Kinetics of C-14 Translocation in Soybean

A kinetic study was made of the translocation of (14)C-photosynthate through soybean stems following pulse labeling and during steady state labeling of the first trifoliolate leaf. The translocation profile proceeded down the stem with little or no change in shape. Following pulse labeling, sucrose accounted for 90 to 95% of the radioactivity in the stem at all times up to 2 hours, at which time less than 3% of the activity was in an insoluble form. Kinetic data on the relative specific activities of sucrose in the leaf and petiole indicated that two-thirds of the petiolar sucrose was in the translocation stream and the remaining one-third was in a stationary pool which slowly accumulated sucrose from the translocation stream. With this assumption, the rate of sucrose efflux from the leaf was calculated to be 22 micrograms per minute, which was equivalent to a sucrose mass flux in the sieve tubes of 20 grams per square centimeter per hour.It is concluded that translocation proceeded without a net radial velocity distribution and with slight loss of translocate, mainly as sucrose, from the translocation stream. The kinetics of (14)C translocation must, therefore, be determined primarily by factors operating within the leaf rather than in the stem. One of these factors may be the presence in the soybean leaf of a specialized paraveinal mesophyll. Since there was no spreading of the profile, the results were inconsistent with an "active diffusion" mechanism of translocation (i.e., a mechanism describable by diffusion equations). The data and conclusions are in substantial agreement with those of Moorby, Ebert, and Evans.