Methods of Assessing Bermudagrass [Cynodon dactylon] Responses to HPPD‐Inhibiting Herbicides

Mesotrione, topramezone, and tembotrione inhibit 4‐hydroxyphenylpyruvate dioxygenase (HPPD), an enzyme integral to carotenoid biosynthesis. Research was conducted to evaluate the efficacy of visual bleaching and chlorophyll fluorescence yield (F v /F m ) measurements for estimating common bermudagrass [ Cynodon dactylon (L.) Pers.] carotenoid and chlorophyll concentrations following 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 ) applications. Measurements of VB and F v /F m were evaluated 3, 7, 14, 21, 28, and 35 d after application. Leaf tissues were sampled on the same dates and assayed for chlorophyll and carotenoid compounds by using high‐pressure liquid chromatography methodology. Carotenoid and total chlorophyll concentrations were regressed upon VB and F v /F m on each evaluation date. While significant ( P < 0.05) relationships were detected on each date, variation explained by linear regression was modest ( R 2 < 0.65); thus, neither VB nor F v /F m assessments are good predictors of carotenoid and chlorophyll concentrations after HPPD‐inhibiting herbicide treatment. Comparisons of R 2 values for VB and F v /F m data suggest no advantage in using F v /F m in place of VB measurements when evaluating HPPD‐inhibiting herbicide activity.