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imi.html
ALSase Inhibitors:
Imidazolinones
Introduction
Developed by American Cyanamid, coincided with development of sulfonylurea herbicides.
Share many characteristics with that family.
Members include imazaquin (Scepter), imazethapyr (Pursuit; 1984), imazapyr (Arsenal;
non-selective), imazamethabenz
Imazaquin is used for broad spectrum weed control (cocklebur, pigweed spp., Pennsylvania
smartweed) in soybeans. It can be applied preemergence, preplant incorporated or
postemergence. Soil applications require rainfall or irrigation to move the herbicides
into the soil water for plant uptake.
Herbicides in this family have exhibited physical incompatibility with 2,4-D and MCPA when
tank-mixed.
The imidazolinones were the second family introduced that have high specific activity: low
(grams) active ingredient application rates per acre.
Chemistry
Despite many similarities between the imidazolinones and the sulfonylureas, structurally
they are quite different.
Physiology and Metabolism of the Imidazolinones
Mode of Imidazolinone Action
The imidazolines act by inhibiting branched chain amino acid (valine, isoleucine,
leucine) biosynthesis. Specifically, they inhibit the catalytic action of acetolactate
synthase (ALSase), also known as acetohydroxyacid synthase (AHASase; the name prefered by
American Cyanamid). Refer to sulfonylurea mode of action discussion.
Mode of Imidazolinone Lethality
Death results from starvation for needed proteins in growth. Refer to sulfonylurea
mode of action discussion.
Uptake and Movement of Imidazolinones in plants
Imidazolinones are readily absorbed by roots and shoots.
They translocate both apoplastically in the xylem and symplastically in the phloem.
They readily move to the plant meristem where they exert their toxic action, growth
stopping fairly quickly.
Basis of Selectivity between Plant Species
In general, imidazolinone herbicides are more active against dicot species relative to
monocot species.
Imazethapyr selectivity is not based on differences in uptake or translocation, nor is it
based on differences in sensitivity of the ALSase target in plant species. It is based on
metabolic detoxification differences between species.
Herbicide Resistance.
-Most weed species found resistant to sulfonylurea herbicides have been shown to also be
resistant to imidazolinone herbicides.
-Past work by Molecular Genetics Co., in Minnesota, found imazaquin resistant corn cell
lines that were subsequently grown to whole plants. These resistant lines were given to
Pioneer Seed Co. which has now developing a high-yielding line of corn seed resistant to
imazaquin. Garst introduced an imidazolinone resistant corn hybrid in 1991. Many feel this
is the solution to imazaquin carryover problems
Fate of the Imidazolinones in the Environment
Soil
There is incomplete information on the fate of many imidazolinone herbicides in the
soil.
The persistence of them in the soil varies, but many of the more commonly used members are
persistent in the soil and can persist until subsequent crops in a rotation are grown,
causing injury.
Imazaquin persistence in the soil
-Probably a function of hydrolysis in the sunlight: Photohydrolysis. This mechanism is not
well characterized.
-Soil pH does not appear to play the dominant role in degradation that it does with the
sulfonylureas.
-Imazaquin persists longer under droughty conditions, with cold weather, and in the
absence of tillage.
-Some breakdown of imazaquin in aerobic soils is microbial, little breakdown occurs under
anaerobic conditions.
-Imazaquin will persist for long periods of time in the soil because photohydrolysis
degradation requires the presence of both light, herbicide and moisture. Typically,
imazaquin is carried into the soil by rainfall where light is not present. If conditions
are dry this will not occur, light will be present but moisture will not be present.
The summer of 1988 in Iowa created conditions that led to extensive imazaquin carryover in
the 1989 maize crop. Imazaquin was widely used in the state, including in the Des Moines
lobe glacial soil region typified by high pH soils originating from calcareous parent
materials. Application of imazaquin in soybeans was preceded, and followed, in most areas
by very dry weather. This also resulted in soybean injury in many, if not most, fields.
Take a look at a case study from near Madrid, Boone Co., Iowa
for how those injury symptoms looked that year. When imazaquin was first registered for
commercial use, it was not registered for weed control in Iowa. Later, the label was
rewritten to include Iowa. Soybean plants were subsequently injured, probably due to
unfavorable plant growth and detoxification metabolism necessary for crop safety. Many
corn growers were concerned with the effect these potential imazaquin residues might have
on their 1989 corn crop. Moldboard plowing was extensively used in many fields in the fall
of 1988 in an attempt to dilute the imazaquin residues in the soil profile and thereby
avoid crop injury in 1989, partially solving the immediate problem but causing extensive
winter wind erosion with this tillage.
Air
Imazaquin is not lost from the soil due to volatility. It can drift off-target and cause
injury to susceptible species such as corn, cotton or vegetable crops.
Water
Imazaquin solubility in water is 60-120 ppm at 25oC.
Animal Toxicity
ALSase does not occur in animals which rely on plants for these essential amino acids.
As such, the primary toxicity of these herbicides is plant specific. Toxicity is similar
to that of the sulfonylureas
Plant Injury Symptomology of the Imidazolines in Plants
Symptoms often develop slowly.
Plants often emerge and symptoms develop after that time.
Susceptible plants stop growing due to meristem inhibition. Meristems can become necrotic
at this point. This inhibition of apical dominance can lead to bud proliferation lower on
the stem of some species (e.g. morningglory, Imopoea spp.).
Soybeans can exhibit internode shortening under stressful growth conditions such as
excessive rainfall or cool temperatures.
Young leaves become deformed, chlorotic (sometimes interveinal) and purple, anthocyanin,
discolorations can occur.
Most of these symptoms are held in common with sulfonylurea herbicides.
Chlorimuron-ethyl injury can sometimes be differentiated from that caused by imazaquin by
better cocklebur and foxtail spp. control with imazaquin, and better velvetleaf control
with chlorimuron-ethyl.
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