Chinese researchers have solved the mystery of
the function of sheaths in flowering orchids.
In a paper published in PLOS ONE,
the team led by Zhong-Jian Liu at the Orchid Conservation & Research Center
of Shenzhen and Laiqiang Huang at Tsinghua University found the perch to be
essential to orchid reproduction.
In many orchids such as Coelogyne rigida, the basal axis of the pendulous,
multi-flowered inflorescence is covered by multiple coriaceous sheaths (bracts)
forming a clavate cylindrical handle whose function has long been a puzzle and
remained mysterious until today.
Researchers have discovered that the
sheaths-wrapped handle serves as the specialized perch (landing platform) to
attract, secure and position foraging sunbird, Aethopyga gouldiae, for orderly
collection and dispersal of pollinaria, attached to different spots of its
beak, resulting in efficient cross-pollination and fruiting. The perch-enabled cross-pollination
by sunbirds accounts for essentially all the seed production of this orchid,
which was largely abolished by sheaths removal (perch damage) that reduced the
visitations by sunbirds markedly both in frequency and duration, revealing an
essential role of the perch in assuring the species' reproductive success by
crossing. Intriguingly, C. rigida, while self-incompatible, has a bi-modal
pollination system. It is also self-pollinated by insects, honeybee and wasp,
that use the floral lip common in orchids as visiting plate, leading to
infertile self-pollination which decreases (wastes) pollinaria and ovules
otherwise available for fruitful cross-pollination, incurring high mating cost
(gamete discounting). However, the insect-mediated fruitless self-pollination
is offset by the efficient cross-pollination by sunbird.
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