Variation in floral scent compounds recognized by honeybees in Brassicaceae crop species

Abstract

Floral scent attracts pollinators. We investigated the floral scent compounds recognized by pollinators in six Brassica crop species, including allogamous species with different genomes and autogamous species with two parental genomes and radish (Raphanus sativus). Biologically active compounds recognized by honeybees were screened from all floral compounds by combined gas chromatography-electroantennogram analysis and their profiles were determined by gas chromatography-mass spectrometry. Fourteen of the 52 compounds were active. All accessions had more than two active compounds, but the compounds greatly differed between the two genera. On the basis of similarities in whether active compounds were presence or absence, their amount and their composition ratio, we divided the Brassica accessions into three to five groups by cluster analyses. Most groups were composed of a mixture of allogamous and autogamous species sharing same genome, indicating that the variation depended on genome, not species. These results suggest that all species require pollinator visits for reproduction, despite their different reproductive systems. However, the inter-genus and intra-specific variations shown by the multiple groups within a species might cause different visitation frequencies by pollinators between genera and among accessions within a species, resulting in insufficient seed production in some accessions or species.

Keywords: Brassicaceae; floral scent compounds; gas chromatography–electroantennogram detection; honeybees; pollinator requirement; reproductive system; seed production.

Fig. 1

Diagram of combined GC-EAD analysis. Dashed arrows mean the flow of carrier gas. Gas chromatogram of extracted sample (A) and oscillograph of honeybee’s antenna (B) were obtained from FID detector and EAD device, respectively.

Fig. 2

Gas chromatogram (A) and oscillograph (B) using long stalked cabbage (Ca-115) sample. Numbers in (A) mean the corresponding compound number in Table 2. Each compound pointed by an arrow in (B) shows high peak, considering as an active compound.

Fig. 3

Dendrograms produced by cluster analysis based on (A) presence or absence, (B) amount (peak area per flower) and (C) composition (%) of all active compounds in the 32 Brassicaceae accessions. The accession numbers (and their genome, in parentheses) correspond to the numbers in Table 1. The two dashed lines indicate separation points at 50% and 75% similarity. The four to six groups were separated at the 50% similarity level and the group number is shown on the left of the dendrogram.