Abstract
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 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.
Highlights
Silicon and sodium are among the most abundant elements in the earth’s crust (Wedepohl, 1995)
While the mechanisms by which Si exerts such effects remain the subject of much exploration, it is clear that the structural properties of polymerized silicates in plant tissues can produce stems and leaves that are stronger and more erect, and better able to, for instance, resist pathogen and herbivore attack, or intercept photosynthetically active light, compared with plants grown without Si (Liang et al, 2015; Meharg and Meharg, 2015)
The slopes of declining plant FW with increasing [external sodium concentration (Na+]ext) were similar for the two cultivars, but because Pokkali had a much greater peak FW, its growth suppression was, percentage-wise, not as pronounced
Summary
Silicon and sodium are among the most abundant elements in the earth’s crust (Wedepohl, 1995). Neither is essential for the growth of most plants, but both can be beneficial under certain conditions (Epstein, 1999; Kronzucker et al, 2013). On plants have been documented for at least two centuries (Davy, 1815; Epstein, 1999). These benefits often take the form of increased biotic and abiotic stress resistances, but include growth enhancements even under benign conditions (Richmond and Sussman, 2003; Ma, 2004; Fauteux et al, 2005; Epstein, 2009; Coskun et al, 2016a). Si provision can trigger changes in gene expression (Van Bockhaven et al, 2013; Che et al, 2016; Debona et al, 2017; Hinrichs et al, 2017), but evidence for consequent changes on plant metabolism is scarce at this time
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