Solution-Flow in the Phloem

Abstract

Translocation profiles along the path were studied using a modified flap-feeding technique for the simultaneous application of THO and "4C-sucrose. A re-evaluation of a mathematical model for phloem transport with reversible lateral exchange of tracer along the path indicates that lower apparent velocities for THO as compared to labeled carbohydrate are primarily due to extensive lateral exchange of THO along the conduction path. Path-chilling experiments support the concept that THO and "C-sucrose exhibit different lateral exchange characteristics. The data presented are consistent with a solutionflow mechanism.The mechanism underlying long distance transport of photosynthate has been a subject of controversy for many years (6, 21, 24). Points of contention center on the fine structure of the sieve tube, the extent and importance of metabolic energy, as well as the underlying driving potential which enables a sourcesink system to function. The most prevalent of the proposed mechanisms is the mass-flow hypothesis adopted from Munch (14). Since mass-flow is set apart from some other proposed mechanisms by its requirement for a "bulk flow" of solution, many workers have looked to isotopic water as a tool in evaluating mass flow; however, experimental results have been contradictory. In the present investigations, the problem of "solution-flow" along the path is approached through the simultaneous appli-

Different Mass Transfer Rates of Labeled Sugars and Tritiated Water in Xylem Vessels and Their Dependency on Metabolism

“…When labeled sugars, dissolved in tritiated H2O, are administered to leaves or petioles, sugars are always transported more rapidly than tritiated H2O in the phloem (1,5,7,9,20). Experiments with bark strips of Salix (16) gave similar results.…”

“…Some authors, therefore, concluded that mass flow is an unlikely (7,9) or even impossible (15,16) transport mechanism. Others have given preference to the mass flow concept (1,5,20).…”

“…The radioactive materials were supplied by the Radiochemical Centre, Amersham, U K. Specific radioactivities of 3H20, 4C-n-sucrose, 4C-_-glucose, 4C-n-fructose, 3H-D-sucrose, and 3H-D-glucose were, respectively, 5 Ci/ml, 10 mCi/mmol, 3 mCi/mmol, 2.9 mCi/mmol, 2 Ci/mmol, and 2. 57,1976 In the present experiments, K (mm2 hr-') is a constant, characteristic for the rate of escape of a compound from the vessel; C0 is the radioactivity applied at the top of the internode; C, the radioactivity collected at the lower end; L (mm) is the length of the internode; Ap (mm2) is the transverse-sectional area of the xylem vessels; V (mm hr-t) is the velocity of flow.…”

Solution-Flow in the Phloem

“…This resulted in the recovery of less THO from the path than was theoretically predicted. However, if one assumes that there is an efflux of xylem water into the lamina, which occurs even when the leaf is maintained in a water saturated atmosphere, then it is likely that THO in the phloem of the blade bundles will be continually flushed out (4). A more representative value for C. may lie in the recoverable activity of the secondary or primary veins of the blade which feed into the petiole or path.…”

“…3, A, scribed here in an effort to demonstrate the similarity of profiles obtained experimentally and theoretically. The 0.6 dm' source leaf of sugar beet was fed using a modified flap feeding method (4) similar to that of Biddulph (2). Velocities were measured for individual plants under study by observing the arrival of "C-translocate at a sink leaf.…”