Re-evaluation of sorbitol metabolism in fruit from rosaceae trees

As fruit sink strength is a limiting factor in fruit production and increased sink strength may enhance photosynthesis, strengthening the fruit sink would be advantageous in terms of fruit production. Enzyme activities, such as those of sorbitol-metabolizing enzymes in Rosaceae fruit, are major factors in the fruit sink strength, and NAD-dependent sorbitol dehydrogenase (NAD-SDH) plays a key role in converting sorbitol into fructose and in the accumulation carbohydrates. However, it is not clear whether only NAD-SDH metabolizes sorbitol translocated from leaves to the fruit. Therefore, we carried out a tracer experiment using ¹⁴C-sorbitol. Disks prepared from apples, Japanese pears, and other pear fruit were incubated with ¹⁴C- sorbitol. The ¹⁴C-sorbitol was converted into ¹⁴C-glucose, while ¹⁴C-fructose and ¹⁴C-sucrose were not detected. These results suggest that in the fruit of Rosaceae trees, sorbitol is converted into glucose when it is taken up into fruit cells in an apoplastic manner. Considerable activities of NADP-dependent sorbitol dehydrogenase and sorbitol-6-phosphate dehydrogenase were detected in apples and peach fruit, although they were lower than those of NAD-SDH, suggesting that a pathway other than that of NAD-SDH may be present.