Sucrose Translocation in the Sugar Beet

In a recent study, Jones et al. (7) reported on the translocation of C14 in tobacco following assimilation of C14O2 by a single leaf (the presentation leaf or source leaf).The labeled CO2 was supplied for varying periods of time (10 mins to 3 hr) after which photosynthesis was continued in normal air for the duration of the experiment (up to 96 hr continuous light). There was a complete turnover of the sucrose in the source leaf in about 24 hours without change in amount. Distribution of labeled translocate, at least to leaves above the source leaf, was completed in about 6 hours. Direct analyses of the total amount of activity distributed to the roots and incorporated in the stem were not made, although, as measured by difference, the greater fraction of the labeled translocate accumulated in these regions. The present work extends this approach to a study of translocation in the sugar beet, with the modification that the test plants were pruned to a highly simplified source-link translocation system to permit a more detailed comparison of the rate of depletion of label from specified pools in the source leaf and the concurrent gain of label in the sink. Specifically we have attempted to answer the following question: Which of the various carbohydrate pools in the source leaf, during a period of steady-state photosynthesis, supplies a major fraction of the translocate? The sugar beet plant is of interest in such studies because of the high invertase activity of its leaves (10). Classically it has been regarded as a plant in which the bulk of the translocate is composed of invert sugars (2, 4, 5, 10, 15, and others). More recent studies, however, employing C14, reveal; a less maverick composition, sucrose now being considered the major constituent of the translocate (13, 14, 17). Additional data in substantiation of this view are presented below