Optical coherence tomography biospeckle imaging for fast monitoring varying surface responses of a plant leaf under ozone stress L. K. T. Srimal* a,c , H. Kadono a , U. M. Rajagopalan b a Graduate School of Science and Engineering, Saitama University, 255 Shimo-okubo, Sakura-ku, Saitama-shi, Saitama 338-8570, Japan; b Laboratory for Integrative Neural Systems, RIKEN Brain Science Institute, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan; c Department of Mechanical and Manufacturing Engineering, Faculty of Engineering, University of Ruhuna, Hapugala, Galle 80000, Sri Lanka ABSTRACT In this study, Optical Coherence Tomography (OCT) was used to investigate the effect of short term ozone exposure on both front and back surfaces of the leaves of Allium tuberosum plant. Plant leaves were subjected to treat with 240ppb level of ozone, and imaging data were acquired from back and front surfaces of the leaves. Variations of the biological activations were monitored based on the biospeckles. Standard deviations (SD) of OCT temporal signal at each point were calculated and used to visualize the effect of ozone exposure. Leaf back surface showed higher SDs in biospeckle OCT signal in comparison to these of the front surface. These observations prove that the short term ozone stress on plant can be investigated successively with biospeckle OCT imaging technique. Keywords: Optical Coherence Tomography, Biospeckles, Ozone stress 1. INTRODUCTION Ozone is a secondary pollutant with a regional distribution, and its disastrous effects on the environment may spread over enormous land area of Japan. The ozone concentrations contaminated environment in Japan is liable to cause a wide range of effects including visible leaf injury, growth and yield reductions 2,10,11 . Conventional techniques for studying these changes in plants require destructive and invasive analysis, thus plant ’s responses to external stress are inferred indirectly. For O 3 stress, biomass, height change, counting, and chemical analysis were commonly performed. These conventional techniques cannot be performed in real-time and in-situ. To overcome above mentioned limitations and to see short term response of plant leaf against O 3 stress, in this study, we employed optical coherence tomography (OCT). OCT is an interferometric technique that detects internal reflected light can provide non-contact two or three dimensional in-vivo tomographic images of the internal tissue structure with very high resolution of few micrometers, and data acquisition takes only a few seconds 8,9,14 . OCT is based on a low- coherence interferometry. Light from the low coherence source is splitted into sample and reference (usually a mirror) arms. The reflected light from the sample and reference arms are combined to interfere. The electric signal detected at the photoreceiver are processed to get an A-scan, representing the depth resolved reflectance profile of the sample with the mirror in the reference arm scanning in Time Domain OCT (TD-OCT). A two-dimensional cross-sectional image of the sample, called B-scan, achieved by scanning the probing beam in the sample arm. This TD-OCT requires a mechanical scanning for A-scan, and the is largely activated. To overcome this difficulty, a spectral domain OCT (SD- OCT) was proposed and is commonly used nowadays 15 . In SD-OCT, which is used in our study, the reference arm length is fixed at a position approximately corresponding to the position of the sample. The spectral interference pattern between the light returned from the reference and that from the sample is collected by a line scan camera in a spectrometer. Finally one scan obtain depth profile of the sample by taking a Fourier transforming the spectral interference pattern. Those mechanical scanning can be eliminated to achieve A-scan. *thanujasrimal@gmail.com; phone 048-858-3873; fax 048-858-3873