278 Auxins in Agriculture 



In the generalized graph tor grains given here the ordinate might 

 be an expression of the percentage of plants killed with a given dosage 

 of an herbicide, or it might be an expression of the degree of crop 

 injury or the reduction in yield following the application of the herbi- 

 cide at a low concentration. In several instances small gains have 

 actually been reported to follow applications of weak auxins during 

 the period of low sensitivity prior to flowering (Shaw and Willard, 

 1949). 



A generalization seems much more difficult in the case of di- 

 cotyledonous plants. A remarkable degree of variation in susceptibility 

 with stage of development is pointed out by Blackman (1950). He has 

 found that the susceptibility of pepperwort and dandelion can vary in 

 strikingly heterogeneous patterns depending on the growth regulator 

 used. For example, as the growing season progresses pepperwort be- 

 comes less and less susceptible to MCPA (2-methyl-4-chlorophenoxy- 

 acetic acid). Yet it shows an increasing susceptibility to 2,4-D until 

 flowering time followed then by decreased susceptibility. The dande- 

 lion shows only small changes in susceptibility to MCPA during the 

 growing season, whereas its susceptibility to 2,4-D increases progres- 

 sively from April to October. We may conclude then from Blackman's 

 work that susceptibility in dicotyledonous weeds may change in any 

 of several different possible fashions during successive stages of devel- 

 opment. 



It appears to be safe to say that almost all plants undergo a 

 period of maximum susceptibility in the early seedling stages, and of 

 low susceptibility after fruiting has occurred. This was first pointed 

 out by Lee and Bewick (1947). The susceptibility of a plant between 

 early seedling stage and fruiting varies with the species and the 

 herbicide used, but in general it would seem that many plants show 

 a trend toward decreased sensitivity as the plant grows older. 



In a study of changes in sensitivity of excised epidermal tissues 

 taken from plants at various stages of development, Biebl (1953) 

 found that tissues from seedlings were generally more susceptible to 

 2,4-D than similar tissues from older plants. No changes in suscepti- 

 bility were found in later stages, suggesting that susceptible stages 

 other than the first one were due to morphological differences within 

 the plant rather than to overall differences common to all tissues. 



Growth Rate 



Another factor in susceptibility is the growth rate of the plant. 

 In discussing the anatomical effects of auxin applications in chapter 

 V, it was pointed out that cells which are young and weakly differ- 



