. Scale bars: Figs. 23,24=1μm and Fig. 25=1,5μm.

Type II:

     Pollen grains oblate spheroidal to suboblate, rarely prolate-spheroidal; 27.26-36.17 x 29.26-38.57 μm; rarely 6-colpate (O. corniculata var purpurea) (Figs. 7-14).
     Colpus membrane with granules irregularly and densely scattered.
     Exine 1.2-3.2 μm thick. Intine almost as thick as exine or thicker. Foot layer thick as tectum, granular in contact with the intine in some cases (O. corniculata) (Fig. 26). Columellae  variable in hight, 0.2-1.2 μm. Endexine discontinuous to absent (Figs. 26-29).
     Taxa included: O. paludosa Saint Hilaire, O. conorrhiza Jacquin, O. corniculata L. var. atropurpurea Planchon and O. corniculata L. var. corniculata.


FIGURES 26-28. Type II. TEM photographs of pollen grain wall of Oxalis species. Section through the wall showing tectum, columellae, foot layer, darkly stained endexine and intine white. Fig. 26. O. corniculata var. corniculata. Fig. 27. O. paludosa. Fig. 28. O. conorrhiza. Fig. 29. SEM photograph of pollen wall section of O. conorrhiza. c= columellae; en= endexine; in= intine; ft= foot layer; t= tectum.
Scale bars: Fig. 26= 0,5μm; Fig. 27= 1μm; Fig. 28= 1,5μm and Fig. 29= 2μm.

Type III:

     Pollen grain oblate spheroidal to prolate spheroidal, rarely subprolate; 23-36.84 x 24.07-40.43 μm. (Figs. 15-20).
     Colpus membrane with granules arranged in a file on the colpus margin and assembled in central small clusters (Figs. 16, 18, 20).
     Exine 0.8-3.5 μm thick. Intine almost as thick as exine. Foot layer thinner than tectum. Columellae  variable in hight 0.28-1.66 μm. Endexine very thin and discontinuous (Figs. 30-32, 34).
     Taxa included: O. hispidula Zuccarini, O. perdicaria Bertero, O. debilis  Humboldt, Bonpland et Kunth.
     In Oxalis debilis (Figs. 17-18) most of the pollen grain are anomalous or abortive. Pollen grains of this species are 8-12 colpate (Fig. 18).


FIGURES 30-35. Types III and IV. TEM photographs of pollen grains of Oxalis species. Section through the wall showing tectum, columellae, foot layer, darkly stained endexine and intine white. Fig. 30. O. perdicaria. Fig. 31. O.debilis. Fig. 32. O.hispidula. Fig. 33. O. niederleinii. Figs. 34-35. SEM photographs of pollen wall section of Oxalis species. Fig. 34. O. hispidula. Fig. 35. O. niederleinii. c= columellae; in= intine; ft= foot layer; t= tectum.
Scale Bars: Fig. 30= 1μm; Fig. 31= 2μm; Figs. 32, 33= 1μm; Fig. 34= 5μm and Fig. 35= 2 μm.

Type IV:

     Pollen grains oblate spheroidal to spheroidal, rarely subprolate; 41.23-48.8 x 45.48-49.6 μm. Amb rounded to triangular (Figs. 21-22).
     Colpus membrane with granules arranged in a row on the colpus margin and assembled in central conspicuous clusters (Fig. 22).
     Exine 1.50-1.83 μm thick. Intine almost as thick as exine. Foot layer thinner than tectum. Columellae very straight, arranged equally distant, variable in hight 0.66-0.75 μm. Endexine very thin and discontinuous (Figs. 33, 35). Taxa included:  O. niederleinii Knuth.

Discussion

     In the present study, four pollen types have been distinguished mainly on the basis of the colpus membrane ornamentation.
     According to Huynh (1969a), pollen grains from 424 species of Oxalis can be ranged into 13 pollen-types. The species studied in this paper could be referred to the group A of the Huynh's classification according to the general morphology of the pollen grain. This author established types within each group; however, he didn't distinguish between the pollen of the long and short stamen filaments, and the sizes are too variable. For this reason, the pollen studied can not be referred to none of the Huynh's types.
     The pollen grain wall section is another feature with diagnostic value in the genus Oxalis. In the type I, the exine is thin and the foot layer is from extremely thin to absent (Figs. 23-25). The endexine is continuous. In types II, III and IV, the exine is thick and the endexine is discontinuous or absent (Figs. 26-35). However, the foot layer is thicker in the type II (Figs. 26-29) than in types III and IV (Figs. 30-32). Type IV is differentiated from type III by the presence of straight columellae that are arranged equally distant (Fig. 33).
     Anormal pollen grains with a variable number of apertures have been observed in O. corniculata var atropurpurea and O. debilis (Figs. 7, 17-18).
     The occurrence of aberrant pollen in the genus Oxalis has been reported previously by Dreyer and Van Wyk (1998). These authors showed that the aberrations always affect the number and arrangement of the colpi, but never alter the exine structure of the grain.
     Dreyer and Van Wyk (op.cit) identified 7 types or categories of pollen in the genus Oxalis, based on aperture number and arrangement. Some individual grains of
     O. corniculata var. atropurpurea could be referred to the category C (6-pantocolpate) (Fig. 7). In O. corniculata var. atropurpurea two types of pollen are observed; one type is 3-colpate and the other one is 6-pantocolpate. In concordance with these observations, Ghosh and Verma (1985) found two types of pollen in this species.
     The majority of  O. debilis  pollen grains are aberrant, with numerous variable forms, and cannot be referred to any category identified by Dreyer and Van Wyk (1998) (Figs. 17-18). This fact might be related to the absence of seed production in this species (Jehlík, 1995).
     In a previous study on orbicule morphology in Oxalis species from Buenos Aires Province  (Rosenfeldt and Galati, in press), four types were distinguished. The taxa included in each of these orbicule types are in concordance with the ones present in pollen types identified in this work.
     The taxa included in the Type I belong to the Section Articulatae, the ones of the Types II and IV are in the Section Corniculatae and the Type III agrees with the Section Ionoxalis (Lourteig, 1983, 2000).
     Our results indicate that the close morphological similarity among Oxalis species is also reflected in their pollen and orbicules.

Acknowledgement

We thank Lic. Marina M. Gotelli for reviewing the English. This work was supported by Grant Nº 2514 from CONICET, Argentina.

References

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Received on September 8, 2005.
Accepted on December 27, 2006.

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