Methods of Assessing Bermudagrass Responses to Three HPPD Inhibiting Herbicides.

Mesotrione, topramezone, and tembotrione are herbicides that inhibit the enzyme 4-hydroxyphenyl pyruvate dioxygenase (HPPD) that is central to carotenoid biosynthesis.  Herbicidal activity following treatment with HPPD inhibitors is commonly evaluated through visual ratings or assessments of chlorophyll fluorescence (Fv/Fm). Minimal information is available regarding the accuracy of these techniques for quantifying physiological changes in carotenoid and chlorophyll pigment concentrations following HPPD herbicide treatment. Research was conducted at the University of Tennessee to determine the accuracy of visual ratings and chlorophyll fluorescence measurements for evaluating physiological changes in carotenoid and chlorophyll pigment concentrations following treatment with mesotrione, topramezone, and tembotrione.

Separate experiments were conducted on mature ‘Riviera’ common bermudagrass (Cynodon dactylon (L.) Pers.) and ‘Tifway’ hybrid bermudagrass (Cynodon dactylon (L.) Pers. x C. transvaalensis Burtt Davy.) maintained at a 2 cm in a glasshouse. Each study was arranged as a 3 x 3 factorial randomized complete block design with three replications. Plants were treated with three rates of mesotrione (0.28, 0.35, and 0.42 kg ha^-1), topramezone (0.018, 0.025, and 0.038 kg ha^-1), and tembotrione (0.092, 0.184, and 0.276 kg ha^-1). Percent visual bleaching and chlorophyll fluorescence data were collected 3, 7, 14, 21, 28, and 35 days after application. Immediately following data collection, leaf material above 2 cm was harvested from individual plants and frozen at -80°C. Chlorophyll and carotenoid pigments were extracted from leaf tissue and quantified via high-performance liquid chromatography.

Chlorophyll, lutein and xanthophyll cycle pigment concentrations were regressed over visual bleaching and chlorophyll fluorescence data.  Comparisons of R-squared values indicated no distinct advantage to using chlorophyll fluorescence in place of visual ratings as a means of evaluating HPPD inhibiting herbicide activity. While several significant linear relationships were detected, R-squared values never exceeded 0.65, suggesting neither evaluation method can accurately quantify physiological changes in carotenoid and chlorophyll pigment concentrations following HPPD inhibiting herbicide treatment.