| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Botanical Institute, University of Copenhagen, Gothersgade 140, DK-1123 Copenhagen K, Denmark
Received for publication June 2, 2000. Accepted for publication January 2, 2001.
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
|---|
Key Words: germination • mycorrhiza • Neuwiedia • orchid • Orchidaceae • protocorm • seedling
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
|---|
] for a thorough review of the classification history of the group). Few taxonomists (e.g., Hutchinson, 1973
) have placed apostasioids outside an orchidaceous group at family or ordinal level. On the other hand, highly contrasting views on whether the group should be included in Orchidaceae in a broad sense or recognized as a separate family have been advanced. Reveal and Hoogland (1991)
went as far as creating a separate family comprising only Neuwiedia, citing evidence for this view by Burns-Balogh and Funk (1986)
.
Orchidaceae in the strict sense are one of the most distinct of all plant families, easily recognized on floral characters. However, it has often been pointed out that the holomycotrophic protocorm stage, enabled by a presumed change of fungal symbiont from Zygomycetes to Basidiomycetes, may be the most important event in the evolution of orchids (Benzing, 1981
; F. N. Rasmussen, 1994, 2000
; H. N. Rasmussen, 1995
). Benzing (1987
, p. 50) wondered why it was not yet established whether apostasioids have this fundamental orchid characteristic. Siebe (1903)
, in his anatomical work on Apostasia and Neuwiedia, did not mention any mycorrhizal infections. This could be because he was working with fragments of herbarium specimens that may have been poorly preserved. The first notion of mycorrhiza in apostasioids seems to be by Stern, Cheadle, and Thorsch (1993)
who mentioned the "occurrence of coils of fungal hyphae in some cortical cells of some species" and depicted infected cortical root cells of Apostasia wallichii. Stern and Warcup (1994)
reported and illustrated infections by septate hyphae in root-tubercles of this species, but protocorms have so far not been observed from this group of orchids. The present paper describes the protocorm and seedling mycorrhiza of Neuwiedia veratrifolia Blume.
|
MATERIALS AND METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
|---|
|
25°C in a portable exsiccator (low pressure drying apparatus) and transferred after 24 h to 70% ethanol.
The FAA fixations were stepwise dehydrated through tertiary butanol and embedded in Paraplast. Sections 10 µm thick were cut on a Leitz Wetzlar 1515 rotary microtome, stained with Safranine O, Fast Green, or Toluidine Blue, and mounted using Depex (Ruzin, 1999
).
Fixations of GA-PFA were prepared for GMA (glycol methacrylate). Sections 3 µm in thickness were cut on an LKB Bromma 2218 Historange microtome (Analytical Instrument Recycle, Golden, Colorado, USA), stained with Toluidine Blue and PAS/ABB (periodic acid-Schiff's reagent/aniline blue black), and mounted using DPX (Ruzin, 1999
). Sections were observed and photographed in a Reichert Jung Polyvar microscope (Mikrovid GmbH, Arnsberg, Germany).
|
RESULTS AND DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
|---|
Seedlings consisted of a subterranean protocorm with an apical, leafy shoot (Fig. 1). The shoot carried a number of scale leaves and one or two small foliage leaves at the top, appearing above ground. Short thick roots developed from the axils of the lowermost scale leaves. Foliage leaf venation, cell pattern of margin, and epidermal cells, as seen in stereomicroscope, correspond to that of adult N. veratrifolia and thus supported the identification. Adult plants do not have corms or tubers.
|
6–15 mm long and 4 mm thick. The surface was covered with hairy warts, forming an annular pattern. Epidermis and the outer layer of the thick cortex (Fig. 3) was little infected except under the warts; the main infection was located in the middle and inner cortex except for the endodermis and pericycle around the stele. Mid-cortex infection appeared to consist of loose or dense pelotons of live hyphae; the fungi were clampless and appeared to have dolipores (Fig. 4). Monilioid cells were regularly seen in the pelotons (Fig. 5). Infection in the inner cortex was disintegrating, the pelotons aggregating in the center of the cell, and collapsing into "clumps" while a secondary infection took place (Fig. 6). Nuclei of these large cells were hypertrophied and amorphous and tended to segregate into several sections (Fig. 5). Hyphae were passing through nuclei or along grooves in the nuclear envelope (Fig. 6).
The anatomical and cytological structure in these protocorms, thus, coincides closely with that of other orchid protocorms with tolypophagy (Rasmussen, 1995
, and sources therein), in which fungal hyphae are believed to become digested with necrotrophic and possibly also biotrophic transfer of organic products from the fungal mycelium to the plant. This provides strong evidence that the seedling biology in Neuwiedia closely resembles that of other orchids. The cytological appearance of the endophytes indicates that they could belong to Rhizoctonia. Isolates based on seedling pelotons were obtained from all three locations, and a closer description of the living cultures is in preparation.
Burgeff (1936)
saw structures similar to the mycotrophic protocorm of N. veratrifolia in, e.g., Cymbidium sp., including the hairy warts. Neuwiedia protocorms also resemble the mycotrophic root tubercles described by Stern and Warcup (1994)
in Apostasia spp. However, such root appendages are not known from Neuwiedia, and there was nothing to suggest that the structures we found were detached from roots of established plants. The proliferation/fragmentation of the hypertrophied nuclei in digestion cells were also observed by Burgeff (1936)
in roots of Platanthera chlorantha and by Stern and Warcup (1994)
in tubercles of Apostasia.
The verification of a protocorm stage in the seedling biology of Neuwiedia strongly corroborates the view that Orchidaceae, also in the broad sense, are indeed a monophyletic group. Neuwiedia may be basal in orchid phylogeny, as indicated by analyses of morphological as well as molecular characters (Judd, Stern, and Cheadle, 1993
; Freudenstein and Rasmussen, 1999
; Cameron et al., 1999
), but the mycorrhiza or protocorm characters are not different from types known from higher orchids. The protocorms seem to match the elongated protocorms described from several genera by Veyret (1974)
.
Whether the orchid clade should be regarded as one family with apostasioids as a subfamily or as an order subdivided into three or more families is not a question of phylogeny, rather an assessment of overall diversity and taxonomic tradition. However, the observation of protocorms and typical orchid mycorrhiza in Neuwiedia leaves only the number of stamens as possible morphological argument for distinction of more than one family of orchids. We recommend the one-family view as advanced in the monograph by de Vogel (1969)
and in recent taxonomic treatments by Dressler (1993)
and Pridgeon (1999)
.
|
FOOTNOTES |
|---|
2 Author for reprint requests (KimAK{at}bot.ku.dk
).
3 Current address: Danish Forest and Landscape Research Institute, Hørsholm Kongevej 11, DK-2970 Hørsholm, Denmark.
|
LITERATURE CITED |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
|---|
———. 1987 Major patterns and processes in orchid evolution: critical synthesis. In J. Arditti [ed.], Orchid biology IV, 34–77. Cornell University Press, Ithaca, New York, USA
Burgeff, H. 1936 Samenkeimung der Orchideen. Gustav Fischer, Jena, Germany
Burns-Balogh, P., and V. Funk. 1986 A phylogenetic analysis of the Orchidaceae. Smithsonian Contributions to Botany 61: 1–79
Cameron, K. M., M. W. Chase, M. W. Whitten, P. J. Kores, D. C. Jarrell, V. A. Albert, T. Yukawa, H. G. Hills, and D. H. Goldman. 1999 A phylogenetic analysis of the Orchidaceae: evidence from rbcL nucleotide sequences. American Journal of Botany 86: 208–224
de Vogel, E. F. 1969 Monograph of the tribe Apostasieae. Blumea 17: 313–350
Dressler, R. L. 1993 Phylogeny and classification of the orchid family. Cambridge University Press, Cambridge, UK
Freudenstein, J. V., and F. N. Rasmussen. 1999 What does morphology tell us about orchid relationships? A cladistic analysis. American Journal of Botany. 86: 225–248
Holmgren, P. K., N. H. Holmgren, and L. C. Barnett. 1990 Index herbariorum. Part I: The herbaria of the world, 8th ed. New York Botanical Garden, Bronx, New York, USA
Hutchinson, J. 1973 The families of flowering plants, 3rd ed. Clarendon Press, Oxford, UK
Judd, W. S., W. L. Stern, and V. I. Cheadle. 1993 Phylogenetic position of Apostasia and Neuwiedia (Orchidaceae). Botanical Journal of the Linnean Society 113: 87–94[CrossRef]
Pridgeon, A. M. 1999 Apostasioideae. In A. M. Pridgeon, P. J. Cribb, M. W. Chase, and F. N. Rasmussen [eds.], Genera orchidacearum, vol. 1, 94–104. Oxford University Press, Oxford, UK
Rasmussen, F. N. 1994 Aspects of orchid phylogeny. In A. M. Pridgeon [ed.], Proceedings of the 14th World Orchid Conference Glasgow 1993, 52–60. Her Majesty's Stationary Office, Edinburgh, UK
———. 2000 Ins and outs of orchid phylogeny. In K. Wilson and D. A. Morrison [eds.], Monocots: systematics and evolution, 426–431. CSIRO (Commonwealth Scientific and Industrial Research Organisation), Collingwood, Australia
Rasmussen, H. N. 1995 Terrestrial orchids: from seeds to mycotrophic plant. Cambridge University Press, Cambridge, UK
Reveal, J. L., and R. D. Hoogland. 1991 Validation of three family names in the Magnoliophyta. Bulletin du Muséum national d'Histoire naturelle, 4e série. 13 section B, Adansonia: botanique, phytochimie 1–2: 91–93
Ruzin, S. E. 1999 Plant microtechnique and microscopy. Oxford University Press, New York, New York, USA
Siebe, M. 1903 Ueber den anatomische Bau der Apostasiinae. Hörning & Berkenbusch, Heidelberg, Germany
Stern, W. L., V. I. Cheadle, and J. Thorsch. 1993 Apostasiads, systematic anatomy, and the origins of Orchidaceae. Botanical Journal of the Linnean Society 111: 411–455[CrossRef]
———, and J. H. Warcup. 1994 Root tubercles in apostasiad orchids. American Journal of Botany 81: 1571–1575[CrossRef][ISI]
Veyret, Y. 1974 Development of the embryo and the young seedling stages of orchids. In C. Withner [ed.], The orchids: scientific studies, 223–265. Wiley, New York, New York, USA
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
