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Ultrastructure and cytochemistry of the hydrophilous pollen of Lepilaena (Zannichelliaceae)
Micron, Vol.]3, No.3, pp.339-340,
]982.
0047-7206/82/030339-02503.00/0 Pergamon Press Ltd.
Printed in Great Britain
ULTRASTRUCTURE
AND
C.A. School
of
CYTOCHEMISTRY OF THE HYDROPHILOUS (ZANNICHELLIACEAE)
MCCONCHIE,
R.B.
KNOX
Botany, University Victoria 3052,
and
S.C.
of Melbourne, Australia
POLLEN
OF
LEPILAENA
DUCKER Parkville,
Aquatic angiosperms have an assemblage of adaptations to enable reproduction in their submerged environment. These include reduction of floral parts (Sculthorpe, 1967), absence of the endothecium layer in the anther and modifications of shape and wall structure of the pollen (Pettitt and Jermy, 1975). In one group of aquatic monocotyledons, the seagrasses, these adaptations involve filiform shape or long chains of spherical grains and loss or reduction of the outer exine (Pettitt et al. 1981). Are these adaptations also found in other aquatic monocotyledons? To answer this question, we have studied the cytology of the pollen grain of two species of Lepilasna. L. bilo~lari8 is pollinated entirely under water, while L. cy~indrocarpa is pollinated on the water surface. The pollen grains of both species are spherical, 34-45 ~m in diameter, and are inaperturate. The degree of development of the exine layer differs. In L. cylindrocarpa, the semi-tectate exine forms a reticulate network over the grain surface, and comprises tectum, bacula and nexine layers (Fig. i). Coating the exine are a series of lamellar or vesicular electron dense components, of tapetal origin. The development of the exine of L. biloculari8 is much reduced, and is in ultrastructural appearance similar to Zannichellia pal~stri8 (Pettitt and Jermy, 1975). L. biloculari8 has a sparse network of spinules over the nexine which are covered by a thick layer of mucilage, evident by its electron dense lamellar appearance (Fig. 2). In both species, immediately beneath the nexine, an electron-opaque layer covers the relatively thin microfibrillar intine, c.0.5 ~m in thickness. The exine is detectable by the same cytochemical techniques which reveal its presence in pollen of terrestrial monocots, and is positively stained by auramine O, scarlet R and Sudan black B (lipids); basic fuchsin (negative charges) and autoflucrescence. The intine stains with calcofluor, PAS, and alcian blue (indicative of acidic and neutral polysaccharides) and Coomassie blue (proteins). A feature of intine staining is that the entire layer reacts uniformly with these dyes. The present study supports the trend for reduction in exine structure in species with hydrophilous pollination. There is a reduction of the exine in L. bilocularis, a species with hydrophilous pollination. In contrast, L. aylindocarpa is surface pollinated and has a well developed exine. These results contrast with hydrophilous and anemophilous species of Potamogeton in which few differences in pollen wall structure were found (Pettitt and Jermy 1975). The question then arises whether differences in pollen wall structure are reflected in adaptations of the stigma which receives the pollen. The stigma of L. cylindrocarpa is spathulate, and floats on the surface, pollen interception being mediated by surface tension (Fig. 3). In contrast, L. biloculari8 stigmas are laciniate, with multi-directional branches enhancing pollen interception underwater (Fig. 4). Further study of the stigma and its pollen interactions is likely to increase understanding of adaptations for underwater pollination.
Pettitt, J.M. and Jermy, A.C. 1975. Pollen in hydrophilous angiosperms. Micron 5: 377-405. Pettitt, J.M., Ducker, S.C. and Knox, R.B. 1981. Submarine pollination. Sci. Amer.244: 94-101. Sculthorpe, C.D. 1967. The Biology of Aquatic Vascular Plants. Edward Arnold Ltd., London. 339
340
C.A.
McConchie, R. B. Knox and S. C. Ducker
Fig. 1 Lepilaena cylindrocar~a radial section of pollen wall; t-tectum; b-baculum; n-nexine; e- electron dense layer; i- intine; p-plasmalemma; pc-pollen coat materials.
Fig. 3 L. cylindrocarpa pollinated spathulate stigma.
Fig. 2 L. biloculari8 radial section of pollen wall; abbreviations as Fig. 1 except s-spinule.
Fig. 4
L. biloculari8 laciniate stigma
]982.
0047-7206/82/030339-02503.00/0 Pergamon Press Ltd.
Printed in Great Britain
ULTRASTRUCTURE
AND
C.A. School
of
CYTOCHEMISTRY OF THE HYDROPHILOUS (ZANNICHELLIACEAE)
MCCONCHIE,
R.B.
KNOX
Botany, University Victoria 3052,
and
S.C.
of Melbourne, Australia
POLLEN
OF
LEPILAENA
DUCKER Parkville,
Aquatic angiosperms have an assemblage of adaptations to enable reproduction in their submerged environment. These include reduction of floral parts (Sculthorpe, 1967), absence of the endothecium layer in the anther and modifications of shape and wall structure of the pollen (Pettitt and Jermy, 1975). In one group of aquatic monocotyledons, the seagrasses, these adaptations involve filiform shape or long chains of spherical grains and loss or reduction of the outer exine (Pettitt et al. 1981). Are these adaptations also found in other aquatic monocotyledons? To answer this question, we have studied the cytology of the pollen grain of two species of Lepilasna. L. bilo~lari8 is pollinated entirely under water, while L. cy~indrocarpa is pollinated on the water surface. The pollen grains of both species are spherical, 34-45 ~m in diameter, and are inaperturate. The degree of development of the exine layer differs. In L. cylindrocarpa, the semi-tectate exine forms a reticulate network over the grain surface, and comprises tectum, bacula and nexine layers (Fig. i). Coating the exine are a series of lamellar or vesicular electron dense components, of tapetal origin. The development of the exine of L. biloculari8 is much reduced, and is in ultrastructural appearance similar to Zannichellia pal~stri8 (Pettitt and Jermy, 1975). L. biloculari8 has a sparse network of spinules over the nexine which are covered by a thick layer of mucilage, evident by its electron dense lamellar appearance (Fig. 2). In both species, immediately beneath the nexine, an electron-opaque layer covers the relatively thin microfibrillar intine, c.0.5 ~m in thickness. The exine is detectable by the same cytochemical techniques which reveal its presence in pollen of terrestrial monocots, and is positively stained by auramine O, scarlet R and Sudan black B (lipids); basic fuchsin (negative charges) and autoflucrescence. The intine stains with calcofluor, PAS, and alcian blue (indicative of acidic and neutral polysaccharides) and Coomassie blue (proteins). A feature of intine staining is that the entire layer reacts uniformly with these dyes. The present study supports the trend for reduction in exine structure in species with hydrophilous pollination. There is a reduction of the exine in L. bilocularis, a species with hydrophilous pollination. In contrast, L. aylindocarpa is surface pollinated and has a well developed exine. These results contrast with hydrophilous and anemophilous species of Potamogeton in which few differences in pollen wall structure were found (Pettitt and Jermy 1975). The question then arises whether differences in pollen wall structure are reflected in adaptations of the stigma which receives the pollen. The stigma of L. cylindrocarpa is spathulate, and floats on the surface, pollen interception being mediated by surface tension (Fig. 3). In contrast, L. biloculari8 stigmas are laciniate, with multi-directional branches enhancing pollen interception underwater (Fig. 4). Further study of the stigma and its pollen interactions is likely to increase understanding of adaptations for underwater pollination.
Pettitt, J.M. and Jermy, A.C. 1975. Pollen in hydrophilous angiosperms. Micron 5: 377-405. Pettitt, J.M., Ducker, S.C. and Knox, R.B. 1981. Submarine pollination. Sci. Amer.244: 94-101. Sculthorpe, C.D. 1967. The Biology of Aquatic Vascular Plants. Edward Arnold Ltd., London. 339
340
C.A.
McConchie, R. B. Knox and S. C. Ducker
Fig. 1 Lepilaena cylindrocar~a radial section of pollen wall; t-tectum; b-baculum; n-nexine; e- electron dense layer; i- intine; p-plasmalemma; pc-pollen coat materials.
Fig. 3 L. cylindrocarpa pollinated spathulate stigma.
Fig. 2 L. biloculari8 radial section of pollen wall; abbreviations as Fig. 1 except s-spinule.
Fig. 4
L. biloculari8 laciniate stigma