Solute Distribution in Sugar Beet Leaves in Relation to Phloem Loading and Translocation

The distribution of solutes in the various cells of sugar beet (Beta vulgaris L.) source leaves, petioles, and sink leaves was studied in tissue prepared by freeze-substitution. The differences in degree of cryoprotection indicated that sieve elements and companion cells of the source leaf, petiole, and sink leaf contain a high concentration of solute. The osmotic pressure of various types of cells was measured by observing incipient plasmolysis in freeze-substituted tissues equilibrated with a series of mannitol solutions prior to rapid freezing. Analysis of source leaf tissue revealed osmotic pressure values of 13 bars for the mesophyll and 30 bars for the sieve elements and companion cells. The osmotic pressure of the mesophyll of sink leaves was somewhat higher.The sharp concentration increase at the membrane of the sieve element-companion cell complex of the source leaf indicates active phloem loading from the free space at this site. Active loading of the phloem is presumably needed to move the sugar from the chloroplasts of the mesophyll to the sieve tubes against the concentration gradient. The osmotic pressure of the mature sieve element-companion cell complex appears to be approximately the same in source leaf, path, and sink leaf tissue. There is a distinct difference in concentration between the mature sieve element-companion cell complex in the sink and the surrounding mesophyll. The solute distribution suggests that sugar is actively accumulated from the free space by the developing sink leaf tissue.The osmotic values observed in the various cells are consistent with the operation of a mass flow mechanism of translocation driven by active phloem loading and by active accumulation of sugar by sink tissues.