Factors Altering Effectiveness 149 



are characteristically the most sensitive to 2,4-D injury (see chapter 

 XVI). In pineapple plants a fairly high carbohydrate level is known 

 to be best for uniform forcing with auxin and it can be assumed that 

 under such conditions the nitrogen level in the plant would not be 

 excessive. 



>- 

 QC 



O 



UJ 



X 



UJ 



13 Days 



TIME AFTER 2,4D APPLICATION 



Fig. 68. Injury to soybean plants by exposure to 20 mg./l. of 2,4-D in a nutrient 

 solution, comparing extent of injury at three levels of nitrogen nutrition (14, 56, 335 

 ppm). Injury was estimated on an arbitrary visual basis (Wolf et al, 1950). 



ENVIRONMENTAL FACTORS 



Environmental factors apparently have a bearing on the respon- 

 siveness of the plant through means which cannot be explained solely 

 on the basis of absorption, translocation, plant developmental stage 

 and nutritive conditions as we understand them. Perhaps the most 

 striking of these unexplained factors is that of temperature. It is 

 well known that maximal auxin effectiveness is obtained at high 

 temperatures and this can undoubtedly be explained in part by the 

 greater absorption which one would expect on the basis of Rice's 



