Pollination biology studies of the endangered orchid Cypripedium japonicum were conducted in its natural habitat using pollinator observation and hand-pollination experiments. The observed fruit set was as follows: artificial outcross-pollinated, 100%; artificial self-pollinated, 100%; pollinator-excluded, 0%; and emasculated flowers, 0%. These results show that this species, although self-compatible, is neither autogamous nor agamospermous. The fruit set for open-pollinated flowers was 14.9%, which suggests that the study population was subject to pollinator limitation. The nectarless flowers of C. japonicum were exclusively visited and pollinated by the queens of two bumblebee species (Bombus ardens and B. diversus diversus). It is probable that the nectarless flowers of C. japonicum attract pollinators through a generalized food deceptive system.
Larger inflorescences in reward-producing plants can benefit plants by increasing both pollinator attraction and the duration of visits by individual pollinators. However, ultimately, inflorescence size is determined by the balance between the benefits of large inflorescences and the increased cost of geitonogamy. At present, little is known about the relationship between inflorescence size and fecundity in deceptive plants. Given that pollinators are likely to leave inflorescences lacking rewards quickly, it seems unlikely that longer pollinator visits and the risk of geitonogamy would be strong selective pressures in these species, which indicates that pollinator attraction might be the most important factor influencing their inflorescence size. Here we examined the pollination ecology of the deceptive orchid Cephalanthera falcata in order to clarify the effects of inflorescence size on the fruit set of this non-rewarding species. Field observations of the floral visitors showed that C. falcata is pollinated by the andrenid bee Andrena aburana, whilst pollination experiments demonstrated that this orchid species is neither autogamous nor apogamous, but is strongly pollinator dependent. Three consecutive years of field observations revealed that fruit set was positively correlated with the number of flowers per inflorescence. These results provide strong evidence that the nectarless orchid C. falcata benefits from producing larger inflorescences that attract a greater number of innate pollinators. Large inflorescences may have a greater positive effect on fruit set in deceptive plants because a growing number of studies suggest that fruit set in reward-producing plants is usually unaffected by display size.
Calanthe discolor is a Japanese terrestrial orchid that is cultivated for its beautiful flowers arranged in racemose inflorescences. Although, its propagation for horticultural purposes has been studied extensively, resulting in the successful production of seedlings little is known about the pollinators and breeding system of C. discolor in its natural habitat. The current study, which combined field observations and pollination experiments, was conducted to gain further insight into the reproduction of this important orchid species. Three bee species: Eucera nipponensis, Osmia cornifrons and Apis cerana japonica, were found to be effective pollinators, transferring the pollinaria on their heads. However, pollination experiments also revealed that this species was self-compatible, although it was neither autogamous nor apogamous. The fruit set for the open-pollinated flowers was less than 10%, suggesting a high degree of pollinator limitation, possibly as a result of the deceptive nature of this species. These results provide evidence that pollinator specificity is the primary mechanism of reproductive isolation between C. discolor and its close relative C. striata, because the latter species is known to be exclusively pollinated by carpenter bee.
Owing to their excessive harvest for ornamentals, medicines, and foods, and the destruction of their natural habitats, many orchids are threatened globally (Hinsley et al. 2018). Consequently, supplemental artificial crossing has been recommended to restore the population of several endangered orchid species (Roberts 2003). However, even when pollination is successful, the resulting orchid seeds remain susceptible to herbivory by seed-feeding insects, which often destroy relatively large portions of their food supply (Janzen 1971). Nevertheless, the impact of seed predators on the reproductive ecology of rare orchid species is often overlooked (but see Kimura et al. 2009), especially when compared with the attention given to the impact of pollinators (Roberts 2003, Gale 2007). While investigating their reproductive biology, we frequently observed the seed-feeding fly Japanagromyza tokunagai laying eggs in the ovaries of various Japanese orchids during or shortly after anthesis (Fig. 1). Less than 1 month from oviposition, the hatched larvae pupated within the fruit capsule. The fly can infest various orchids (Sasakawa and Matsumura 1998), whose flowering season differs significantly. Orchids are the only known hosts of
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