Day-night periodicity of exudation in detopped tobacco

Exudate was collected periodically from the root systems of detopped tobacco plants. Volume, cations, and ⁴²K or ⁸⁶Rb transfer were measured. According to measurements of K by ⁴²K and by the flame photometer, when concentrations of KCl and KNO₃ were lower than 10⁻² m, the K in the exudate came mostly from 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 RbNO₃, however, may not pass through such a pool. The addition of 10⁻² m KNO₃ into the external solution during 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.