Phenoxyalkylcarboxylic acids and derivatives 
The phenoxyalkylcarboxylic acids are seldom applied as chlorinated acids, Applica- 
tion is usually made in the form of esters, organic amine salts, and other salts. Evidence 
froni many sources suggests that conversion to the free acid is necessary for the ex- 
pression of herbicidal activity. Esters of 2,4-Darehydrolyzed to the free acid by enzymes 
obtained from a wide variety of plant tissues (31,44). Evidence has been obtained in vivo 
to indicate that the isopropyl ester of 2,4-D is hydrolyzed in barley leaves and that the 
acid moiety of the original molecule is then translocated to other parts of the plant (11). 
A new principle for the selective control of weeds in crops was established with the 
discovery that plant species differ in their capacity to B-oxidize the lengthy side chain of 
chloro-substituted phenoxyalkylcarboxylic acids, Certain ones ofthe long-chainchemicals 
are nonphytotoxic unless the plant can degrade the side chain to yield the corresponding 
acetic acid derivative. The literature pertaining to B-oxidationof chemicals in plants has 
been recently reviewed by Wain (61). 
The L-amino acid derivatives of 2,4-D are more active against intact plants than the 
corresponding D-derivatives (39,49). Conversion to the free acid probably occurs by 
enzymatic hydrolysis of the peptide linkage in plants or possibly in microorganisms of 
the soil. Soil microorganisms are responsible for the activation of at least one nontoxic 
chemical, Plants are not killed by sodium 2,4-dichlorophenoxy-ethyl sulfate [sesone] ap- 
plied to the foliage. However, this chemical is converted to 2,4-D in the soil and the toxic 
compound may then be absorbed through the roots (60), The amount of chemical available 
in toxic form for growth-controlling actions partly depends on the plant's ability to convert 
inactive chemicals to toxic ones. 
A second obstacle keeping the herbicide from its site(s) of action may be absorption 
of the free acid at sites not involved in the growth response. Experiments with 2-methyl- 
4-chlorophenoxyacetic acid [MCPA] show that this compound is adsorbed to materials 
obtained from plant tissues (7,8). The resistance of plants to MCPA seemed to be cor- 
related to their ability to absorb the chemical (7). 
There is now considerable evidence that the 2,4-D molecule itself is metabolized by 
plants (2,9,15,16,18,32,35,36,37,38,64,65,66,67). Metabolism of 2,4-D includes (a) minor 
alterations in structure of the parent molecule (35,65), (b) partial degradation with loss 
of the acetic acid side chain (18,36,64,65,66,67), and (c) conjugation of 2,4-D or its 
metabolites with plant substrates (9,15,16,35,38). The quantity and identity of the complex 
2,4-D conjugates formed by plants vary among species (9,15,16), but the selective action 
of the herbicide does not appear to becorrelated with differences in the reaction of 2,4-D 
with plant constituents in susceptible and resistant plants (15). Corn and beans behave 
similarly in their ability to degrade 2,4-D. Both species slowly release the carbon atoms 
of the2,4-D side chain as CO? (66). If degradation of the 2,4-D molecule represents a 
detoxification mechanism, it seems unlikely that this type of detoxification can be re- 
sponsible for the selective action of 2,4-D. Plants have a limited capacity to produce 
CO2 from 2,4-D, and the CO2 released per day probably represents a very small per- 
centage of the 2,4-D applied in the field, Plants can also alter the structure of 2,4-D 
without loss of the acetic side chain (35,65). Hydroxylation of the ring has been sug- 
gested as a possible process in formation of one of the non-2,4-D acids recovered from 
plant extracts (35), Evidence for ring hydroxylation has been obtained for certain 
phenoxy -aliphatic acids (68). 
Investigations on the fate of 2,4-D in plants are in agreement that a relatively small 
percentage of the herbicide can be recovered from metabolically active plants as the 
unaltered molecule. It is not known whether 2,4-D or one of its metabolized forms is the 
toxic principle responsible for the major biological and biochemical changes observed 
in plants treated with 2,4-D. Consequently, the various types of metabolism of the 
herbicide in plants could represent either activation or detoxification mechanisms. 
25 
