Auxins and the Process of Aging in Root Cells 



169 



an interpretation concerning the phenomenon of root growth and 

 aging in relation to the strength of native auxins. Figure 2 gives the 

 essential points of this theory. The speed of growth passes a maxi- 

 mum at the moment when the level of auxins, which increase with 

 increasing age, becomes supra-optimal. When the root is young, lAA 

 treatment induces an increase in elongation, whereas exposure to 

 light inhibits growth. These facts can be explained if, at this moment, 

 the roots possess only very low levels of auxins. If the roots are old, 

 however, this phenomenon is reversed, for auxin treatment inhibits 

 the elongation while illumination stimulates the growth. Needless 

 to say, the period of stimulation is extremely brief and it can be 

 supposed that in certain cases it is practically non-existent (4,32). 



Auxin Destruction 



Data on lAA destruction, measured by a colorimetric technique 

 (15,26), are plotted in Figure 3. They indicate that lAA destruction 

 is greater in old than in young tissues. The suggestion can be made 

 that lAA-oxidases determine the endogenous auxin level: high en- 

 zyme activity meaning low auxin content (old cells) and vice versa. 

 Meanwhile, recent observations do not exactly confirm this hypothesis. 

 If the lAA destruction is expressed in terms of growth gradients, 

 simple relations between auxin catabolism and endogenous auxins are 

 not so evident. [ See P. E. Pilet, Gradients de croissance et problemes 

 auxiniques. Bui. Soc. Bot. Suisse. 70: (in press) 1960. ] 



In both regions, this lAA destruction increases with the in- 

 creasing age of the roots. The fact, however, that lAA destruction 

 increases with age would seem at first sight to contradict the material 

 discussed above (which shows that the auxin content increases with 

 increasing age). Nevertheless, it can be supposed that these two 



Table 1. Auxin content (acid fraction free auxins) in ng 

 IAA/100 mg fresh weight of root segments. 



