Acta Hortic. 1315. ISHS 2021. DOI 10.17660/ActaHortic.2021.1315.66 Proc. I Int. Conf. and X National Horticultural Science Congress of Iran (IrHC2017) Eds.: K. Arzani and K. Funnell 445 Effect of drought stress on some morphological and physiological characteristics of Echinacea spp. S. Heidari 1 , R. Fotouhi Ghazvini 1 , M. Kafi 2 and M. Zavareh 3 1 Department of Horticultural Sciences, Faculty of Agricultural Sciences, University of Guilan, Rasht, Iran; 2 Department of Horticultural Sciences, College of Agriculture and Natural Resources, University of Tehran, Tehran, Iran; 3 Department of Agronomy, Faculty of Agricultural Sciences, University of Guilan, Rasht, Iran. Abstract Drought stress is one of the most important environmental stress of plants in arid and semi-arid regions that limit productivity. To evaluate the effects of drought stress on two-year-old seedlings of Echinacea purpurea and Echinacea angustifolia, an experiment was conducted with four levels of irrigation: 25, 50, 75, and 100% of field capacity (FC) as a completely randomized factorial. Several morphological, biochemical, and developmental traits were measured after three months of treatment. Growth indices such as plant height, shoot weight, root weight, shoot dry weight, and root dry weight, were reduced at lower FC in both species. Flowering was not observed in E. angustifolia under 50 and 25% FC treatments, and the best flowering quality and longevity was observed in E. purpurea under 75% FC. Protein content decreased in both species when plants were exposed to lower irrigation levels. The lowest protein content was seen in E. angustifolia under 25% FC. Reducing radical scavenging activity was observed in both species during drought stress, while the highest antioxidant capacity was found in E. purpurea under 75% FC. Proline content was highest in E. angustifolia under 25% FC. Based on flower quality, longevity, and antioxidant capacity, it seems that E. purpurea is more tolerant than E. angustifolia to drought stress. Keywords: coneflower, antioxidant, floriculture, xeriscaping INTRODUCTION Among environmental stresses, drought has been recognized as the most frequent stress limiting plant productivity (Hossain et al., 2016). Water stress influences the size of plant roots and shoots, and biochemical responses (Fazeli et al., 2007). Different species may use various mechanisms for drought stress tolerance and complete their life cycle (Muscolo et al., 2015). Within plants, there are two main systems used to cope with drought stress, including drought escape (i.e., completing life cycle before water deficit) and drought resistance (i.e., improving normal functionality even at low tissue water potential). One of the ways that plants improve functionality is by producing protective solutes (Wang et al., 2012). Two biochemical substances that can be monitored as being affected by environmental stresses, and especially drought stress, are protein and proline content (Cao et al., 2017; Liu et al., 2011). Antioxidant capacities of plants also change within plants under stress (Bettaieb et al., 2011), with species showing different antioxidant capacities under different levels of environmental stress (Khoshkhui et al., 2012; Lin et al., 2006). Ornamental perennials are an important part of urban green spaces and private gardens; however, they are not always properly watered and may suffer from drought stress (Chylinski and Lukaszewska, 2010). Some regions with humid, cold, flood, or saline environments have been planted with Echinacea sp. as stress-tolerant plants (Sabra et al., 2012; Asadisanam et al., 2015; Chapman and Auge, 1994). While few studies have reported drought tolerance of Echinacea species, this study aimed to evaluate some important indices like growth, flowering, protein, proline, and antioxidant capacity of two Echinacea species under various irrigation levels.