Differential Responses in Crop Plants 



R. S. DUNHAM 



ONE of the most important characteristics of 2,4-dichlorophenoxy- 

 acetic acid is its selective action. Although the reasons for this 

 selectivity are not fully understood, it has been the basis for a wide 

 practical use. Selective action becomes a distinct advantage because it 

 permits the spraying of certain weeds growing in tolerant crops. Studies 

 of the practical application of selective herbicides are necessarily con- 

 cerned with the reaction of the crop as well as the weeds. 



Research in this phase of investigation soon showed that not only were 

 there differential responses of certain species classed as weeds and others 

 grown as crops but that there were different reactions among crop plants 

 and, even further, that varieties and strains of the same crop might 

 show varying tolerance or resistance to applications of herbicides. Later 

 it was learned that the same variety or strain may be influenced in 

 reaction by environmental conditions. It is the purpose of this paper 

 to discuss such differential responses among field-crop plants, particularly 

 the small grains, flax, and corn. The different phases of the problem will 

 be discussed separately by reviewing the pertinent literature and pre- 

 senting short summaries of new information from unpublished experi- 

 ments of the writer and co-workers. 



Selective Action of 2,4-D 



Species and varietal differences. — Norman (39) points out that selectivity 

 is only an apparent one based on different degrees of susceptibility to 

 a particular dosage appUed in a particular way, and Crafts (10) states 

 that even tolerant plants succumb provided the concentration is high 



Editor's Note: Paper No. 2503 of the Scientific Journal Series, Department of 

 Agriculture, University of Minnesota, St. Paul, Minnesota. 



