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 coord ination of the processes responsible for these two plant functions. To this end, we studied the coordination of alkaloidal and photosyntheti c responses to simulated herbivory in the context of changes in leaf n itrogen in plants grown under a range of nitrate supply rates in two e xperiments. In the first experiment, damage-induced changes in leaf ni cotine, total nitrogen, nitrate, and photosynthetic rate (PR) were mon itored in same-aged undamaged leaves of young Nicotiana sylvestris pla nts grown in pots. In the second, the alkaloidal response to damage wa s uncoupled from damage by growing plants in pots for >150 d, causing them not to respond to leaf damage with increased nicotine concentrati ons (''uninducible''). We propose that the changes in PR and nicotine content induced by damage reflect the allocation of resources to regro wth and defense, respectively, and examine the predictions of the opti mal 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 r equirements as is predicted by the carbon/nutrient (C/N) theory. If PR reflects the fitness value of the leaf and damage reflects a high pro bability of future damage, we interpret the OD theory to predict that PR and nicotine content should be correlated, and that damage should i ncrease the amount of nicotine allocated for a given PR. Nicotine, nit rogen, and PR increased in a coordinated fashion in response to leaf d amage in the inducible plants. In both experiments, PR and nitrogen we re highly correlated, but damage did not affect the PR-nitrogen relati onship. In the first experiment with inducible plants, nicotine and ni trogen were also highly correlated. However, damage significantly incr eased the slope of the nicotine-nitrogen relationship 1.6-fold. Simila rly, nicotine and PR were significantly correlated and the slope of th e nicotine-PR relationship increased significantly (1.9-fold) in respo nse to damage. These results are consistent with the predictions of th e 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 wi th a correlated increase in leaf nitrogen content. Nicotine and PR wer e not significantly correlated in undamaged plants, but were significa ntly 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.