The cytosolic Na⁺ : K⁺ ratio does not explain salinity‐induced growth impairment in barley: a dual‐tracer study using ⁴²K⁺ and ²⁴Na⁺

Abstract

ABSTRACT, from a similar baseline, and to a similar extent, at both low and high K + supply. We compare our results to those of other groups, and conclude that the maintenance of the cytosolic Na + : K + ratio is not central to plant survival under NaCl stress. We offer alternative explanations for sodium sensitivity in relation to the primary acquisition mechanisms of Na + and K + .

Sodium as nutrient and toxicant

“…In addition to this well-recognized osmotic effect, a second parsimonious explanation may lie in the disruption of potassium homeostasis-one that may reasonably and fruitfully supplant alternative, to date ill-substantiated , hypotheses, such as those of "toxic" Na + fluxes or cytosolic K + /Na + ratios (Maathuis and Amtmann 1999;Davenport and Tester 2000;Yao et al 2010; see below). The decline in cytosolic [K + ] ([K + ] cyt ) under saline conditions is well documented (Hajibagheri et al 1987(Hajibagheri et al , 1988Binzel et al 1988;Schroeppel-Meier and Kaiser 1988;Speer and Kaiser 1991;Hajibagheri and Flowers 2001;Carden et al 2003;Kronzucker et al 2006; see also Fig. 1) and is attributable to sodium's effects on K + transport Kronzucker and Britto 2011).…”

“…1b), offering an alternative, or additional, explanation of enhanced K + release under Na + exposure (Britto et al 2010;Coskun et al 2013). Combined, the above effects of impaired K + influx and enhanced efflux are expected to result in blockage of K + translocation to the shoot (Erdei and Kuiper 1979;Botella et al 1997;Kronzucker et al 2006;Munns and Tester 2008), with obvious consequences for downstream events such as photosynthesis and stomatal function.…”

“…Other than these instances, little evidence for Na + -coupled K + uptake exists in terrestrial plants Rodríguez-Navarro and Rubio 2006;Corratgé-Faillie et al 2010;Schulze et al 2012), although it may play a significant role in aquatic angiosperms and algae . A far more common observation is that Na + , at already modest (below-saline) concentrations, inhibits K + -influx systems, both in the high-and low-affinity transport ranges for K + (Rains and Epstein 1967a, b, c;Cheeseman 1982;Jeschke 1982;Kochian et al 1985;Benlloch et al 1994;Schachtman and Schroeder 1994;Santa-María et al 1997;Flowers and Hajibagheri 2001;Fuchs et al 2005;Martínez-Cordero et al 2005;Kronzucker et al 2006Kronzucker et al , 2008Nieves-Cordones et al 2007;Wang et al 2007), and can, additionally, stimulate K + efflux (Shabala et al 2006;Britto et al 2010;Coskun et al 2013), thus depressing K + -utilization efficiency in a two-pronged fashion.…”

“…1) and is attributable to sodium's effects on K + transport Kronzucker and Britto 2011). Na + has been shown to suppress K + influx in both its high-and low-affinity ranges, particularly at millimolar concentrations (Cheeseman 1982;Jeschke 1982;Schachtman and Schroeder 1994;Rubio et al 1995;Gassmann et al 1996;Maathuis et al 1996;Santa-María et al 1997;Martínez-Cordero et al 2005;Kronzucker et al 2006Kronzucker et al , 2008Nieves-Cordones et al 2007). Some studies have reported only weak Na + effects (Epstein 1961;Epstein et al 1963; see also Seemann and Critchley 1985), or even stimulations of K + influx by Na + (Rubio et al 1995;Spalding et al 1999), but such studies are in the minority.…”

“…a Under non-osmotic challenges, Na + may inhibit (red) or stimulate (green) K + transport (blue arrows) (Santa-María et al 1997;Thiel and Blatt 1991;Shabala et al 2006). Cytsolic concentrations of K + and Na + ([K + ] cyt and [Na + ] cyt , respectively) and resting membrane potentials can alter with long-term Na + stress (Hajibagheri et al 1988;Flowers and Hajibagheri 2001;Carden et al 2003;Kronzucker et al 2006). b Sudden salinity stress can result in membrane disintegrity (osmotic shock) and the release of cellular contents, including K + and water (Nassery 1975;Cramer et al 1985;Britto et al 2010;Coskun et al 2013) K + provision (but not complete deprivation; Evans and Sorger 1966), or when K + uptake is partially inhibited, including by Na + itself (see above, and Figs.…”