Graduate Thesis Or Dissertation

Factors affecting selective control of wild oat and downy brome in wheat by trifluralin

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  • Trifluralin (a, a, a - trifluoro -2, 6 -dinitro-N, N-dipropyl-ptoluidine) is effective in controlling downy brome (Bromus tectorum L.) and wild oat (Avena fatua L.) but does not severely affect the growth of wheat (Triticum aestivum L.). Studies were conducted using germination chamber and greenhouse facilities to investigate possible factors contributing to this selective action of trifluralin. Dose response studies were carried out in two different media, sand and soil, to determine the comparative phytotoxicity of trifluralin to downy brome, wild oat, and wheat. The concentration of trifluralin needed to reduce the growth 50% (GR₅₀) was similar for all three species in the sand medium. GR₅₀ values determined from the results of the soil bioassay were much lower for downy brome and wild oat than for wheat. A double petri dish technique was used to determine the effective site of uptake of trifluralin by the three species. The results indicated that root uptake was of minor importance in reducing shoot growth of all species. Exposure of wild oat shoots to trifluralin caused severe reduction in their growth. Treatment of both root and shoot of downy brome and wheat resulted in greater reduction of shoot growth than the additive effect of treating shoots and roots of separate plants. This may have been caused by the experimental technique used. The double dish technique isolated the first few mm of the coleoptile nearest the seed when the shoot was exposed to trifluralin and not when the root or the root and the shoot was exposed. The region reported to have the greatest sensitivity to trifluralin, the coleoptile node, is located close to the seed in downy brome and wheat, but is found just below the soil surface in wild oat. Thus, the coleptile node of wheat and downy brome was not subjected to the trifluralin treatment when the shoot was exposed which may explain the limited growth reduction observed from the shoot exposure. Experiments were conducted in which the depth of seed placement and the depth of trifluralin incorporation were varied in such a manner as to allow the seeds to be placed (a) above the trifluralin, (b) in the center of the treated band, or (c) below the herbicide. When the seeds of wild oat and downy brome were placed in or below the trifluralin, shoot growth was severely reduced. When wheat was planted below the trifluralin-treated zone, it escaped serious injury with the amount of injury being reduced as the distance between the treated zone and the wheat seed was increased. The morphological differences existing between wheat and wild oat regarding the location of the coleoptile node may be one factor allowing selective control of wild oat in wheat. The coleoptile node is located next to the seed in both downy brome and wheat so this is not a factor in the selective control of downy brome. One possible factor may be the seed size of downy brome as it relates to the vigor of the germinating seedling. Downy brome germinating below the trifluralin zone would be subjected to such stress just to emerge from this depth that perhaps the additional stress imparted by the trifluralin would be enough to prevent it from establishing as a viable plant.
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