Demonstration of Two Translocation Mechanisms in Studies of Bidirectional Movement

Abstract

A study of microautoradiograms showing the simultaneous movement of C14 photosynthate and P32 from separate leaves on the bean plant revealed 2 distinctly different patterns of bidirectional movement. In the 1st upward movement of C14 tracer and downward movement of p32 occurred in separate phloem bundles: older mature portions of the bundles were utilized. In the 2d pattern the 2 tracers moved in opposite directions through the same phloem bundles. The younger phloem cells lying adjacent to the cambium were involved in this type of movement. (The rate of movement for both patterns was well in excess of 38 cm/hour.) The 1st pattern fits the characteristics of a pressure flow mechanism ideally while the 2d requires an activated mechanism. The conflicts which have plagued translocation data for so many years seem resolvable with evidence for the 2 mechanisms. It seems unjustified to hope further for the simplicity that would exist if but a single mechanism were operative in translocation.