Characterization of volatiles in strawberry varieties 'Elsanta' and 'Sonata' and their effect on bumblebee flower visiting

The bumblebee Bombus terrestris is a general pollinator in Europe and also highly suitable as greenhouse pollinator of various agricultural and horticultural crops such as tomato, sweet pepper or strawberry. Here, we highlight the importance of volatile emissions for the attractiveness and pollination efficiency of strawberry varieties, starting from the unique observation that two related strawberry varieties are pollinated to a different extent when cultivated together. In a flight cage Fragaria x ananassa var. 'Sonata' was pollinated more frequently (> 2 times) and with longer-duration visits (11.2 vs. 6.7 s) by B. terrestris as compared to var. 'Elsanta.' To investigate whether this visitation difference could be attributed to differential production of flower volatile compounds, we quantified and analyzed the floral emissions of both varieties. Samples of var. 'Elsanta' contained more green leaf volatiles such as E-2-hexenal (0.53 vs. 0 ng/3 flowers), Z-3-hexenol (2.26 vs. 0.20 ng/3 flowers) and Z-3-hexenyl acetate (2.15 vs. 0.46 ng/3 flowers) which are known to play a role in plant defense. In a third series of experiments, we determined olfactory responses of B. terrestris to some similar individual synthetic green leaf volatiles presented in a Y-tube olfactometer. B. terrestris workers responded in an aversive manner to these volatile compounds compared to purified air. Since the floral bouquet of var. 'Elsanta' contains more green leaf volatiles, bumblebees will exhibit a preference for var. 'Sonata.' Our observations suggest that the pollination preference for 'Sonata' is due to being 'less repellent' instead of 'more attractive' than 'Elsanta,' with variety-specific flower emissions lying at the basis