Effect of Monosilicic and Polysilicic Acids on Cd Transport in Rice, a Laboratory Test

Silicon (Si) has been reported to enhance the plant tolerance to cadmium (Cd) toxicity, while the Si–Cd interactions remain poorly investigated. In a laboratory study, the transport of monosilicic and polysilicic acids in the apoplast and symplast of roots, stems and leaves of rice as well as the effect of Si supplementation on the external solution-to-root-to-stem-to-leaf time-dependent Cd transport through apoplastic and symplastic pathways were investigated. The Cd translocation was very fast and mainly through apoplastic pathway. For 24 h the Cd concentration increased 38–50 times in apoplast and 6–8 times in symplast. In Cd-exposed plants, Cd entry into apoplast was inhibited during the first 2 days, but its massive flow was observed on the 4th day. In Si-supplied plants, the ability of apoplastic barriers to impede Cd root-to-leaf transport was higher during all time of examination. Silicon reduced the Cd transport through the apoplast of roots, stems, and leaves by 50–90%, while the symplast transport of Cd was influenced by Si only in stems. In the apoplast and symplast, soluble Si was presented in the form of both monomers and polymers of silicic acid. Exposure to Cd initiated the uptake by roots and fast redistribution of Si from leaves to roots via mono- and polysilicic acids movement in apoplast and symplast.