78 Fundamentals of Auxin Action 



cally confirmed by Gordon and Nieva in their pineapple studies. They 

 observed that the rate of auxin production in leaf breis rose with time 

 as extraction proceeded until the supply of the neutral precursor (in- 

 doleacetaldehyde) was essentially exhausted, at which time the auxin 

 production rate fell off. These changes, graphically shown in figure 

 38, provide evidence that the formation of indoleacetic acid takes 

 place at the expense of the neutral aldehyde precursor. One is struck, 

 however, by the fact that the amount of auxin formed was considerably 



0.1 





O 0.01 



I- 

 < 



a: 



,9 0.001 



o 



[tl 0.000^ 



< 

 q: 



0.000,01 - 



2 4 6 8 10 12 DAYS 



TIME 



Fig. 38. The rates of formation of indoleacetic acid and of indoleacetaldehyde from 

 tryptophan in leaf discs of pineapple (Gordon and Nieva, 1949). 



greater than the amount of indoleacetaldehyde disappearing. This 

 discrepancy is particularly notable in view of the findings discussed 

 below, that two mols of aldehyde are required for the production of 

 one mol of the acid. 



Several lines of evidence indicate that the neutral precursor is 

 indeed indoleacetaldehyde. In the first place, it is readily oxidized by 

 Schardinger's enzyme which is approximately aldehyde-specific. The 

 neutral precursor in pineapple has been shown to form a bisulfite 

 addition product from which it can be regenerated by neutralization. 



