Herbicide-resistant sunflower plants, isolated polynucleotides that encode herbicide-resistant and wild-type acetohydroxyacid synthase large subunit (AHASL) polypeptides, and the amino acid sequences of these polypeptides, are described. Expression cassettes and transformation vectors comprising the polynucleotides of the invention, as well as plants and host cells transformed with the polynucleotides, are described. Methods of using the polynucleotides to enhance the resistance of plants to herbicides, and methods for controlling weeds in the vicinity of herbicide-resistant plants are also described.