36 



Metabolism by Plants 



A herbicide has been defined as a compound which deranges the phy- 

 siology of a plant over a period long enough to kill it (Van Overbeek, 

 1964). Thus, those plants which can more effectively metabolize or 

 inactivate the herbicide generally exhibit less sensitivity to the 

 herbicide. Numerous studies indicate that plants exhibit varying abili- 

 ties to either metabolize or inactivate 2,4-D once it has entered the 

 plant (Weintraub et al . 1952; Fang, 1958; Crafts, 1964; Van Overbeek, 

 1964; Ashton and Crafts, 1973; Feung et al . , 1978). The most common 

 mode of metabolism appears to be decarboxylation and hydrolysis to 

 free phenol (Crafts, 1964; Ashton and Crafts, 1973). One of the first 



investigations into the metabolism of 2,4-D by plants was conducted by 



14 

 Weintraub et al . (1952) utilizing radioactive 2,4-D with C in either 



the carboxyl , ethylene, or ring positions. These studies indicated that 



14 

 COo was readily released from the carboxyl position and was not 



released from phenol ring positions. Weintraub et al . (1952) also 

 documented the presence of a wide variety of metabolities. Ashton and 

 Crafts (1973) suggested that a plant's ability to oxidize carboxyl and 

 ethylene carbon atoms of 2,4-D correlated with its tolerance to 2,4-D. 

 Other mechanisms of 2,4-D metabolism or inactivation by plants include 

 (1) ring hydroxylation followed by oxidation of hydroxyls to car- 

 boxyl s with ring splitting (Crafts, 1964; Ashton and Crafts, 1973; Feung 

 et al . , 1978); (2) complexing with proteins (Fang, 1958; Crafts, 1964; 

 Van Overbeek, 1964); (3) complexing with amino acids followed by ring 

 hydroxylation (Feung et al . , 1978); (4) sugar conjugation (Feung et al . , 

 1978); and (5) ring hydroxylation followed by sugar conjugation (Feung 

 et al . , 1978; Weed Science Society of America, 1979). 



