Allocation of 15N from Nitrate to Nicotine: Production and Turnover of a Damage‐Induced Mobile Defense

Abstract

Nicotine is thought to be an excellent example of a mobile" (sensu Coley et al. 1985) plant defense metabolite: it is synthesized in the roots, transported to leaves, and reported to be metabolically labile, with a half—life of <24 h. In a companion paper (Ohnmeiss and Baldwin 1994), we demonstrated that nondestructive damage dramatically increased the whole—plant allometric accumulation of nicotine in Nicotiana sylvestris (Solanaceae) and that damaged plants accumulated a larger proportion of their total nitrogen in nicotine pools than did undamaged plants. These damage—induced accumulations could arise from either increased nicotine production and/or decreased turnover with unaltered production. Here we ask how much of the change in whole—plant nicotine pools that is induced by damage is a result of increased nicotine production, with nitrogen derived from 15NO3 that was acquired, reduced, and assimilated after damage. In three experiments, we examine the rates of induced and constitutive nicotine production over 2 d, over 8 d under two nitrate supply rates, and over the lifetime of the plant. By examining 15N—labeled nicotine pool sizes at different times after damage, we estimate the magnitudes of nicotine turnover and its importance in the induced changes in whole—plant nicotine pools. Two days after damage, damaged plants had accumulated 4.8 times the nicotine pool of undamaged plants, and 57% of the increase in the nicotine pool of damaged plants was synthesized with N from 15NO3 acquired after damage. Damage increased the rate of nicotine—N15 production from 2.0 mg/h in undamaged plants to 6.3 mg/h in damaged plants. No evidence for nicotine turnover was found in either damaged or undamaged plants. The 8—d experiment conducted under two different nitrate supply rates confirmed these results. Furthermore, the 8—d experiment documented that N from 15NO3 acquired after damage was allocated to nicotine production in constant proportions within damaged and undamaged plants, independently of nitrate supply rate; 3.0—3.1 and 5.7—6.1% of the total 15N pool in undamaged and damaged plants, respectively, was used for nicotine production. Therefore, leaf damage doubles the allocation of recently acquired nitrogen to nicotine production in plants that do not differ in nitrate uptake or growth. In an experiment lasting 41 d after damage, we found that the damage—induced increase in total nicotine pools quantified after 5 d remained unchanged over the lifetime of the plants and that in undamaged plants the nicotine—15N pool produced after 5 d was similarly unchanged at 41 d. In damaged plants the nicotine—15N pool produced after 5 d decreased by 33.3% after 41 d. This decrease in the nicotine—15N pools of damaged plants could be due to either metabolism or volatilization. We conclude that nicotine is not the metabolically labile secondary metabolite that it has been previously reported to be and that the changes in pool sizes induced by damage and nitrogen stress are principally due to changes in production and not turnover. Previous studies examined the metabolism of exogenously fed nicotine, rather than the metabolism of endogenously produced nicotine, and this difference in experimental protocol may account for the differences in results. These results demonstrate that the induced patterns of nicotine accumulation are not consistent with the predictions of the carbon/nutrient theory and are consistent with those of the optimal defense theory.