Membrane fluxes, bypass flows, and sodium stress in rice: the influence of silicon

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 (24)Na(+), 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 (24)Na(+) fluxes (influx and efflux), or on Na(+)-stimulated plasma-membrane depolarization, in either IR29 or Pokkali. We conclude that, while Si can reduce Na(+) translocation via bypass flow in some (but not all) rice cultivars, it does not affect unidirectional Na(+) transport or Na(+) cycling in roots, either across root cell membranes or within the bulk root apoplast.