Tolerance of Oenothera elata Transplants to Preemergence Herbicides

Greenhouse experiments were conducted in the winter of 2010 at Texas Tech University to evaluate the response of Oenothera elata transplants to preemergence herbicides. Herbicides were applied 2 days prior to transplant of plugs (3.5 cm in diameter) into 11.3 liter pots filled with a Brownfield sandy clay loam (loamy, mixed, superactive, thermic Arenic Aridic Paleustalfs) with a pH of 8.0 and organic matter content of 1.2%. Fertilizer (14N-14P₂O₅-14K₂O) was applied at time of transplant at a rate of 24 kg N/ha. Irrigation was applied through an automated system throughout the study to ensure optimal growth and prevent stress. Pots were maintained by hand weeding in order to eliminate competition. Eight herbicide treatments and a non-treated control were examined. Treatments were applied using a CO₂ backpack sprayer equipped with XR8004VS nozzle tips and calibrated to deliver 375 L/ha at 221 kPa. Preemergence herbicide treatments included trifluralin + isoxaben at 2.8 kg ai/ha, pendimethalin at 0.67 kg ai/ha, prodiamine at 1.68 kg ai/ha, dithiopyr at 0.56 kg ai/ha, metolachlor at 2.13 kg ai/ha, oryzalin at 2.24 kg ai/ha, isoxaben at 0.56 kg ai/ha, and oxadiazon at 3.4 kg ai/ha. The trial was arranged in a randomized complete block design with 5 replications of treatments. Percent phytotoxicity was assessed visually 3, 5, 7, 14, 21, and 28 days after transplant (DAT) using a scale of 0 (no phytotoxicity) to 100% (plant death). 0. elata plants were harvested 28 DAT, separated into above and below ground biomass, dried, and weighed. Data were subjected to analysis of variance and mean separation using Fisher’s Protected LSD at the 0.05 significance level. Oryzalin and dithiopyr exhibited 49 and 47% O. elata phytotoxicity, respectively, 7 DAT. All other treatments exhibited ≤ 16% phytotoxicity and were similar to the non-treated check. Trifluralin + isoxaben, oryzalin, and dithiopyr phytotoxicity increased to 87, 64, and 63, respectively, 28 DAT. All other treatments exhibited ≤ 17% phytotoxicity and were similar to the non-treated check. Oryzalin, dithiopyr, metolachlor, and trifluralin + isoxaben exhibited significant reductions in shoot weight (67 to 78%) compared to the non-treated check 28 DAT. Prodiamine and trifluralin + isoxaben exhibited significant reductions in root weight (55 to 68%) compared to the non-treated check 28 DAT.

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