Isoxazolidones
I. General Information about the Isoxazolidones
A. Clomazone (Command) by FMC Corp. in 1984; first used in Iowa 1986
B. Clomazone used in soybeans (also pumpkins, peas, sweet potatoes) applied preemergence
or preplant incorporated. Use now limited to incorporated treatments.
C. Control of foxtails, velvetleaf, lambsquarters, purslane, panicum grasses, crabgrass;
weak on pigweeds, sunflower
D. Chemistry:
1. Azoles:
2. Isoxazoles ( 1 nitrogen azoles):
3. Isoxazolidone general structure:
4. Clomazone:
II. Physiology of the Isoxazolidones
A. Mode of Isoxazolidone Action
1. Uncertain, but probably inhibition of either isopentyl pyrophosphate isomerase
(IPPIase) or between farnesyl-PP and geranylgeranyl-PP, an enzyme that acts at the
beginning of the isoprenoid pathway responsible for biosynthesis of chlorophyll,
carotenes, xanthophyll and other pigments
2. General scheme of isoprenoid biosynthesis:
B. Mode of Isoxazolidone Lethality
1. Lack of leaf pigment development: chlorphyll accumulation, phytylation and membrane
integration are reduced
2. Photooxidation of leaves, loss of photosynthetic capacity
2. Stunting, death by starvation for energy for growth
C. Uptake and Movement of the Isoxazolidones in plants
1. Soybeans: soil absorption by roots and underground shoot tissues, apoplastic movement
to shoots with transpiration stream; diffusion into symplast in leaves and shoots
2. Maize: less translocation to shoots but enough to cause injury
3. Little or no foliar absorption and uptake (but injury symptoms can occur)
4. No downward phloem movement
D. Basis of Selective Toxic Activity between Plant Species
1. Soybeans (resistant) and velvetleaf (susceptible)
a. Hydroxylation
b. Cleavage at ... -N-CH2 ...
c. Conjugation of metabolites: aglycone + glucose
d. Metabolism:
e. Velvetleaf metabolizes clomazone faster that soybeans
f. Velvetleaf and soybean differences in susceptibility
1) Source 1: not based on differences in uptake, translocation
2) Source 2: Soybeans also may translocate less clomazone apoplastically to target sites:
second basis of resistance
g. Best hypothesis of differences in susceptiblity between soybean and velvetleaf:
velvetleaf forms a unique metabolite with greater phytotoxicity, metabolic bio-activation
2. Maize: susceptible
a. No cleavage of ... -N-CH2 - ...
b. Less conjugation
3. Soybean cell culture work: differences in the site of action (IPPIase) is probable
selective mechanism in soybean cell genotypes
4. Peppers (resistant) and tomatoes (susceptible)
a. No differences in uptake
b. Sugar conjugates formed
c. Peppers greater translocation to roots, more root metabolism
d. Tomatoes greater translocation to shoots, more shoot metabolism
e. Selectivity mechanism not metabolism, unknown
III. Fate of the Isoxazolidones in the Environment
A. Soil
1. Adsorption to soil colloidal fraction; organic matter content is key, clay content less
important
2. Degradation in soil primarily microbial; hydrolysis and photolysis play small role in
breakdown
3. little effect of soil pH
4. 24-84 day half-life in soil depending on soil type and organic matter: carryover
potential to subsequent small grain crops, alfalfa; carryover to maize potential in some
years
5. Soil degradation: greater rates in flooded soils
6. Soil degradation: soil temperature, soil type biggest factors
B. Air
1. Readily volatilizes (similar to trifluralin): vapor pressure 0.000144 mm Hg at 25°C
2. soil water replaces clomazone on adsorption sites
3. high off-target movement potential (drift) if not incorporated into soil, or if sprayed
on wet soil
C. Water
1. fairly water soluable; fairly mobile in soil profile, can leach
IV. Plant Injury Symptomology of the Isoxazolidones in Plants
A. White leaves
B. Bleached leaves usually all white, sometimes interveinal
Clomazone Plant Injury Slides
1 foxtail seedling emergence bleaching
2 soybean seedling at emergence bleaching to cotyledons
3 soybean seedling 8 days later: cotyledon bleaching
4 newspaper headline June 7, 1986; 1st year of Command use; pre-e and wet soil
5 corn carryover injury: spike
6 corn carryover injury: 2 leaf
7 corn carryover injury: 3 leaf
foxtail seedling bleaching
8 corn carryover injury: 3 leaf bleaching and green
9 corn row 10 days after treatment and planting
10 corn row 14 DAT
11 corn row 22 DAT
12 corn row 33 DAT
13 corn row 26 DAT: white leaves gone due to wind