Influence of Gibberellic Acid on lAA Disappearance 643 



Dr. Andreae: I should like to ask what justification is there to 

 equate loss of lAA from solution with destruction? We found with 

 similar experiments that as little as one-fifth to as much as all of the 

 lAA lost from solution accumulates in the tissues as indoleacctylaspar- 

 tic acid. 



Dr. Housley: One may certainly account for the disappearance of 

 lAA from solution other than by its destruction; the example you 

 mention is conjugation with some other entity. In our preliminary 

 experiments we frequently obtained little or no disappearance from 

 solution (i.e., no decrease in absorption) over a period of 6 or more 

 hours; however, if suitable amounts of manganese ion and 2,4-di- 

 chlorophenol were incorporated in the solution, lAA began to dis- 

 appear rapidly. This dependence upon addition of these addenda 

 for lAA disappearance led us to suppose that the acid was being re- 

 moved from solution primarily as a result of its destruction by the 

 lAA oxidase enzymes. 



Dr. Purves: I'd like to describe one experiment in connection 

 with the theory put forth by Dr. Housley. In etiolated pea epicotyl 

 sections under certain conditions you can get a very low lAA opti- 

 mum, and can get a concentration curve showing an lAA optimum 

 at lO'^M with concentrations above lO'^M being inhibitory to the 

 growth rate. That is, the growth produced in the presence of high 

 auxin concentration is actually lower than that of the controls. Now, 

 if gibberellin is to act by virtue of an auxin-sparing mechanism, you 

 would expect the treatment under these conditions with GA leading 

 to an increased auxin content would lead to a further inhibition in 

 the presence of inhibitory lAA concentrations. However, we find 

 that the increment of growth produced by gibberellin is almost as 

 great in the presence of inhibitory auxin concentrations as in the 

 absence of auxin, suggesting that gibberellin cannot act by an auxin- 

 sparing mechanism. 



Dr. Housley: Dr. Purves' work might suggest that gibberellin is 

 not acting by an lAA-sparing mechanism alone and perhaps that 

 more than one mechanism in his experimental system is operating. 

 Such a mechanism, for example, could involve chemical combination 

 of gibberellin with endogenous auxin of pea sections forming a com- 

 plex with novel growth properties as has been postulated by Phillips, 

 Vlitos, and Cutler (Contr. Boyce Thompson Institute. 20, 111-120. 

 1959) to account for their gibberellic acid — endogenous auxin studies 

 on pea. 



With respect to Dr. Purves' suggestion that gibberellin cannot 



