FIRE, NITROGEN, AND DEFENSIVE PLASTICITY IN NICOTIANA-ATTENUATA

Nicotiana attenuata is a post-fire annual that utilizes jasmonate-indu cible nicotine production as an inducible chemical defense which, in t urn, 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 (pinyon-juniper woodlands) and examine how variation in nitrogen source and supply rate influence the patterns o f allocation to growth and inducible and constitutive nicotine product ion. Available soil nitrogen increases dramatically (40-fold) immediat ely after a fire and consists principally of ammonia which is subseque ntly oxidized to nitrate during post-fire succession. We simulate thes e changes in nitrogen availability in hydroponic culture and use allom etric techniques to characterize changes in allocation. In two experim ents, we alter (1) nitrate supply rates 8-fold and (2) the ratio of am monia:nitrate under consistent nitrogen supply rates. In both experime nts, we increase the allocation to nicotine by treating roots with met hyl jasmonate (MJ), the methyl ester of the plant's internal wound sig nal, jasmonic acid, which increases nicotine production in the roots a fter shoot herbivory. MJ treatments decrease whole plant (WP) growth, increase root:shoot ratio, and increase WP nicotine pools in all nitro gen 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 al location patterns contradicts a key prediction of carbon nutrient (C/N ) theory. With N-15-pulse-chase techniques, we demonstrate that plants preferentially utilize ammonia for nicotine production over nitrate w hen both nitrogen sources are available. This preferential use of ammo nia may allow N. attenuata to reduce the biochemical costs of producin g nicotine in the post-fire environment.