The Free Space of the Xylem Translocation Pathway of the Tomato Stem

Abstract

In this paper a method is presented for measurement of the apparent free space (AFS) directly involved in the xylem vessel translocation. The AFS of xylem vessel bundles was measured by perfusing tomato internodes with solutions of a non-absorbed (inulin-carboxylic acid), a weakly absorbed (α-aminoisobutyric acid), and a strongly absorbed compound (L-α-alanine). The xylem cross-section actually involved in the longitudinal flow of these substances plus the area which is in rapid (10 to 15 min), diffusional, reversible equilibrium with the flow area was found to amount to 2·5 times the cross-section of the xylem vessels. Since all apoplasmic compartments of the xylem constitute only 70% of the AFS of the xylem vessels, 30% of the AFS seems to be situated outside the xylem.