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 herbicides soon after the latter's introduction. As in most weeds, triazine resistance is conferred by a point mutation in the chloroplast psbA gene that negatively affects the fitness of its carrier. To assess levels of triazine resistance in S. vulgaris field populations, we adopted a PCR-RFLP-based molecular diagnostic test recently developed for the triazine resistance-conferring 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 significant linear correlation was found between phytotoxic symptoms and biomass reduction. Variability in phenotypic response was not only found between populations or inbred lines of S. vulgaris but also within replicates of the same inbred line. No clear relationship, however, was found between the DNA restriction pattern and the phenotypic response to triazine application, thereby throwing doubt on the use of such molecular diagnostic tests to track triazine resistance in S. vulgaris. Our results indicate that the chloroplast genome of S. vulgaris is polymorphic and that the level of polymorphism may be variable within single leaves of individual plants. We discuss the possible genetic basis of this polymorphism and its consequence for the acquisition and inheritance of chloroplast-based traits.