Theories on allelochemical concentrations in plants are often based upon the relative carbon costs and benefits of multiple metabolic fractions. Tests of these theories often rely on measuring metabolite concentrations, but frequently overlook priorities in carbon partitioning. We conducted a pulse-labeling experiment to follow the partitioning of CO-labeled photosynthate into ten metabolic pools representing growth and maintenance (amino acids, organic acids, lipids plus pigments, protein, residue), defense (phenolic glycosides, methanol:water and acetone-soluble tannins/phenolics), and transport and storage (sugars and starch) in source and importing sink leaves of quaking aspen (Populus tremuloides). The peak period of C incorporation into sink leaves occurred at 24 h. Within 48 h of labeling, the specific radioactivity (dpm/mg dry leaf weight) of phenolic glycosides declined by over one-third in source and sink leaves. In addition, the specific radioactivity in the tannin/phenolic fraction decreased by 53% and 28% in source and sink leaves, respectively. On a percent recovery basis, sink leaves partitioned 1.7 times as much labeled photosynthate into phenolic glycosides as source leaves at peakC incorporation. In contrast, source leaves partitioned 1.8 times as much C-labeled photosynthate into tannins/phenolics as importing sink leaves. At the end of the 7-day chase period, sink leaves retained 18%, 52%, and 30% of importedC photosynthate, and labeled source leaves retained 15%, 66%, and 19% of in situ photosynthate in metabolic fractions representing transport and storage, growth and maintenance, and defense, respectively. Analyses of the phenolic fractions showed that total phenolics were twice as great and condensed tannins were 1.7 times greater in sink than in source leaves. The concentration of total phenolics and condensed tannins did not change in source and sink leaves during the 7-day chase period.

Red oak (Quercus rubra), a mesic species, and chestnut oak (Quercus prinus), a xeric species, were grown in a greenhouse with and without fertilizer (F+ and F-, respectively) and subjected to a 10-week drydown (W-) or kept well watered (W+). In both species, fertilized seedlings exhibited greater reductions in mean net photosynthesis (A), leaf conductance (g(wv)), leaf water potential (Psi(leaf)) and water use efficiency (WUE) during the drydown than unfertilized seedlings. In the W- treatments, red oak showed greater reductions in A, g(wv) and Psi(leaf) than chestnut oak. Differential fertilization of the seedlings of both species had a greater effect on tissue water relations than differential watering. During the latter weeks of the drydown, there was no osmotic adjustment in red oak, but chestnut oak in the F+/W- treatment had significantly lower osmotic potentials at full and zero turgor than seedlings in any of the other treatments. The results indicate that high nutrient availability does not improve the drought tolerance of these two oak species.

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We tested the hypothesis that soil nitrogen availability could alter the efficacy of the cry1A(a) d-endotoxin of Bacillus thuringiensis (B.t.) through variation in phytochemistry. Extracts of the cry1A(a) d-endotoxin were administered to second-instar gypsy moth (Lymantria dispar) larvae on foliage from hybrid Populus plants (clone NC5339) grown in sand culture for 10 weeks with 10, 25, 50, and 120 ppm nitrogen applied as a modified Hoagland's solution. Poplar biomass increased with increasing nitrogen availability. Net rates of photosynthesis responded positively to increased nitrogen availability in fully expanded leaves only. Concentrations of condensed tannins and salicortin increased as nitrogen availability decreased. Larvae feeding on fully expanded leaves encountered twice the condensed tannin concentration but experienced less than half the mortality of larvae feeding on halfway expanded leaves. Low nitrogen availability increased larval mortality and prolonged the development times of second-instar larvae feeding on both halfway and fully expanded leaves treated with the B.t. cry1A(a) d-endotoxin. Nitrogen concentrations in fully expanded leaves were negatively correlated with larval mortality. Relationships among larval performance, condensed tannins, and phenolic glycosides were weak or nonexistent. These results suggest that the efficacy of the B.t. d-endotoxin will be enhanced when short-rotation poplar plantations are grown on soils of low fertility, but this protection will be at the expense of biomass productivity.