As emphasized in Chapter 7 of Kozlowski and Pallardy (1997), carbohydrates are the most important constituents of woody plants. They comprise about three-fourths of the dry weight of woody plants and are the primary energy storage compounds and the organic substances from which most other organic compounds are synthesized.
The amount of carbohydrates translocated in woody plants exceeds that of all other solutes combined. Carbohydrate transport may occur downward, upward, and laterally. When radioactive sucrose is applied to the cambial region, it moves along the rays to the inner sapwood (Holl, 1975). Sugar uptake from xylem vessels by contact ray cells also occurs and is followed by transport along the rays to the phloem (Van Bel, 1990). Lateral movement of sugar occurs almost entirely through the ray symplast (Sauter and Kloth, 1986). Norway spruce needles transported carbohydrates downward in the shoot axis as well as laterally along the rays (Fig. 3.1). Sugars were transported via the leaf trace phloem, then to the base of the stem in the sieve cells of the latest increment of secondary phloem. On the way down sugars moved radially from the sieve cells into phloem parenchyma cells, the cambium, rays, the inner periderm, and some cells of the pith and cortex, including the epithelial cells around resin ducts (Langenfeld-Heyser, 1987).
Most carbohydrates are transported from sources (suppliers) to sinks (utilization sites) during the growing season. However, some carbohydrates may be loaded into the phloem and transported basipetally along the shoot
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