Cation Effects on Chloride Fluxes and Accumulation Levels in Barley Roots

Accumulation of Cl^- by excised barley roots, as of K⁺, approaches a maximum level at which the ion influx and efflux rates become equal. The rate of Cl^- influx at this equilibrium is close to the initial rate while the efflux rate increases with time from zero to equality with influx. The Cl^- fluxes are independent of simultaneous exchange flux of the cations, but depend on the nature and concentration of the salt solutions from which they originate. The Cl^- content at equilibrium, however, is largely independent of the external concentrations. The approach to equilibrium reflects the presence of the cation. Cl^- flux equilibrium is attained more rapidly in KCl than in CsCl or CaCl₂. This is presumably an effect of much slower distribution of Cs⁺ and Ca⁺⁺ than of K⁺ within the roots. Accumulated Cs⁺ appears to form a barrier to ion movement primarily within the outermost cells, thereby reducing influx and ultimately efflux rates of both Cl^- and cations. Slow internal mixing and considerable self-exchange of the incoming ions suggest internal transport over a series of steps which can become rate-limiting to the accumulation of ions in roots.