Post-pollination emission of a repellent compound in a sexually deceptive orchid: a new mechanism for maximising reproductive success?

The flowers of the sexually deceptive orchid Ophrys sphegodes are pollinate d by pseudocopulating males of the solitary bee Andrena nigroaenea. We inve stigated the changes in odor emission and reduced attractiveness that occur after pollination in these plants. We analyzed floral odor of unpollinated and pollinated flowers by gas chromatography and compared relative and abs olute amounts of electrophysiologically active compounds. Headspace odor sa mples of O. sphegodes flowers showed a significant increase in absolute and relative amounts of all-trans-farnesyl hexanoate after pollination. Flower extracts also indicated an increase of farnesyl hexanoate after pollinatio n. The total amount of the other physiologically active odor compounds decr eased slightly. Farnesyl hexanoate is a major constituent of the Dufour's g land secretion in females of the pollinator bees, A. nigroaenea, where it f unctions in the lining of the brood cells. Furthermore, this compound lower s the number of copulation attempts in males. In dual-choice tests, we show ed that flowers artificially scented with an amount of farnesyl hexanoate e qual to the increased amount after pollination were significantly less attr active than flowers treated with solvent only. We propose that the increase d production of farnesyl hexanoate in pollinated flowers is a signal to gui de pollinators to unpollinated flowers of the inflorescence, which represen ts a new mechanism in this pollination system.