Responses Of Invasive Plant Secondary Metabolites To Herbivory And Environmental Factors

Exotic invasive plants have been stressed by changing biotic and abiotic environment factors during their invasion process.The different biotic interaction strength and abiotic environments between native and invasive ranges,and within the invasive range will lead to changes in plant physiological traits and defensive strategies evolution.In this study,we examined how plants respond physiologically to multiple biotic and abiotic environments during their invasion process and along latitudinal or elevational gradients using two invasive plants,Phytolacca americana and Triadica sebifera.1.We sampled individuals of the invasive plant Phytolacca americana at 15 field sites spanning 10 degrees of latitude from 25.72° to 36.15° N in central and southern China and measured traits related to growth(plant height,canopy width,number of branches and stem diameter),reproduction(fruits per raceme and racemes per plant)and anti-herbivory defense(leaf,stem,root and fruit saponin).We found growth architecture was significantly related to latitude: number of stems increased,while stem diameter decreased with increasing latitude.Reproductive architecture was also significantly related to latitude: with increasing latitude,plants produced fewer fruits per raceme,but more racemes per plant.We also found defense in reproductive tissue(fruit saponin)increased with increasing latitude.Our study indicates that the effect of latitude on growth,reproduction and defense traits may depend on a combination of abiotic and biotic pressures.2.We investigated biotic(herbivory)and abiotic(temperature,precipitation and solar radiation)effects on secondary chemicals(tannins and flavonoids)of leaves of Triadica sebifera along latitudes in its native range(China)and invasive range(US).We found higher herbivory and tannins and lower flavonoids in native range than those in introduced ranges.The foliar concentrations of tannins and flavonoids both decreased with increasing latitudes in invasive range,showed a similar latitudinal pattern with temperature and solar radiation.However,both foliar concentrations of tannins and flavonoids were highest at intermediate latitudes in native range,and showed a similar latitudinal pattern with herbivory.3.We conducted a meta-analyses of previous common garden experimental chemical(tannins and flavonoids)results.We found higher and tannins and lower flavonoids in native populations than those in introduced populations,which suggest genetic variations in responses to differing herbivory and climate between the ranges.While there was no effect of latitude on the concentrations of tannins or flavonoids for either native or invasive populations,indicating that variations of chemicals along latitudes in either range are plastic responses,associated with changing environments: flavonoids increased with solar radiation and temperature while tannins increased with leaf area consumed,precipitation,and temperature as latitudes decrease.4.We conducted field surveys of herbivores and their damages on Triadica sebifera and performed chemical(tannins and flavonoids)analysis of both damaged and undamaged leaves along an elevational gradient.Tannins and flavonoids showed elevational trends,and those responses might have been driven by herbivory and abiotic environments.Plants allocated more tannins and flavonoids to damaged leaves than undamaged ones under the higher herbivory at low elevations,whereas at high elevations,plants had more flavonoids in undamaged leaves,likely responding to stronger abiotic stresses.Overall,the ratio of flavonoids to tannins increased as the elevation increased,suggesting that plants may be able to trade off these chemical responses to the changing environments along elevational gradients(i.e.,low herbivory and a harsh climate at high elevations).5.We conducted transplant experiment at three sites using seedlings and rhizosphere soils from same Triadica sebifera population and another group of rhizosphere soils from the nearest local Triadica sebifera population to examine the effects of herbivory,climate factors(temperature and precipitation)and soil conditions on variations of plant secondary metabolites.We found that climate affected all secondary metabolites in plant leaves and roots,and climates and soil conditions had an interactive effect on almost all secondary metabolites in leaves and roots,except for leaf flavonoids.However,herbivory only affected leaf flavonoids,which indicates that plant secondary metabolites in native Triadica sebifera is more responsive to soil and climatic conditions than herbivory.Moreover,the analysis of relationships between plant secondary metabolites and climatic conditions(temperature and precipitation),with using the data from the same soil group of different locations,showed that climate factors played a key role in the variations of plant secondary metabolites.6.We examined the effects of aboveground chewing and sucking herbivores on indirect defenses in leaves and direct defenses in both leaves and roots of tallow tree,Triadica sebifera.We also included foliar applications of methyl jasmonate(MeJA)and salicylic acid(SA).We found that chewing herbivores and MeJA increased aboveground defense chemicals but SA only increased belowground total flavonoids.Herbivory or MeJA increased aboveground indirect defense(extrafloral nectar)but SA decreased it.PCA analysis of defense types showed similar plant responses to chewing herbivory and MeJA and to sucking herbivory and SA.There was a trade-off between increasing total root phenolics and tannins(MeJA,chewing)versus latex and total root flavonoids(sucking,SA).In conclusion,our results showed the important role of secondary chemicals in plant physiological and ecological strategies responding to varying environments,which implicate that invasive plants could trade off abiotic tolerance and chemical defense to herbivory to facilitate invasion success across heterogeneous environments.