CONSTRAINTS ON AN INDUCED DEFENSE - THE ROLE OF LEAF-AREA

Folivory results in both leaf damage and the loss of photosynthetic ca pacity. Leaf damage activates the production of induced defenses, but diminished photosynthetic capacity resulting from lost leaf area may i mpair a plant's ability to respond defensively. Because damage-induced nicotine production in Nicotiana sylvestris (Solanaceae) is an energy -demanding, active process, we predicted that the loss of leaf area wo uld constrain this plant's ability to produce an induced nicotine pool . We examined our prediction in an experiment which combined leaf punc ture and removal protocols, quantified induced nicotine pools on a who le-plant basis, and accounted for losses in the nicotine pool due to r emoved leaves and lost growth potential. In contrast to our prediction , leaf removal did little to diminish the growth-corrected estimates o f the induced nicotine pool in plants with sufficient damage ''cue''; only when plants had lost 88% or more of their leaf area did the induc ed nicotine pool decline significantly. These results demonstrate that the induced defense is relatively insensitive to current photosynthet ic capacity. In contrast to the size of the induced nicotine pool, the concentration of nicotine in the remaining shoot tissues continued to rise as puncture damage increased over all defoliation levels tested. The mechanisms responsible for inducible nicotine production may have evolved as a means of providing shoot tissues with protection that is proportional to the amount of damage incurred while keeping productio n costs constant for the remaining plant parts.