Transcript Document

Sulfentazone absorption by plant roots increases as soil or
solution pH decreases.
2
Witt
1
1
Vencill University
J.A.
W.W.
and , W. K.
2University of Kentucky, Lexington, KY.
of Georgia, Athens;
Sulfentrone Uptake- excised roots
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l
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l
Root absorbed pesticides follow either a
symplastic (lipophilic) or apoplastic
(hydrophilic) route to the xylem.
Weak acid pesticides, when protonated, tend
toward the symplast.
Ion trapping occurs when a protonated
molecule enters a cell and become ionic due to
the difference in pH between the cytosol and
the symplast.
Ion trapping results in the accumulation of
weak acids within a cell.
Materials and Methods
Whole Plant Uptake – formulated sulfentrazone
l TN 90 tobacco was grown in float trays and
transplanted into sulfentarzone treated soil.
l Soil pH was either 5.8, 6.5. or 7.2.
l Water use as an indicator of plant injury.
Bq/g
Background
Whole Plant Uptake – 14C-sulfentrazone
l Transplants were placed in hydroponic solution
containing 16 kBq sulfentrazone for 24 hours.
l Solution was buffered to pH 5.8, 6.5, 7.2.
l Roots and shoots were separated and combusted in
a biological oxidizer. 14CO2 was detected using LCS.
60
60
50
50
40
40
pH 6.5
30
pH 5.5
20
pH 7.5
pH 7.5
30
pH 6.5
20
pH 5.5
10
10
0
0
0
50
100
0
150
Introduction
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Most weak acid herbicides have a pKa between
2 and 4.
Hypothesis
Decreasing soil pH will cause sulfentrazone be
become protonated (more lipophilic),
subsequently increasing uptake and herbicide
injury.
Objectives
Determine if root absorption of sulfentrazone
and glyphosate is affected by solution pH.
14C-glyphosate
Excised rootsand
*This method removes differences in uptake due to
differences in transpiration.
l Cotton seeds (Fibermaxx 989 BG/RR) were
germinated on wet paper towels for 5 d.
l The bottom 4 cm, cut into 1cm segments, was placed
into a glass vial.
l Herbicide was added to each vial and shaken for 10,
20, 30, 45, 60, 90, and 120 minutes.
l Root segments were washed for 1 min with ice cold
solution to remove herbicide from apoplastic spaces.
l Herbicide extraction was accomplished by placing the
root segments in scintillation fluid for 24 hours.
l 14C was quantified using LSC.
Total water use (ml)
Sulfentrazone becomes 10-fold less soluble
when pH is decreased from 7.5 and 6.0. The
solubility of glyphosate (pKa 5.6) decreases
only slightly between pH 7 and 2.5.
14C-sulfentrazone
Solutio
n pH
900
800
pH 5.8
700
pH 6.5
600
pH 7.2
Determine if greater sulfentrazone injury is
observed when soil pH is decreased.
Roots Sho
ots
Bq/g fresh
weight
500
0
0.21 0.42 0.63 0.84 1.05
Sulfentrazone Rate (kg ai ha-1)
7.2
37
2.5
6.5
41
3.2
5.8
57
2.6
LSD
(0.05)
11
NS
Injury - Sulfentrazone
Summary
3
l
2.5
2
pH 4.8
1.5
l
pH 5.6
pH 7.2
1
0.5
l
l
0
l
150
400
Plant Dry Weight (g)
l
Sulfentrazone is a weak acid herbicide (pKa
6.56).
100
Sulfentrazone uptake – whole plants
1000
Materials and Methods
50
Minutes
Minutes
Injury - Sulfentrazone
l
Glyphosate Uptake- excised roots
Bq/g
1
Ferrell*
0
0.21 0.42 0.63 0.84 1.05
l
Sulfentrazone Rate (kg/ha)
l
l
Decreased soil pH decreased water use and dry
weight of tobacco.
Decreased solution pH increased uptake by
tobacco.
Decreased pH decreases the solubility of
sulfentrazone, but increases soil sorption.
Decreased pH scarcely affects the solubility of
glyphosate.
Decreased pH increased uptake of sulfentrazone,
but not glyphosate.
Solubility, as affected by pH, regulates
sulfentrazone uptake.
Sulfentrazone injury to crop plants could be due to
lower soil pH.