Abstract
Grasses accumulate silicon in the form of silicic acid, which is precipitated as amorphous silica in microscopic particles termed phytoliths. These particles comprise a variety of morphologies according to the cell type in which the silica was deposited. Despite the evident morphological differences, phytolith chemistry has mostly been analysed in bulk samples, neglecting differences between the varied types formed in the same species. In this work, we extracted leaf phytoliths from mature plants of Sorghum bicolor (L.) Moench. Using solid state NMR and thermogravimetric analysis, we show that the extraction methods alter greatly the silica molecular structure, its condensation degree and the trapped organic matter. Measurements of individual phytoliths by Raman and synchrotron FTIR microspectroscopies in combination with multivariate analysis separated bilobate silica cells from prickles and long cells, based on the silica molecular structures and the fraction and composition of occluded organic matter. The variations in structure and composition of sorghum phytoliths suggest that the biological pathways leading to silica deposition vary between these cell types.
Highlights
Grasses are silicon accumulators, concentrating silicic acid ( Si) from the soil solution through the activity of Si transporters (Ma et al, 2006; Ma et al, 2007; Sakurai et al, 2015)
Higher magnifications of phytoliths extracted by microwavedassisted digestion (MAD) (Figures 2D–G) and sulphuric acid-hydrogen peroxide-nitric acid extraction (SONE) (Figures 2H–K) revealed spherical loosely aggregated particles in long cells only when extracted by SONE (Figure 2J)
scanning electron microscopy (SEM) indicated that the long cells behaved differently under the two extraction methods, in accordance with the Raman PCA that could discriminate between the extractions based on long cells spectra
Summary
Grasses are silicon accumulators, concentrating silicic acid ( Si) from the soil solution through the activity of Si transporters (Ma et al, 2006; Ma et al, 2007; Sakurai et al, 2015). Organic molecules are trapped within phytoliths (Perry, 1985; Harrison, 1996; Elbaum et al, 2009; Parr and Sullivan, 2010; Gallagher et al, 2015; Asscher et al, 2017) and possibly reflect the chemical environment in which the silica formed (Perry and Keeling-Tucker, 2000). These organic entities can be studied by nuclear magnetic resonance (NMR) (Ravera et al, 2016). A seminal study of the hairs in the grass Phalaris canariensis demonstrates that plant silica has a significant fraction of surface silanol groups (Mann et al, 1983; Perry and Mann, 1989)
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