Diffusion of tritiated water into roots as influenced by water status of tissue

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Abstract
Root segments of Vicia faba and Zea mays were osmotically equilibrated to different degrees of water stress by means of mannitol solutions. The segments were transferred to identical solutions in which tritium hydroxide (THO) was present and the time course of diffusion was determined. Self diffusion of water into Vicia segments 3-4 mm from the root apex is increased by increased water stress. Segments further from the apex are less permeable to THO and show a similar response to stress. Stress has little or no effect on diffusion in Zea roots. Exchange of water occurs primarily through the cylindrical walls of root segments rather than through cut surfaces. Diffusion constants are reduced to about 10% of that of free self diffusion in pure water. The results in general appear to agree with those obtained by other workers using mass flow techniques on Vicia roots. A tortuosity factor may be invoked to help explain the changes in self-diffusion constants.