Measurement of Unloading and Reloading of Photo-assimilate within the Stem of Bean

There is growing evidence that the phloem transport pathway is leaky with passive unloading into the apoplast and active reloading from the apoplast. In this paper we report in vivo measurements of unloading and reloading in immature stems of bean expressed as a percentage per unit length of the photo-assimilate flow within the transport pathway. Gross unloading was up to 6% cm-1 and net unloading up to 4% cm-1. Time constants of bulk flow through 10 cm of stem were about 4-6 min whereas those of net unloading were 8-13 min and of gross unloading, at the most, 7-8 min.