- Italian ryegrass populations have been identified with evolved resistance to glyphosate in orchards with a history of glyphosate use. Two of these populations were selected to investigate the inheritance of glyphosate resistance. The mechanisms involved in the herbicide resistance were an altered target site for the population SF and reduced herbicide translocation for the population OR1. Mendelian inheritance studies and dose response experiments were conducted on the two populations. Four F₁ families were formed by reciprocal crosses between each of the glyphosate resistant populations (SF and OR1) and the susceptible population (S) C1. Eight backcross families (BC₁) were formed between the F₁ individuals from each family and the susceptible population C1. Most of the F₁ families resulting from SF and C1 had susceptible:resistant ratios of approximately 1:1. Similar trends were observed in the backcross families concluding that glyphosate resistance due to target site mutation in the SF Italian ryegrass population is likely conferred by a single, nuclear, partially-dominant gene. For population OR1, there was significant variation in the susceptible:resistant ratios in the F₁ families. Chi-square analysis for backcross families failed to fit the model for a single major gene suggesting that the glyphosate resistance due to reduced herbicide translocation in the Italian ryegrass population OR1 is multigenic.
Italian ryegrass glyphosate resistant populations OR1, OR2, and OR3, the population MG, and three susceptible populations C1, C2, and C3 were selected to conduct dose-response experiments, ammonia accumulation assays, and enzymatic studies to quantify their sensitivity to glufosinate. The glufosinate rates required to reduce the growth by 50% (GR₅₀) were 0.15, 0.18, and 0.21 kg ai ha⁻¹ for the susceptible populations C1, C2, and C3, respectively, and for the resistant populations OR1, OR2, OR3, and MG, the GR₅₀ values were 0.49, 0.42, 0.40, and 0.45 kg ai ha⁻¹ respectively, resulting in an average resistance index of 2.4. The same trend was observed in ammonia accumulation studies between 48 and 96 hours after glufosinate treatment. The susceptible populations accumulated between 1.5 to 2.5 times more ammonia than the resistant populations. The glufosinate concentration required to reduce glutamine synthetase enzyme activity by 50% (I₅₀) was not different for the resistant OR1, OR2, and OR3 and susceptible populations. However, a different response was observed for the population MG. The I₅₀ values ranged from 3.1 to 3.6 µM for the resistant populations OR1, OR2, and OR3, and from 3.7 to 4.3 µM for the susceptible populations. The population MG had an I₅₀ of 10.7 µM resulting in a resistant ratio 2.6-fold higher than the average of the control populations C1 and C2. Eighty-three percent of the plastidic GS gene from the resistant population MG and the susceptible C1 was cloned and sequenced. One amino acid substitution was found in the population MG that may be responsible for the reduced enzyme sensitivity. These results are the first reports of target site and non target site based glufosinate resistance in a weed species.