Auxin Transport in Roots: IX. Movement, Export, Resorption and Loss of Radioactivity from IAA by Zea Root Segments

The reasons underlying the initial increase and subsequent decrease in the amount of radioactivity in the receiver block at the apical end of a Zea root segment supplied with a basal donor block containing labelled IAA have been investigated. The phenomenon was observed in segments supplied with IAA-1-14C, IAA-2-14C and IAA-5-3H. An acropetal polarity in the movement of radioactivity into the receiver blocks was observed using donor blocks containing IAA-5-3H at concentrations as low as 10-10M. The decrease in the amount of radioactivity in the receiver block begins after 6—8 h of transport at 25° C, and is unaffected by renewal of the donor block every 2 h, or the presence of 2% sucrose in the donor and receiver blocks. The net export of radioactivity into the receiver block at the apical end of the segment virtually ceases after 6—8 h of transport at 25° C, and is not prolonged by the presence of 2% sucrose in the donor and receiver blocks. At 10° C, net export of radioactivity continues for at least the first 50 h of transport, and the amount of radioactivity in a continuously applied receiver block continues to increase over this period. Receiver blocks removed from the apical end of segments after 8 h of transport and placed on planchettes show little or no decrease in the amount of radioactivity they contain as a function of time, in marked contrast to those left in contact with the segment. There is a marked, and metabolically dependent, resorption of radioactivity from the receiver block at the apical end of the segment after about 8 h of transport at 25° C; most of the resorbed radioactivity remains in the apical 2—4 mm of the segment. There is a loss of radioactive CO2 from segments supplied with a basal donor block containing 10-6M IAA-1-14C at 25° C, the emission beginning after 6—8 h of transport. Segments similarly supplied with 10-6M IAA-2-14C did not begin to lose radioactive CO2 until after about 10—12 h of transport. The ability of the segments to transport radioactivity in a polar manner declines with time after they are excised from the root, regardless of whether their cut ends are kept in the intervening period in contact with plain agar blocks, or ones containing unlabelled IAA at 10-6M. By the 6th h after excision at 25° C no transport of radioactivity through the segments and into the receiver blocks could be detected in either the acropetal or basipetal direction. The decrease in radioactivity in the receiver block after transport