Weed management programs in corn typically include herbicides applied both preemergence (PRE) and postemergence (POST) for season-long weed control. Mesotrione is a new triketone herbicide registered for PRE and POST control of broadleaf weeds in corn. Triketone herbicides function through inhibition of the enzyme p-hydroxyphenylpyruvate dioxygenase. Mesotrione applied PRE did not adequately control common lambsquarters (Chenopodium album L.), smooth pigweed (Amaranthus hybridus L.), common ragweed (Ambrosia artemisiifolia L.), or morningglory species (Ipomoea spp.) in conventional tillage corn, but control of these weeds was generally improved in no-till corn. Mesotrione combinations with acetochlor did not always improve control of broadleaf weeds, but increased control of smooth pigweed and giant foxtail (Setaria faberi Herrm.). POST applications of mesotrione at 105 g ai/ha controlled most annual broadleaf weeds except common ragweed, but did not control giant foxtail. The addition of atrazine at 280 g ai/ha to mesotrione, however, improved control of common ragweed. Tank-mixtures of glyphosate, imazethapyr, or imazethapyr plus imazapyr with mesotrione improved control of giant foxtail in herbicide-resistant corn. Corn injury was usually low from PRE and POST mesotrione applications in non-genetically modified corn, however, greater injury occurred in glyphosate-resistant varieties. Corn treated with mesotrione combinations generally yielded similar to corn treated with commercial standards. Mesotrione applied POST also suppressed the perennial weeds horsenettle (Solanum carolinense L.) and Canada thistle [Cirsium arvense (L.) Scop.]. Additions of atrazine increased the rate of plant tissue necrosis on these perennial weeds as compared to the slower bleaching symptoms associated with mesotrione applied alone. In general, Canada thistle plants were more susceptible to mesotrione in the rosette stage of growth than when bolting. Absorption, translocation, and metabolism of 14C mesotrione in Canada thistle was generally low. However, higher levels of absorption and translocation and lower root metabolism of mesotrione in rosette stage plants compared to bolting stage plants may explain why Canada thistle is more susceptible to mesotrione in the rosette stage of growth. The changes in symptomology and increased control from mesotrione plus atrazine tank-mixtures is likely due to the interrelationship between the modes of action of atrazine and mesotrione. / Ph. D.

Mesotrione is the first triketone herbicide registered for use in turfgrass. Triketones prevent carotenoid biosynthesis by inhibiting the enzyme p-hydroxyphenylpyruvate dioxygenase (HPPD). Although mesotrione controls many species of grass and broadleaf weeds, it is best know for selective control of perennial grasses like creeping bentgrass (Agrostis stolonifera L.). Field trials conducted at Virginia Tech and Blacksburg Country Club determined that several programs that integrate herbicide treatment and turf seeding effectively transitioned creeping bentgrass contaminated golf roughs back to a tall fescue [Schedonorus phoenix (Scop.) Holub] monoculture. However, mature weeds require multiple mesotrione applications for effective control. This requirement is a major limitation to mesotrione's competitiveness in turfgrass markets. Several greenhouse and laboratory studies were conducted to evaluate scenarios where mesotrione rates were titrated and applied daily to mimic ascending, descending, and intervallic time-release patterns. These patterns were applied following an initial treatment to foliage or soil to mimic a potential sprayable or granular time-release formulation. These scenarios effectively controlled five targeted weed species equivalent to the standard of two broadcast sprays, regardless of initial application placement or release pattern. However, both time-release treatments and the standard injured tall fescue based on leaf counts, plant weights, and visual phytotoxicity ratings. Additional growth chamber studies found that changes in relative humidity from 50 to 90% caused a 4- to 18-fold increase in plant phytotoxicity with a concomitant decrease in photochemical efficiency when mesotrione was applied to foliage of smooth crabgrass (Digitaria ischaemum (Schreb.) Schreb. ex Muhl.). Furthermore, white tissue was found predominately in the two youngest leaves when mesotrione was applied to soil, but in older leaves when applied only to foliage. Laboratory studies were conducted to evaluate interspecific differences in 14C mesotrione absorption and translocation between two plant species when applied to foliage or roots. Annual bluegrass (Poa annua L.) absorbed 2- to 4-fold more radioactivity than Kentucky bluegrass (Poa pratensis L.). Both species absorbed less radioactivity through roots than through foliage and root absorbed radioactivity was more often exuded into Hoagland's solution while foliar absorbed radioactivity was often found in other foliage. / Ph. D.

Ce travail de thèse vise à décrire l'écodynamique d'herbicides -tricétones synthétiques (sulcotrione et mésotrione) et naturel (leptospermone) et à estimer leur impact écotoxicologique sur la communauté bactérienne de sols agricoles. Les processus impliqués dans la dissipation de ces herbicides (adsorption et biodégradation) ont été étudiés dans des microcosmes de sol. Deux souches bactériennes, Bradyrhizobium sp. SRl dégradant la sulcotrione et la mésotrione, et Methylophilus sp. LS1 dégradant la leptospermone ont été isolées. Une banque de 12000 mutants de Bradyrhizobium sp. SRl a été construite et deux mutants Sul• ont été sélectionnés, mais les gènes interrompus ne codent pas pour les enzymes de dégradation de la sulcotrione. L'impact écotoxicologique des tricétones synthétique (sulcotrione) et naturelle (leptospermone) sur la communauté bactérienne du sol a été estimé à l'aide d'outils de métagénomique et de métabolomique. La leptospermone modifie de manière transitoire la diversité et la composition de la communauté bactérienne, en accord avec sa rémanence dans les sols. La sulcotrione ne modifie ni la diversité ni la composition de la communauté bactérienne. La combinaison des approches de métagénomique et de métabolomique est prometteuse pour évaluer l'impact écotoxicologique des herbicides sur les microorganismes du sol. / This work aims to describe the ecodynamics of synthetic (sulcotrione and mesotrione) and natural (leptospermone) -triketone herbicides and to estimate their ecotoxicological impact on the bacterial community in arable soils. The processes involved in the dissipation of these herbicides (adsorption and biodegradation) have been studied in soil microcosms. Two bacterial strains, Bradyrhizobium sp. SRl able to degrade sulcotrione and mesotrione, and Methylophilus sp. LS1 degrading leptospermone, have been isolated. A bank of 12 ooo mutants of Bradyrhizobium sp. SR1 was established allowing the selection of two Sul•mutants but interrupted genes didn't code for enzymes degrading sulcotrione. The ecotoxicological impact of synthetic (sulcotrione) and natural (leptospermone) triketones on soil bacterial community was estimated using metagenomics and metabolomic tools. Leptospermone transitory modified the diversity and composition of the bacterial community, in accordance with its persistence in soil. Sulcotrione did not modified neither the diversity nor the composition of the bacterial community. The combination of metagenomics and metabolomics is promising for the assessment of ecotoxicological impact of herbicides on soil microorganisms.