Sources of Sucrose Translocated from Illuminated Sugar Beet Source Leaves

A search for source leaf sucrose pools that differed in their relation to export was carried out in photosynthesizing leaves of Beta vulgaris L. The time course of depletion of [(14)C]sucrose in a leaf in unlabeled CO(2) following steady state labeling provided evidence for two distinct sucrose pools. After the start of the light period, leaf blade sucrose remained constant although it exchanged between the two pools. Newly synthesized sucrose destined for export passed through one pool more rapidly than through the other. All of the leaf blade sucrose appeared to exchange with export sucrose. Modeling and regression analysis of [(14)C]sucrose data provided a means for estimating the size of the two pools. From 20 to 40% of the sucrose was calculated to be present in the pool that provided the less direct path to export; this was likely vacuolar sucrose. The remainder of the sucrose in the blade was probably in the cytoplasm and veins. Added amounts of leaf blade sucrose, produced in response to elevated CO(2), appeared to be stored mainly in the vacuolar compartment.