Functional Optical Coherence Tomography in the study of plant-sound interactions

Apart from water, light, carbon dioxide, temperature, and humidity, sound has also been established to play an essential role in plant growth. Sound of a particular frequency appropriate to a plant species has been proven to increase yield and immunity and used in fields. However, speedy evaluation methods remain a challenge. We reported earlier the application of laser biospeckles in the speedy evaluation of the response of plants to sound stimulation under different frequencies. In this study, we have considered the possibility of functional Optical Coherence Tomography (fOCT) in investigating the changes in the internal structures in response to sounds of different frequencies, 100 Hz and 10kHz. SD-OCT system (wavelength: 836 nm and depth resolution: 5.4 mu m) was used to acquire OCT structural images from the rear side of the leaf of a month-old arugula plant in-vivo before and after exposure to sound for a minute. Functional OCT signals were calculated as a ratio of the OCT reflectivity obtained after sound exposure to that before. Results showed a clear dependence of the fOCT signal on the sound frequency, with 100 Hz showing an increase while 10kHz showing a decrease. A closer examination of the anatomy revealed regions of significant changes nearly correspond to stomatal locations. Our confocal observations confirmed the existence of stomatal changes following a minute exposure to sound. The application of OCT in the study of plants under exposure to sound has never been reported before. Our results extend the possibility of the application of OCT to the field of plant photonics.