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Summary. Exudate was collected periodically from the root systems of detopped tobacco plants. Volume, cations, and 42K or 88Rb transfer were measured. According to measurements of K by 42K and by the flame photometer, when concentrations of KCl and KN03 were lower than 10-2 M, the K in the exudate came mostly froni a pool in the plant rather than from the external solution. With higher external KNO3 solutions, within a few hours nearly all of the K in the exudate came directly from the external solutions. Studies with 86Rb lead to the same conclusion. In contrast the maximum proportion of K in the exudate that came from KCl in the external solution was reached usually in many hours after detopping and amounted to from 50 to 75 %. The higher the external concentration the faster it was reached. These data for KCl are indicative of the 42K passing through a K pool in the root cells. K and Rb from high concentrations of KNO3 and RbNO,, however, may not pass through such a pool. The addition of 10-2 M KNO3 into the external solution dutring exudation essentially eliminated the effect of periodicity at least for a period of time and under the conditions of the experiments. Hydrochloric acid, mercuric chloride, anaerobiosis, and 2,4-dinitrophenol had the same effect and each resulted in a massive final exudation that usually persisted for 1 to 3 days before stopping. These results all lead to a hypothesis that periodicity is regulated at the tonoplast.Detopped root systems of many plant species exhibit diurnal fItlctuations in exudation volume (2,3,4

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Cycloheximide drastically reduced the rate of root pressure exudation in detopped tobacco (Nicotiana tabacum L.), and the effect was more pronounced for nitrate salts in the external solution than for some other salts or when the roots were in water. Diurnal periodicity was greatly decreased, and its phase was changed. Effects began within an hour. Cation uptake was reduced by relatively low levels of cycloheximide. The effects of cycloheximide seemed to be reversible. Mild phosphorus deficiency resulted in decreased exudation rates from detopped tobacco and in no response to nitrate. Periodicity of exudation was not greatly affected by phosphorus deficiency, however. Gamma ray irradiation with a 60Co source at levels (up to 40-50 kilo-roentgens) which are considered disruptive of moderately large molecules had relatively little effect on the exudation rate. Higher levels of irradiation, which disrupt most protein molecules, decreased exudation and obscured peri-odicity. The results indicate either that new protein (or peptide) synthesis is needed for the rapid nitrate transport or that the deficiency and inhibitor disrupt cellular membranes. Phosphorus deficiency increased the sensitivity of the plants to inhibition by irradiation of the exudation process.