*4.11. Measurement of Na*<sup>+</sup> *and K*<sup>+</sup> *Concentrations*

Sodium and potassium ions in shoots and roots were quantified by a wet digestion method [90]. Dried, finely powered plant samples (50 mg) were digested in HNO3/H2O<sup>2</sup> solution (2:1) in a microwave oven for 4–5 min until the solution became clear. The digested solution was shaken gently and filtered through 0.2-µm filters (Whatman, Maidstone, England), and the solid fraction was discarded. The contents of Na<sup>+</sup> and K<sup>+</sup> in the extract were quantified by atomic absorption spectrophotometry (Z-6100, Hitachi, Tokyo, Japan).

For localization of sodium and potassium ions, we prepared samples according to the protocol of Mitsui et al. [91]. Harvested basal portions of shoots were immediately frozen and embedded in OCT compound medium (Sakura Finetek USA, Inc., Torrance, CA, USA), which contained 10.24% w/w polyvinyl alcohol, 4.26% *w*/*w* polyethylene glycol, and 85.50% *w*/*w* of a nonreactive ingredient. Then 5-µm sections were scanned with an electron probe microanalyzer (EPMA-1605; Shimadzu, Kyoto, Japan).
