Ontogenetic Patterns Of Iridoid Glycosides As A Strategy Of Chemical Defense In Scrophularia Ningpoensis

Radix Scrophulariae, the dry root of Scrophularia ningpoensis Hemsl., is one of the famous traditional herb medicines, "Zhe ba wei". Iridoids and phenylpropanoid glycosides are considered as its characteristic bioactive components. Iridoid glycosides are one of the major defense chemicals in plants in response to herbivores, pathogens and other evironmental stresses, which are thus of vital significance for plant fitness. However, few studies have focused on the ecological functions in terms of chemical defense in S. ningpoensis, even less on their changing patterns across plant whole life cycles. In the present research, we evaluated the defense strategies of S. ningpoensis across multiple age stages during two critical ontogenetic periods (seedling and adult stages) by analysing content variation of four iridoid glycosides in different organs; we also analysed the changing patterns of iridoid glycosides under different light intensity conditions. The main results are summerized as follows:1) We analysed the content variation of four iridoid glycoside across four age classes during seedling stage in S. ningpoensis. The level of secondary metabolites increased with plant ages during plant vegetable stages, which are consistent with the predictions of GDBH, rejecting the plant age hypothesis which considered a decrease of defense level with plant development; Content variation of four iridoid glycosides showed significant genetic difference in four populations; We didn’t detect the trade-offs between-growth and defense, which are contradicted with GDBH; The contents of the four iridoid glycosides showed different trajectories during seedling stage, indicating they might play different roles in the process of plant development, also this chemical dynamics might be an adaptive strategy of seedlings in facing of the limited acquirement of resources.2) The contents of four iridoid glycosides continued to decline across four age classes during the matured stage, which indicate S.ningpoensis adopted a defense strategy different from that in the seedling stage. This pattern fit the predicted model of annual herbs proposed by Boege and Marquis (2005). It can be explained by the fact that S. ningpoensis has long been cultivated as an annual plant during domestication despite its perennial traits. In contrary to seedling stage, we detected significant trade-offs between growth and defense in the matured stage, supporting the Carbon/Nutrient Balance Hypothesis (GDBH). These results imply that plant defense strategies may differ among different age stages.3) We detected the greatest IGs content in, which support the predictions by the optimal defense hypothesis (ODH). This theory assumes that plants allocate more defense chemicals to organs with higher fitness values. This hypothesis was further favored by the decreased IGs content after flower removing, which also improved the total biomass and the root biomass. This effect only applied in the fully blooming period. These results indicated a significant trade-off between vegetative and reproductive growth.4) We observed the highest net photosynthetic rate under the 50% light intensity condition, leading to the highest IGs level, which were consistent with predictions proposed by the GDBH. Although shading treatment increased "quality" (content of secondary metabolites) of S. ningpoensis, the production of total biomass and the root biomass decreased.In conclusion, ontogenetic patterns of secondary metabolites in S. ningpoensis do not follow the traditionanl linear model, but consistent with a non-linear model-proposed by Boege and Marquis (2005). The ontogenetic pattern of secondary metabolites varied depending on plant life form (annual, perennial, etc), population genetic differentiation, type of defense chemicals and resource availability. Our study highlights ontogeny as an important factor in explaining the changing pattern of plant secondary metabolites, and emphasizes importance of temporal and spatial scales. The results also provide the scientific suggestion for Good Agricultural practice of S. ningpoensis...