Summary• The dependence of seeds of terrestrial orchids on specific fungi for germination provides a means of locating these fungi in the wild and to investigate the role of appropriate fungi in the germination of orchid seed and development of seedlings under natural field conditions.• Seed baits, comprising orchid ( Caladenia arenicola ) seed enclosed in fine nylon mesh, were placed at sample points along four transects through two orchid populations in bushland in Western Australia. Seed germination was scored and compared with adult orchid plant distribution and soil factors.• A small fraction of available seed (< 1%) germinated to a stage of tuber formation where survival over the subsequent dry season would have been possible. Germination increased in the vicinity of adult C. arenicola plants, but other factors, such as soil potassium levels and presence of leaf litter, were also correlated with seed germination.• The measurement of the spatial variability in germination events within an orchid habitat demonstrated the availability of new recruitment sites. This information is required to assess the natural recruitment capacity and the potential for orchid reintroduction in natural habitats.
The impact of seed drying, seed storage and development of testing procedures
for seed viability assessment was undertaken for a selection of common taxa
with congeners that are rare and endangered (Caladenia,
Diuris, Pterostylisand
Thelymitra). Freshly collected seed showed significantly
lower levels of germination compared with seed that had been subjected to
drying over silica gel for 24 h. Seed dried over silica gel for 24 h and
plunged into liquid nitrogen exhibited a further increase in germination
levels. Germination of seed stored at 4, 18 or 22˚C for 1 year was
substantially higher than freshly collected seed (4 weeks after dehiscence),
but germination was highest overall after storage of dried seed in liquid
nitrogen (–196˚C). Mycorrhizal fungi that promote the germination
and growth of plants were also successfully preserved in liquid nitrogen. The
use of cryoprotectants on fungal isolates had no observable deleterious
effects on fungal regeneration. Histochemical staining procedures
(tetrazolium, fluorescein diacetate and Evans blue) substantially
overestimated seed viability, relative to symbiotic seed germination, for most
seed treatments indicating a need for re-evaluation of the effectiveness of
staining procedures for testing viability. The implications of the long-term
ex situ storage of orchid seed and fungal symbionts for
the conservation of endangered orchids is discussed.
The conservation of wild orchid populations may depend on the establishment of propagated orchids to field sites to help sustain depleted populations if natural recruitment is not successful. However, very little is known about biotic factors which influence the establishment of terrestrial orchid seedlings in natural habitats. An experiment was established to measure the survival of six orchid species during their first growing season following transplantation to a West Australian urban bushland with a Banksia and Eucalyptus canopy and understorey dominated either by weeds or native vegetation. Symbiotically germinated orchid seedlings raised in the laboratory for 5 months before planting were established in adjacent field sites with high or low weed cover. There was a gradual mortality of seedlings at field sites throughout the growing season, primarily owing to insect grazing, and this was not affected by the enclosure of seedlings by wire mesh or shade cloth. Overall rates of survival varied from 49% for Microtis media R.Br., a species capable of growing in disturbed habitats, to 21% for Caladenia arenicola Hopper & A.P.Brown, the most common native orchid at these sites. However, not all surviving seedlings produced a tuber, so their expected rate of survival after the next dry season was reduced further. The factors having the greatest impact on seedling survival were site aspect (slope and canopy cover), weed cover and orchid species respectively. Orchid seedling survival was not well correlated with the presence of existing orchids of the same species at the same sites or the presence of compatible fungi in soil at these sites (simultaneously measured by orchid seed baiting).
The establishment of five species of temperate terrestrial orchids (Caladenia arenicola Hopper & A.P.Brown, Diuris magnifica D.L.Jones, D. micrantha D.L.Jones, Pterostylis sanginea D.LJones & M.A.Clem. and Thelymitra manginiorum ms) in natural habitat through in situ seed sowing, or by planting of seedlings and dormant tubers, was evaluated. Seed of the Western Australian temperate terrestrial taxa, Caladenia arenicola and Pterostylis sanguinea germinated best when sown into soil inoculated with mycorrhizal fungi at field sites but failed to develop the tubers necessary for surviving summer dormancy. However, seedling survival improved when actively growing symbiotic seedlings were transferred to natural habitat during the growing season. Caladenia arenicola and P. sanguinea seedlings survived the initial transfer to field sites but only P. sanguinea survived into the second growing season. Highest survival was obtained by translocating dormant tubers of C. arenicola and Diuris magnifica, with D. magnifica persisting at the site 5 years after translocation. However, outplanted C. arenicola survived for only 2 years. In another trial, where seedlings and dormant tubers of a rare orchid Thelymitra manginiorum were translocated into eucalypt woodland, 18% persisted after 5 years. The rare orchid D. micrantha exhibited the highest survival rates, with greater than 80% of tubers surviving 5 years after transfer of mature dormant tubers to field sites. This study highlights the benefit of using optimised methods for seedling production by symbiotic germination and nursery growth to produce advanced seedlings or dormant tubers to maximise the survival of translocated plants. It also demonstrates the need to consider different strategies when dealing with individual species.
Effective plant conservation involves careful con- sideration and difficult choices when investing limit- ed resources to conservation programs and policies. The conservation practice must integrate the under- standing of existing and future environmental threats, taxonomic distinctiveness, numbers of individuals in populations, reproductive biology, ex situ propagation and the maintenance of evolutionary processes influ- encing population distribution patterns.
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