Changes in specific activities of peroxidase, chitinase and phenylalanine ammonia-lyase and phenolic content in cucumber leaves inoculated with Podosphaera fusca, the causal agent of powdery mildew.

In this investigation, the effects of powdery mildew disease [caused by Podosphaera fusca (syn. Sphaerotheca fuliginea)] on specific activities of several defense-related enzymes and phenolic content were studied in cucumber leaves. Spore suspension of the fungus was sprayed on cucumber (cv. Super Dominus) plants in greenhouse and leaves from both inoculated and non-inoculated control plants were sampled at 0, 24, 48, 72 and 144 hours after inoculation (HAI). Spore germination and tissue colonization of P. fusca were microscopically studied on the inoculated surface of leaf samples. Further, Phenolic content (PHE) and specific activities of peroxidase (POX), chitinase (CHI) and phenylalanine ammonia-lyase (PAL) were spectrophotometrically measured in leaf extracts. Time-course of disease progress on the leaf surface showed that maximum spore germination occurred within 24 HAI and host penetration and disease development process began during 24-48 HAI. Evaluation of enzyme activities showed that POX specific activity in inoculated plants significantly increased at 72 HAI onwards and reached 2.5 times of that of control at 144 HAI. CHI specific activity showed a transient reduction in inoculated plants between 48-72 HAI and thereafter increased significantly in relation to control. PAL specific activity in inoculated plants was not significantly different from that of control. PHE in inoculated plants showed a significant increase compared to control at 48 HAI and thereafter. Comparison of time-course of disease progress with changes in enzyme activities indicated that POX activity had an increasing trend during disease progress whereas CHI activity showed a transient decrease at the early stages and then increased during the later stages of infection: PAL activity did not show any changes during the infection and PHE increased at the early stages of infection process and remained constant at rest of the time.

Effects of Acibenzolar-S-methyl on the specific activities of peroxidase, chitinase and phenylalanine ammonia-lyase and phenolic content of host leaves in cucumber-powdery mildew interaction.

One of the properties of systemic acquired resistance in plants is its concomitance with the biochemical changes including enhancement of activities of defense-related enzymes. In this study, the effects of Acibenzolar-S-methyl (ACI) on the some of defense responses of the cucumber plants (healthy or inoculated with spore suspension of Podosphaerafusca, the causal agent of cucumber powdery mildew) were surveyed via in vivo tests. Changes of defense responses in ACI-treated cucumber plants, inoculated with pathogen or not, were studied and compared with those of non-treated control plants. Results indicated that specific activity of peroxidase increased significantly in treated plants. Increase in enzyme activity was higher in pathogen-inoculated than non-inoculated plants, thus pathogen attack stress to plant plays a role in enhancement of enzyme activity. Specific activity of phenylalanine ammonialyase showed no changes in ACI-treated non-inoculated plants, but in inoculated plants it Increased due to interaction between ACI treatment and pathogen attack stress. Specific activity of chitinase increased in both inoculated and non-inoculated ACI-treated plants at 24 hours after treatment onwards, and pathogen attack stress did not affect it. Phenolic content of ACI-treated plant tissues, despite of small fluctuations, did not show any definite pattern of changes.