Sucrose Compartmentation in the Palisade Parenchyma of Vicia faba L.

Intracellular sucrose compartmentation in the palisade parenchyma of Vicia faba L. leaflets was investigated by comparing the specific radioactivity of photosynthetically labeled [(14)C]sucrose in samples enriched in vacuole to that in samples enriched in cytoplasm. Brief centrifugation of leaflet punches was used to sediment most of the palisade parenchyma cytoplasm in the adaxial ends of the cells. The punches were quick-frozen, freeze-substituted, and embedded in methacrylate. Samples enriched in cytoplasm or in vacuoles were obtained from paradermal sections. After pulse-labeling, the sucrose specific radioactivity in vacuole-enriched samples was fairly constant. Sucrose specific radioactivity in cytoplasm-enriched samples was about 2.5 times that in vacuole-enriched samples initially and declined thereafter. Earlier interpretation of intracellular sucrose compartmentation (Plant Physiol 1975 55: 704-711) had predicted larger specific activity differences (up to 20 times) between the cytoplasm and vacuole. The difference between the actual and predicted behavior is ascribed to the observed extent of cross-contamination in samples and, more importantly, to the confinement of sucrose to extrachloroplastic regions of the cytoplasm.