A microautoradiographic study of 14C-labelled picloram distribution in soybean following root uptake

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Abstract
A microautoradiographic technique was evaluated with a view to determining the distribution of 14C-labelled picloram (4-amino-3,5,6-trichloropicolinic acid) in soybean (Glycine max (L.) Merr.) tissue following root uptake. The method involved embedding the tissue in a gelatin medium, rapidly freezing at −78 °C, sectioning in a cryostat at −20 °C, mounting the sections on cover slips coated with dry photographic emulsion in the dark, and photographically exposing the section to the emulsion at −25 °C. After processing, the cover slip was mounted with the autoradiograph above the section. Examination of microautoradiographs indicated that comparatively little radioactivity was retained in the roots and the amount that was present appeared to be associated mainly with the protoplasm of the root cortical cells. In the stem, radioactivity was present in both the xylem and phloem with comparatively little movement into the other tissues. Considerably more radioactivity was present in the younger apical leaves than in the older primary leaves. In the primary leaves the radioactivity was present only in the xylem whereas in the apical leaves it was present in both xylem and phloem.