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Physiological Response of Johnsongrass to Herbicides

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)

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Physiological Response of Johnsongrass to Herbicides

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  1. Physiological Response of Johnsongrass to Herbicides J. A. Ferrell, H. J. Earl, W. K. Vencill University of Georgia

  2. 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)

  3. Objectives • Determine time required for glyphosate, clethodim, imazapic and nicosulfuron to render SORHA physiologically noncompetitive

  4. 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

  5. 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

  6. 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

  7. SORHA plants before treatment

  8. Materials and Methods • Photosynthesis and Stomatal Cond. • Li-Cor 6400 • PPFD = 1200 µmol m-2 s-1 • CO2 concentration = 400 µmol mol-1

  9. 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

  10. Materials and Methods • SPAD • 5 measurements per leaf • Experiment conducted twice • Randomized Complete Block Design with 5 replications • All data normalized to the control

  11. Abbreviations • gs – stomatal conductance • AN – net carbon assimilation • Fv/Fm – chlorophyll fluorescence • SPAD – chlorophyll meter

  12. Effect of Herbicide on AN Relative AN

  13. AN50 Values

  14. Regrowth Biomass

  15. Fv/Fm, SPAD, gs, as indicators of AN

  16. Relationship of Fv/Fm to AN

  17. Relationship of SPAD to AN SPAD

  18. Relationship of gs to AN

  19. gs50 Values

  20. gs50 Values

  21. 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

  22. Any Questions?

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