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Floral Scent in Salix L. and the Role of Olfactory and Visual Cues for Pollinator Attraction of Salix caprea L.
(2007)
- The chemical composition of floral scent of different Salix species as well as its inter- and intraspecific variability were analysed. The role of floral scent for attracting flower visitors was examined in S. caprea by means of electroantennographic studies and bioassays. The relevance of different pollen vectors for the reproductive success of this willow species was examined by pollination experiments. The floral scent from 34 Salix species was collected using a dynamic headpace MicroSPE method and analysed by gas chromatography coupled with mass spectrometry. Isoprenoids and aromatic compounds were identified as typical floral scent compounds. Particularly 1,4-dimethoxybenzene and trans-β-ocimene were responsible for the interspecific variation. In some species gender-specific differences in the floral scent pattern were determined. The circadian rhythm of floral scent emission was studied in S. caprea. Generally, a larger quantity of floral scent was emitted during the day than at night. Primarily, lilac aldehyde was responsible for significant circadian differences. It is produced in higher quantities at night, which could be interpreted as an adaptation to nocturnal flower visitors – e. g. Orthosia gothica, which responded strongly to lilac aldehyde in bioassays. Insect species from the orders Coleoptera, Diptera, Hemiptera, Hymenoptera, and Lepidoptera were identified as flower visitors of S. caprea. The visiting frequency was highest for Hymenoptera (primarily honeybees, bumblebees, and solitary wild bees) and Lepidoptera (many nocturnal and only few diurnal species). During the day primarily bees and butterflies dominated, whereas during and after dawn moths were the only flower visitors. Altogether, nocturnal flower visitors had a lower frequency than diurnal ones. Floral scents of S. caprea and S. atrocinerea were tested for their electrophysiological activity on the oligolectic wild bee Andrena vaga by gas chromatography coupled with electroantennography. Altogether 16 floral scent compounds of both Salix species were physiologically active. The main component 1,4-dimethoxybenzene led to the strongest antennal signals. 1,4-Dimethoxybenzene and lilac aldehyde are main floral scent compounds of S. caprea. 1,4-Dimethoxybenzene dominated day and night, but its proportion decreased over night, while the proportion of lilac aldehyde increased at night. Both compounds elicited antennal signals in both Apis mellifera, the most frequent diurnal visitor, and Orthosia gothica, the most frequent nocturnal visitor. In biotests, the honeybee was attracted most strongly by 1,4-dimethoxybenzene; in contrast, O. gothica was attracted most strongly by lilac aldehyde. Biotests highlighted that male S. caprea is more attractive to A. mellifera than female ones. Considering the relatively high similarity of floral scent of both genders, this is most likely due to visual cues. Because of their conspicuously yellow-coloured pollen, the male catkins are obviously visually more attractive than the greenish female catkins. The genders differed also in the sugar composition of nectar. While females produced hexose-rich nectar, males had sucrose-dominated nectar. The higher visit frequency to male S. caprea may be of ecological importance, since it increases the probability that flower visitors collect sufficient pollen – of possibly several male individuals – before visiting a female individual. Thus not only the probability for successful pollination and fertilisation, but also the genotypic variability might increase within a population.
