Content And Structure Of Condensed Tannins In Polygonum Viviparum And Synthesis Of Tannin-¹⁵N Protein Complex

In the traditional nitrogen cycle model in terrestrial ecosystems, inorganic nitrogen is the main source of plant nitrogen nutrient in soil. However, in recent years, a number of research have found that in areas like Arctic tundra and alpine meadow where nitrogen mineralization is severely limited, organic nitrogen in soil may also be a good contributor to plant nutrition. Most of these research have focused on the aspects of plants directly absorbing free amino acid in soil, whereas the research on the decomposition and utilization of complicated organic nitrogen such as tannin-protein complex is very limited.Condensed tannins is a kind of secondary metabolites exists widely in plant kingdom. Tannin can not only prevent plants from disease infection and decrease the intake of herbivores in terrestrial ecosystems; they also play an important role in nutrient cycling. Stable complex formed by tannins and protein in litter or in soil is resistant to most of the soil organisms. Research has shown that the complex may be decomposed by mycorrhizal fungi of plants that are rich in tannins, which ensures the advantage of this plant acquiring nitrogen in nutrient competitions with neighboring plants.In this study, Polygonum viviparum was used as our research subject. This plant is rich in tannins and is usually the dominant species in communities in the alpine meadow in Maqu county, which located in the Gannan Tibetan autonomous prefecture in Gansu Province. The tannin-¹⁵Nprotein complex was synthesized after the extraction of tannins from the plant and the acquisition of stable ¹⁵N-isotope-labeled hypha protein cultivated by fungi. The experiment measured the content, monomer type and protein precipitation ability of condensed tannins in P. viviparum, as well as the C:N ratio in the tannin-15 N protein complex. This study could provide experimental materials for future investigations on plants rich in tannins absorbing complicated organic nitrogen in soil. The main results are as follows:（1）The type of tannins in P. viviparum is mainly condensed tannins. The organs of P. viviparum contain different levels of condensed tannins with the order of root>seed>flower>fresh leaf>stem>litter. Tannin content in litter was up to 30.82 mg STE/g DW（mg sorghum tannin equivalent/g dry weight）. The condensed tannin in P. viviparum has high protein precipitation abilities. The identified monomers of condensed tannin are catechin（C）, gallocatechin（GC）, epicatechin（EC） and epigallocatechin gallate（EGCG）.（2） ¹⁵N-labeled fungal protein can be obtained via the cultivation of rhizoctonia solani at the temperature of 25 oC and the speed of 120 rpm in incubator Modified Richard medium contains H₂O, K₂HPO₄·7H₂O, FeSO4·7H₂O, Agar and MgSO4·7H₂O, with sucrose served as the carbon source and KNO₃ as the nitrogen source. Our results show that the growth rate of the treatment with ¹⁵N-KNO₃ as the nitrogen source was slower than that of the control with 14N-KNO₃ as the nitrogen source.（3） Tannin-¹⁵Nprotein complex was synthesized by P. viviparum tannins and fungal protein, and the C: N ratio of the complex is 10.52 ± 0.73.The following conclusions were obtained from this study: P. viviparum is rich in tannins and these compounds have high protein precipitation abilities. Tannin-¹⁵N protein complex can be attained by fungus cultivation and compound complexion. In future studies, injecting¹⁵N-labeled complex into Polygonum viviparum dominated soil will help us to understand how the mycorrhizal fungi of Polygonum viviparum could decompose the organic complex and transfer nitrogen to its host plant. Results of this study will provide useful experimental material for the research of alpine meadow plants absorbing organic nitrogen, which is of great importance for the maintenance of ecosystem stability and sustainable development of the Qinghai-Tibet Plateau.