Studies on the mechanism of solute translocation in the phloem

Abstract

Following a brief introduction to this field of research (Part 1), physiological and ultramicroscopical studies are presented (Part 2), in which the influence of thermal and chemical treatments upon solute translocation in the phloem is analysed. It is felt that the results of these studies find their plainest interpretation in terms of those theories which invoke a mass flow of sieve tube sap and sometimes, more specifically, in terms of the Pressure-Flow Hypothesis of Munch. Thus it would appear (Section 2.2) that the 'path' region was remarkably insensitive to metabolic inhibitors (with the exception of extremely concentrated KCN where an indirect influence would seem probable). Thermal treatments to this region (Sections 2.3 and 2.4) influence translocation rate in a manner that appears to be mediated by changes in the physical characteristics of the system, such as the viscosity of the sap and the hydraulic conductivity of the conduit.

Becoming despondent with the lily which neither toils nor spins but notwithstanding accomplishes so much in so small a body, attention was directed to a consideration of models of the sieve tube (Part 3), in which hitherto impossible measurements became straightforward. The first of these were not satisfactory for one reason or another (Section 3.2), but a later 'Working Model' drew attention to certain interesting possibilities, although problems of scale ruled out quantitative predictions. A rather different approach, involving hydrodynamic equations and computer methods, (Sections 3.4 and 3.5) permitted the consideration of the flow of solution along conduits which were of the scale of the sieve tubes and in which the probable and possible characteristics of the sieve tube sap and the sieve tubes themselves could be investigated. This work led to rather more valuable suggestions concerning the water relations of the phloem tissues, and perhaps to a more rigorous feasibility test of Munch's Hypothesis than had been made before; from this the Hypothesis emerged unscathed.

Before drawing the thesis to a conclusion (Part 4), it was felt that mention should be made (Section 4.2) of an attempt to correlate the work of Parts 2 and 3; while largely unsuccessful, this avenue offers a most interesting prospect for subsequent study. Some new material was introduced (Section 4.3) in which the possibility of a further role for the sieve plate is advanced. This is followed by a discussion (Section 4.4) of certain less straightforward possibilities that should be considered in an analysis of sap flow in the phloem.