INFLUENCE OF GIBBERELLIN AND AUXIN 333 



subjecting the plant to another dark period, showed that the floral 

 stimulus does not appear in its final form, but first in the form of some 

 labile precursor; this precursor is then converted to the floral stimulus 

 proper, this "stabilization" requiring high-intensity light, or at least 

 being greatly promoted by such a light. Thus, five partial reactions 

 have been proposed: (1) the "first" high-intensity-light reaction; (2) 

 pigment conversion; (3) "preparatory reaction"; (4) synthesis of the 

 floral stimulus precursor; (5) stabilization of the stimulus (second 

 high-intensity-light reaction). All these reactions occur in the leaf; 

 export of the floral stimulus to the buds does not, as a rule, begin until 

 several hours after the end of the dark period. 



It ought to be stated that, the evidence for the above reactions being 

 strictly physiological in nature, it is entirely possible that any of these 

 reactions, perhaps with the exception of pigment conversion, is of a 

 compound character. On the other hand, it is equally possible that 

 some of the reactions are actually identical. Thus, it seems conceivable 

 to me that the two high-intensity-light reactions are the same, and I 

 am also not entirely convinced that the separation of the "preparatory 

 reaction" and the floral stimulus (precursor) synthesis is sufficiently 

 justified. 



These questions are important for the final interpretation of the 

 "locus" of auxin action in photoinduction of short-day plants. At 

 present, we can say three things. First, auxin affects some of the 

 reactions which occur in the leaf; secondly, auxin does not seem to 

 interfere with the "timing mechanism" of induction, but rather with 

 floral stimulus synthesis; thirdly, its action seems to depend on the 

 (first) high-intensity-light reaction. 



Salisbury (1955) and Salisbury and Bonner (1956) showed in 

 Xanthium that auxin applied after the floral stimulus has left the leaf 

 does not reduce the flowering response any more and may even 

 promote it (see later). They also showed that auxin application does 

 not affect the critical length of the dark period, and that auxin must 

 therefore interfere with the later events of the dark period, that is, with 

 some of the events in flower hormone synthesis. Lockhart and Hamner 

 (1954) found that auxin application greatly enhances the flower- 

 reducing action of the "second dark period" (see above). Hamner and 

 Nanda (1956), on the other hand, showed that the inhibitory effect of 



