Movement of Water and Solutes in Sieve Tubes of Willow in Response to Puncture by Aphid Stylets. Evidence against a mass flow of solution

Abstract

Experiments are described in which bark strips of willow were sealed to polythene tubes having two compartments. This allowed investigations to be made of the transport along the sieve tubes of tritiated water, "C-Iabelled sugars, and '-P-phosphates from one compartment, towards a stylet situated in the bark over the other compartment.Although activity from both "C and ^^p was detected in the stylet exudate usually within 1 hour from isotope application, tritium activity was never detected even after a period of 8 hours in most experiments, though in certain cases, very low activities were detected after 4 hours. Suhsequent experiments in which stylets were sited over both compartments showed that tritium activity moved laterally into the punctured sieve element more rapidly than either '''C or '-P. Experiments vising both live and dead bark in which stylets were not employed, showed that within 4 hours tritium activity had moved by diffusion along the whole length of a bark strip, therefore after this time tritium activity could have moved into the stylet exudate by a diffusional process.The lack of rapid longitudinal movement of ti-itiated water along the sieve tubes, indicates that the transport process is unlikely to be a mass flow of solution.

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

“…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).…”

A mechanism of translocation based on high proton mobility and K+ counterflux at negatively charged surfaces in sieve elements

“…13-16. The lateral leakage of water+ (Peel et al, 1969) is due to the repetitive movement of the K layer in the loops formed by phloem-rays-xylem. This serves to concentrate the phloem sap and at the same time keeps the negatively charged surfaces in the xylem elements wet.…”

Long Distance Transport in Macrocystis integrifolia

“…Experiments with THO to prove that water does indeed move with the other translocated materials in the sieve tubes, however, have given contradictory results. Some authors claim to have demonstrated a phloem transport of THO (1, 4, 5, 23), whereas others have denied it and claimed that water was relatively immobile (8,14,15). Also, there is no agreement whether the solvent (water) and organic or inorganic solutes move together at the same or at different velocities (4,5,23).…”

“…Also, there is no agreement whether the solvent (water) and organic or inorganic solutes move together at the same or at different velocities (4,5,23). The possibility of lateral exchange of THO along the conduction pathway has been alluded to and may underlie some of the contradictory data in the literature (4,14,15,24).The studies referred to above have all been done with higher vascular plants. We thought it would be instructive to investigate the movement of tritiated water in kelps which represent a differ-…”