Effect of nitrogen application rate on physiology of flue-cured tobacco plants with or without inoculation of Phytophthora parasitica var. nicotianae and the severity of black shank

The effect of different nitrogen (N) application rates (N 71, 142, 213 and 284 mg /kg) on polyphenol oxidase (PPO) and catalase (CAT) activity and the contents of phenols, flavonoids, soluble sugars, free amino acids, and total nitrogen, as well as the incidence and severity of black shank in flue-cured tobacco (Nicotiana tabacum L. cv. Hongda) plants, was studied in a pot culture experiment with or without inoculation of the black shank pathogen (Phytophthora parasitica var. nicotianae). The results showed that inoculation of black shank pathogen led to higher PPO and CAT activities, and higher contents of phenols, flavonoids, soluble sugars, free amino acids, and total nitrogen in the tobacco plants 5 and 15 days after inoculation (DAI) compared to non-inoculated plants. A similar tendency was found under the different N application rates. Increasing N amount caused higher contents of N and free amino acids, but decreased contents of flavonoids, phenols and soluble sugars and lowered CAT activity, in both inoculated and non-inoculated tobacco plants 0, 5 and 15 DAI. The highest and lowest nitrogen amounts induced significant differences in the above-mentioned biochemical indices. Activity of PPO was higher in the N 142 and 213 mg /kg treatments than in the N 71 and 284 mg /kg treatments. In addition, increased severity of black shank was observed with increasing N application rate. Overall, the results indicated that N application rate affected not only the initial resistance (pre-infection of the pathogen), but also the induced resistance (post-infection), which in turn resulted in the different occurrences of black shank. Optimal N management is recommended to decrease the severity of black shank in the commercial production of N. tabacum cv. Hongda.