We examined the effects of ambient, non-stressing ultraviolet (UV)-B (280-315nm) level combined with different intensities of photosynthetic active radiation (PAR, 400-700nm) on the accumulation of the lignan (-)-hinokinin, in leaves and stems of Hydrocotyle leucocephala. Plants were exposed in sun simulators under almost natural irradiance and climatic conditions to one of four light regimes, i.e. two PAR intensities (906 and 516μmolm(-2)s(-1)) including or excluding UV-B radiation (0 and 0.4Wm(-2)). Besides hinokinin, we identified three chlorogenic acid isomers, one other phenolic acid, 12 quercetin, and five kaempferol derivatives in the H. leucocephala extracts. Hinokinin was most abundant in the stems, and its accumulation was slightly enhanced under UV-B exposure. We therefore assume that hinokinin contributes to cell wall stabilization and consequently to a higher resistance of the plant to environmental factors. Quercetin derivatives increasingly accumulated under UV-B and high PAR exposure at the expense of kaempferols and chlorogenic acids, which was apparently related to its ability to scavenge reactive oxygen species. In general, the concentration of the constituents depended on the plant organ, the leaf age, the light regimes, and the duration of exposure. The distribution pattern of the compounds within the examined organs was not influenced by the treatments. Based on the chemical composition of the extracts a principal component analysis (PCA) enabled a clear separation of the plant organs and harvesting dates. Younger leaves mostly contained higher phenylpropanoid concentrations than older leaves. Nevertheless, more pronounced effects of the light regimes were detected in older leaves. As assessed, in many cases the individual compounds responded differently to the PAR/UV-B combinations, even within the same phenylpropanoid class. Since this is the first report on the influence of light conditions on the accumulation of lignans in herbaceous plants, it opens many perspectives for a more precise elucidation of all involved biochemical and molecular processes.
Keywords: Light intensity; Lignans; Oxidative stress; Phenylpropanoids; Spectral quality.
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