Movement of Growth Regulators in Plants

Summary The synthetic auxin 2,4‐dichlorophenoxyacetic acid (2,4‐D), labelled with carbon‐14 in the carboxyl group, was applied in blocks of agar gel to either the apical or the basal ends of segments cut from the petioles of young primary leaves of Phaseolus vulgaris, and the carbon‐14 appearing in receiving blocks at the other end was measured. When application was made to the apical ends, all the carbon‐14 accumulating in the receiving blocks appeared to be still in the form of 2,4‐D. This basipetal transport had a velocity of 0.6–1.0 mm/hour, and continued even when the concentration of 2,4‐D in the receiving blocks had become higher than that remaining in the donor blocks. Acropetal movement was much smaller. The ratio of acropetal to basipetal movement was approximately constant with time at low concentrations of 2,4‐D. At a high concentration, the ratio was increased; with longer segments it was decreased. More 2,4‐D entered the segments from blocks applied to the apical than to the basal ends. It is concluded that 2,4‐D is moved through these petiole segments by active polar transport.