Effect of nitrogen on herb production, secondary metabolites and antioxidant activities of Hypericum pruinatum under nitrogen application

Abstract The optimization of growing conditions using germplasm potentials is an effective way to manage the production and quality of medicinal plant raw materials. The present study aimed to evaluate the effect of mineral nitrogen on the Hypericum pruinatum plant yield, the accumulation of secondary metabolites and their antioxidant activities. Mineral nitrogen fertilizer containing 33% ammonium was applied at five rates, which corresponded to 0, 30, 60, 90 and 120 kg N/ha. The results showed that the yield of herb material increased with increasing doses of nitrogen. Application of nitrogen enhanced the accumulation of naphtodianthrones and emodin, and did not have any effect on phloroglucinol derivatives. Meanwhile, the increased nitrogen supply significantly reduced the accumulation of major phenolics, such as catechin, chlorogenic acid, hyperoside, quercitrin, isoquercitrin and total phenolic content. The results of the antioxidant capacity assay using the HPLC cupric-ion reducing antioxidant capacity (CUPRAC) method with post-column detection indicated that hydroxycinnamic acids and flavonoids, such as quercetin, isoqercitrin and hyperoside, were responsible for most of the antioxidant activity of plant extracts. Overall, antioxidant activity was reduced from non-fertilized samples to samples exposed to high nitrogen dose, although it was variable for individual compounds. Samples with a greater abundance of phenolics had the highest antioxidant capacity. Application of medium rates of nitrogen (60–90 kg N/ha) should be considered as the optimal mode for reconciling the sufficient yield and high quality of herb material. The results of this study emphasize the importance of controlled growth conditions as an effective means of optimizing the accumulation of secondary metabolites and expressing their biological activities.