Transport of hexoses by the phloem of Ricinus communis L. seedlings

The sieve-tube sap of Ricinus communis L. seedlings has been analysed to determine whether or not hexoses can be taken up by the phloem. Under natural conditions, i.e. with the endosperm attached to the cotyledons, glucose and fructose occurred only in trace amounts in the sieve-tube sap. Incubation of the cotyledons with hexoses in the concentration range 25-200 mM caused a rapid and substantial uptake of hexoses into the phleom, where they appeared eventually in the sieve-tube sap at the same concentration as in the incubation medium. Phloem loading of glucose, 3-O-methyl-glucose and sorbitol occurred easily, whereas fructose was less well loaded. glucose and to a larger extent fructose were also transformed to sucrose, which was loaded into the phloem. The loading of hexoses into the sieve tubes as observed in the experimental exudation system also occurred in the intact seedling, but transloction in the latter soon came to a standstill, probably because of lack of consumption by the sink tissues. These results indicate that the virtual absence of hexoses in the sievetube sap under in-vivo conditions is not because of the inability of the phloem-loading system to transport the monosaccharides but because of the absence of sufficiently high concentrations in the apoplast.