Translocation of C 14 Metabolites in the Phloem of the Bean Plant

Abstract

Relatively little is known of the nutritional interrelationships of plant parts. Currently, however, considerable effort is being expended in such studies, and satisfactory methods for following specific metabolites from their sight of synthesis to their destination have been developed. Kursanov's (9) recent review includes this area of translocation phenomenon.The general conclusion to be reached from these studies is that there exists a demonstrable localized sink area for the metabolites produced in a particular source area, and during the course of the movement ,of the metabolite between them there is little lateral transport and general mixing with substances moving in an adjacent source-sink channel (7, 9).Assimilates from mature leaves are said to move to immature ones of the same orthostichy, but there is little movement between mature leaves (7, 9).It is now generally understood that the assimilate flow from a single leaf may be divided, on reaching the stem, into an upward moving and a downward moving component. When the flow from 2 leaves is considered, if 1 is located above the other, the downward moving component from the upper leaf must pass the upward moving component from the lower leaf. This has been loosely referred to as bidirectional movement since a given internode supports movement in opposite directions. How this occurs was the subject of a former study (1), but the details of the movement are better clarified herein.Zimmerman, in 1959 (11), rightly stated that bidirectional movement in a sense incompatible with mass flow had not been demonstrated. This would probably require that the movement be bidirectional within the smallest functional conduction unit, i.e., within files of sieve cells.What is required for a better understanding of the nutritional interrelationships of parts and for a clarification of the mechanics of metabolite movement, is a successful linking of detailed flow patterns with detailed phloem anatomy. Anything less has not proved to be of critical value.The present report is concerned with the information that has been gained from a direct comparison of high resolution autoradiograms of assimilate flow 1 Received June 16, 1964. with the fluoresced phloem tissue through which the flow occurred. This permitted the precise identification of the particular phloem bundles involved in the transport of metabolite between a selected source area and its normal receiving sights.