TRANSLOCATION OF 14C METABOLITES IN CARROT ROOT

Four-month-old carrot plants exposed to ¹⁴CO₂ for 1 hr in light were harvested successively at 1, 5, 10, 26, and 78 hr after initial exposure. Half-mm thick transverse slices from 3 and 10 cm below the crown were frozen quickly, freeze-dried, and autoradiographed on film. Radioactivity was first localized in a ring surrounding the cambium. The radioactive region extended centrifugally along radii as discrete loci to half the phloem thickness in 10 hr, in a pattern similar to radial rows of callose-stained sieve elements. Median longitudinal sections stained for callose demonstrated the presence of anastomosing sieve tube strands between the more vertical sieve tubes of differing age. Radioactive materials did not move across the cambium for 5–10 hr. These data fit with the decreased growth, in earlier studies, of uniform phloem explants removed from increasing distances from the cambium and of the lesser growth than phloem of similar xylem explants.