Effects of nitrogen and phosphorus fertilization rates on the biomass, essential oil yield and composition, polyphenol contents and antioxidant activity of Vitex negundo Linn. grown under field conditions

107

Vitex negundo Linn is an edible aromatic medicinal plant, which is distributed throughout tropical Asia and Africa. The shoots of V. negundo possess various medicinal properties, such as pesticidal, anti-fungal, anti-bacterial and anti-inflammatory activity. According to the Chinese Pharmacopoeia, V. negundo is used as a medicinal plant for the treatment of cough, flu, asthma and stomachache. The main bioactive compounds present in V. negundo essential oil are β-caryophyllene and eremophilene. β-caryophyllene possesses anti-inflammatory, anti-depression and anti-anxiety activities, eremophilene possesses anti-microbial, anti-cancer and immunomodulatory properties. To date, although many studies have reported on the bioactivities of V. negundo, information on the cultivation and fertilizer management of V. negundo still remains limited. Therefore, the objective of this study was to evaluate the effects of nitrogen and phosphorus fertilization rates on the biomass, essential oil yield and composition, polyphenol contents and antioxidant activities in V. negundo. This study was conducted under field conditions, eight treatments namely nitrogen control (no fertilizer；NCK), phosphorus control (no fertilizer；PCK), three different rates of nitrogen (50, 100, 200 kg-N ha^(-1)) and three different rates of phosphorus (50, 100, 200 kg-P ha^(-1)). After 180 days of cultivation, soil sample and shoots of V. negundo were collected for chemical analysis. The results showed that at 200 kg-P ha^(-1) phosphorus, V. negundo had the highest biomass and essential oil yield. The essential oil compositions of V. negundo showed that 200 kg-P ha^(-1) treatment had the highest number of volatile components (45 compounds), followed by 42 compounds in 100 kg-P ha^(-1) treatment, 40 compounds in 200 kg-N ha^(-1) treatment and 50 kg-P ha^(-1) treatment. In addition, 200 kg-P ha^(-1) treatment also showed the highest contents of eremophilene (23.44%) content. Total phenolic and total flavonoid contents in shoots of different fertilizer treatments were not significantly different. However, with the increasing rates of nitrogen and phosphorus fertilization, the contents of total phenols in shoots of different nitrogen and phosphorus treatments appear to decrease, and then increased. Similar result also found in the contents of total flavonoid in shoots of different phosphorus treatments. The changes of total phenolic and total flavonoid contents in shoots could be due to the different nitrogen and phosphorus fertilization rates. Antioxidant activity studies of V. negundo shoot extracts and essential oils showed that there were no significantly difference in DPPH（1,1-diphenyl-2-picrylhydrazyl）radical scavenging activity. Antioxidant activity studies of V. negundo essential oils also showed that there were no significantly difference in the FRAP (Ferric reducing antioxidant power) assay. However, the FRAP assay values of the V. negundo shoot extracts were found to decrease, and then increased with increasing nitrogen and phosphorus fertilization rates, its values were similar to the total phenolic and total flavonoid contents, suggesting that the amount of polyphenols might contribute to the antioxidant capacity of V. negundo shoots. In conclusion, this study indicates that 200 kg-P ha^(-1) phosphorus fertilization rates has a positive effect on the biomass, essential oil yield and bioactive compounds of V. negundo.