Shifts in herbicide tolerance in paddy field periphyton following herbicide application

Herbicide susceptibility was examined in various algal strains isolated from herbicide-treated and untreated paddy fields. Diatom strains showed obvious tolerance to the herbicides simetryn and pretilachlor irrespective of herbicide exposure, while the susceptibility of green algae mostly reflected their history of herbicide exposure. Cross-resistance was also induced in some algae. Rapid development of herbicide tolerance indicated that the tolerance seemed to be conferred by tolerant strains already present in the communities. Thus, changes in genetic composition within a population seem to be the main mechanism by which paddy periphyton gain herbicide tolerance.