Competition for canopy cover between accessions of Phalaris minor that are susceptible and resistant to ACCase inhibiting herbicides

Herbicide resistance has the potential to impose fitness costs in plants. In Mexico, Phalaris minor Retz. has evolved resistance to acetyl-coa carboxylase (ACCase) inhibiting herbicides. However, it is unknown whether these changes may involve other ecological costs for resistant individuals under non-selective conditions. The aim of this study was to determine whether the evolution of resistance has reduced the ability of P. minor to compete for resources at the seedling stage. De Wit's replacement series experiments (susceptible:resistant 100:0, 50:50, 0:100) were performed under two initial conditions. In the first experiment, we examined the effect of differences in germination time between resistant and susceptible genotypes on canopy cover. In the second experiment, germination was synchronized, and all the replacement series began at the same time. Delayed germination of resistant accessions reduced their ability to compete for canopy space, competition intensity index, and relative productivity in relation to susceptible accessions. When germination was synchronized, the resistant accessions had the same canopy cover and productivity as susceptible accessions. The delayed germination may cause displacement of resistant individuals in the absence of the selective factor (herbicide).