Different mass transfer rates of labeled sugars and tritiated water in xylem vessels and their dependency on metabolism.

Solutions of (14)C-sugars in tritiated water or solutions of (14)C- and (3)H-sugars were perfused by gravity through the xylem vessels of excised tomato internodes (Lycopersicon esculentum) mostly during 2 hours.Mass flow of a solution in plant vessels is found not to be in conflict with different mass transfer rates of sugar and water molecules. It is explained by individual lateral escape rates from the vessel for each single compound. The escape of tritiated water can be ascribed to diffusion, while the escape of sugars is apparently linked to the metabolism of the surrounding parenchyma cells (Q(10) sucrose uptake = 3.5). Lateral escape rates of sugars from the xylem vessels are in the proportion of sucrose-glucose-fructose (2.4:1.0:0.6). The accessibility of xylem parenchyma cells and their differential permeability to sugars control the longitudinal mass transfer of sugars in the xylem vessels.As sucrose and glucose do not compete for uptake from the vessels by the contiguous cells, separate uptake systems for both may be postulated.