Partitioning of C-14-labeled photosynthate to allelochemicals and primary metabolites in source and sink leaves of aspen: evidence for secondary metabolite turnover

Theories on allelochemical concentrations in giants are often based upon th e relative carbon costs and benefits of multiple metabolic fractions. Tests of these theories often rely on measuring metabolite concentrations, but f requently overlook priorities in carbon partitioning. We conducted a pulse- labeling experiment to follow the partitioning of (CO2)-C-14-labeled photos ynthate into ten metabolic pools representing growth and maintenance (amino acids, organic acids, lipids plus pigments, protein, residue), defense (ph enolic glycosides, methanol:water and acetone-soluble tannins/phenolics). a nd transport and storage (sugars and starch) in source and importing sink l eaves of quaking aspen (Populus tremuloides). The peak period of C-14 incor poration into sink leaves occurred at 24 h. Within 48 h of labeling, the sp ecific radioactivity (dpm/mg dry leaf weight) of phenolic glycosides declin ed by over one-third in source and sink leaves. In addition, the specific r adioactivity in the tannin phenolic fraction decreased by 53% and 28% in so urce and sink leaves, respectively. On a percent recovery basis. sink leave s partitioned 1.7 times as much labeled photosynthate into phenolic glycosi des as source leaves at peak C-14 incorporation, In contrast, source leaves partitioned 1.8 times as much C-14-labeled photosynthate into tannins/phen olics as importing sink leaves. At the end of the 7-day chase period, sink leaves retained 18%, 52%, and 30% of imported C-14 photosynthate, and label ed source leaves retained 15%, 66%, and 19% of in situ photosynthate in met abolic 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 g reater 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.