Centipedegrass is a low-maintenance, warm-season grass used throughout the southern United States (Baird et al., 1989; Beard, 1973). Centipedegrass is established vegetatively and by seed; however, centipedegrass is slow to germinate and weed competition can delay turf coverage. Seeded centipedegrass requires 10 to 14 d to germinate, and large crabgrass (Digitaria sp.) populations can delay centipedegrass stolon development (Brede, 2000; Gannon et al., 2004). Slow establishment and growth rate often allows for increased weed competition (Gannon et al., 2004).
Weed control options for newly established centipedegrass are limited (Gannon et al., 2004; Porter 1996). Triazine herbicides such as atrazine and simazine are viable options for centipedegrass weed control. Gannon et al. (2004) demonstrated centipedegrass tolerance to atrazine (2.2 kg·ha−1) and simazine (3.4 kg·ha−1) at seeding stage; however, both provide limited residual weed control. McElroy and Walker (2009) found that atrazine reduced centipedegrass photosystem II (PS II) efficiency when applied postemergence and should only be applied preemergence to seedling centipedegrass. Triazine herbicides inhibit photosynthesis by competing with plastoquinone (PQ) for the quinone B binding domain of the D1 protein. As a result, electron flow between PS II and PS I is inhibited. The subsequent accumulation of reactive oxygen species (ROS) and singlet and triplet chlorophyll overwhelms the carotenoid quenching mechanism and degradation of thylakoid membranes occurs (Hess, 2000).
Previous research has shown mesotrione tank-mixture with atrazine improves weed control compared with either herbicide applied alone (Abendroth et al., 2006; Armel et al., 2003, 2005; Johnson and Young, 2002; Willis et al., 2007). Mesotrione, a carotenoid biosynthesis inhibitor, is an effective herbicide for turfgrass weed control. Mesotrione competitively inhibits 4-hydroxyphenylpyruvate dioxygenase, which prevents the biosynthesis of α-tocopherol and PQ (Prysbilla et al., 1993). α-Tocopherol scavenges ROS that are detrimental to the thylakoid membrane (Trebst et al., 2002). PQ transfers electrons from PS II to PS I reaction centers and is a cofactor in phytoene desaturase, a crucial enzyme of the carotenoid biosynthesis pathway (Norris et al., 1995). Carotenoids are components of light harvesting complexes where they transfer light energy to the photosynthetic reaction center and act in photoprotection by quenching free radicals, singlet oxygen, and other ROS (Siefermann-Harms, 1987). Thus, the combined effect of free radical and reactive oxygen generation by triazine herbicides and depletion of tocopherol and carotenoid antioxidants induces herbicidal synergism.
Mesotrione has been reported to control creeping bentgrass (Agrostis stolonifera) as well as other common turfgrass weeds postemergence, including crabgrass, goosegrass (Eleusine indica), nimblewill (Muhlenbergia schreberi), ground ivy (Glechoma hederacea), common purslane (Portulaca oleracea), black medic (Medicago lupulina), and dandelion (Taraxacum officinale) (Beam et al., 2006; Giese et al., 2005; Johnson and Young, 2002; Reicher et al., 2006).
Previous research found tank-mixing low rates of atrazine with mesotrione improved weed control (Armel et al., 2005; Johnson and Young, 2002). Similarly, Willis et al. (2007) reported simazine plus mesotrione tank-mixture controlled white clover (Trifolium repens) greater than mesotrione alone. Mesotrione applied as an early postemergence herbicide on newly seeded centipedegrass does not reduce turf cover (McElroy and Walker, 2009). Improved weed control and centipedegrass injury because of tank-mixture of mesotrione and atrazine is well documented; however, tank-mixture of other PS II-inhibiting herbicides, such as simazine, with mesotrione is limited. Therefore, it is necessary to evaluate centipedegrass response to simazine plus mesotrione tank-mixture.
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