Apoplastic Phloem Unloading in the Stem of Bean

Sucrose has been found in the apoplast of bean stems at a concentration of 25–60 mM with an axial concentration gradient in the appropriate direction for Munch translocation. Removal of the epidermis from a 50 mm length of stem enabled the washout of labelled photosynthate from the apoplast. The rate of label washout was strongly dependent on temperature, and the rate increased on blockage of phloem pathways to the main sink for that assimilate. Washout did not reduce when the bathed tissue was plasmolyzed. We propose that sucrose is unloaded from the phloem into the apoplast, and a sucrose concentration is maintained there by a balance of sucrose uptake into sink tissue or reloading into the phloem. It is proposed that the apoplastic pool of photosynthate can act to buffer sudden changes in phloem contents when there are rapid changes in source-sink configuration.