Investigations into differential glyphosate sensitivity between two horseweed (Conyza canadensis) growth types

Abstract

Horseweed [Conyza candensis (L.) Cronquist] grows in one of two distinct growth phenotypes, so called “rosette” and “upright” growth types, and they have recently been observed co-occurring in Michigan fields. Previous research found that “upright” plants from two glyphosate-resistant populations were 3- and 4-fold less sensitive to glyphosate than their rosette siblings. Further experiments were conducted to investigate whether differential glyphosate sensitivity of the growth types was due to glyphosate retention, absorption, or translocation. The total amount of glyphosate retained on C. canadensis leaf surface was similar for both growth types; however, on a per weight and area basis the upright growth type retained 21 and 18% less glyphosate, respectively. Glyphosate absorption was up to 85%, 168 HAT, and was not different between the rosette and upright growth types or between the susceptible (S) and resistant (R) biotypes. Additionally, there was no difference in translocation between the two growth types within each biotype at any time point. Interestingly, at 168 HAT 14C-glyphosate translocation was higher in the S rosette compared with the two growth types from the R biotype; however, the S upright-type was similar to both R growth types. Thus, glyphosate resistance in the R biotype may be due to an alternative mechanism instead of impaired translocation which has been cited as the primary mechanism of glyphosate resistance in C. canadensis. These results suggest that reduced glyphosate retention on a per weight and area basis of the upright growth type may contribute to increased glyphosate tolerance due to a diluted concentration of glyphosate in the plant. However, another factor is likely related to the mechanism of resistance within the R biotype that is contributing to a 3-fold difference in glyphosate sensitivity between the two growth types, such as alterations in EPSPS gene expression or changes in undescribed metabolism genes.