Hormonal Mechanism of Growth Inhibition by Radiation 553 



excised internode sections of a dwarf pea (treated with lAA) grow 

 much more than the same internode on the intact plant (even if lAA 

 is added to the plant). Gibberellic acid added to excised internodes 

 had only a relatively small effect. (If the intact plant is treated with 

 gibberellic acid, the growth of the internode is 6 times as great as 

 maximum section growth.) 



The critical question here is: How can the excised internode grow 

 more than it does on the plant, when no gibberellic acid is added? 

 Their conclusion is that an inhibitor, formed in some other part of 

 the plant, prevents elongation in the intact plant, while excision of 

 the internode removes the source of this inhibitor. Clearly this inter- 

 node is capable of greater extension, even without added gibberellic 

 acid, than normally takes place in the plant. Thus, some form of 

 growth-limiting effect from another part of the plant must act on the 

 intact internodes. 



RADIATION 



STEM GROWTH 



RADIATION 



RECEPTOR 



leaf 

 growth 



node 

 formation 



y 



hook 

 opening 



nnevalonic acid ( ? ) 



dwarf 

 nnutants 



GIBBERELLIN 



+ auxin 



+ other growth 

 factors 



HIGH PLASTICITY 



water 



high OP in eel 



CELL EXTENSION 

 (GROWTH) 



Fcg. 3. Suggested mechanism by which visible radiation limits stem elongation. 



