Abstract
Herbicide resistance is an evolutionary process that may involve changes in physiological and ecological traits related to plant fitness. This type of resistance occurs in littleseed canarygrass ( Phalaris minor ). This study was aimed to determine the effect of resistance to ACCase-inhibiting herbicides on fitness of seed accessions of littleseed canarygrass. Seed characteristics are important because they may determine the outcome of competition between susceptible (S) and resistant (R) biotypes. Experiments were performed to determine whether differences between S and R accessions in embryo size, dormancy, longevity, and germination speed might occur. Differences in plant fitness associated to herbicide resistance were found in seed physiological characteristics. The embryo size of R accessions was reduced up to 50 % (on a dry weight basis), which resulted in an increase in the number of days required to germinate and a reduction in the germination speed. Damage to membranes, which is related to longevity, was more severe in the R accessions. The observed decrease in seed vigor could affect the establishment of seedlings, which may explain the low success of R accessions under herbicide selection pressure.