Benzothiadiazole and nitrogen source modify the nitrogen metabolism in cucumber infected with Pseudomonas syringae pv. lachrymans

The influence of nitrogen source and benzothiadiazole (BTH) pretreatment on nitrogen metabolism-related enzyme activities was studied in cucumber plants infected with Pseudomonas syringae pv. lachrymans (Psi). The plants were grown hydroponically in media containing nitrate and urea at different ratios: 100% NO3- (100N), 75% NO3-/25% urea (75N) and 50% NO3-/50% urea (50N) and pretreated with BTH prior to Psl inoculation. In the presence of urea in the nutritional medium the activities of most enzymes in the control plants decreased. Disease development was influenced by nitrogen nutrition and the weakest symptoms occurred in the 75N plants. The BTH-induced disease restriction was dependent on the type of nitrogen nutrition and was most visible in the 50N plants (about 2-fold reduction of infected leaf area compared to the plants not pretreated with BTH). The nitrogen metabolism was modified by BTH and this effect was stronger than that of urea. Irrespective of the type of nitrogen nutrition, in the BTH-treated plants, the activities of nitrate reductase, ferredoxin-and NADH-dependent glutamate synthase decreased whereas those of NADH-dependent glutamate dehydrogenase (NADH-GDH), aminotransferases and NADP-dependent isocitrate dehydrogenase were enhanced compared to the controls. After infection, in all nutritional variants, the strongest effect of BTH pretreatment was observed for NADH-GDH on the 2 day, when its activity increased by 85-256% compared to the infected non-pretreated plants. Our results concerning the glutamate-metabolizing enzymes suggest that BTH treatment combined with low dose of urea may have a positive influence on the nitrogen management leading to the inhibition of disease symptom progress.