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?