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

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]) 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 and lower K) than HN plants. In addition, counterion effects on ∆∆Ψ were pronounced in LN plants (with ∆∆Ψ decreasing in the order: Cl > SO > HPO ), 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 and V values suggested that a single, dominant Na-transport mechanism was operating under each nutritional condition, with K values of 1.2 and 16 mM for LN and HN plants, respectively. Comparing saturating patterns of depolarization to linear patterns of 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.