Although not well studied, the function of plastocyanin (the electron donor for PSI) is affected by
potassium cyanide as well as by mercuric chloride. These compounds probably interfere with the cy-
tochromeb 6 ƒ complex [5,22] (see Figure 1).
RESISTANCE Triazines are metabolized in naturally resistant crops by several pathways, including 2-
hydroxylation, conjugation with glutathione, and N-dealkylation of lateral chains, followed by oxidation.
For example, corn uses these three routes. The resistance to substituted ureas is due to limited absorption,
rapid degradation by cytochrome P-450 monooxygenase, or oxidation [23–25].
Currently, transgenic plants are being obtained by the incorporation of a bacterial gene that codifies
an enzyme that specifically produces the hydrolysis of bromoxynil.
Resistance to triazine should be due to mutations in the gene psbAthat produces a change of serine
to glycine and causes D1 not to be susceptible to triazines [26]. This gene is codified by chloroplast DNA;
therefore the resistance to triazine is maternally inherited [27]. Other mutations confer different resistance
spectra on triazines, ureas, and uracils. Thus, some weed biotypes have been made resistant upon in-
creasing the expression of glutathione S-transferase and detoxify atrazine upon conjugating it with gluta-
tion [28]. Other biotypes are resistant to all PSII inhibitors because of the activity of cytochrome P-450
monooxygenase [29,30].
B. Uncouplers
Uncouplers act by dissociating ATP synthesis of electron transport by dissipating the energized state of
the thylakoid membrane. There are many uncouplers, such as ammonium, CCCP (carbonyl cyanide m-
chlorophenyl-hydrazone), FCCP [carbonyl cyanide p(trifluorometoxy) phenyl-hydrazone] and grami-
cidin, that are not herbicides. The only herbicide known that acts purely as an uncoupler in photophos-
phorylation is perfluidone, which has this capacity when the pH is about 8. This herbicide is protonated
in the interior of the thylakoid and deprotonated in the stroma, thus breaking the proton gradient and pre-
venting ATP synthesis [31,32].
C. Energy Transfer Inhibitors
Herbicides of this group inhibit electron transport as well as ATP formation in coupled systems. How-
ever, if an uncoupler that dissipates the proton gradient is added, inhibition of electron flow is relieved
but without ATP formation. The inhibition by these compounds is on the phosphorylation but in a step af-
ter the action of uncouplers.
Several types of phenylureas have been reported to act as energy transfer inhibitors of photophos-
phorylation under certain conditions [5,33]. Nitrophen and related diphenyl ethers may inhibit by bind-
ing directly to the coupling factor and preventing the ADP exchange [5,34]. Nonherbicidal compounds
that behave as energy transfer inhibitors include the antibiotic Dio-9, phlorizin, DCCD, TBT, etc. [4,35].
D. Inhibitory Uncouplers
The herbicides affecting electron transport as well as the proton gradient are found in this group. Like
DCMU, they inhibit electron transport from water and do not inhibit reduction of NADPwith ascorbate
plus DPIP, but unlike DCMU they inhibit noncyclic phosphorylation. Cyclic phosphorylation assayed in
the absence of oxygen is also inhibited by treatment with herbicides of this group. With compounds such
as dinitrophenol, an uncoupler action is observed at approximately pH 6; however, electron transport is
inhibited at a pH higher than 8 [36]. Herbicides of this group have been provided that affect the perme-
ability of thylakoid and mitochondrial membranes [37]. To this group belong: acylanilides, bro-
mofenoxim, dinitrophenols, imidazoles, pyridinols, etc. [32,38].
E. Electron Acceptors
Compounds classified within this group are able to compete by electrons with some components of the
electron transport chain. Certain bipyridyliums can compete by electrons with the acceptor of PSI and
have herbicidal activity (site D in Figure 1). They are bivalent cations that intercept the electrons between
ferredoxin and NADPand then reduce oxygen to superoxide. The superoxide reacts with a wide range
PHYSIOLOGICAL MECHANISMS OF HERBICIDE ACTIONS 777