Lateral auxin transport in stems and roots.

The lateral movement of auixin has been stuidied with bioassays and also by application of '4C-IAA. Both techniqutes have demonstrated that the tupper hal,f of a horizontally positioned stem or root contains at least 30 to 45 % of the total auxin present in the tissuie (2). However, such an auxin gradient is difficult to reconcile with the Cholodny-WVent theory of root geotropism becatuse it is too snmall to accouniit for the growth inhibition which occulrs in the uindlerside of a root experiencing a geotropic curvatuire (2). \Ve shall present evi(lence that the auixini (lifferential actuially' is conisiderably larger than these experiments indicate, an(I dliscliss several problems which 'have obsculred this fact and hiinldered the measuiremenlt of lateral aulxin transport. Previotusly we reported (3) that in 8 experiments, each tising 20 sections and 1 ,LUM 14C-IAA, the specific activity of the upper half of horizontally positioned etiolated pea stem segments was 37.5 ± 2.7 % that of the lower half. In an equlal nuimber oif experiments the specific activities of the uipper quarter, middle half, and lower quarter relative to that of the intact section were 0.58 40.07, 0.99 + 0.07, and 1.46 ± 0.10 respectively (3). From these values the distribtution of 14C-IAA across the diameter of the stem has been calculated (fig 1). The data show the specific activity to be fairly uiniform throughotut the middle of the stem, only rising or falling markedly at the ouiter stirfaces. Apparently each cell passes the atixin it receives to the next lower cell so that IAA is (lisplace(d from the upper to the lower suirface with little change in concenitration in the center of the sectioni where most of the tisstue mass resides. ConiseqLuentlv when sectioins are halve(d the large (lifferential between the tipper and(I lower suirfaces is obscuired by the lack of a gradienit in the central bulk of tissue included in each sample. The magnitude of the gradient between the sturfaces can be estiimated by extrapolating the curves ('dotted lines, fig 1) and may be as large as 10:1. The problem o-f the 'central tissue mass can be circtumvented by utilizing hollow cylinders stuch as Avena andl corn coleoptiles, btut this approach introduces new difficulties. It is well established that auxin does not spread laterally in these tissues except under the influence of gravity; otherwise the Avena and corn curvature tests would not work. Consequiently when coleoptiles are positioned horizontally, IAA should only migrate at the flanks from the upper to the lower half of the coleoptile (see fig 2). In addition a net movement of IAA from U1 to U. and L1 to L, wotul(d be expected, giving rise to the finial distribultioIn illuistrated in figure 2. If this interpretationi is correct the lower half of a split coleoptile muist contain a mass of flank cells in which there is Ino gradient as well as a zone (L1) partially depleted of auxin, whereas the tipper half shouil(d containi an atuxin rich zone (U,). Therefore, we believe that atlxin gradients U, :U9 ain(d L1 :L., are greater 'than the differential meastured between the tipper aind lower halves of split coleop-