J. VAN OVERBEEK 23I 



What do we know about the physiology of flower induction in the 

 pineapple? In the first place we know that as minute amounts as 50 

 micrograms of synthetic auxins per plant will cause the vegetative 

 growing point of the pineapple to change promptly into a floral apex 

 (48). We also know that auxin exists inside the pineapple plant, and that 

 the highest concentrations of native free auxin are found in the growing 

 point (54). This auxin has been identified as indoleacetic acid (22). The 

 immediate precursor of this indoleacetic acid in the pineapple plant is 

 indoleacetaldehyde (23), which is stored in considerable quantities in the 

 bases of the youngest leaves (54). Since these leaf bases are located in 

 close proximity to the apex, it is likely that free auxin of the growing 

 point originates from the precursor in the leaf bases of the youngest 

 leaves. It is also known that the pineapple plant contains enzyme 

 systems which convert tryptophan into indoleacetaldehyde and indole- 

 acetic acid. Furthermore from the pineapple leaf an enzyme system has 

 been isolated which inactivates indoleacetic acid (23). It is clear, there- 

 fore, that the pineapple possesses an active auxin metabolism. This is 

 again manifested by the curious geotropic flower induction of the 

 Cabezona variety. When the vegetative plant is put on its side, it will 

 not only right itself by a normal geotropic process, but in addition it 

 will go into the flowering stage completely out of season and without the 

 benefit of treatment with chemicals. The phenomenon has been in- 

 terpreted as being controlled by the native auxin (50). 



Under natural conditions flowering is brought about by a drop in 

 night temperature during the winter (51). Photoperiodic effects seem 

 of minor consequence in the pineapple. At present experimental data 

 are lacking which link temperature and auxin induced flower induction. 

 It has been suggested that this link may be found in the plant's organic 

 acid metabolism (51). Also lacking is a link between these two types of 

 flower induction and the induction by unsaturated hydrocarbons. No 

 evidence was found that ethylene treatment increases the auxin level of 

 the plant; Cooper (12) found no change, while Carl Leopold (personal 

 communication) found a slight but consistent decrease in the free auxin 

 level. 



Conclusions on the physiology of flower formation in the pineapple. — 

 From the evidence available one is confronted with the facts that, on the 

 one hand, an active auxin mechanism exists in the plant and that auxin 



