148 



Fundamentals of Auxin Action 



It should be pointed out here that the nature of the growth 

 regulator used has a strong bearing on the relative susceptibility at 

 any given stage of development. Blackman (1950) points out, for 

 example, that the Hoary Pepperwort is most susceptible to 2,4-D 

 injury at the flowering stage; whereas the greatest sensitivity to 

 2-methyl-4-chlorophenoxyacetic acid occurs at the pre-flowering stage 

 (see table 8). 



TABLE 8 



Changes in Sensitivity of Hoary Pepperwort {Cardaria drabd) to MCPA and 



TO 2,4-D WITH Various Stages of Development 



(Blackman, 1950) 



The stage of development of plants as it bears on herbicide sus- 

 ceptibility and selectivity is discussed further in chapter XVI. 



Nutritive Condition 



The nutritive condition of the plant is a powerful factor in deter- 

 mining the responsiveness to auxin application. This phenomenon has 

 been studied by Wolf et al (1950), who grew soybeans at various 

 nitrogen levels and measured the degree of injury caused by 2,4-D 

 applications to the roots. The relative degree of injury obtained at 

 each of the nitrogen levels is shown diagrammatically in figure 68. One 

 can see that at low nitrogen levels the soybeans showed little injury 

 following 2,4-D application, whereas at increased levels of nitrogen 

 the toxicity became much more pronounced. There is evidence for 

 believing that 2,4-D causes a mobilization of nitrogen into the stems 

 and roots of plants, apparently at the expense of the leaf proteins 

 (Freiberg and Clark, 1952). It is possible that the availability of nitrog- 

 enous materials for such a mobilization may account for the in- 

 fluence of nitrogen supply on susceptibility to 2,4-D. 



Differences in plant constituents which exist at various stages of 

 development may be in large part responsible for the differences in 

 sensitivity with age of the plant. For example, the soluble nitrogen 

 content of rice plants reaches the highest values in the seedling stage 

 and at the time of bolting (Sircar and De, 1948), and these two stages 



