Herbicides 



277 



FACTORS IN SUSCEPTIBILITY 



Stage of Development 



Perhaps the most outstanding factor which influences the suscep- 

 tibility of plants to auxin herbicides is the stage of development of 

 the plant at the time of treatment. Judging from the work on suscepti- 

 bility by Blackman's group (summarized by Blackman, 1950) and the 

 work in America pioneered by Shaw and Willard (1949), it would 

 appear that susceptibility of grains follows a consistent general pat- 

 tern during the various stages of growth. Such a pattern is presented 

 in a diagrammatic fashion in figure 114. There is a very low degree of 

 susceptibility in the ungerminated seed. Upon germination and the 

 commencement of growth, sensitivity to auxins increases at once and 

 an extremely high sensitivity is maintained through the early stages 

 of seedling growth. This high degree of sensitivity falls off in the case 

 of corn at the 6- to 8-leaf stage (Rodgers, 1952), in the case of oats, 

 wheat and barley at the time the plants are five inches in height and 

 have two leaves (Olson et al, 1951). During the tillering stages of 

 wheat and barley, susceptibility remains low. Upon flower initiation 

 susceptibility becomes rather high again and remains so until the 

 stages of rapid growth associated with bolting have passed. After 

 heading the susceptibility again drops off. 



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 UJ 

 O 

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Dry Seedling 

 Seed 



Tillering 



Flower 

 Initiation 



Flowering Fruiting 



STAGE OF DEVELOPMENT 



Fig. 114. A schematic diagram of changes in relative susceptibility of grains to 

 auxin herbicides applied at various stages of development. 



