Incorporation of 14C-Photosynthate into Major Chemical Fractions of Source and Sink Leaves of Cottonwood

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Abstract
The incorporation and distribution of photosynthetically fixed (14)CO(2) was followed for 48 hours in a recently matured source leaf (LPI 7) and in young expanding source and sink leaves (LPI 4) of cottonwood (Populus deltoides Bartr.). The major chemical constituents of leaf laminae and petioles were separated by sequential solvent extractions and enzyme hydrolyses. Two hours after labeling, about 80% of the (14)C was found in water-alcohol-soluble constituents in the mature source lamina as compared to about 45% in those of the young expanding leaf. In both mature and expanding source leaves the water-alcohol-soluble constituents decreased while the CHCl(3)-soluble and -insoluble compounds increased with time. After 48 hours, 7 and 37% of the total (14)C was recovered from structural carbohydrates and from protein + CHCl(3)-soluble fractions, respectively, in the mature source leaf; and 4 and 65%, respectively, in the young source leaf. When the distribution of (14)C among major chemical fractions was calculated on per cent dpm/mg basis, the data showed that a young sink leaf incorporated over twice as much (14)C into structural carbohydrates as a young source leaf (11% versus 4%). However, when calculated on an absolute dpm/mg basis, activity in this fraction of the young source leaf exceeded that in the sink leaf by a ratio of about 11:1 (9528 versus 845 dpm/mg). Thus, most of the material for synthesis of structural carbohydrates was derived from in situ photosynthate.The distribution of (14)C in chemical fractions recovered from petioles was similar to that recovered from their respective laminae, except that petioles incorporated greater amounts (up to 24% of total (14)C) into structural carbohydrates. In contrast to lamina tissue, most of the photosynthate for synthesis of structural carbohydrates in the petioles of young developing leaves was imported from mature leaves farther down the stem.