174 Fundamentals of Auxin Action 



such as abscission, flowering, apical dominance, prolonging dormancy, 

 and modification of herbicide effects. 



An interesting possibility is the use of anti-auxins as systemic 

 fungicides. The work of Crowdy and Wain (1950, 1951) indicates that 

 several materials, such as isobutyric derivatives of phenoxy acids, and 

 2,4,6-trichlorophenoxyacetic acid, are effective in preventing infection 

 by some virus and fungal pathogens. These anti-auxins may them- 

 selves be toxic to the pathogen or they may modify the constitution 

 of the plant in such a way as to prevent infection. 



Epinasty Agents 



Compounds which cause swelling of cells along the vascular 

 strands of leaves, stems and roots are commonly spoken of as epinastic 

 agents. An example of such distortion — the epinastic response — is 

 shown in figure 54. These agents include compounds such as carbon 

 monoxide, unsaturated hydrocarbons (ethylene, acetylene), and many 

 auxins and related compounds. Epinastic agents which are not auxins 

 can often bring about responses which are commonly thought of as 

 being auxin functions. For example, ethylene can stimulate root for- 

 mation, can induce parthenocarpic fruit-set in tomato, and can stimu- 

 late flower initiation in the pineapple plant, as well as cause epinastic 

 distortion of leaves and stems. Ethylene, however, cannot stimulate 

 growth in the absence of auxin. Epinastic agents, then, are not neces- 

 sarily functional as auxins. 



Many auxins, particularly some of the chlorinated phenoxyacetic 

 acids, are powerful epinastic agents. It should be noted that epinastic 

 activity of different compounds will vary from one species to another. 

 For example, 2,4-D produces strong epinastic effect upon tomato 

 foliage, but is much less active on the foliage of tobacco (Zimmerman, 

 1951). 



The nature of the epinastic effect is not at all clear. It may some- 

 times be the stimulation of a general swelling of cells in contrast to 

 an orderly linear elongation, and sometimes only a poisonous effect 

 in altering cell size. From the evidence of the effects of ethylene on 

 the pea test (Michener, 1938), it appears that auxins must be present 

 for epinastic responses to at least some compounds. 



