INVESTIGATING THE MECHANISMS OF GLUFOSINATE RESISTANCE IN ITALIAN RYEGRASS POPULATIONS. Wilson V. Avila*, Elena Sanchez, Carol Mallory-Smith; Oregon State University, Corvallis, OR (55)
Glufosinate is a broad spectrum post-emergence herbicide used in vineyards, orchards, and cropping systems with the Liberty-Link® trait. Glufosinate is a potent inhibitor of the enzyme glutamine synthetase (GS) which is essential for recycling the ammonia that is released during nitrate reduction, amino acid degradation and photorespiration. GS is a nuclear-coded enzyme that is present in the cytoplasm and plastid, with the plastidic isoform more prevalent in Poaceae species. Italian ryegrass is a troublesome weed in orchards and major cereal crops and has evolved resistance to at least five different herbicide chemical groups. In an herbicide screening test, two Italian ryegrass populations (OR1 and MG) showed a differential pattern of sensitivity to glufosinate. The OR1 population was collected from a hazelnut orchard and also is glyphosate resistant. The mechanism of glyphosate resistance in OR1 is due to reduced herbicide translocation. The MG population was collected from a wheat field and also is resistant to ALS inhibitors. Dose-response, ammonia accumulation and enzyme activity studies were conducted to test the sensitivity of the two populations to glufosinate. A control population (Gulf) was included in the experiments. The rates of glufosinate required to reduce growth by 50% (GR50) were 0.48 and 0.43 kg ai ha-1 for OR1 and MG, respectively; whereas, the GR50 value for the control population was 0.15 kg ai ha-1. The control population accumulated on average three times more ammonia than the resistant population OR1 and two times more than the MG population at 48, 72, and 96 hours after glufosinate treatment. There were no differences in GS enzyme activity between the control population and the resistant population OR1. However, the MG population was different and had a less sensitive enzyme. These results confirm that both populations evolved resistance to the herbicide glufosinate and two different mechanisms of resistance are likely involved. In the case of the OR1 population, the reduced herbicide translocation mechanism that is responsible for glyphosate resistance appears to be involved in the resistance to glufosinate. However, an altered target site may be responsible for glufosinate resistance in the MG population.