Use correct sanitation to avoid the expansion of resistant biotypes.
Integrate different methods (chemical, mechanical, or cultural) of weed control.II. PHOTOSYNTHESIS INHIBITORS
The herbicides of this group bind to specific sites of chloroplasts and generally lead to slow death of the
plant; however, on occasion, the plant experiences a more rapid death because of the production of toxic
secondary products. Photosynthesis inhibitors can be classified according to their mode of action into the
following groups.
A. Electron Transport Inhibitors
Photosynthetic electron transport can be inhibited by inactivation of one of the redox mediators or by
competition at the action site of this mediator.
Most of the inhibitors classified here have their action site near photosystem II [3–5] (Figure 1). This
is observed in the inhibition of noncyclic electron transport; on the contrary, the cyclic transportation
around photosystem I is maintained as well as the reduction of NADPas long as an artificial electron
donor is added with a redox potential between those of the two photosystems (for example, DPIP ascor-
bate) [6].
The reaction center of photosystem II (PSII) is formed by two proteins of 32 kDa (D1 and D2), both
of which are found in the interior of the thylacoid membrane. Near them are localized the transducer pig-
ment P680, a molecule of pheophytin, the complex acceptor of electrons of PSII consisting of two
quinones (QAand QB), and a nonheme iron atom (Figure 2). When pigment P680 is excited by the arrival
of a photon, the electron of higher energetic level passes to the pheophytin, which immediately reduces
to QA; then QAis reoxidized in a tenth of a millisecond by QB, which has the capacity to accept two elec-
trons. Once QBreceives the two electrons, captures two protons and loses affinity to its union site, being
substituted for an oxidized quinone QB[7–9].
PHYSIOLOGICAL MECHANISMS OF HERBICIDE ACTIONS 775
Figure 1 A scheme showing photosynthetic electron flow within the chloroplast from water to NADP.
Open arrows indicate light reactions, solid arrows indicate dark reactions. A, Site of action of ureas, triazines,
and phenol-type herbicides. B, Site of inhibition by analogues of plastoquinone. C, Site of action for compounds
that interfere with cytochrome bfcomplex. D, Site of diversion of electron flow by bipyridyls.