¹ From the Mineral Nutrition Laboratory, Soil and Water Conservation Research Division, Agricultural Research Service, United States Department of Agriculture, Beltsville.

Dr. Edwards' present address is the Department of Soil Science and Plant Nutrition, University of Western Australia, Nedlands, Australia

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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