Light duration alters carbon partitioning among carbohydrates (CHOs) in source leaves. The current experiments were designed to determine how light duration affected the metabolism of newly fixed and reserve CHOs in various organs of apple and whether longer durations favored sorbitol (sor) synthesis. One-year-old `Gala' apple plants that experienced a 1-, 4-, 7-, 10-, or 14-h photoperiod for 7 d were exposed to 14CO2 for 15 min. Individual CHO concentrations and the activity of newly-fi xed [14C]CHOs in leaves, stems, and roots were analyzed during different intervals after labeling. In source leaves, sor increased significantly, whereas sucrose (suc) did not change as light duration increased from 1 to 10 h, resulting in increased sor/suc ratios from 2.6 in the 1-h to 4.3 in the 10-h light duration. The increased sor/suc ratios may be due primarily to the preferential carbon partitioning into sor in longer light durations. Longer light durations enhanced the sor export rate from source leaves, resulting in higher sor in stems and sink tissues. In roots, starch increased significantly over increasing light durations. A major portion of starch in roots may be converted from newly fixed CHO. Our results suggest that light duration alters the metabolism of sor and other CHOs in source and sink tissues of apple and that the changes in CHO concentrations result from different rates of carbon synthesis, partitioning, and export.
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