Chemical evidence for intrinsic 'Si' within Equisetum cell walls.

This contribution provides the first detailed chemical evidence for the association of 'Si' with soluble carbohydrate and proteinaceous components of Equisetum arvense cell walls. For Equisetum telmateia and E. arvense, the presence of intrasilica organics trapped within phytolithic silica deposits and organics packaged within silica coated particles on the outside of spores have previously been identified. The current paper shows that 'Si' is also found intimately associated with cell wall polymers that can be released using mild extraction procedures (CDTA and Na(2)CO(3)/NaBH(4)) that do not solubilise the mineral phase. The isolates comprise both protein and carbohydrate components with increases in 'Si' (up to slightly more than 1% by weight) being particularly linked to increased levels of protein within the extracts. The general composition of the cell wall isolates associated with 'Si' was very different to that previously found for intrasilica and spore related material with much lower levels of charged amino acids, particularly basic amino acids being detected. The range of monosaccharides detected was much wider than for the other silicified materials investigated. It is possible that 'Si' in some form could act to crosslink the cell wall polymers thereby providing a modest improvement in rigidity/stability of the cell wall against biotic and abiotic stresses. This study demonstrates that distinct differences are to be found between extra- and intrasilica organics in the cell wall of E. arvense.