The influence of externally applied organic substances on phloem translocation in detached maize leaves

Solutions of organic substances show differing influences on the direction of phloem transport of (14)C-labeled assimilates in predarkened maize leaf strips, when externally applied to one end of the strip. One group of substances "pushes" the assimilates away from the site of application. Examples of this group are 75 mM solutions of sucrose, trehalose, maltose, D-glucose, D-fructose, glucose-6-phosphate, raffinose and galactose. There is strong evidence that "pushing" substances are taken up from the apoplast and loaded into the phloem. Another group of substances attracts the assimilates, it seems to "pull" the assimilates in direction to the site of application. Examples of this second group are 75 mM solutions of arabinose, melibiose, myo-inositol, D-mannitol, polyethylene glycol 2000, and Na2-EDTA (ethylene-diaminetetraacetate). The "pulling" substances obviously are not taken up into living cells. It is assumed that they accumulate in the apoplast and build up a water stress (water potential), which is counteracted by an increase of solute concentration in the parenchyma, thus creating a sink for assimilates. A third group of substances shows inert behaviour, having no perceptible influence on phloem transport, at least not, when applied as 75 mM solutions. At concentrations of more than 300 mM, inert substances tend to attract assimilates like those of the second group. Inert substances are xylose, sorbose, 2-deoxy-D-glucose, mannose and sorbitol.