Plasma-membrane electrical responses to salt and osmotic gradients contradict radiotracer kinetics, and reveal Na + -transport dynamics in rice ( Oryza sativa L.)

Main conclusion A systematic analysis of NaCl-dependent, plasma-membrane depolarization (∆∆Ψ) in rice roots calls into question the current leading model of rapid membrane cycling of Na + under salt stress. To investigate the character and mechanisms of Na + influx into roots, Na + -dependent changes in plasma-membrane electrical potentials (∆∆Ψ) were measured in root cells of intact rice ( Oryza sativa L., cv. Pokkali) seedlings. As external sodium concentrations ([Na + ] ext ) were increased in a step gradient from 0 to 100 mM, membrane potentials depolarized in a saturable manner, fitting a Michaelis–Menten model and contradicting the linear (non-saturating) models developed from radiotracer studies. Clear differences in saturation patterns were found between plants grown under low- and high-nutrient (LN and HN) conditions, with LN plants showing greater depolarization and higher affinity for Na + (i.e., higher V max and lower K m ) than HN plants. In addition, counterion effects on ∆∆Ψ were pronounced in LN plants (with ∆∆Ψ decreasing in the order: Cl − > SO 4 2− > HPO 4 2- ), but not seen in HN plants. When effects of osmotic strength, Cl − influx, K + efflux, and H + -ATPase activity on ∆∆Ψ were accounted for, resultant K m and V max values suggested that a single, dominant Na + -transport mechanism was operating under each nutritional condition, with K m values of 1.2 and 16 mM for LN and HN plants, respectively. Comparing saturating patterns of depolarization to linear patterns of 24 Na + radiotracer influx leads to the conclusion that electrophysiological and tracer methods do not report the same phenomena and that the current model of rapid transmembrane sodium cycling may require revision.