Coordination of Photosynthetic and Alkaloidal Responses to Damage in Uninducible and Inducible Nicotiana Sylvestris

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 (ᵤninducible₎. 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.