Aspects of the physiological ecology of the Western-Australian ruderal orchid, Microtis media R.Br, with special reference to the functions of its mycorrhizal fungi

Wei-Han Lim

    Research output: ThesisDoctoral Thesis

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    This study focuses on the common mignonette orchid, Microtis media (, which in recent decades, has successfully expanded beyond its natural habitats, often into novel and disturbed settings such as garden and horticultural beds, and will explore aspects of the physiology of M. media that may have contributed to the success of the orchid. Particular emphasis was placed on elucidating the functionality of mycorrhizal fungi associating with M. media.

    Investigations into the basic eco-physiology of M. media have highlighted the lack of reproductive constraint within the species, with seed production occurring through autogamy and allogamy, and vegetative reproduction occurring through the formation of daughter tubers. It was also determined that there were no barriers to M. media seed germination, with the lack of photo-inhibition on germination, and the occurrence of asymbiotic seed germination on water agar. However, the lack of normal development in these asymbiotic seedlings also indicated that associations with mycorrhizal fungi were essential in order to allow for the normal development of M. media protocorms.

    Characterizing the symbiotic germination process of M. media highlighted the extremely rapid rate of M. media protocorm development, after being colonized by an appropriate mycorrhizal symbiont. Within four weeks of sowing, M. media seeds developed into photosynthetic stage five protocorms, and the in-vitro flowering of multiple symbiotic seedlings was observed by 16 weeks of age, often followed with the formation of seeds by autogamy. This accelerated development also shows that M. media is able to mature rapidly in the presence of a compatible fungal partner under optimal conditions, which may explain the rapid proliferation observed in novel settings.

    Molecular identification of mycorrhizal fungi associating with M. media revealed that the orchid predominantly associated with species of the fungal genus Tulasnella, which are cosmopolitan soil saprophytes and orchid symbionts. Despite being from the same genus, the Tulasnella isolates of this study varied in their abilities to promote M. media germination and growth, potentially reflecting differences in ecological functionality.

    The ability to associate with multiple Tulasnella isolates has the potential to allow for the ruderal expansion of M. media if these are widespread mycorrhiza. The proliferation of M. media in the modified mulched environments of garden beds would suggest that not only is Tulasnella a genus of disturbance tolerant fungi, but that the formation of a symbiosis with M. media is also a robust process in disturbed areas.

    Investigations of the phosphorus (P) physiology of M. media revealed evidence of preferential accumulation of the element in its seeds, which can be inferred to be an adaptation to maximise seedling survivability. Demonstration of the ability of the mycorrhizal symbiont of M. media to utilise both organic and inorganic forms of the element, and to facilitate the transport of P to the developing protocorms and to adult plants establishes the likely functional importance of the symbiosis in the essentially P-starved natural environments colonized by the orchid.

    Analysis of carbon flows in M. media demonstrated the bidirectional flow of nutrients in the orchid-mycorrhizal symbiosis. ‘Up-flow’ of carbon (assimilated from cellulose) from mycorrhizal fungi to developing protocorms of M. media was established. In addition, the ‘down-flow’ of carbon (fixed by photosynthesis) from orchid to mycorrhizal fungi establishes the bi-directional movement of nutrients between M. media and its mycorrhizal symbiont, and is indicative of the mutualistic nature of this symbiosis. This study also demonstrated the inter-generational sharing of carbon resources in M. media, where carbon assimilated by photosynthesizing adults is transferred or shared with non-photosynthetic protocorms by way of the mycorrhizal symbiont.

    It is concluded that the fungal pathways identified in this study should contribute significantly to the ability of the orchid to exploit the minimal phosphorus reserves in bushland soil. It is proposed that the mycorrhizal association may thus provide a fundamental contribution to the success of M. media as a colonist of both natural and disturbed soil systems throughout Western-Australia. Further studies will be required to determine the relative importance of other attributes of its biology, in particular the shortening of the period between germination to seed-set, and aspects of its pollination biology.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Publication statusUnpublished - 2015

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