METHODS OF ASSESSING THE ACTIVITY OF HPPD-INHIBITING HERBICIDES. M. Elmore*, J. Brosnan, D.A. Kopsell, and G.K. Breeden, University of Tennessee, Knoxville, TN (21)


   Mesotrione, topramezone, and tembotrione are herbicides that inhibit 4-hydroxyphenyl pyruvate dioxygenase (HPPD). Weed control following treatment with HPPD inhibitors is commonly evaluated through visual ratings or assessments of chlorophyll fluorescence. Minimal information is available regarding the accuracy of these techniques for estimating carotenoid and chlorophyll pigment concentrations after HPPD inhibiting herbicide treatment. Research was conducted in 2009 to determine the accuracy of visual ratings and chlorophyll fluorescence (Fv/Fm) measurements for evaluating changes in carotenoid and chlorophyll pigment concentrations in bermudagrass (Cynodon dactylon L. Pers. cv. ‘Riviera’) leaf tissue following treatment with mesotrione, topramezone, and tembotrione.
   In July 2009, mature bermudagrass plants were transferred to 10 cm pots in a glasshouse (Knoxville, TN) and maintained at a 2 cm height of cut. After acclimating for 4 weeks, plants were treated with three rates of mesotrione (0.28, 0.35, and 0.42 kg/ ha), topramezone (0.018, 0.025, and 0.038 kg/ha), and tembotrione (0.092, 0.184, and 0.276 kg/ha) forming a 3 x 3 factorial, randomized complete block, design with three replications. A non-treated control was also included for comparison. Percent visual bleaching (VB) and Fv/Fm data were collected 3, 7, 14, 21, 28, and 35 days after application (DAA). VB was rated using a 0 (dark green leaf tissue) to 100% (white leaf tissue) scale, while Fv/Fm data were collected using a hand-held pulse modulated fluorometer. Immediately following VB and Fv/Fm data collection, leaf material above 2 cm was harvested and frozen at -80°C. Chlorophyll and carotenoid pigments were extracted from harvested tissue and quantified via high-performance liquid chromatography.  
   Chlorophyll, lutein, β-carotene, and xanthophyll cycle pigment concentrations were regressed over VB and Fv/Fm data. With the exception of zeaxanthin, both VB and Fv/Fm were linearly associated with all carotenoid and chlorophyll concentrations measured between 7 and 28 DAA; few significant relationships were detected at 3 and 35 DAA. R2 values never exceeded 0.65 on any date, suggesting neither evaluation method can accurately estimate carotenoid and chlorophyll pigment concentrations following HPPD inhibiting herbicide application. Comparisons of R2 values indicated no distinct advantage to using Fv/Fm measurements in place of visual ratings to evaluate HPPD inhibiting herbicide activity.