A TISSUE-AUTORADIOGRAPHIC STUDY OF THE TRANSLOCATION OF C14-LABELLED SUGARS IN THE STEMS OF YOUNG SOYBEAN PLANTS

Autoradiography was used to determine the tissues involved in the downward translocation of C14-labelled photosynthate and of C14-labelled glucose, fructose, or sucrose introduced into soybean plants through the cut petiole of a primary leaf. Sections of lyophilized, paraffin-embedded tissue were autoradiographed in such a way as to prevent significant movement of water-soluble compounds. Cross sections, 10 μ thick, were taken from petiole and stem at various distances from the primary node. In 5 to 10 minutes, the bulk of the C14 from photosynthate or from sugars that were introduced through the cut petiole was concentrated in and around the vascular tissues. These tissues were located on the same side of the stem as the petiole through which the C14 had entered. Within these tissues the bulk of the C14 was concentrated in some columns of cells with little or no C14 in others. The C14 of photosynthate was localized in the phloem while that of sucrose introduced through the cut petiole was concentrated predominantly in the pith and in the adjacent xylem. The C14 of fructose or glucose was localized mainly in the xylem. This comparative study has demonstrated that all translocation need not take place in the phloem, but some may take place in such tissues as the pith and xylem.