Abstract
The hemibiotrophic oomycetes are significant threats to a wide range of
Cucurbitaceae species, causing substantial losses of plant productions.
Particularly, Phytophthora melonis evokes severe symptoms, thus
dramatically limiting yield in cucumber. However, information about
cucumber–P. melonis interaction is still limited. This study
explored changes in the activities of phenylalanine ammonia-lyase (PAL),
peroxidase (POX), catalase (CAT), superoxide dismutase (SOD), and
polyphenol oxidase (PPO) in cucumber roots of two resistant genotypes
(Soheil and Ramezz), one moderately resistant genotype (Baby) and three
highly susceptible genotypes (Extrem, Mini 6-23 and Yalda), over the
time courses of 7, 14 and 21 days after inoculation (DAI). The results
indicated that the activities of defence‐related enzymes differed
between the resistant and highly susceptible genotypes. Although, the
defense-related enzymatic activities were elevated sharply in the
resistant and moderately resistant genotypes after inoculation, but no
significant correlations were present between the activity trends of
PPO, SOD and CAT and resistance characteristics. Moreover, no
significant changes in enzyme activities were found in the control
plants, non-inoculated plants of the six genotypes during the testing
period. Altogether, the resistance of cucumber to P. melonis is
related to POX and PAL activities, but does not show relationship with
PPO, SOD and CAT activities. Studying the physiological metabolic
pathways of POX and PAL appears to be an important direction in research
to elucidate resistance to P. melonis in cucumber genotypes.
KEYWORDS : Phenylalanine ammonia-lyase, Peroxidase, Catalase,
Superoxide dismutase, and Polyphenol oxidase.