Transfer of Rubidium from the Xylem to the Phloem in Wheat Internodes

Abstract

Summary Wheat ( Triticum aestivum L., var. «Arina») was cut below or above the flag leaf node. The detached shoots were incubated standing in a solution containing 2 mM RbCl and 2 mM SrCl 2 . Source/sink relations were modified by steam-girdling the stem below the ear (phloem interruption) or by removing the ear or the flag leaf lamina. Strontium (mobile in the xylem, but immobile in the phloem) was distributed in the detached shoots as expected from transpiration data. Rubidium on the other hand accumulated in the stem and in the leaf sheath of illuminated plants, but only minor quantities were detected in the leaf lamina and in the ear. The node was not a prerequisite for the removal of rubidium from the transpiration stream. The rubidium content of the ear was further decreased by removing the leaf lamina, and the flux of Rb (but not of Sr) was essentially blocked by steam-girdling below the ear. The results from these experiments suggest that Rb was transferred efficiently from the xylem to the phloem in wheat internodes. The mechanisms involved, the regulatory interactions, the selectivity and the physiological significance must be investigated in future experiments. Such transfer processes might be important for the channelling of solutes in maturing wheat.