5.4 ǀ Silicon
Low Si levels are also a feature of highly-weathered acid soils. Although a ‘beneficial’ rather than an essential nutrient, it is taken up in large amounts by rice plants, and applications to rice under Fe toxic conditions have been found to reduce Fe uptake and produce fewer leaf symptoms (Becker, Ngo & Schenk, 2020; dos Santos et al., 2020; Dufey, Gheysens, Ingabire, Lutts & Bertin, 2014). Its specific role in plant metabolism is not well understood, but probably involves a stabilising effect on cell walls as an inert deposition in lignified cells and also by modulating lignin biosynthesis (Broadley et al., 2012). Becker et al. (2020) found decreased Fe2+uptake on addition of Si to nutrient culture, which they attributed to enhanced formation of the root exodermal Casparian strip. Chalmardi, Abdolzadeh & Sadeghipour (2014) found Si application improved antioxidant enzyme activity under Fe toxic conditions.