Biogeographic differences in the allelopathy of leaf surface extracts of an invasive weed

Abstract

Allelopathy, the release of chemicals byplants that inhibit the germination and growth ofcompeting species, can be an important trait forinvasive success. However, little is known aboutpotential biogeographical differences in allelopathydue to divergent regional eco-evolutionary histories.To test this, we examined the allelochemical potentialof the highly invasive species Centaurea solstitialisfrom six world regions including native (Spain,Turkey) and non-native ranges (Argentina, Chile, California and Australia). Seeds from several populationsin each region were collected and grown undercommon garden conditions. Allelopathic potential andchemical composition of three leaf extract concentrationsof C. solstitialis from each region: 0.25%, 0.5%0.75% (w/v-1) were assessed on the phytometerLactuca sativa. The main allelochemicals present inthe leaf-surface extract were sesquiterpene lactonesthat varied in major constitutive compounds acrossregions. These leaf extracts had strong inhibitoryeffects on L. sativa seed germination and net growth.Summed across regions, the 0.25% concentrationsuppressed germination by 72% and radicle elongationby 66%, relative to the controls. At the 0.5% concentration, no seeds germinated when exposed toextracts from the non-native ranges of Argentina andChile, whereas germination and radicle growth werereduced by 98% and 89%, respectively, in theremaining regions, relative to controls. Germinationand seedling growth were completely inhibited at the0.75% concentration extract for all regions. Some nonnativeregions were characterized by relatively lowerconcentrations of allelochemicals, suggesting thatthere is biogeographical variation in allelopathicexpression. These findings imply that rapid selectionon the biochemical signatures of an exotic invasiveplant species can be highly region-specific across theworld.