Abstract
Poaceae plant cell walls are composed of phenolic compounds such as lignin and hydroxinnamic acids which are known to affect the potential uses of such feedstock. They are not uniformly distributed within plant organs and this distribution could explain differences in key properties such as their milling behaviour or hydrolysis patterns. Based on the autofluorescence properties of these components, specific spectral signatures of plant cell walls have been assessed according to plant tissues or organ localisation and related to known differences in tissue composition [1,2]. Indeed, autofluorescence spectra could be relevant for the presence of different components (ferulic acid/para-coumaric acid, lignin) and their amount or relative proportions, but also for their structure and interactions in the cell walls. The UV wavelength range is of great importance and the fluorescence signal of ferulic acid in maize stem after UV excitation has been shown to be in shorter excitation wavelength than para coumaric acid [2]. The aim of this work was to focus on UV excitation fluorescence to investigate the heterogeneity of plant cell walls at the microscopic scale, both in excitation and emission mode. Two well-studied reference samples (wheat grain and maize stem) were used to cover a wider range of variability. Cross sections were imaged with a filter microscope to obtain multispectral images. Large field of view at high spatial resolution (pixel size=1.06µm) were investigated in accordance to plant structure size and heterogeneity. In this case, acquisition is carried out to obtain pseudo-excitation spectra, focusing in the UV range. The fluorescence emission was then recovered in four wavelength bands in blue and green for 4 excitation ranges in UV, centred around 340nm, 360nm, 380nm and the last one covering the whole UV range. The possible influence of pH was also evaluated in relation to known fluorescence shift.