122 L. J. Audits and J. K. BakJish 



batch of DCA-grown segments in which cell lengths were studied, 

 lAA at this concentration produces no appreciable reduction in cell 

 extension. 



There are two possible reasons for this lack of response to lAA. 

 First, it might be argued that DCA, being an auxin antagonist and 

 being retained in the adapted segment, completely antagonizes auxin 

 action and therefore prevents its inhibition of cell extension. This 

 seems unlikely, since previous work by Audus and Shipton (2), has 

 shown no evidence for such auxin antagonism in roots, in which 

 DCA and 2,4-D inhibitions seem strictly additive. Secondly, it could 

 be due to the complete destruction of the applied lAA by heightened 

 lAA-oxidase, since DCA-adapted roots have about twice the oxidase 

 content of normal roots, a difference which is highly significant 

 (Table 1, p. 114). Some of this stimulation may be caused by a direct 

 activation of the oxidase already present as shown by in vitro studies, 

 in which a 40 per cent increase in activity was obtained by the 

 adapting concentration of 10"^ g/ml- Evidence for lAA-oxidase in- 

 duction is thus less convincing than for TIBA. Lack of lAA response 

 would seem then to be more likely the result of heightened lAA- 

 oxidase activity. 



It is interesting then to compare the cell extension behavior of 

 DCA-grown segments ^vith those from lAA-grown and TIBA-grown 

 roots in which lAA-oxidase activity is also increased. The differences 

 are mainly in the maturer basal cells, which, unlike the correspond- 

 ing lAA- and TIBA-grown cells, extend much less than normal, 

 owing, we have presumed, to depressive DCA retained at the groAvlh 

 centers. By implication this suggests that TIBA has no direct inhibit- 

 ing action on roots at this concentration, its sole effect being exerted 

 via lAA metabolism (3). 



CONCLUSIONS 



It may be concluded from the experiments described that adapta- 

 tion of roots to exogenous growth substances takes place in the cells 

 of the meristem before extension commences and is the residt of in- 

 duced changes in the balance of enzyme complexes or of endogenous 

 growth factors or of both, and that these changes persist during sub- 

 sequent extension and therefore determine its pattern. ^\'ilh some 

 adapting molecules (e. g., lAA, TIBA, and DCA), a marked aug- 

 mentation of lAA-oxidase activity may play an important part in 

 lAA-response changes, Init, as in 2,4-D adaptation, a real change in 

 the sensitivity of the growth centers is undoubtedly involved. Here 

 the picture may be complicated by the competitive action of the 

 adapting molecules which remain adsorbed to the growth centers 



