Optimal Defense (OD) theory predicts that the within-plant allocation of secondary metabolites that function as defenses will be positively correlated with the fitness value of particular plant parts. Here, we experimentally examine this prediction by exploiting our understanding of the mechanisms of wound-induced nicotine production in Nicotiana sylvestris (Solanaceae) to manipulate the patterns of nicotine allocation and to determine their fitness consequences. In two perturbation experiments conducted over three stages of ontogeny (rosette, elongation, and flowering), we wounded or removed leaves of different ages (young, mature, or old) and determined the effects on nicotine allocation (whole-plant and within-plant) and fitness (lifetime viable seed production). OD theory predicts that, as leaves age and their fitness value decreases, the allocation of defense to particular leaves and the fitness consequences of their removal should be positively correlated. We found that (1) leaf removal results in a significant decrease in seed mass at the elongation stage, but not at the rosette or flowering stages; (2) the relative value of leaves decreases from young and mature to old leaves; (3) leaf damage significantly increases the whole-plant nicotine contents of rosette-stage plants, but not of elongation- or flowering-stage plants, and after damage, younger leaves are more heavily defended than older leaves at the elongation and flowering stages; and (4) regardless of ontogenetic stage, plants distribute nicotine among leaves in accordance with their relative fitness value, thus supporting OD theory predictions. Leaf value increases after N fertilization at the flowering stage, but is not changed if adjacent leaves are removed at earlier growth stages. Moreover, plants are capable of sending their root-synthesized nicotine to specific leaves after damage; at the elongation stage, the new and young leaves receive greater proportional allocations of nicotine than other leaves. The cessation of significant whole-plant nicotine inductions at later stages in ontogeny is not due to the root's decreased ability to respond to the plant's wound signal, jasmonic acid (JA) with increased nicotine biosynthesis, but rather to the decline in a leaf's sensitivity to wounding and in its ability to export JA from the leaves to roots. Ontogeny has profound effects on the use of this induced defense. Plants mount systemic nicotine inductions at the rosette stage that rely on large increases in de novo nicotine synthesis, switch to the selective targeting of nicotine to the new and young leaves at the elongation stage without large increases in de novo nicotine synthesis, and allocate to reproductive structures, but not leaves, at the flowering stage. These changes are consistent with the predictions of OD theory.
Nicotine is thought to be an excellent example of a m obile" (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 1 5 NO 3 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 1 5 N—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 1 5 NO 3 acquired after damage. Damage increased the rate of nicotine—N 1 5 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 1 5 NO 3 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 1 5 N 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— 1 5 N pool produced after 5 d was similarly unchanged at 41 d. In damaged plants the nicotine— 1 5 N pool produced after 5 d decreased by 33.3% after 41 d. This decrease in the nicotine— 1 5 N 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.
summary In Nicotiana sylvestris Spegazzini and Comes (Solanaceae), we examined the relationships among wounding, endogenous leaf jasmonic acid (JA) pools, and whole‐plain (WP) nicotine accumulation over a range of wounding intensities and spatial distributions, in order to explore optimal defence (OD) theory predictions. We quantitatively wounded one or four leaves and then quantified: (1) JA in damaged and undamaged leaves 90 min after wounding; (2) WP nicotine concentration after 5 d (the times when JA and nicotine attain the largest wound‐induced concentrations). We find: (1) statistically significant, positive relationships on a leaf‐by‐leaf basis among the number of leaf punctures, endogenous leaf JA, and WP nicotine accumulation; (2) that young, undamaged leaves have a higher concentration of JA than do older, undamaged leaves, and produce a greater amount of JA per puncture than older leaves, but that all leaves have the same JA content (ng JA per leaf); and (3) that a damaged leaf produces less JA when other leaves in the canopy are wounded than when it is the onh wounded leaf in the canopy, but that when it is the only wounded leaf, the phylotactically adjacent, undamaged leaves do not increase their JA concentrations. The observation that younger leaves produce more JA per puncture than do older leaves is consistent with OD theory predictions. The observation that a small amount of damage localized to a single leaf is as effective as a larger amount of damage dispersed across the canopy in increasing leaf JA and WP nicotine accumulation shows the plant's ability to differentiate between dispersed and localized damage. Because the quantity of JA in a wounded leaf 90 min after wounding is a reliable indicator of the WP nicotine response to wounding, this trait provides insight into how plants integrate information about environmental insults and tailor their defence responses.
Optimal Defense (OD) theory predicts that the within-plant allocation of secondary metabolites that function as defenses will be positively correlated with the fitness value of particular plant parts. Here, we experimentally examine this prediction by exploiting our understanding of the mechanisms of wound-induced nicotine production in Nicotiana sylvestris (Solanaceae) to manipulate the patterns of nicotine allocation and to determine their fitness consequences. In two perturbation experiments conducted over three stages of ontogeny (rosette, elongation, and flowering), we wounded or removed leaves of different ages (young, mature, or old) and determined the effects on nicotine allocation (whole-plant and within-plant) and fitness (lifetime viable seed production). OD theory predicts that, as leaves age and their fitness value decreases, the allocation of defense to particular leaves and the fitness consequences of their removal should be positively correlated. We found that (1) leaf removal results in a significant decrease in seed mass at the elongation stage, but not at the rosette or flowering stages; (2) the relative value of leaves decreases from young and mature to old leaves; (3) leaf damage significantly increases the whole-plant nicotine contents of rosette-stage plants, but not of elongation- or flowering-stage plants, and after damage, younger leaves are more heavily defended than older leaves at the elongation and flowering stages; and (4) regardless of ontogenetic stage, plants distribute nicotine among leaves in accordance with their relative fitness value, thus supporting OD theory predictions. Leaf value increases after N fertilization at the flowering stage, but is not changed if adjacent leaves are removed at earlier growth stages. Moreover, plants are capable of sending their root-synthesized nicotine to specific leaves after damage; at the elongation stage, the new and young leaves receive greater proportional allocations of nicotine than other leaves. The cessation of significant whole-plant nicotine inductions at later stages in ontogeny is not due to the root's decreased ability to respond to the plant's wound signal, jasmonic acid (JA) with increased nicotine biosynthesis, but rather to the decline in a leaf's sensitivity to wounding and in its ability to export JA from the leaves to roots. Ontogeny has profound effects on the use of this induced defense. Plants mount systemic nicotine inductions at the rosette stage that rely on large increases in de novo nicotine synthesis, switch to the selective targeting of nicotine to the new and young leaves at the elongation stage without large increases in de novo nicotine synthesis, and allocate to reproductive structures, but not leaves, at the flowering stage. These changes are consistent with the predictions of OD theory.
Defense and regrowth after herbivore attack are not mutually exclusive alternatives for most plants, yet few studies have examined the coordination of the processes responsible for these two plant functions. To this end, we studied the coordination of alkaloidal and photosynthetic responses to simulated herbivory in the context of changes in leaf nitrogen in plants grown under a range of nitrate supply rates in two experiments. In the first experiment, damage—induced changes in leaf nicotine, total nitrogen, nitrate, and photosynthetic rate (PR) were monitored in same—aged undamaged leaves of young Nicotiana sylvestris plants grown in pots. In the second, the alkaloidal response to damage was uncoupled from damage by growing plants in pots for >150 d, causing them not to respond to leaf damage with increased nicotine concentrations ( u ninducible ) . We propose that the changes in PR and nicotine content induced by damage reflect the allocation of resources to regrowth and defense, respectively, and examine the predictions of the optimal defense (OD) theory regarding these responses. We have previously established that neither constitutive nor induced nicotine production is a passive consequence of a nitrogen imbalance in excess of growth requirements as is predicted by the carbon/nutrient (C/N) theory. If PR reflects the fitness value of the leaf and damage reflects a high probability of future damage, we interpret the OD theory to predict that PR and nicotine content should be correlated, and that damage should increase the amount of nicotine allocated for a given PR. Nicotine, nitrogen, and PR increased in a coordinated fashion in response to leaf damage in the inducible plants. In both experiments, PR and nitrogen were highly correlated, but damage did not affect the PR—nitrogen relationship. In the first experiment with inducible plants, nicotine and nitrogen were also highly correlated. However, damage significantly increased the slope of the nicotine—nitrogen relationship 1.6—fold. Similarly, nicotine and PR were significantly correlated and the slope of the nicotine—PR relationship increased significantly (1.9—fold) in response to damage. These results are consistent with the predictions of the OD theory. Despite this coordination, alkaloidal and photosynthetic responses can be uncoupled. Regardless of nitrate supply rate, damaged uninducible plants exhibited no significant increase in nicotine content, but significantly increased their PR in response to damage with a correlated increase in leaf nitrogen content. Nicotine and PR were not significantly correlated in undamaged plants, but were significantly correlated in damaged plants. Unless other defenses are activated in uninducible plants, these results may reflect a priority of growth over defense in uninducible plants with slow growth rates or reduced rooting volume.
