THE SYNTHESIS OF FLOWERING HORMONE 167 



the leadership of Richard Lincoln (57), is very excited by a successful 

 extract from cocklebur. The substance E of Nitsch (see Chapter 4) 

 is also interesting. Unfortunately, we now have some rather serious 

 doubts about the meaning of success in this experiment. It may very 

 well be, as with the gibberellins (see Chapter 4), that a substance 

 extracted from plants may lead to flowering when applied to vegeta- 

 tive plants, even though it was not itself a flowering hormone. It 

 may simply trigger the production of the hormone in some way, or 

 the plant may fail to flower because it lacks something besides the 

 flowering hormone. Such a substance would be of considerable 

 interest, but it might not fit the rigorous definition for the flowering 

 hormone upon which we must insist: a specific material which is 

 synthesized in one location in response to the proper environment 

 and translocated to the tips, causing flowering. 



2. The Principle of Antimetabolite Action 



In order not to become embroiled in this time-worn problem of 

 extracting the flowering hormone, I tried the more indirect approach 

 of applying chemicals to whole plants and observing the effects on 

 flowering. But what compounds should be tested ? 



A number of chemicals are known to inhibit certain biochemical 

 reactions. In some cases this inhibition is quite a specific thing. A 

 classic example is the inhibition of respiration by malonic acid, which 

 competes with succinic acid for a position on the enzyme succinic 

 dehydrogenase. Competition occurs because the structures of succinic 

 and malonic acids are very similar, and both may become attached 

 to the enzyme. Malonic acid cannot be dehydrogenated, however, 

 and thus the reaction is blocked. 



Let us assume that malonic acid takes no part in any phase of the 

 plant's biochemistry except to block production of fumaric acid by 

 dehydrogenation of succinic acid. Then if we apply malonic acid to 

 a cocklebur plant and flowering is inhibited, we might conclude 

 quite logically that flowering some way depends upon the conversion 

 of succinic to fumaric acid. If on the other hand, malonic acid 

 blocks some other reaction, then our conclusion would not be valid. 

 Furthermore, a plant cell might metabolize the malonic acid so that 

 it never has a chance to interfere with any reaction. In the cocklebur 

 (or the Japanese morning glory) the last possibility seems to be the 

 correct one, since application of malonic acid does not inhibit 



