[Translocation of(14)C-labeled assimilates in light and darkness inVicia faba]

Abstract

By methods involving autoradiography, historadiography of water soluble activity, girdling, and honey dew-chronograms, the uptake, assimilation and translocation of(14)C-labeled tracers have been studied.Vicia faba, grown in aerated nutrient solution, was treated with NaH(14)CO3, sucrose-(14)C-(U), and urea-(14)C. Translocation experiments have been carried out in a cooling room at 14°C. Only phloem-mobile activity is translocated when the leaf is fed with H(14)CO 3 (-) in the light; in the dark there is no assimilation, but incorporated(14)C is translocated. Application of tracers to the root gives different results depending on illumination and the kind of tracer applied.In the light (6000 Lux) the administration of any one of the above tracers gives radioactive honey dew, which drops from aphids (Acyrthosiphon pisum Harris) feeding on the third primary leaf. In the dark, sucrose-(14)C is not assimilated; H(14)CO 3 (-) is assimilated for several hours according to the production of labeled honey dew. Urea-(14)C gives labeled honey dew every time.Steam girdling the upper part of the stem has no effect on translocation when the plant is illuminated; all tracers move as in non-girdled plants and appear in the honey dew, taken up above the girdle. Girdling the stem between the root and the illuminated green plant (epicotyl) has the following effects: Sucrose-(14)C and H(14)CO 3 (-) are taken up by the root, the radioactivity is spread over the plant but it does not enter the sieve tubes, and the honey dew is not labeled. When the tracer is urea-(14)C, the honey dew contains radioactivity.It is to be concluded that a certain substance, which is formed in the illuminated green parts of the plant, is necessary for the incorporation of sucrose and HCO 3 (-) into assimilates able to move in sieve tubes and to pass a girdle. This substance enters the root via the sieve tubes. Assimilates able to move in xylem and phloem may be called "ambimobile".