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ISHS Acta Horticulturae 969: VI International Symposium on Edible Alliaceae

INVESTIGATION OF DISEASED ONION BULBS USING DATA PROCESSING OF OPTICAL COHERENCE TOMOGRAPHY IMAGES

Authors:   S. Landahl, L.A. Terry, H.D. Ford
Keywords:   OCT, Allium cepa, Botrytis allii, neck rot, non-destructive, interferometric
DOI:   10.17660/ActaHortic.2012.969.35
Abstract:
Optical coherence tomography (OCT) is a non-destructive interferometric technique to probe the internal structures of semi-transparent samples. Onion (Allium cepa) tissue was investigated using a Thorlabs OCT system based on a broadband 930 nm source. The specified depth resolution was about 7 µm and the lateral resolution about 10 µm. A ‘live’ real-time display of the sample was observed, with selected images stored on request. Each image comprised 500 pixels laterally (4 mm) and 150 pixels in depth (ca. 0.3 mm). Trials were carried out with stored New Zealand (NZ)-grown onion bulbs (‘Elk’ and ‘Red Keeper’) imported to the UK and with recently harvested ‘SS1’ onions (UK). NZ-bulbs were stored for 3.5 months at 4°C. UK-bulbs were stored for 1 month at 20°C. Half of the onions were treated with 100 µl sterile water and the other half were inoculated with conidial solution (NZ: 70000 spores/ml, UK: 30000 spores/ml) containing Botrytis allii (IMI # 292066) the causal pathogen of neck rot disease. On the last outturn, bulbs were cut and photographed. After that a number of laminae (n=5 to 9) were measured by means of OCT (n=4 bulbs). Image processing was applied within Matlab software to automatically extract features from the images to make a more quantitative assessment of tissue changes. Original images were enhanced, filtered and segmented into cells. Cell areas and centroids were measured and recorded. Acquired OCT images showed clear differences between healthy and diseased tissue. In neck rot affected tissue, detachment of inner cell walls was seen in regions of infected tissue. In infected regions average cell size was smaller and cells were less round. OCT provided high resolution images from onion tissue, to a depth of about 0.5 mm. Different types of tissue from various sites were mapped. Immediate acquisition of images with histological resolution has been demonstrated. It would require complex preparation, and take possibly several hours to acquire similar images with a conventional microscope. A qualitative link has been shown between the appearance of the OCT images and the incidence of disease in the tissue.

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