Herbivory Pressure Drives Constitutive And Induced Resistance Of Invasive Plants

It is significantly important to study the interaction between invasive plants and herbivorous insects to reveal the mechanism of plant invasion.In introduced range,exotic plants generally released from specialist herbivores,are still attacked by a variety of generalist herbivores,resulting in a change in the defense strategy of exotic plants.Moreover,different exotic plant species suffer different generalist herbivore pressure.Herbivore pressure affects plant constitutive and induced defense strategy.However,it remains largely unclear how variation of herbivore pressure determines constitutive and induced defense strategies of exotic plant species,and thus the effect on plant invasion.Firstly,we conducted a large-scale field survey and common-garden experiments to explore the differences in herbivore pressure and herbivore community between exotic plant species and native species.We then manipulated greenhouse experiments to clarify the effects of herbivore pressure on constitutive and induced defense of exotic plants.We finally identified the regulation mechanism of secondary metabolites on constitutive and induced resistance of invasive plants through secondary metabolic analysis.We investigated plant growth rate to elucidate the effect of plant constitutive and induced defense strategy on plant competitiveness.The main results are as follows:(1)A field survey of 28 samples from Henan,Shandong,Hebei and Jiangsu provinces showed that exotic plants suffered significantly less herbivore pressure than native species.Meanwhile,there was a significant difference in herbivore pressure among exotic plant species,ranging from 3.67% to 83.07%.Herbivore biomass was also dramatically different among exotic plant species,and Lepidoptera was the dominant herbivore.Correlation analysis showed that there was a positive correlation between herbivore biomass and herbivore pressure on exotic plants.(2)We conducted common-garden experiments using 14 dominant invasive plants in the field survey and their native congeners.The results showed that herbivore pressure of invasive plants was significantly lower than that of native plants by 70.9% in the same habitat.Different plants suffered different herbivore pressure,herbivore pressure of invasive plants ranged from 0.31% to 27.64% and that of native plants ranged from 8.01% to 45.05%.Herbivore biomass was also dramatically different among exotic plant species,and Lepidoptera was the dominant herbivore.Correlation analysis showed that there was a positive correlation between herbivore biomass and herbivore pressure on exotic plants.(3)Correlation analysis results showed that there was a positive correlation between constitutive resistance and herbivore pressure of invasive plants,but no correlation between constitutive resistance and herbivore pressure of native plants.There was no difference in larvae weight gains on healthy invasive plants and native plants.And there was a negative correlation between induced resistance and herbivore pressure of invasive plants,but there was no correlation between induced resistance and herbivore pressure of invasive plants.Herbivory-induced invasive plants had 22.2% higher defense than herbivory-induced native plants.(4)Correlation analysis showed that phenolic concentrations in 14 healthy invasive plants was positively correlated with herbivore pressure,while phenolic concentrations in herbivory-induced invasive plants was negatively correlated with herbivore pressure.There was no correlation between phenolic concentrations in healthy native plants and herbivore pressure,between phenolic concentrations in herbivoryinduced native plants and herbivore pressure.There was no difference in phenolic concentrations between invasive plants and their native plants in control,but phenolic concentrations of invasive plants were twice that of native plants in herbivory-induced.Correlation analysis showed that there was no correlation between triterpenoid concentrations in healthy invasive or native plants and herbivore pressure,between triterpenoid concentrations in herbivory-induced invasive or native plants and herbivore pressure.(5)In order to elucidate the effect of plant constitutive and induced defense strategy on plant growth,we measured the biomass of invasive plants and their native congeners,and examined the relationship between defense and growth.The results showed that the biomass of invasive plants was significantly higher than that of native plants.The correlation analysis between induced defense intensity and growth showed that there was a significant positive correlation between induced defense intensity and plant growth rate.The stronger the induced defense,the greater growth potential of plants.These results highlight that herbivore pressure changes constitutive resistance and induced resistance strategy of exotic plants.With the increase of herbivore pressure,constitutive resistance of exotic plants increased and induced resistance decreased,and vice versa.Total phenolics played an important role in constitutive and induced resistance strategy of exotic plants.Invasive plants adopt a defense strategy of low constitutive resistance and high induced resistance,which improves their competitiveness compared with native plants.The results of our study have an important theoretical value for clarifying the influence of herbivore pressure on constitutive and induced resistance strategy of invasive plants,and provide a basis to further reveal the invasion mechanism of exotic plants.