Nectar Sugar Enhancement in Response to Bee Buzzing in Rhododendron x pulchrum: Sound-sensing Organs and Sensitivity Range

Plants can perceive and respond to airborne sounds, but our current understandings of this phenomenon remain limited. Flowers are known to exhibit increased nectar sugar concentration in response to pollinator sounds; however, there has been only a single report from Oenothera drummondii. To test the generality of pollinator-sound-induced nectar enrichment in bee-pollinated plants, we examined nectar responses to airborne sounds, particularly those produced by bee buzzing, in Rhododendron x pulchrum and Lamium amplexicaule. We exposed the flowers to playback sounds of bee buzzing, a similar synthetic sounds signal (200 Hz) and those with higher frequency (5000 Hz) and compared nectar sugar concentrations among these sound-treated and silent control flowers. We found an approximately 10% increase in nectar sugar concentration in response to playback sounds of bee buzzing and synthetic sounds signals at bee-like frequency (200 Hz), while no such response was observed under higher-frequency stimuli or silent condition in R. x pulchrum. A similar response to bee sounds was also observed in L. amplexicaule. Furthermore, these responses were observed at a sound pressure level of 65 or 100 dB, but not at 50 dB in R. x pulchrum, indicating a sound sensitivity threshold. We experimentally removed petals, stamens, or pistils (alone or in combination) from flowers and exposed them to 200 Hz sounds and silent treatments and found that both petals and stamens were necessary for such sound responses, suggesting that these floral organs are involved in acoustic sensing. Our findings provide new insights into floral sound sensing and suggest that acoustic responses to pollinator visitations may be more widespread among bee-pollinated flowering plants.