THE MOVEMENT OF IAA‐C 14 IN THE HYPOCOTYL OF PHASEOLUS VULGARIS

The amount of IAA-C14 transported basipetally through excised hypocotyl sections was strongly affected by the pH of the donor blocks, less so by the pH of the receivers. The effect of donor pH was mostly on uptake. The small amount of acropetal movement was not noticeably affected by pH. Sucrose added to the donor resulted in increased basipetal transport. The time-course of C14 movement into basal receivers followed a linear course from 1.5 to 3 hr as expected, but there was no net loss from the donors until after 30-45 min. The usual type of velocity calculation, which assumes uptake starting from zero time, would therefore be lower than the true value. Basipetal transport through segments cut from various positions in the hypocotyl and from seedlings of various ages was maximal in 6-8-day-old hypocotyl segments cut 25-30 mm below the cotyledons. Acropetal movement was minimal at all positions of all ages tested. FEW INVESTIGATIONS of auxin transport consider the development of the transport system. Still fewer consider the transport system of the entire plant. Several papers from this laboratory have considered developmental changes in auxin movement in such organs as young bean hypocotyls (Jacobs, 1950a, b), aging bean petioles (McCready and Jacobs, 1963a, b; Jacobs, McCready, and Osborne, 1966), and Coleus stems and petioles (Naqvi, 1963; Thompson and Jacobs, 1966; Werblin and Jacobs, 1967). Because these studies had been helpful in understanding organ development, we decided to try a broader investigation of the whole plant. In petioles of the primary leaf the polarity of auxin movement declined with age, mostly as a result of increased acropetal movement of auxin. This was true for both IAA and 2,4-D and was closely correlated with a decline in the elongation of the segments during the experiment (McCready and Jacobs, 1963b). Auxin movement in the hypocotyl, measured with the Avena curvature bioassay, showed a steady increase in the amount of basipetal movement through sections cut from the tip of progressively older seedlings. There was a declining gradient from the tip to the base of "8-day" hypocotyls. No detectable acropetal movement was found (Jacobs, 1950a). Correlated studies of elongation supported the hypothesis that basipetal auxin movement through the hypocotyl tips was limiting the elongation of the region near the bend (Jacobs, 1950b). f Received for publication 1 August 1968. Research supported by an NSF grant to W. P. Jacobs and by funds of Higgins Trust Fund allocated to Princeton University. 2 Present address: Botany Department, University of Hawaii, Honolulu, Hawaii 96822. The relative contribution of velocity and flux was not investigated. Furthermore, the development of auxin movement was followed only in the tip region of the hypocotyl. In order to correlate the earlierwork fromthis laboratory on bean petioles and roots, we studied the transport of labelled auxin along the hypocotyl during germination until xylem differentiation and lignification were well advanced. MATERIALS AND METHODS-Seedlings of Phaseolus vulgaris L. 'Red Kidney' were grown in vermiculite at 78 F and 807% relative humidity in the dark. At various days after planting they were used for transport tests. (These "ages" are not exactly equivalent to those used in Jacobs [1950a, b] as the seedlings used here grew faster than the earlier ones.) Transport techniques followed McCready (1963) and McCready and Jacobs (1963a) with the following modifications. Segments averaging 4.99 mm long were cut from the "tip" region of the hypocotyl or from progressively lower levels. If not otherwise stated, segments were from 6-dayold seedlings, position 2 (2.5-3.0 cm below the cotyledons). Segments were always in a vertical position during transport. Cylinders of agar, cut from longer cylinders molded in precision-bore glass tubing, were added to each cut surface to act as donor blocks (containing 1 mg per liter of methylene-labeled IAA-C14) or receiver blocks (usually containing phosphate-citrate buffer). All transport experiments were run in the dark (partly to obviate the 25% loss of radioactivity as a result of photo-oxidation). Radioactivity was measured with a liquid scintillation counter, adding the donor or receiver blocks directly to 15 ml of scintillation fluid in the counting vial. The fluid was a standard toluene-PPO-POPOP