Day-Night Periodicity of Exudation in Detopped Tobacco

Exudate was collected periodically from the root systems of detopped tobacco plants. Volume, cations, and (42)K or (86)Rb transfer were measured. According to measurements of K by (42)K and by the flame photometer, when concentrations of KCl and KNO(3) 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 KNO(3) solutions, within a few hours nearly all of the K in the exudate came directly from the external solutions. Studies with (86)Rb 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 (42)K passing through a K pool in the root cells. K and Rb from high concentrations of KNO(3) and RbNO(3), however, may not pass through such a pool. The addition of 10(-2)m KNO(3) 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.