Seasonal dynamics of constitutive levels of phenolic components lead to alterations of antioxidant capacities in Acer truncatum leaves

Abstract

Acer truncatum leaves (ATL) have long been used in China as a substitutional tea with health benefits. However, little is known about the antioxidant capacities as well as its bioactive components. The present study was to investigate constitutive phenolic compositions and evaluate antioxidant activities of ATL through its whole growing season. The results showed that all seasonal ATL samples contained remarkable phenols, tannins and flavonoids, and exhibited high antioxidant capacities. Constitutive levels of total phenols, tannins and flavonoids, and three individual phenolic compounds, namely 1,2,3,4,6-penta-O-galloyl-β-d-glucose (PGG), methyl gallate (MG), and quercetin-3-O-l-rhamnoside (QR), and DPPH and ABTS+ scavenging capacities were all significantly higher in both the beginning (S4) and ending (S11) months than those in the middle months (S5–S10), resulting in a consistent seasonal saddle-shaped pattern. Since S4 and S11 showed significant differences in oxygen radical absorbance capacity (ORAC) values but not in total flavonoids, and there was no significant correlation between ORAC and total flavonoids, it can be inferred that total flavonoids contributed less to the ORAC values than total phenols and tannins did. Correlation analyses further revealed that total phenols, tannins and flavonoids were the main contributors to antioxidant ability, and PGG, MG and QR were found to be the key components of phenols (including tannins and flavonoids) in ATL. Gallic acid was first discovered in ATL, and high levels of PGG and QR (both higher than 1g/100g d.w. in April) were corroborated through HPLC analysis. Impressive antioxidant abilities of S4 and S11 were further demonstrated by cellular antioxidant assay, and flavonoids showed stronger antioxidant capacity in the cellular level than other phenolic compositions. In conclusion, seasonal dynamics of constitutive levels of phenolic components lead to alterations of antioxidant capacities in ATL, and November and April were the ideal harvesting time with highest levels of phenols and antioxidant capacity.