Movement to the Xylem Exudate of 86Rubidium Accumulated in the Apex of Corn Roots

Apical segments of roots of corn (Zea mays, L.) were excised and mounted in experimental salt solutions containing (86)RbCl and CaCl(2). Xylem exudates were collected hourly beginning at the 21st hour. At the 24th hour, experimental solutions were replaced with solutions of other chlorides (nonlabeled) of Ca(2+) +/- some monovalent cation.Following replacement of the labeled solutions, the rate of (86)Rb(+) release to the exudates from root segments declined by about the same proportional amount regardless of the initial steady-state output rate or the kinds of salts used to replace the RbCl. The content of (86)Rb(+) in the roots also declined with time, but to a relatively less degree. Loss of accumulated (86)Rb(+) to the various external solutions following replacement was less than 6% in 6 hours. It is concluded that movement of accumulated Rb(+) to the exudate is dependent on the concentration of Rb(+) in the root but is largely independent of exchange between accumulated ions and other ions in transit across the root.