Swords into plowshares? Nicotiana sylvestris does not use nicotine as a nitrogen source under nitrogen-limited growth

Whether or not a plant can recover its investment of resources in a chemical defense is central to the "mobile-immobile" metabolite dichotomy of the resource availability theory. Biochemical measures of metabolite turnover have been used to estimate this trait, but they do not address the ecological question of resource recovery. Numerous studies have found that many Nicotiana species, which normally produce the nitrogen-intensive defense metabolite, nicotine, can rapidly take up and metabolize exogenously administered nicotine from hydroponic solutions. However, Baldwin et al. (1994) found no evidence for turnover of endogenously produced nicotine in pulse-chase experiments using ¹⁵NO₃ as the biosynthetic precursor in N. sylvestris. Given that the capacity to metabolize nicotine exists, we asked (1) whether N. sylvestris could metabolize exogenously fed nicotine and sustain growth under nitrogen-limited conditions and (2) whether leaf damage alters the plants' ability to use nicotine as a nitrogen source. We fed plants with sufficient nicotine in hydroponic culture to increase their nitrogen pools by 70% at the time of nicotine feeding; in 6-10 consecutive harvests over 28-35 days, we measured the biomass of roots, leaves and stems, and the total nitrogen pools of these plant parts as well as the pools of nicotine, nornicotine and myosmine of these plant parts in undamaged nicotinefed and control plants and finally, in a separate experiment, in nicotine-fed damaged and undamaged plants. Nicotine feeding increased nicotine pools by 1.2 times, which was not sufficient to significantly increase total nitrogen pools at the end of the experiment. Nicotine-fed plants rapidly demethylated their acquired nicotine pools to nornicotine, but did not process the alkaloid pool further than myosmine over the duration of the experiment. Leaf damage significantly increased the nicotine pool, but did not significantly alter the processing of the exogenously acquired nicotine. We conclude that N. sylvestris does not recover the nitrogen invested in nicotine even under nitrogen-limited growth, that the rapid metabolism of exogenously introduced nicotine is likely a detoxification pathway, and that these plants are homeostatic with regard to their nicotine pools.