Relationship between concentration of free IAA in coleoptile sections and the magnitude of their growth response

The results of our experiments indicate that the growth of 5 day old Zea mays L. coleoptile sections is not necessarily related to the amount of internal free IAA. [3H]IAA was applied, in the presence and absence of NPA, to 1-cm coleoptile sections showing low responsiveness and high responsiveness to applied IAA. Following 120 min incubation, total growth of the sections and their accumulation of radiolabel were measured. This accumulation was corrected to express only free IAA. In general, all treatments showed that growth and accumulation increased with increasing concentration of applied IAA. However, when growth was expressed in terms of IAA accumulated, it was found to decrease as accumulation of applied IAA increased