Evaluating herbicide resistance in invasive Japanese knotweed (Reynoutria japonica)
Herbicide-resistant weeds are a pervasive problem in agricultural systems, but they are rarely considered in less anthropogenic ecosystems. Aggressive treatment of invasive plants with herbicides represents a selective force that may be sufficient to induce rapid evolution of herbicide resistance. Adirondack Park in upstate New York has mounted a large-scale effort to eradicate Japanese knotweed (Reynoutria japonica) using the herbicide glyphosate. After years of treatment, plants from several sites showed persistent re-growth. This experiment assesses the possibility of resistance in these sites by comparing chemical responses of historically treated and historically untreated stands of R. japonica to glyphosate. Rhizomes were collected from treated and untreated sites (n=65 for each group). These rhizomes were grown in the greenhouse for 80 days, during which height was measured at several points. After 80 days, leaves were collected before and after spraying the plants with different concentrations of glyphosate (0, 28mM, 113mM). Change in shikimic acid was used to determine glyphosate susceptibility. Plants that accumulate more shikimate in response to glyphosate are more susceptible. Change in shikimate was measured with a spectrophotometer. Plants were then cut at the stem and allowed to grow back for 40 days.
Plants from historically treated sites grew more slowly and to smaller heights than those from historically untreated sites. This indicates that there is either a fitness cost to glyphosate resistance, or a persistent effect of glyphosate. Change in shikimate after glyphosate treatment did not follow the expected pattern for resistance. Plants from treated sites accumulated about four times as much shikimate as those from untreated sites under moderate concentrations (0.437±0.207mM and 0.095±0.184mM respectively). Shikimate accumulation under high glyphosate concentrations was indistinguishable for the two groups (0.725±0.229mM and 0.883±0.184mM). Low shikimate accumulation in untreated sites under moderate concentrations points to the possibility of a constitutive resistance to glyphosate in R. japonica. After 40 days of re-growth, plants displayed a pattern of reduced growth for plants that had been treated with higher concentrations of herbicides, but this pattern was noticeably less prominent in plants from treated sites. In particular, two treated sites in particular showed almost no reduction in re-growth even under high glyphosate conditions. R. japonica does not display the expected pattern for chemical herbicide resistance, but may have evolved to respond to glyphosate under strong selection.