The simplest floral fragrance and unusual osmophore structure in Cyclopogon elatus (Orchidaceae).

WIEMER, PÍA; MORÉ, M.; BENITEZ-VIEYRA, S.; COCUCCI A. A.; RAGUSO, R.; SÉRSIC, A.

PLANT BIOLOGY

Año: 2007

1435-8603

Abstract Background and aims To explore how floral features associated with scent production (complexity and volatility) and scent gland localization might differ from what is known in euglossine- pollinated and sexually-deceptive orchids, we studied gland morphology and anatomy as well as the chemical composition of the floral fragrance in the sweat-bee-pollinated Cyclopogon elatus, a terrestrial orchid that uses scent solely as a secondary attractant. This is apparently the first such analysis for any Cyclopogon species and one of very few studies in which odour and osmophore are characterized in an orchid with nectar as reward. Methods Structures responsible for floral scent production were localized with neutral red staining and histochemical assays for lipids and starch. Their morphology and anatomy were studied with SEM and LM-thin sections, respectively. Fragrance samples were collected using SPME fibres, and analyzed with GC-MS. Key results Anatomical evidence suggests that two parallel oval-shaped patches of unicellular trichomes on the abaxial surface of the labellum are osmophores. These were rich in stored lipids while the parenchyma surrounding the vascular bundles contained starch. Only freshly-open flowers produced odours, while buds and withered flowers were scentless. The chemical composition of the odour was dominated (>99.8 %) by a single compound, trans-4,8-dimethyl-nona-1,3,7-triene (DMNT). Conclusions Gland anatomy and position on the outside of the perianth are unusual for scent glands in general. The fragrance differed from well studied orchid systems in that it lacked the fragrance complexity of euglossine-pollinated orchids and the low volatility hydrocarbons of sexually deceptive orchids. The presence of DMNT, a nearly ubiquitous compound in herbivore-induced vegetative emissions and one of the major floral volatiles of Yucca, is not surprising in view of hypotheses about the evolutionary origin of flower scents, suggesting that wound volatiles were utilized as kairomonal attractants by florivores whose activities resulted in pollination.   Keywords: Osmophore, floral scent, GC-MS, trans-4,8-dimethylnona-1,3,7-triene, DMNT, pollinator attraction, Cyclopogon elatus (Sw.) Schltr, Halictidae,