VARIABILITY IN METRIBUZIN TOLERANCE AMONG RUDERAL AND AGRESTAL POPULATIONS OF SOLANUM-PTYCANTHUM DUN

Solanum ptycanthum Dun. is an annual weed of both agrestal and ruderal habitats in southern Ontario, which is the northern limit of its distribution. Plants of S. ptycanthum are introduced annually to the tomato-production areas of Ontario via the importation of tomato transplants from Georgia and North Carolina. The objective of this study was to test for genetic differences in tolerance to the herbicide metribuzin among agrestal and ruderal populations from southwestern Ontario and agrestal populations from Georgia and North Carolina. We compared growth responses among and within populations to low (0.4 kg a.i. ha(-1)) and high (0.9 kg a.i. ha(-1)) doses of metribuzin applied postemergence in greenhouse trials. Only the Ontario agrestal populations had been previously exposed to metribuzin. Ontario populations, regardless of habitat of origin, were significantly more tolerant of low and high doses of metribuzin than Georgia populations. Ruderal populations were more susceptible than Ontario agrestal populations to the higher dose of metribuzin. North Carolina seedlings were as tolerant as Ontario agrestal populations. These results suggest a genetic basis for tolerance differences unrelated to previous exposure. Overall intra-population variability in response to metribuzin was low, although the Ontario agrestal populations showed significant within-population variability in height and leaf growth in response to low and high doses of metribuzin. This suggests that the ability to respond to selection may differ among populations. Lack of significant levels of genetically based plasticity also suggests that herbicide-dependent selection within populations would be minimal.