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Movement to the Xylem Exudate of ⁸⁶Rubidium Accumulated in the Apex of Corn Roots ¹

^a Department of Botany, University of Florida, Gainesville, Florida 32611

Apical segments of roots of corn (Zea mays, L.) were excised and mounted in experimental salt solutions containing ⁸⁶RbCl and CaCl₂. 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 Ca²⁺ ± some monovalent cation.

Following replacement of the labeled solutions, the rate of ⁸⁶Rb⁺ 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 ⁸⁶Rb⁺ in the roots also declined with time, but to a relatively less degree. Loss of accumulated ⁸⁶Rb⁺ 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.