| Summary: | Seasonal priorities for the use of current photosynthate were studied in the leaves of Betula glandulosa Michx. and Ledum groenlandicum Oeder from northern Quebec. A sequential extraction scheme was developed to separate ('14)C-labelled leaf tissues into several classes of organic compounds. Data were analysed in terms of carbon use priority (CUP) defined as the ('14)C activity in a compound category as percentage of the total activity in the leaf. Developing leaves represented the strongest sink for current photosynthate among evergreen leaf age classes. In both species the priorities for structural components, leaf protection and metabolic components decreased while the importance of cellular lipids and storage carbohydrates increased with leaf age. The deciduous leaves had higher overall carbon use priorities for metabolic components, cellular lipids and storage carbohydrates. The priorities for structural components and leaf protection were higher in the evergreen leaves. Leaf protection was an important consideration in both species. Both B. glandulosa and L. groenlandicum leaves (1) allocated photosynthate to antiherbivore compounds when leaf succeptibility was highest, (2) used both quantitative and qualitative type defenses, and (3) reduced carbon use for quantitative type defenses during active growth.
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