Assessment of two biotypes of Solanum ptycanthum that differ in resistance levels to imazamox

Glasshouse and laboratory experiments were conducted on acetolactate synthase (ALS) homozygous resistant Solanum ptycanthum biotypes from Illinois (IL-R) and Indiana (IN-R), and homozygous susceptible biotypes from Illinois (IL-S) and Indiana (IN-S). Genetic similarity of biotypes was assessed by random amplified polymorphic DNA (RAPD) markers, which determined that the Illinois biotypes are more similar to each other than to the IN-R biotype. ALS enzyme activity from the IL-R and IN-R biotypes had I₅₀ values of 362 and 352 μM imazamox respectively. Dose-response experiments using three- to four-leaf-stage plants of the IL-R and IN-R biotypes had GR₅₀ values of 242 and 69 g ae ha^-1 imazamox respectively. Whole-plant and ALS enzyme results are different than previously reported values in the literature, which was attributed in the current study to the original IN-R population having individuals that were segregating for ALS resistance. Metabolism studies showed no difference in percentage [¹⁴C]imazamox remaining between the IL-R and IN-R biotypes up to 72 h after treatment. The IL-S biotype metabolised [¹⁴C]imazamox approximately two times faster than the IL-R and IN-R biotypes and this trait was heritable. Response of F₃ plants containing homozygous ALS-resistant alleles from the IL-R biotype in a genetic background of 50% Illinois and 50% Indiana biotypes suggests that genetic factors other than an altered target site or metabolism may also contribute to the magnitude of resistance at the whole-plant level in resistant biotypes.