Mechanical wounding stimulates nicotine synthesis in tobacco plants. In the practice of tobacco production, most nitrogen (N) is taken up before removal of the shoot apex, while nicotine is mainly synthesized afterwards. Since N is required for nicotine synthesis, it is interesting to know whether plants can use N taken up before removal of the shoot apex to synthesize nicotine after wounding. To address this question, a hydroponics culture experiment was carried out, in which N was supplied as NH(4)NO(3) at two levels (1 mM and 6 mM) in pre-culture, and N was either withdrawn or replaced by (15)N after removing the shoot apex for the next seven days. Removal of the shoot apex caused a marked increase in nicotine concentration in various organs, also when plants grew under low-N conditions and showed symptoms of N deficiency. Increased nicotine accumulation even occurred when N was withdrawn from the growth medium before the apex was removed, indicating that tobacco plants can use N taken up previously to synthesize nicotine after mechanical wounding. The amount of N used for nicotine synthesis accounted for 5-6% of the total N, irrespective of treatment. Although most of the nicotine in intact plants and plants with the apex removed was synthesized de novo, as evidenced by the data when N was replaced by (15)N-labeled NH(4)NO(3), a large amount of the N absorbed before the N replacement was incorporated into the newly formed nicotine. The proportion of nicotine-(15)N to total nicotine-N was almost the same as that of (15)N to total N in various organs. The results show the utilization of remobilized N taken up before excision of the shoot apex for nicotine synthesis afterwards, and highlight the importance of N cycling within plants, both when grown under N-sufficient and N-deficient conditions.
Keywords: 15N-isotope nitrogen; mechanical wounding; nicotine concentration; nicotine synthesis; nitrogen deficiency; removal of the shoot apex; tobacco (Nicotiana tabacum L.).