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Provision of silicon (Si) to roots of rice (Oryza sativa L.) can alleviate salt stress by blocking apoplastic, transpirational bypass flow of Na⁺ from root to shoot. However, little is known about how Si affects Na⁺ fluxes across cell membranes. Here, we measured radiotracer fluxes of ²⁴Na⁺, plasma membrane depolarization, tissue ion accumulation, and transpirational bypass flow, to examine the influence of Si on Na⁺ transport patterns in hydroponically grown, salt-sensitive (cv. IR29) and salt-tolerant (cv. Pokkali) rice. Si increased growth and lowered [Na⁺] in shoots of both cultivars, with minor effects in roots; neither root nor shoot [K⁺] were affected. In IR29, Si lowered shoot [Na⁺] via a large reduction in bypass flow, while in Pokkali, where bypass flow was small and not affected by Si, this was achieved mainly via a growth dilution of shoot Na⁺. Si had no effect on unidirectional ²⁴Na⁺ fluxes (influx and efflux), or …