Visualizing changes in internal layers of Chinese chives leaf under short-term ozone exposure with biospeckle optical coherence tomography

Ozone (O₃) stress can cause physiological changes as well as changes in the growth patterns of plants. Conventional techniques for studying these changes in plants require destruction of the sample and a subsequent analysis, thus the plant’s responses to external stress are inferred indirectly. In this paper, we show that a biospeckle optical coherence tomography (bOCT) biospeckle imaging can be used to monitor the response of Chinese chives leaves to O₃ stress. A Fourier-domain OCT operating at a wavelength of 836 nm with a depth of resolution of 6 µm was used. The bOCT biospeckle image was obtained by calculating the standard deviation of the OCT signal along the time axis at each point of the selected portion of the cross-sectional image from the successively acquired OCT images. In the bOCT biospeckle images, the internal layer structure could be clearly visualized, especially the mesophyll cells with high contrast in comparison to the structural OCT image. In addition, an analysis of the average autocorrelation functions of the biospeckle signals revealed that the correlation length decreased in the mesophyll layer during O₃ exposure, possibly indicating the increase in movements of internal structures within the leaf. Our results suggest that biospeckle imaging can be a suitable technique for fast and non-destructive monitoring of environmental stresses on plants, in contrast to conventional techniques that require a few days’ to a few weeks’ time, with destruction followed by subsequent analysis, thus paving the way for significant time saving.