WEED control: applied botany 415 



rapidly translocated, highly selective, and effective through the soil as well 

 as through the foliage. Production skyrocketed to 20 million pounds in 1948 

 and has continued to rise since then. Meanwhile, production of 2,4,S-T has 

 reached 5 million pounds per year, the use of MCP to control weeds in cer- 

 tain crops is increasing, 2,4-D and 2,4, 5-T propionic acids in various formula- 

 tions are coming into use for woody-plant control, and a whole series of 

 chlorinated phenoxy butyric acids is being tested. 



These compounds have brought a new principle to the field of weed con- 

 trol ; in contrast to the older, highly toxic chemicals, they are all growth regu- 

 lators: they act slowly; in many cases they bring about severe formative 

 effects; apparently they kill by disturbing plant metabolism. Physiologically 

 they present a dilemma, for their effective use depends upon translocation 

 through living tissues, and at the same time they must be toxic enough to 

 destroy roots. 



Selectivity of the phenoxy compounds has been studied from many angles, 

 but it still is not clearly understood. In general, plants of the grass family 

 tolerate the chemical, whereas broad-leafed plants are susceptible. Selectivity 

 must relate to the specific properties of the protoplasm. Tests have shown 

 that the grasses do not absorb and translocate these compounds as readily 

 as do the broad-leafed plants, and yet the differences are not great enough to 

 account for the observed selectivity. It has been suggested that 2,4-D acts on 

 or through some specific enzyme system in the plant, and yet many studies 

 have failed to discover which enzyme is involved. And overshadowing the 

 whole subject of selectivity is the fact that it is only relative; grasses may be 

 killed in their seedling stages; cereal crops affected in their seedling stages 

 show injury to their inflorescences; and a number of broad-leafed plants, 

 including strawberries, flax, coffee, Russian knapweed, Aruba, and others, are 

 tolerant. Furthermore, MCP is more selective than 2,4-D, particularly on 

 certain legumes. And many woody species respond to 2, 4, 5-T that are resistant 

 to 2,4-D. Recently it has been shown that 2,4,5-trichlorophenoxypropionic 

 acid will kill a number of oak species that are not affected by the acetic acid 

 compounds. 



A number of new growth regulators, the gamma chlorophenoxy butyric 

 acids, are currently being tested. Working with a series of gamma phenoxy 

 alkylcarboxylic acids, Synerholm and Zimmerman (1947) found that alter- 

 nate compounds in a homologous series caused leaf epinasty in tomato. 

 The compounds having activity were the acetic, butyric, caproic, and hep- 

 tanoic derivatives, and these workers proposed that these were being degraded 

 to the acetic acid by /3-oxidation, the mechanism responsible for the break- 

 down of fats in the animal body. In careful studies Wain and his coworkers 

 at Wye College (1955) have shown that this behavior is consistent in wheat 

 cylinder growth tests for a number of homologous series of gamma phenoxy 

 acids. However, in the pea curvature test and the tomato epinasty test the 



