Halohydroxybenzonitriles
I. General Information
A. Halohydroxybenzonitriles first synthesized in 1896 in Germany. Herbicidal activity
reported in 1960.
B. Bromoxynil (Buctril, Rhone-Poulenc Agrochimie, France); ioxynil (TOTRIL, MATE, May and
Baker,j Canada) not registered in U.S., but with greater activity than bromoxynil
C. Postemergence weed control (0.25-1 lb. a.i./acre)
1. Crops: wheat, barley, oats, rye, tricale, corn, sorghum, garlic, onions, seedling
alfalfa, flax, mint, newly seeded turf
2. Weeds: many dicots, especially those missed by phenoxy herbicides
D. Chemistry
1. Nitriles:
2. Bromoxynil:
2. Ioxynil:
II. Physiology of the benzonitriles
A. Mode of Action
1. Inhibitor of photosynthesis
a. photosystem II, D-1 protein binding
b. similar to atrazine
2. Respiration inhibitor
B. Mode of Lethality
1. Contact action on leaves, injury often visible within 24 hours
2. Gradual increase in affected area until significant leaf loss
3. Death by starvation from lack of photosynthetic area
C. Uptake, Movement and Translocation
1. Relatively little movement in plant once absorbed
2. Some apoplastic movement, and to some extent symplastically, in plants
D. Basis of selective action between plant species
1. Differences between R and S species may be function of several factors: spray
retention, absorption, translocation, metabolism
2. Bromoxynil may be hydrolyzed to benzoic acid, although studies are not conclusive
a. slow hydrolysis of the nitrile group to the corresponding amide and benzoic acid
derivatives
b. perhaps dehalogenation and conjugation also occurs
c. perhaps bromoxynil converted to benzoic acid derivative by decarboxylation
3. Resistance in cotton and tobacco:
a. Strain of bacterium Klebsiella pneumoniae isolated which uses bromoxynil as a
primary nitrogen source
b. contains nitrilase enzyme to break bromoxynil down to ammonia and
inactive benzoic acid moeities:
c. gene transferred to tobacco conferring 10-20X resistance
d. gene transferred to cotton via Agrobacterium and expressed as a dominant gene; R cotton
metabolizes bromoxynil to non-toxic metabolites
2. Weed resistance
a. some populations with negative cross-resistance to triazines: i.e. triazine resistant
weeds are more sensitive to bromoxynil than triazine susceptible populations
III. Fate of the benzonitriles in the environment
A. Soil: Short soil life: half-life is about 1 week
B. Water: Oil soluable amine and ester formulations rainfast fairly quickly after
application; sodium salt can be washed off (water soluable)
IV. Plant injury symptomology
A. Contact type injury to leaves: blistered or necrotic spots, later extensive destruction
of leaf tissue
B. Chlorosis may appear later around necrotic spots