Allometric Limits to the Induced Accumulation of Nicotine in Native Tobacco(Phenotypic Plasticity and its Evolutionary-Ecological Implications)(INTERNATIONAL SYMPOSIUM : Differentiation Patterns of Plant Populations and Adaptive Mechanisms)(<Special Issue
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The induced defenses of plants are, by definition, examples of adaptive phenotypic plasticity ; however, they have rarely been considered as such due largely to the difficulty of defining the character states of these dynamic chemical responses. Nicotine functions as an inducible defense in Nicotiana sylvestris and is one of the few induced defenses which is understood on a whole-plant basis. Nicotine biosynthesis is restricted to the roots and increases after wounding or herbivore attack to the shoot. Newly synthesized nicotine is transported throughout the plant ; induced plants are better protected from subsequent herbivore attack in laboratory trials. Root nicotine production is activated by an endogenously-produced signal, jasmonic acid (JA), which is synthesized in response to leaf damage and either directly through transport or indirectly via another, as yet unknown signal, increases JA pools and nicotine biosynthesis in the roots. The addition of JA or its methyl ester (MJ) to plants stimulates nicotine production in the same way that leaf damage does. In this study I use MJ to elicit the induced defense without wounding and describe a new character state for this induced defense : the allometrically determined setpoint. Plants were induced with 45 μg of MJ, once or in different combinations of 2 or 3 times during a 21-day period of vegetative growth, allowed 7 days between elicitations for the response to relax, and harvested during one of 4 harvests in order to quantify biomass and whole-plant nicotine pools. For plants induced only once, the largest induced change in whole-plant nicotine pools occurred during the second induction period, when elicitation increased nicotine pools by 5.99 mg, approximately 87% more than either the first or third induction periods. However, plants induced once had the same whole-plant nicotine pools at the end of the experiment regardless of whether they were induced during the highly responsive second period or during the less responsive first and second induction periods. Sequential elicitations consistently increased nicotine pools, but after 3 inductions, the nicotine pools were not significantly higher than those in plants which had been induced twice ; plants can only increase their nicotine pools in response to 2 inductions when grown under these conditions. These results suggest that this plant has allometrically-determined setpoints for this induced defense which are determined by the number of times a plant's wound-signal cascade is stimulated, rather than by the quantity of nicotine synthesized during an induction episode.
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