32 

 (Ashton and Crafts, 1973; Mullison, 1982) and the rate and type of for- 

 mulation applied (Weaver, 1972). At low application rates, responses are 

 typical of those caused by plant growth substances and may induce 

 rooting, blossom set, ripening of fruit, and delaying preharvest drop 

 (Wort, 1964b; Weed Science Society of America, 1979). At higher appli- 

 cation rates, 2,4-D, typically causes epinasty or downward twisting 

 and bending of stems and petioles and curling of leaves (Cardenas et 

 al., 1968; Weaver, 1972; Black and Buchanan, 1980; Mullison, 1982). 

 Young leaves cease expanding due to cell elongation, photosynthesis is 

 reduced, and chlorosis may occur (Turkey et al . , 1945; Van Overbeek, 

 1964; Aston and Crafts, 1973). As the herbicide is translocated through 

 the plant, mature parenchyma cells tend to first swell and then divide 

 radially more rapidly producing callus tissue and root primorida which 

 results in the blockage of phloem tissue and the cessation of assimilate 

 transport in the phloem (Turkey et al . , 1945; Van Overbeek, 1964; Ashton 

 and Crafts, 1973). Meristem activity is inhibited and new organ or 

 lateral bud growth may occur (Weaver, 1972). Growth of mature or 

 primary roots is inhibited and roots may lose the ability to take up 

 water and salts (Van Overbeek, 1964; Wort, 1964a; Cardenas et al . , 1968; 

 Mullison, 1982). The progression of these responses ultimately leads to 

 the withering, collapse, and death of 2,4-D sensitive species due to a 

 combination of new and irregular leaf and root growth and inadequate 

 nutrition because of phloem blockage (Ashton and Crafts, 1973). At high 

 application rates, 2,4-D functions as a contact herbicide, does not 

 translocate throughout the plant and thus may not completely kill 

 meristematic tissue (Ashton and Crafts, 1973). 



