41 6 CRAFTS 



2,4, 5-T, the 2,4-dichloro-5-methyl, and the 2-methyl-5-chloro series failed to 

 produce the response, whereas the 4-chloro, the 2,4-D, the 2-methyl-4-chloro, 

 the 3,4-D, and the 3-methyl-4-chloro series produced the normal alternation 

 in activity. This suggested to Wain that possibly some plants lack the neces- 

 sary oxidation enzyme system to degrade the higher homologues down to 

 the acetic acid compound where the above three ring substituents are present. 

 Testing proved this to be true, and Wain reasoned from this situation that it 

 might be possible to use certain of the butyric, caproic, and octanoic deriva- 

 tives as selective toxicants against weeds in crops. Selectivity in this case 

 would depend upon the enzyme make-up of the weed and crop plants, the 

 weeds being able to degrade the herbicide to the acetic compound whereas 

 the crop would be unable to do so. 



Trials have proved that the above mechanism of selectivity is effective. 

 Some weeds that respond are annual nettle (Urtica urens), Canada thistle 

 (Cirsium arvense), fumitory (Fumaria officinalis), knotgrass {Polygonum 

 aviculare), and mustard {Sinapis arvensis). Some crops that tolerate these 

 compounds are celery, carrot, parsnip, flax, red clover, white clover, and 

 alfalfa. Undoubtedly more crops that are tolerant and weeds that are sus- 

 ceptible will be discovered as testing proceeds. 



The above work opens a whole new field for weed-control research. Not 

 only do plants differ in size, structure, and appearance, they also vary in 

 chemical composition and in their enzyme systems. This variation provides 

 the basis for many new selectivities dependent upon the presence or absence 

 of specific enzymes. One can scarcely visualize the limits of the application of 

 this principle. Obviously, specific herbicides for specific weeds are to be de- 

 sired, and from our knowledge of the biochemistry of plants, it seems hopeful 

 that a great number can be found. I know of no area in the fields of bio- 

 chemistry and plant physiology that offers greater promise to the researcher. 



Mechanisms of action. Auxin physiologists for years have sought the bio- 

 chemical mechanism by which lAA affects growth in plants. Because the 

 formative (auxin) effects of weed killers seem related to the toxic effects 

 (Weintraub, 1952), physiologists interested in the mode of herbicidal action 

 of the 2,4-D class of herbicides have tried to draw a parallel with that of 

 auxin. 



In 1938, Koepfli et al. set forth the following structural requirements for 

 auxin activity: (a) a ring system as the nucleus, (6) a double bond in this 

 ring, (c) a side chain, (d) a carboxyl group, or a group readily converted into 

 a carboxyl, on the side chain, with at least one carbon atom removed from the 

 ring, and (e) a particular space relation between the ring and the carboxyl 

 group. 



During the years since 1938 exceptions to most of these requirements have 

 been found and many modifications have been proposed. Theories suggested 



