Relative Fitness of Susceptible and Acetyl-CoA carboxylase Resistant Alopecurus myosuroides Biotypes: Germinability and Seedling Pre-Emergence Growth under Salinity and Drought Stress Conditions

Fitness costs of herbicide resistance may be more evident and increase under stress environmental conditions. To test this hypothesis, a herbicide-resistant (R) and a herbicide-susceptible (S) phenotype with controlled and homogenized genetic background were isolated within a non–target-site resistant (NTSR) black-grass population. Accumulated maximum germination (Gmax) and time to reach the Gmax (G50) of biotypes were examined under non- and abiotic-stress conditions (salinity at 16 dSm-1, drought at -0.8 Mpa). In addition, seedling pre-emergence growth of the biotypes were evaluated at non-stress condition. There was no difference in Gmax between the R and S biotypes, whereas the Gmax of R phenotype was 28% and 49% lower than that of the S phenotype under salinity- and drought-stress conditions, respectively. The R phenotype germinated around seven to 16 hours later than the S phenotype under all non- and abiotic-stress conditions, i.e. the R phenotype had higher G50 than the other phenotype. The experiments of seedling pre-emergence growth showed that the R and S biotypes had similar root and shoot length under non-stress condition. The results clearly showed that the germinability fitness costs of NTSR black-grass increase when the plants are grown under salinity- and drought- stress conditions.