Fig. 7.

Model outlining Na⁺ transport scenarios in IR29 and Pokkali rice, under saline conditions, and the role of Si. The large magnitudes of unidirectional Na⁺ influx (ϕ_oc) and efflux (ϕ_co) depicted in (A) are responsible for root Na⁺ cycling; we hypothesize that this cycling occurs mainly in the apoplast. The much smaller resultant net flux into the plant (ϕ_{net plant}, defined as the difference between ϕ_oc and ϕ_co) branches into still smaller fluxes leading to Na⁺ accumulation in root cells (cytosol and vacuole), the root apoplast, and in the shoot, where it arrives both through bypass flow and via the symplasm into the xylem and then via the transpiration stream to the shoot (B–E). In IR29, there is a large contribution of apoplastic bypass flow of Na⁺ to the shoot under control (–Si) conditions (B), but bypass flow and Na⁺ accumulation in the shoot are both significantly decreased by Si treatment (C). In Pokkali, in contrast, there is low apoplastic bypass regardless of Si condition (D, E). In this cultivar, Si decreases shoot [Na⁺] by virtue of its stimulation of shoot growth, and the resulting dilution of incoming Na⁺ (represented by checkered fill of ϕ_{shoot via bypass}, the flux to the shoot via apoplastic bypass; E). ϕ_{shoot via symplasm}, flux via the symplasm to the xylem and then to the shoot; ϕ_{root vacuole}, flux to the root vacuole; ϕ_{root cytosol}, flux to the root cytosol; ϕ_{root apoplast}, flux to the root apoplast.