THE SYNTHESIS OF FLOWERING HORMONE 175 



synthesis in the leaf. The fact that it is effective when applied during 

 the period of hormone synthesis must be some sort of coincidence 

 arising from a delay in its action. 



What is the evidence that 5-FU is effective only in the bud and not 

 in the leaf as well ? Tracer studies seem to indicate that 5-FU will 

 move readily from the leaf to the bud (in small amounts, at least), 

 but not from the bud to the leaf. Orotic acid will reverse the effect 

 of 5-FU on flowering, and its incorporation into ribonucleic acid 

 (RNA) in the bud is inhibited by 5-FU. Thus it seems clear that 

 5-FU is inhibiting processes in the meristem related to conversion to 

 the reproductive condition. 



Is this the only action of 5-FU and similar compounds ? We are 

 not yet certain. Sometimes bud applied 5-FU will move to the leaf, 

 at least in trace amounts. Perhaps more important, in preliminary 

 experiments leaf applied 5-FU (which clearly inhibits flowering) has 

 inhibited RNA synthesis in the leaf but not in the bud. When the 

 problems are finally resolved, the anti-nucleic acid compounds may 

 tell us a lot about the biochemistry of flowering. 



3. Hormone Synthesis and the Critical Dark Period 



Could time measurement be a matter of the time it takes to 

 synthesize a certain minimal amount of flowering hormone ? Prob- 

 ably not, for reasons discussed in Chapter 8 and also because time 

 measurement and hormone synthesis are influenced in different ways 

 by chemicals. Cobaltous ion inhibits during the critical dark period 

 but not later, and furthermore, it extends the critical night. But how 

 can we tell that compounds which are effective during hormone 

 synthesis are not also inhibiting time measurement ? 



The first approach is to determine the critical night for control 

 plants and for plants treated with the compound in question. 

 Cobaltous ion causes an extension of the critical dark period, as 

 shown in Fig. 9-12. Most other compounds fail to increase the 

 critical night, and we can probably eliminate them as having an 

 influence upon time measurement. Yet two compounds, ethionine 

 and picolinic acid, do extend the critical night. Is it possible that 

 these compounds influence only synthesis of hormone and not timing ? 

 If a certain amount of the compound inhibited the synthesis of a 

 certain amount of flowering hormone, then it might take longer to 

 produce the first effective amount of hormone, and critical dark 



