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Post-pollination pulling power in Platanthera
As we have noted before in ContentSelect, the Orchidaceae provide an amazing range of plant-pollinator relationships. One of the variants is the provision of a reward to the pollinator, as studied by Malgorzata Stpiczynska at Lublin, Poland (pp. 191-197) in Platanthera chlorantha. This white-flowered species is pollinated by moths, attracted initially by the floral scent. However, the flowers also secrete nectar into the long labellar spurs from which the moths are able to drink. This reinforces the reward once the pollinator has arrived, and thus nectar production is seen as a mechanism for prolonging the visit of the pollinator, thereby increasing the probability of successful pollination. But the story is more complex than this. Nectar production is energetically expensive in terms of synthesis, transport and secretion, and thus increased pollination success is a trade-off against the metabolic cost of holding the pollinators’ attention. Furthermore, in P. chlorantha, nectar production is very copious: individual flowers often producing in excess of 8 microlitres (means of 6.86 and 7.84 in successive years). Nectar secretion starts about a day and a half before anthesis but, contrary to expectation and despite the supposed metabolic cost, it does not stop once the flower is pollinated. Thus, for a period of about 5 days after pollination, flowers continue to provide a nectar reward to moths attracted to the inflorescence; indeed, nectar flow increases after pollination. Eventually, it is the post-pollination senescence and death of the flower that cuts off nectar flow (and concomitantly increases nectar production in neighbouring unpollinated flowers). What then is the trade-off against the cost of production? There is evidence from other species that multiple pollinations increase fertilization and seed set efficiency (even though only one pollen grain is directly involved in fertilization), and this clearly provides a selective rationale for the post-pollination nectar production reported in P. chlorantha.
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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