空心莲子草响应南方菟丝子寄生的生长-防御权衡

为探讨全寄生植物南方菟丝子(Cuscuta australis)防治入侵植物空心莲子草(Alternanthera philoxeroides)的可行性,以二者野外天然生长的种群为研究对象,分析南方菟丝子寄生对空心莲子草生长及防御的影响,阐明空心莲子草在受到寄生胁迫时如何权衡自身生长与防御的关系,进而发展出一套应对南方菟丝子寄生的生长-防御策略。结果显示:(1)南方菟丝子寄生显著改变空心莲子草茎的形态,茎直径和平均节间长均增加,茎直径变化极显著(P0.01);(2)南方菟丝子寄生显著减少空心莲子草叶片数,但同时显著增加后者茎的分枝数,而茎上的节是潜在的无性繁殖体,故有利于空心莲子草的克隆繁殖;此外,南方菟丝子寄生显著降低了空心莲子草的根、茎、叶生物量和总生物量,抑制空心莲子草的生长;(3)南方菟丝子寄生显著增加空心莲子草茎的单宁、总酚、三萜皂苷含量,增强其防御能力;(4)南方菟丝子寄生的空心莲子草的生物量与茎部木质素、三萜皂苷、单宁和总酚含量均呈现显著负相关性(P0.01),对照组则不存在相关性;且寄生组较对照组相比,生物量的相对百分比显著低于对照组(P0.01),而用于防御的次生代谢产物总含量的相对百分比显著高于对照组(P0.01)。以上结果表明,受到南方菟丝子寄生胁迫后,空心莲子草改变自身的生长-防御策略,减少营养生长投入而将更多的资源投向克隆繁殖,同时增强对"防御"物质的投入,增强其防御能力,以利于后代生存和繁衍。

The trade-off between growth and defense in Alternanthera philoxeroides parasitized by Cuscuta australis

The uncontrolled range expansion of invasive plant species has become a worldwide problem in this century. Finding suitable biological control measures for these invasive species has become a focus for many biologists. Using biological control species that are not native to the invaded community can generate further problems in the community. Thus, in recent years, scientists have started to look for enemies that are (1) able to inhibit growth in an invasive species, and (2) native to the invaded region. Parasitic plants are one example of such novel native enemies. Several studies provide support for the use of native parasitic plants as potential biological control agents for invasive plants. However, the exact response of invasive plants to the parasitic plants is still poorly known. In this study, the relationship between a common invasive plant Alternanthera philoxeroides (Amaranthaceae) and its native parasitic plant Cuscuta australis (Convolvulaceae) was investigated. A. philoxeroides is a notorious invasive weed that originates from South America. It is widely distributed in China and has had a marked negative impact on local biodiversity and the economy of China. C. australis, a native holoparasitic plant in China, has been found to naturally parasitize invasive A. philoxeroides in the field. The growth of the parasite depends on assimilation of the host's nutrients and water. A field survey was conducted to investigate the trade-off between growth and defense in A. philoxeroides in response to parasitization by C. australis . Biomass of A. philoxeroides and the secondary metabolite composition in the stems with or without C. australis were measured. Results showed the following: (1) Root mass, stem mass, leaf mass, total biomass and leaf number of A. philoxeroides were significantly reduced in plants parasitized by C. australis, but the number of stem nodes significantly increased. Clonal reproduction of A. philoxeroides mainly relies on the stem nodes; thus, the results indicate a significant inhibition of growth and an increased investment in clonal reproductive ability. (2) Parasitization by C. australis significantly increased the secondary metabolite contents in stems of A. philoxeroides, including lignin, total phenols, tannins and tri-terpenoid saponin. These four types of secondary metabolite play important roles in the plant when under stress. The increase in secondary metabolites indicates an enhanced defense capability in the host. (3) The total biomass of A. philoxeroides parasitized by C. australis was significantly reduced, while the relative percentage content of secondary metabolites significantly increased. Moreover, a significant negative correlation was found between total biomass and secondary metabolite content in stems of A. philoxeroides parasitized by C. australis. Overall, this study suggests that in response to parasitism by C. australis, A. philoxeroides alters its balance in investment between growth and defense, with less investment in growth and more on clonal reproduction and defense. Such a trade-off strategy between growth and defense may help invasive plants mitigate the negative effects of new native enemies in the invaded community. Nonetheless, native parasitic plants do provide a novel enemy for invasive plant species and may provide a less risky but effective way to control invasive plants.