Isolation and Identification of Allelopathic Substances from Forsythia suspensa Leaves, and Their Metabolism and Activity

Abstract

The fruit of Forsythia suspensa (Thunb.) Vahl has been used in traditional Chinese medicine as "Forsythiae fructus". The species is also grown in parks and gardens, and on streets and building lots, as an ornamental plant, but it requires pruning. In this study, the allelopathic activity and allelopathic substances in the leaves of pruned branches of F. suspensa were investigated to determine any potential application. The leaf extracts of F. suspensa showed growth inhibitory activity against three weed species; Echinochloa crus-galli, Lolium multiflorum, and Vulpia myuros. Two allelopathic substances in the extracts were isolated through the bioassay-guided purification process, and identified as (-)-matairesinol and (-)-arctigenin. (-)-Matairesinol and (-)-arctigenin, which showed significant growth inhibitory activity at concentrations greater than 0.3 mM in vitro. The inhibitory activity of (-)-arctigenin was greater than that of (-)-matairesinol. However, both compounds were more active than (+)-pinolesinol which is their precursor in the biosynthetic pathway. The investigation suggests that F. suspensa leaves are allelopathic, and (-)-matairesinol and (-)-arctigenin may contribute to the growth inhibitory activities. Therefore, the leaves of the pruned branches can be applied as a weed management strategy in some agricultural practices such as using the leaf extracts in a foliar spray and the leaves in a soil mixture, thereby reducing the dependency on synthetic herbicides in the crop cultivation and contributing to developing eco-friendly agriculture.

Keywords: arctigenin; eco-friendly agriculture; lignan; matairesinol; pinolesinol; pruned waste; soil additive.

Figure 1

Leaves and flowers of F. suspensa.

Figure 2

Before and after the pruning of F. suspensa.

Figure 3

Effects of the leaf extracts of F. suspensa on the growth of the roots and coleoptiles of E. crus-galli, L. multiflorum and V. myuros. Concentration (mg leaf equivalent extract/mL) corresponds to the amount of the extracts obtained from 10, 30, 100 or 300 mg leaves per mL. Mean ± SE from 4 independent experiments with 10 seedlings for each determination are shown. Asterisks indicate significant differences between control and treatment: ***, p < 0.001.

Figure 4

Effects of the F. suspensa leaf extracts on the growth of the roots and coleoptiles of cress. Other conditions were as described in Figure 3. Asterisks indicate significant differences between control and treatment: *, p < 0.05, ***, p < 0.001.

Figure 5

Purification process of compounds 1 and 2 from the leaf extract of F. suspensa.

Figure 6

Effects of the fractions separated by a silica gel column chromatography on the root and hypocotyl growth of cress. Concentration of tested samples corresponded to the extract obtained from 0.3 g of F. suspensa per mL. Other conditions were as described in Figure 3. Asterisks indicate significant differences between control and treatment: **, p < 0.01, ***, p < 0.001.

Figure 7

HPLC chromatogram and chemical structures of compounds 1 and 2. HPLC condition; column: reverse-phase, Inertsil ODS-3, 4.6 mm i.d. × 250 mm in length (YMC Ltd., Kyoto, Japan), solvent: 70% aqueous methanol (flow rate, 0.8 mL), detection: 220 nm.

Figure 8

Chemical structures of (-)-matairesinol and (-)-arctigenin.

Figure 9

Effects of (-)-matairesinol on the growth of the roots and hypocotyls/coleoptiles of cress and L. multiflorum. Means ± SE from 4 independent experiments with 10 seedlings for each determination are shown. Different letters on the symbols in the same panels indicate significant differences at the p < 0.05 level.

Figure 10

Effects of (-)-arctigenin on the growth of the roots and hypocotyls/coleoptiles of cress and L. multiflorum. Other conditions were as described in Figure 9.

Figure 11

Effects of (+)-pinoresinol on the growth of the roots and hypocotyls/coleoptiles against of cress and L. multiflorum. Other conditions were as described in Figure 9.

Figure 12

The biosynthetic pathway of (-)-matairesinol and (-)-arctigenin in the Forsythia species. PLR: pinoresinol/lariciresinol reductase, SIRD: secoisolariciresinol dehydrogenase, OMT: O-methyltransferases.