EVIDENCE FOR DIFFERENT KINDS OF CONCURRENT TRANSLOCATION OF PHOTOSYNTHETICALLY ASSIMILATED C14 IN THE SOYBEAN

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Abstract
When the primary leaf of a soybean plant was allowed to carry on photosynthesis in C14O2, a small amount of labelled carbon was translocated downward into the root at a velocity of at least 5040 cm per hour. This velocity was 50 times greater than the velocity at which sucrose was translocated in the phloem. The rapidly translocated C14 was not moved as carbon dioxide. When the time of translocation was increased from 30 seconds to 10 minutes there was an increase in C14 content of the root without any increase in C14 content of the lowest part of the stem. Part of the C14 was translocated through a steam-girdled stem, but under these conditions there was no accumulation in the root. This indicates that the rapid translocation of C14 occurred in both living and dead tissues of the stem. These results were correlated with the localization of C14 in tissue autoradiographs of cross sections of the stem to show that there were at least three pathways of downward translocation: (a) a slow translocation in the phloem, (b) a rapid translocation in the xylem, and (c) a rapid translocation in either the cortex, the phloem, or the pith.