不同生境下空心莲子草响应模拟昆虫采食的生长和化学防御策略

外来植物往往可以入侵多种生境并受到多种昆虫的采食,而不同生境条件将可能会影响这些入侵植物对昆虫采食的防御策略。以入侵我国的克隆植物——空心莲子草为研究对象,分别选择生长在水生生境、水陆两栖生境和陆生生境中的无性个体(分株),通过50%去叶处理模拟昆虫采食,分析不同生境下空心莲子草对模拟昆虫采食处理的生长及化学防御响应的差异。模拟昆虫采食处理显著抑制了陆生生境、水陆两栖生境以及水生生境下空心莲子草的根、茎、叶和总生物量,但对3种生境下空心莲子草的生物量分配(根冠比、根生物量分配、茎生物量分配和叶生物量分配)均无显著影响。陆生生境下空心莲子草根、茎和总生物量显著高于水陆两栖生境和水生生境,根冠比显著低于水陆两栖生境和水生生境。模拟昆虫采食处理显著降低了空心莲子草的木质素含量,而对单宁和总酚含量影响不显著。生境对木质素含量无显著影响,但陆生生境下空心莲子草单宁含量显著高于水陆两栖生境和水生生境,且总酚含量显著高于水陆两栖生境,表明陆生生境中空心莲子草具有更强的防御能力。空心莲子草木质素含量与总生物量无显著相关性,但在模拟采食情况下,其总酚含量与总生物量呈显著负相关,而无论模拟昆虫采食处理存在与否,空心莲子草单宁含量与总生物量均呈显著正相关。因此,空心莲子草存在昆虫介导的生长和化学防御之间的权衡,在昆虫采食的情况下可通过减少生长来增加对化学防御物质的投入,但生境对空心莲子草这种生长-防御权衡的影响十分有限。

Effects of simulated insect herbivory on the growth and chemical defense of Alternanthera philoxeroides in different habitats

Alien invasive plant species can invade various habitats and be grazed upon by different insects. Therefore, habitat conditions may affect the strategies of alien invasive plant species to defend against insect herbivory. We conducted a greenhouse experiment in which we grew ramets of the invasive herb Alternanthera philoxeroides collected from three different habitats (aquatic, semi-aquatic and terrestrial) and subjected them to two levels of simulated leaf herbivory, i.e. no defoliation (control) and 50% leaf removal. Simulated leaf herbivory significantly reduced root biomass, stem biomass, leaf biomass and total biomass of A. philoxeroides, but had no significant effect on root/shoot ratio, root mass ratio, stem mass ratio and leaf mass ratio. Root biomass, stem biomass, leaf biomass and total biomass of A. philoxeroides were higher in terrestrial environments than in semi-aquatic and aquatic environments, but root/shoot ratios in terrestrial habitats were significantly lower than in semi-aquatic and aquatic habitats. Simulated leaf herbivory significantly decreased lignin content of A. philoxeroides, but had no significant effect on the contents of tannins and phenols. Habitat types had no significant effect on lignin content of A. philoxeroides, but tannin content was significantly higher in terrestrial habitats than in semi-aquatic and aquatic habitats, and phenol content was also significantly higher in terrestrial habitats than in semi-aquatic habitats. These results suggest that the chemical defense of A. philoxeroides to insect herbivory may be higher in terrestrial habitats than in aquatic and semi-aquatic habitats. Total biomass of A. philoxeroides was significantly negatively correlated with phenol content under simulated leaf herbivory, and was significantly positively correlated with tannin content under both control and simulated leaf herbivory treatments. However, total biomass was not correlated with lignin content. These results indicate that insect herbivory can trigger a trade-off between growth and chemical defense in A. philoxeroides, i.e., herbivory results in investing in chemical defense at the cost of reduced growth, and habitat conditions have little effect on such a trade-off.