PHLOEM LOADING IN PEACH - SYMPLASTIC OR APOPLASTIC

Sorbitol and sucrose are the two main soluble carbohydrates in mature peach leaves. Both are translocated in the phloem, in peach as in othe r rosaceous trees. The respective role of these two soluble carbohydra tes in the leaf carbon budget, and their phloem loading pathway, remai n poorly documented. Though many studies have been carried out on the compartmentation and export of sucrose in sucrose-transporting species , far less is known about sorbitol in species transporting both sucros e and sorbitol. Sorbitol and sucrose concentrations were measured in s everal tissues and in sap, in 2-month-old peach (Prunus persica L. Bat sch) seedlings, i.e. leaf blade, leaf main vein, petiole, xylem sap co llected using a pressure bomb, and phloem sap collected by aphid style ts. The sorbitol to sucrose molar ratio depended on the tissue or sap, the highest value (about 7) found in the leaf main vein. Sorbitol con centration in the phloem sap was about 560 mM, whereas that of sucrose was about 140 mM. The lowest sorbitol and sucrose concentrations were observed in xylem sap collected from the shoot. The volume of the lea f apoplast, estimated by infiltration with H-3-inulin, represented abo ut 17% of the leaf blade water content. This volume was used to calcul ate a global intracellular concentration for each carbohydrate in the leaf blade. Following these simplifying assumptions, the calculated co ncentration gradient between the leaf's intracellular compartment and phloem sap is nil for sorbitol and could thus allow for the symplastic loading of the phloem of this alditol. However, infiltration of C-14- labelled source leaves with 2 mM p-chloromercuribenzenesulfonic acid ( PC-MBS), a potent inhibitor of the sucrose carrier responsible for phl oem loading in sucrose-transporting plants, had a significant effect o n the exudation of both labelled sucrose and sorbitol from the phloem. Therefore, in peach, which is a putative symplastic loader according to minor vein anatomy and sorbitol concentration gradients, apoplastic loading may predominate.