The quantity and kind of radioactive compounds in the blade, main rib, and petiole of sugar beet leaves were determined at intervals up to 3 hours after a 1 minute period during which the blade photosynthetically assimilated C14O2. During the first 5 minutes, radioactive compounds were isolated from the main rib but these were due to the direct assimilation of C14O2by the rib and not to translocation into the rib. After 8 minutes, radioactive sucrose from the blade reached the rib and began to move down the petiole. This movement continued at a steady rate of about 1 of the assimilated C14per minute for about 1 hour. The concentration of C14per 2ensp;cm section of petiole was a linear function of the distance, and this gradient moved down the petiole with an apparent velocity of 50 to 135ensp;cm per hour. After 1 hour, the entire length of the petiole contained radioactive sucrose at a relatively uniform concentration per unit section. Although the blade still contained 40 to 50 of the assimilated C14in sucrose, movement out of the blade was reduced to a very low rate after 1 hour, but radioactive sucrose continued to leave the petiole for an additional hour. The absence of significant metabolism of the radioactive sucrose in the petiole, combined with the changes in gradient of that sucrose as a function of time, suggest that the petiole exerts a relatively passive role in the translocation process.
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