Physiological and yield responses of purple coneflower (Echinacea purpurea (L.) Moench) to nitrogen sources at different levels of irrigation

Echinacea purpurea (L.) is one of the most important medicinal plants in the world showing different biochemical reactions as affected by drought stress and nitrogen fertilizer. The purpose of this study was to determine the effect of nitrogen on soluble protein, enzyme activities, carotenoids metabolism, greenness and biological yield of the Echinacea purpurea under different levels of irrigation. The experiment was conducted in a research field in Iran during 2013–2015. Irrigation treatments included irrigation after 25, 50 and 75% soil water depletion, and nitrogen sources were no nitrogen (N), nitroxin, 40 kg N ha−1, 40 kg N ha−1 + nitroxin and 80 kg N ha−1. Nitroxin is a biofertilizer including Azotobacter and Azospirillum. The activity of catalase (11.9–21.3 mmol g−1fw min−1), peroxidase (40.1–48.9 µmol g−1fw min−1), polyphenoloxidase (0.79–3.63 mmol g−1fw min−1) increased in both years under drought stress conditions. The lowest greenness (50.1–45.9) was achieved from no-application of nitrogen in the irrigation treatment after 75% water depletion. The results demonstrated the beneficial effects of nitrogen sources on physiological reactions, especially peroxidase, polyphenol oxidase and carotenoids metabolism. It is concluded that irrigation of E. purpurea based on 50% water depletion and 80 kg nitrogen as well as the combination of nitroxin and 40 kg nitrogen treatments should be an appropriate choice for 2 years.