Physiological Response of Johnsongrass to Herbicides
J. A. Ferrell, H. J. Earl, W. K. Vencill
University of Georgia
SORHA Competition
• 1:1 inverse-linear relationship between corn yield and SORHA biomass (Perry et al
1983)
• Soybean yield reduction 43% (McWhorter and Hartwig 1972)
• Cotton yield reduction 80% after 12 weeks (Keeley and Thullen 1989)
Objectives
• Determine time required for glyphosate, clethodim, imazapic and nicosulfuron to render SORHA physiologically noncompetitive
Physiologically non-competitive
• 50% reduction in net carbon assimilation (AN)
– Biomass accumulation most common parameter to describe competitiveness
– Biomass accumulation directly linked with AN
– 50% reduction in AN was a comparative indicator of competitiveness
Objectives
• Determine time required for glyphosate, clethodim, imazapic and nicosulfuron to render SORHA physiologically noncompetitive
• Determine value of quantitative measurements to predict changes in AN
– stomatal conductance– chlorophyll fluorescence– SPAD readings
Materials and Methods
• SORHA grown in greenhouse for 20 wk
• Herbicide Treatments– Glyphosate 840 g ai ha-1
– Clethodim 140 g ai ha-1 + COC– Nicosulfuron 35 g ai ha-1 + COC– Imazapic 70 g ai ha-1+ NIS
• Plants treated at ~50 cm
SORHA plants before treatment
Materials and Methods
• Photosynthesis and Stomatal Cond.– Li-Cor 6400
• PPFD = 1200 µmol m-2 s-1
• CO2 concentration = 400 µmol mol-1
Materials and Methods
• Photosynthesis and Stomatal Cond.– Li-Cor 6400
• PPFD = 1200 µmol m-2 s-1 (red and blue diodes)
• CO2 concentration = 400 µmol mol-1
• Chlorophyll fluorescence
– 30 minute dark adaptation
– 3 measurements per leaf
Materials and Methods
• SPAD – 5 measurements per leaf
• Experiment conducted twice
• Randomized Complete Block Design with 5 replications
• All data normalized to the control
Abbreviations
• gs – stomatal conductance
• AN – net carbon assimilation
• Fv/Fm – chlorophyll fluorescence
• SPAD – chlorophyll meter
Effect of Herbicide on AN
0
0.2
0.4
0.6
0.8
1
1.2
0 5 10 15
Days
glyphosate
nicosulfuron
clethodim
imazapicRe
lati
ve
AN
AN50 Values
Herbicide
Rate
(g ai ha-1)
AN50
(days)
Glyphosate 840 4.3 a
Clethodim 140 6.5 b
Nicosulfuron 35 6.2 b
Imazapic 70 6.0 b
Regrowth Biomass
Herbicide
Rate
(g ai ha-1)
AN50
(days)
Regrowth
(g)
Glyphosate 840 4.3 a 0.01 a
Clethodim 140 6.5 b 0.03 a
Nicosulfuron 35 6.2 b 0.09 a
Imazapic 70 6.0 b 0.05 a
Fv/Fm, SPAD, gs, as indicators of AN
Relationship of Fv/Fm to AN
0
0.1
0.2
0.3
0.40.5
0.6
0.7
0.8
0.9
0 10 20 30 40 50
AN (µmol m-2 s-1)
Fv/
Fm
control
glyphosate
nicosulfuron
clethodim
imazapic
Relationship of SPAD to AN
0
10
20
30
40
50
60
0 10 20 30 40 50
AN (µmol m-2 s-1)
control
glyphosate
nicosulfuron
clethodim
imazapic
SP
AD
Relationship of gs to AN
0
0.1
0.2
0.3
0.4
0.5
0.6
0 10 20 30 40 50
AN (µmol m-2 s-1)
gs (
mo
l m-2
s-1
) glyphosate
nicosulfuron
clethodim
imazapic
control
gs50 Values
Herbicide
Rate
(g ai ha-1)
AN50
(days)
gs50
(days)
Glyphosate 840 4.3 a 4.3 a
Clethodim 140 6.5 b 6.1 b
Nicosulfuron 35 6.2 b 6.5 b
Imazapic 70 6.0 b 6.1 b
gs50 Values
Herbicide
Rate
(g ai ha-1)
AN50
(days)
gs50
(days)
gs50
r2
Glyphosate 840 4.3 a 4.3 a 0.96
Clethodim 140 6.5 b 6.1 b 0.96
Nicosulfuron 35 6.2 b 6.5 b 0.97
Imazapic 70 6.0 b 6.1 b 0.95
Conclusions
• Glyphosate renders SORHA physiologically non-competitive more quickly than nicosulfuron, clethodim, or imazapic
• SPAD and Fv/Fm were poorly related to AN
• gs highly correlated with AN
Any Questions?