Escaping costs of induced resistance in the invasive plant, Alliaria petiolata

Background/Question/Methods

Hypotheses of plant invasion, such as the Evolution of Increased Competitive Ability Hypothesis (EICA), assume that resistance to herbivores is costly, yet no tests of EICA have demonstrated this to be the case while testing other aspects of this hypothesis.  In particular, the expression and costs of induced resistance or inducible defense traits have been little examined in any invasive plant. We examined continental, population and environmental variation in the expression and costs of induced defense traits in Alliaria petiolata, an important European invader of North America forests that largely escapes herbivory in its introduced range.  We used jasmonic acid to induce first-year plants of five native and seven invasive populations of A. petiolata in the greenhouse to examine whether induction was costly, whether populations varied in the magnitude of induction or its costs, and whether any variation could be attributed to the continental origin of the populations. We then examined the extent to which the expression and costs of induced defense traits varied across a soil nutrient gradient in a single population. 

Results/Conclusions

While absolute levels varied among populations, the induction of trypsin inhibitor, a marker jasmonate-inducible defense trait, was strong and largely uniform across populations.  Trichome densities varied qualitatively and quantitatively across populations, and were generally inducible by jasmonic acid, which may have been related to reductions in leaf area caused by jasmonic acid treatment.  Jasmonate induction was substantially costly to leaf growth and dry biomass production, the magnitude of which varied little among populations.  The continental origin of the populations explained little variation in any trait.  Trypsin inhibitor was strongly inducible across a nutrient gradient, but induction was substantially more costly to leaf growth at low than at moderate or high levels of soil nutrients.  These results demonstrate that A. petiolata displays chemical defense traits that are strongly and uniformly inducible by jasmonic acid across populations, that jasmonate induction is substantially costly, and that costs of induction are contingent on soil nutrient availability.  In the absence of significant herbivory in the introduced range, escaping the need to express induced defense traits (along with escaping tissues loss itself) may benefit fitness of invasive plants even in the absence of any evolutionary change in resistance in these plants.