Changes in Metabolism During Growth and 



Its Inhibition 



K. V. THIMANN, W. D. BONNER, JR., AND 

 G. S. CHRISTIANSEN 



IN THE growth of isolated sections of Avena coleoptiles it was observed 

 some years ago (5) that iodoacetate acts as a powerful inhibitor. A 

 more careful study of this inhibition and the circumstances surrounding 

 it has revealed some interesting facts. The procedure is simple; seedlings 

 are grown on moist filter paper and the coleoptiles decapitated; three 

 sections each 3 mm. long are mounted on combs floating in indoleacetic 

 acid solution containing i per cent sucrose. After growth in darkness 

 at ifC they are measured, usually at 7, 24, and 48 hours. All conditions 

 are rigidly controlled, for there are numerous possible sources of vari- 

 ability. One of these is the age of the plants from which the sections are 

 cut (10). The younger the plants, the more vigorously the sections grow. 

 However, younger plants are also less sensitive than the older plants 

 to inhibition by iodoacetate. In Figure i (from 10) the growth of the 

 uninhibited controls is in each case set at 100 per cent. It will be seen 

 that coleoptiles 54 hours old yield sections which are only incompletely 

 inhibited even at concentrations as high as iq-^M. At the other extreme 

 the sections from coleoptiles 120 hours old show threshold inhibition 

 even at lo-^M, and at 5 X io~%4 are 100 per cent inhibited. The curves 

 for the intermediate ages show the complication of growth promotion 

 at low concentrations of iodoacetate, but in other respects the sensitivity 

 is intermediate between the two extremes. Thus in general the resistance 

 of the plants to iodoacetate decreases steadily with age. 



A characteristic feature of the iodoacetate inhibition is that it is 

 completely prevented by the presence of certain organic acids (5,10). 



