PHOTOSYNTHESIS 
(CH50)p 
\ 
| as CATALYZED 
S SS’ S$ S$ | ReactiONS 
Ee i a ! OF PLANT 
= II we | 44] SUBSTRATES 
mS? SAS! 1S (S) 
s 
1 eg | GROWTH 
FH FR OFZ) ON OP FACTORS & 
METABOLITES 
MACROMOLECULES 
SUBCELLULAR STRUCTURES 
INTACT CELLS 
POSSIBLE "SITES OF HERBICIDAL ACTION" 
ENZYMES 
2.SUBCELLULAR STRUCTURES 
3. MEMBRANES 
BN-10804-x 
Figure 4 
Some herbicides are known to act by inhibiting photosynthesis (position 1, figure 4) 
and essentially causing the cell to starvetodeath. Herbicides may also uncouple oxidative 
phosphorylation (position 2) and thereby eliminate many of the biosynthetic reactions 
of the cell, Many herbicides inhibit enzymes catalyzing specific reactions. If this inhibi- 
tion occurs in a metabolic pathway, illustrated bythe block at position 3, the plant may not 
be damaged, for growth factor 2 can still be synthesized through other pathways. However, 
if the metabolic blockoccurs in position 4, then growth factor Fn becomes growth limiting. 
The growth-controlling action of herbicides is not necessarily of a biochemical 
nature. Physical or chemical effects on cell membranes, spindle fibers, or other sub- 
cellular structures may disrupt the normal physiological processes of these highly 
organized cellular components. Most herbicides are known to inhibit many biochemical 
or physiological processes, Controlof growth probably results from the cumulative effects 
of these many inhibitions. 
No attempt will be made to review the results of investigations on the mechanisms 
and sites of action of herbicides but a few examples will be discussed to illustrate the 
interrelationships with the metabolism of herbicides. 
The results of recent investigations on mechanisms of action of herbicides may 
illustrate the importance of basic investigations. In 1947, it was demonstrated that 
nonphytotoxic chemicals could be metabolized within plants to phytotoxic compounds (58). 
This important scientific breakthrough resulted in new concepts on the selective action 
of herbicides (62). Fundamental studies in 1954 showedthat certain plants can metabolize 
the herbicidally inactive 4-(2,4-dichlorophenoxy)butyric acid [4-(2,4-DB)] through the 
process of beta-oxidation to the herbicidally active 2,4-dichlorophenoxyacetic acid [2,4- 
Di. Therefore, weeds and crop plants capable of beta-oxidizing 4-(2,4-DB) to 2,4-D will 
be killed by 2,4-D, while plants not capable of beta-oxidizing 4-(2,4-DB) are tolerant to 
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