Complex relation between triazine-susceptible phenotype and genotype in the weed Senecio vulgaris may be caused by chloroplast DNA polymorphism

The weed Senecio vulgaris acquired high levels of resistance to triazine he rbicides soon after the latter's introduction. As in most weeds, triazine r esistance is conferred by a point mutation in the chloroplast psbA gene tha t negatively affects the fitness of its carrier. To assess levels of triazi ne resistance in S. vulgaris field populations, we adopted a PCR-RFLP-based molecular diagnostic test recently developed for the triazine resistance-c onferring region of the psbA gene of other weeds, including Brassica napus, Chenopodium spp. and Amaranthus spp., and compared these molecular results to the phenotypic response after triazine application. A highly significan t linear correlation was found between phytotoxic symptoms and biomass redu ction. Variability in phenotypic response was not only found between popula tions or inbred lines of S. vulgaris but also within replicates of the same inbred line. No clear relationship, however, was found between the DNA res triction pattern and the phenotypic response to triazine application, there by throwing doubt on the use of such molecular diagnostic tests to track tr iazine resistance in S. vulgaris. Our results indicate that the chloroplast genome of S. vulgaris is polymorphic and that the level of polymorphism ma y be variable within single leaves of individual plants. We discuss the pos sible genetic basis of this polymorphism and its consequence for the acquis ition and inheritance of chloroplast-based traits.