Genetic variation for tolerance to defoliation in Cirsium arvense

The agricultural weed, Cirsium arvense (creeping, Canada, or Californian thistle), is notorious for its ability to tolerate defoliation by mowing, herbivores, or herbicides, which is facilitated by the growth of adventitious shoots from its extensive clonal root system. In an outdoor potted plant experiment, we examined the tolerance of 36 unique genotypes of C. arvense to defoliation by establishing pairs of clonal replicates that were assigned to a clipped, or unclipped treatment. Three clippings were applied from spring to early summer to simulate repeated mowing. The average final percentage reduction caused by the repeated clipping treatment was 18%, 72%, 32%, and 50% for shoot biomass, root biomass, number of shoots, and maximum plant height, respectively. Nearly all individual genotypes were negatively affected by the clipping. However, some genotypes overcompensated, and achieved greater shoot biomass, number of shoots, or increased height than their unclipped counterparts. No genotypes were able to overcompensate, or fully tolerate, the lost root biomass due to repeated clipping. Genetic variation for tolerance to defoliation was detected for the number of shoots, plant height, and for relative growth rate. With regard to relative growth rate, significant genetic variation was not detected until after the third clipping event, indicating that genotypes were equally tolerant to a moderate degree of defoliation, but upon more severe defoliation, genetic differences were evident. Since repeated defoliation is a recommended control technique for this weed, selection for more tolerant genotypes is possible, and should be taken into account for the management of this weed.