34 

 which prevented RNA catabolism. The presence of high levels of ribo- 

 nucleases in other resistent grasses has furthered this hypothesis 

 (Ashton and Crafts, 1973). Wort (1964a) reviewed the effects of 2,4-D 

 on cellular enzymes and documented effects on the activity of 16 enzymes 

 including amylase, ascorbic acid oxidase, IAA oxidase, invertase, 

 phosphorylase, and proteolytic enzymes. These changes were postulated 

 to be caused by changing (1) the cellular conditions such as pH or 

 hydration under which enzymatic progress occurs; (2) the supply of 

 material for enzyme formation; and/or (3) the supply of energy necessary 

 for endergonic reactions through alteration of ATP production (Wort, 

 1964a). In support of these latter two mechanisms, Mostafa and Fang 

 (1971) hypothesized seven sites where 2,4-D either inhibits or regulates 

 respiratory breakdown of glucose thus increasing or decreasing the 

 concentration of various metaboli ties. Various studies have documented 

 inhibition of the Hill reaction and oxidative phosphorylation; however, 

 Ashton and Crafts (1973) consider these effects to be of secondary 

 importance. 



Much of the discussion surrounding 2,4-D mode of action deals with 

 which response is the cause and which is the effect of auxins and auxin- 

 like substances (Audus, 1972). Morgan and Hall (1962) documented that 

 2,4-D treated cotton plants exhibited an increased release of ethylene, 

 a gaseous plant hormone which can cause epi nasty • Ashton and Crafts 

 (1973) attributed the initial epinastic effects to cell elongation and 

 not to an ethylene effect; however, subsequent cell divisions and cell 

 proliferation are perhaps caused by ethylene, resulting in phloem 

 blockage and eventually death. 



