1 │ INTRODUCTION
Phytophthora melonis , a hemibiotrophic organism belongs to oomycote class and one of the causal agents of damping-off disease, is considered a potentially destructive disease of cucumber (Cucumis sativus L.; 2n = 2x = 14) across Iran and other parts of the world, that causes economic losses (Wu et al., 2014; Nazavari et al., 2016). Therefore, it is vital to elucidate the mechanisms of plants in response to damping-off. Typical symptoms of the disease are root and root collar rot, stem lesions, foliar blight, fruit rot and plant death (Hatami et al., 2013; Bagheri et al., 2020). Although current control measures forP. melonis are based on the use of fungicides (Lamichhane et al., 2017), identification of disease-resistant genotypes may be an effective strategy and environmentally friendly in modern crop production (Moghaddam et al., 2019; Hashemi et al., 2019). To date, analyses of the interaction between P. melonis and C. sativus have been limited on screening resistant genotypes to damping-off (Nazavari et al., 2016; Hashemi et al., 2019) and no cucumber genotype immune toP. melonis has been reported. In the never ending struggle against pathogens, plants gradually developed a series of complex defense mechanisms involving several factors like defense-related enzymes and inhibitors which lead to prevent infection of pathogens (Andersen et al., 2018). However, plant response during the P. melonis - C. sativus interactions or, more specifically, the underlying factors that increase susceptibility and/or resistance of the cucumber plant is not fully understood. So, identification and application of candidate resistance genes and defense enzymes associated with P. melonis responses can be an efficient method to build up the host resistance in cucumber genotypes. In 2019, three defense-related genes of CsWRKY20 , CsLecRK6.1 andLOX1 genes were reported to be involved in the resistance to theP. melonis infection in cucumber (Hashemi et al., 2019). Complex network of defense-related enzymes, such as peroxidase (POX; EC1.11.1.7), superoxide dismutase (SOD; EC1.15.1.1), catalase (CAT; EC1.11.1.6) and phenylalanine ammonia lyase (PAL; EC4.3.1.5) are promoted the scavenging of ROS and related to resistance inducement in plants (Prasannath and De Costa, 2015; Xie et al., 2017; Khatediya et al., 2018). PAL has a crucial role in flavonoid productions and lignin biosynthesis, which play a key role in the phenylpropanoid biosynthetic pathway (Yusuf et al., 2018). PAL is one of the most extensively studied enzymes with respect to plant responses to environmental stresses, including pathogen infection (Huang et al. 2010; Kim & Huang 2014). Polyphenol oxidase (PPO) and peroxidase (POX) are important oxidative enzymes found in most plant species that catalyze the formation of lignin and other oxidative phenols, thereby contributing to the formation of defense barriers by structural reinforcement, to protect against pathogens (Li & Steffens, 2002; Jiang et al., 2019). In cucumber increasing in the activities of peroxidase were reported following the inoculation with Fusarium oxysporum (Zhao et al., 2012). Amaral et al (2019) reported that high activities of antioxidant enzymes played a major role in both basal and induced resistance of cabbage to black rot. Moreover, regarding the oxidative burst, CAT and SOD are useful biochemical indicators of disease resistance (Su et al., 2019). The activities of different defense-related enzymes vary in different plants after pathogen attack and are highly complex (Siddique et al., 2014; Su et al., 2016). To the best of our knowledge, no comprehensive enzymic analyses have yet been accomplished on cucumber plants to compare defence‐related enzymes in resistant and susceptible cucumber genotypes upon inoculation with P. melonis . Therefore, the present study estimated the activities of PAL, POX, CAT, SOD and PPO quantitatively and investigated their roles in response to P. melonis inoculation in the cucumber genotypes. A better insight in cucumber defense responses can help to establish biochemical characteristics for the selection of resistant cucumber sources and provide a theoretical basis for disease control and breeding of cucumber plants with higher resistance toward damping-off.