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The structure and chemical composition of plant tissues revealed by high resolution attenuated total internal reflectance imaging
F. J. WARREN (1), C. H. Edwards (2), B. B. Perston (3), G. Mandalari (4), P. G. Royall (2), P. J. Butterworth (2), K. W. Waldron (4), P. R. Ellis (2). (1) University of Queensland, Brisbane, Australia; (2) King's College London, London, United Kingdom; (3) PerkinElmer, Beaconsfield, United Kingdom; (4) Institute of Food Research, Norwich, United Kingdom

Detailed studies of plant cell microstructure are essential to inform our understanding of many processes for example, plant breeding programs, the effects of processing on plant cells and the behaviour of plant-based foods during digestion. Traditional methods for the study of plant cell microstructure involve sectioning for microscopical analysis. To provide contrast and identify specific features (e.g. DNA) staining is required, adding to the cost and degree of technical difficulty, and increasing the likelihood of artefacts. The last 15-20 years have seen the introduction of powerful infrared (both mid and near infrared) imaging techniques that allow detailed chemical structural data to be obtained with minimal sample preparation. A major limitation of infrared (IR) imaging has been the lack of spatial resolution due to the long wavelength of the IR region. In this paper, we use an optimised form of infrared imaging in attenuated total reflectance (ATR) mode to study the cellar microstructure of durum wheat grain (Triticum durum L.). Plant tissues were embedded in epoxy resin for ATR imaging. Tissue sections for light microscopy were also cut from the same block. The IR method was found to have sufficiently high resolution (1.5 µM) to identify individual cell walls. From the differences in IR spectra between cells of the wheat caryopsis, it was possible to identify individual cells belonging to endosperm, pericarp and testa, and show that these spectra were the same as those obtained by dissected wheat tissue. Furthermore, it was shown that the intra-cellular contents of wheat tissue could be monitored throughout an in vitro digestion process to investigate the role of cell wall encapsulation on digestion of plant tissues, the first in-situ imaging of such a system to be reported.

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