Fire, nitrogen, and defensive plasticity in Nicotiana attenuata

Nicotiana attenuata is a post-fire annual that utilizes jasmonate-inducible nicotine production as an inducible chemical defense which, in turn, can utilize 6% of a plant' s nitrogen budget and be costly to seed production. We characterize the nitrogen pools of burned soils in the plant' s native environment (piñyon-juniper woodlands) and examine how variation in nitrogen source and supply rate influence the patterns of allocation to growth and inducible and constitutive nicotine production. Available soil nitrogen increases dramatically (40-fold) immediately after a fire and consists principally of ammonia which is subsequently oxidized to nitrate during post-fire succession. We simulate these changes in nitrogen availability in hydroponic culture and use allometric techniques to characterize changes in allocation. In two experiments, we alter (1) nitrate supply rates 8-fold and (2) the ratio of ammonia:nitrate under consistent nitrogen supply rates. In both experiments, we increase the allocation to nicotine by treating roots with methyl jasmonate (MJ), the methyl ester of the plant' s internal wound signal, jasmonic acid, which increases nicotine production in the roots after shoot herbivory. MJ treatments decrease whole plant (WP) growth, increase root:shoot ratio, and increase WP nicotine pools in all nitrogen environments. Overall, source and supply rate of nitrogen have no effect on either the constitutive or induced allometric relationships of nicotine accumulation and growth. This remarkable homeostasis in allocation patterns contradicts a key prediction of carbon nutrient (C/N) theory. With ¹⁵N-pulse-chase techniques, we demonstrate that plants preferentially utilize ammonia for nicotine production over nitrate when both nitrogen sources are available. This preferential use of ammonia may allow  N. attenuata to reduce the biochemical costs of producing nicotine in the post-fire environment.