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.

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