Composition and protein precipitation capacity of condensed tannins in purple prairie clover (Dalea purpurea Vent.)

Authors: HUANG, QIANQIAN - Yangzhou University; HU, TIANMING - Northwest A&f University; XU, ZHONGJUN - Agriculture And Agri-Food Canada; JIN, LONG - Agriculture And Agri-Food Canada; MCALLISTER, TIM - Agriculture And Agri-Food Canada; ACHARYA, SURYA - Agriculture And Agri-Food Canada; Zeller, Wayne; MUELLER-HARVEY, IRENE - University Of Reading; WANG, YUXI - Agriculture And Agri-Food Canada

Submitted to: Frontiers in Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/25/2021
Publication Date: 9/28/2021
Citation: Huang, Q., Hu, T., Xu, Z., Jin, L., McAllister, T.A., Acharya, S., Zeller, W.E., Mueller-Harvey, I., Wang, Y. 2021. Composition and protein precipitation capacity of condensed tannins in purple prairie clover (Dalea purpurea Vent.). Frontiers in Plant Science. 12. Article 715282. https://doi.org/10.3389/fpls.2021.715282.
DOI: https://doi.org/10.3389/fpls.2021.715282

Interpretive Summary: Condensed tannins (CT) are a class of specialized metabolites found in several forage plants and are composed of oligomers and polymers of flavan-3-ol subunits. Benefits of consumption of CT-containing forages by ruminants include protection of protein during rumen digestion leading to improved protein utilization, mitigation of methane production, and protection against parasites because CTs can have anthelminthic activity. Purple prairie clover (PPC) is a native legume widely distributed across the North American prairie that contains a high concentration (up to 94 g/kg dry matter) of CT. Previous studies have shown that PPC CT had mild effects on in vitro ruminal fermentation and that PPC was a highly palatable and nutritive prairie forage for ruminants, despite its high CT content. However, the composition and plant distribution (stems versus leaves versus flowering heads) of PPC CT were unknown. The objectives of this study were to determine the concentration and composition of CT in different tissues of PPC at different maturities and to determine their protein-precipitating capacity. PPC flowering heads contained the highest CT concentration, followed by leaves and stems. PPC CT flavan-3-ol subunit composition varied somewhat between leaves and stems or flowering heads. The mean degree of polymerization (mDP, a measure of how many subunits make up a CT) was highest for CT in stems and lowest for CT in leaves and increased as the plant matured. The protein-precipitating capacity (an indicator of biological activity) of PPC CT varied with stage of maturity and CT from leaves had a greater protein-precipitating capacity than CT from stems or flowering heads. The contrasting protein-precipitating capacities of PPC CT from distinct plant tissues of differing maturity is likely a reflection of the chemical composition of the CT. These findings could play an important role in selection of harvest times to maximize CT content for impact on ruminant production and provide guidance in selecting for CT content and composition in PPC breeding studies.

Technical Abstract: The objectives of this study were to determine the concentration and composition of condensed tannins (CT) in different tissues of purple prairie clover (PPC; Dalea purpurea Vent.) at different maturities and to determine their protein-precipitating capacity. The compositions of CT were elucidated after thiolysis with benzyl mercaptan followed by high-performance liquid-chromatography and 1H-13C HSQC NMR spectroscopy. Results indicated that PPC flowering heads contained the highest CT concentration. Purple prairie clover CT consisted mainly of epicatechin (EC) and epigallocatechin (EGC) subunits. Condensed tannins in leaves were composed of more EC and less EGC than CT in stems and flowering heads at both early flowering and late flowering head stages. The mean degree of polymerization was the highest for CT in stems and increased with maturity. Condensed tannins isolated from PPC leaves at the early flowering head stage exhibited the greatest biological activity in terms of protein precipitation. Overall, condensed tannins in PPC were predominantly procyanidins and the concentration and composition varied among plant tissues and with maturity.