Role of Pesticides in Produce Production, Preservation, Quality, and Safety 367
11.2.7.2 Herbicide Selectivity
Herbicide selectivity is the susceptibility or tolerance of different plants to herbicide
application. Nonselective herbicides kill all plants, whereas selective herbicides kill
only certain plants (preferably only the weeds and not the crops). Selectivity may
be based on various factors including mainly biochemical and morphological dif-
ferences between the weed and crop plants and also physicochemical properties of
the herbicide [1].
Selectivity based on biochemical differences relies mainly on different enzymatic
systems (metabolic insensitivity) or detoxification rates of herbicides (metabolic ability)
of crops and weeds [71]. For instance, the synthetic auxin 4-(2,4-dichlorophe-
noxy)butyric acid (2,4-DB) needs the enzyme beta-oxidase for its conversion to the
active herbicide 2,4-D [5]. Several species of leguminous plants are resistant to 2,4-DB
because they largely lack beta-oxidases, thus, this herbicide can be safely used for
selective postemergence weed control of these crops. Another example is the tolerance
of corn plants to triazines, chloroacetamides, and thiocarbamates, which is attributed
to their rapid detoxification by corn glutathione S-transferase (i.e., their fast conversion
to innocuous metabolites by conjugation with glutathione). This detoxification can be
accelerated using so-called herbicide safeners (also considered as pesticides), such as
benoxacor or flurazole, which enhance metabolism of certain herbicides mainly by
inducing the detoxification enzyme production. The possibilities of biochemical selec-
tivity may be significantly extended using modern biotechnology methods that can alter
the herbicide’s action site. For example, “Roundup Ready” soybeans produce an excess
of 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), the enzyme that is inhibited
by glyphosate (Roundup), and therefore an appropriate glyphosate application does not
affect these genetically modified soybeans [71].
Morphological differences between crops and weeds mainly include the size and
orientation of the leaves, character of the leaf surface, and rooting depth. Dicotyledonous
plants (e.g., broad-leaved weeds) have a larger surface area and the meristematic tissue
is exposed to the herbicidal spray, whereas the leaves of monocotyledonous plants
(e.g., grasses) are narrow and in upright positions, which reduces the application
area and access to the meristematic regions. This difference accounts, for example,
for the selective toxicity of synthetic auxins (2,4-D and others) toward broad-leaved
weeds [1]. The character of the leaf surface determines the herbicide’s penetration
ability. Generally, the more waxy and hairy the surface, the more difficult the
absorption of a foliage-applied herbicide. Similarly, the more deeply rooted the plant
is, the more difficult the uptake of a soil-applied herbicide.
Physicochemical properties of herbicides may also play an important role, par-
ticularly water solubility and mobility in soil, in the case of uptake by the roots. For
instance, the selectivity of triazines is a result of their low water solubility combined
with a fairly high degree of adsorption onto soil colloids. Thus, they penetrate only
a few centimeters downward in the soil, so deep-rooted plants are not affected,
whereas shallow-rooted or germinating weeds are killed [1]. On the other hand, the
foliage-applied bipyridylium herbicides (diquat and paraquat) and glyphosate
strongly bind to soil colloids, and they are useful in directed applications because
only those plants hit by the spray are affected and there is no uptake from the soil.