Nicotine is thought to be an excellent example of a "mobile" (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 15 N0 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 15 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 15 N0 3 acquired after damage. Damage increased the rate of nicotine-N 15 production from 2.0 J.tg/h in undamaged plants to 6.3 J.tg/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 15 N0 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 15 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-' 5 N pool produced after 5 d was similarly unchanged at 41 d. In damaged plants the nicotine-15 N pool produced after 5 d decreased by 33.3% after 41 d. This decrease in the nicotine-15 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 prod...
Although little is known about the patterns of chemical defense allocation in reproductive tissues, optimal defense theory predicts a high constitutive allocation due to the tissues' high fitness value. To examine this prediction, we quantified the short- and long-term changes in the nicotine pools of reproductive tissues in response to both floral and leaf damage. Recently opened flowers (stage 5 capsules) do not alter their nicotine pools within a day in response to herbivory byManduca sexta larvae or mechanical damage to the corolla. Similarly, leaf damage during both vegetative and reproductive growth does not influence the nicotine pools of the first three stage-5 capsules produced. However, the nicotine pools of capsules produced later in reproductive growth were significantly larger (1.2- to 1.9-fold) on plants with leaf damage. These differences in floral nicotine pools were a result of both increases in nicotine pools of capsules on damaged plants and decreases in the nicotine pools of capsules on undamaged plants during reproductive growth. Leaf damage did not affect the rate of capsule maturation or the mass of stage-5 capsules at any time during reproductive growth. An allometric analysis of nicotine pools and biomass of reproductive parts in all stages of development from damaged and undamaged plants demonstrates that damaged plants allocated a significantly larger quantity of nicotine to reproductive parts in all stages of development than did undamaged plants. Given that nicotine is thought to be synthesized in the roots and transported to leaves and reproductive parts, nicotine could be allocated to reproductive parts in proportion to the number of developing capsules on a plant. We excised the first 27 stage-5 capsules on plants with and without leaf damage, with the expectation that plants with fewer capsules would allocate a larger amount of nicotine to the remaining capsules. In contrast to the prediction of this passive allocation model, floral excision did not affect nicotine pools on plants with or without leaf damage. These results demonstrate that the allocation of nicotine to reproductive parts is more strongly influenced by damage to vegetative rather than reproductive tissues. Reproductive parts are constitutively defended over the short term, but the set points for defense allocation are apparently increased by damage to vegetative tissues during reproductive growth. The decrease in allocation of nicotine to reproductive parts in undamaged plants during reproductive growth suggests an optimization of resource allocation as plants realize their potential fitness.
Pyrethrins are a class of potent insecticides produced byChrysanthemum cinerariaefolium. Simulated herbivory does not affect concentrations of pyrethrins in damaged and undamaged expanding and fully expanded leaves, or flowers of greenhouse or field-grown plants.
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