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Bark is an essential component of tree branches, yet its role in controlling branch-leached dissolved organic matter (DOM) characteristics remains unknown in forests. Here, we collected branches (about 1.5 cm in diameter) of two evergreen coniferous trees, two deciduous broadleaf trees, and three evergreen broadleaf trees from a subtropical forest in southern China, and subsequently used a bark removal experiment to determine the effects of bark on branch-derived DOM quantity and bioavailability. Regardless of tree type, the presence of bark reduced tree branch-leached dissolved organic carbon (DOC), dissolved total nitrogen (DTN), and dissolved total phosphorus (DTP) productions. Moreover, DOC, DTN, and DTP productions leached from the branches containing bark were always much lower than the expected values summed from barks and the branches without bark. The presence of bark increased DOM aromaticity in the broadleaf tree branch leachates but reduced DOM aromaticity in the coniferous tree branch leachates. During 42 days of incubation, the presence of bark decreased broadleaf tree branch-leached DOM bioavailability and the relative increments of aromaticity, whereas the opposite trends were observed for the coniferous tree branch-leached DOM. Tree branch-derived DOM bioavailability correlated negatively with SUVA254 values, but exhibited no relationship with either DOC:DTN ratio or DOC:DTP ratio. These observations highlight that tree bark can prevent DOM leaching from branches and regulate branch-leached DOM bioavailability via its effect on DOM aromaticity in subtropical forests. [ABSTRACT FROM AUTHOR]

Chukrasia tabularis is an economically important tree and widely cultured in the southeast of China. Its barks, leaves, and fruits are consumed as a traditional medicine and perceived as a valuable source for bioactive limonin compounds. The extracts from root barks of C. tabularis showed significant anti‐inflammatory effect. The aim of this research was to explore the material basis of C. tabularis anti‐inflammatory activity, and to purify and identify anti‐inflammatory active ingredients. By a bioassay‐guided isolation of dichloromethane fraction obtained two novel phragmalin limonins, Chukrasitin D and E (1 and 2), together with 12 known limonins (3–14). The chemical structure of these compounds is determined on the basis of extensive spectral analysis and chemical reactivity. In addition, the activities of these isolated limonins on the production of nitric oxide (NO), tumor necrosis factor alpha (TNF‐α), and nuclear factor kappa B (NF‐κB) in RAW264.7 cells induced by lipopolysaccharide (LPS) were evaluated. Limonins 1 and 2 indicated significant anti‐inflammatory activity with IC50 values of 6.24 and 6.13 μM. Compound 1 notably inhibited the production of NF‐κB, TNF‐α and interleukin 6 (IL‐6) in macrophages. The present results suggest that the root barks of C. tabularis exhibited anti‐inflammatory effect and the limonins may be responsible for this activity. [ABSTRACT FROM AUTHOR]