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.