Photoperiod Alters Diurnal Carbon Partitioning into Sorbitol and Other Carbohydrates in Apple

The current experiments were designed to determine the diurnal carbon partitioning among sorbitol, sucrose, and starch in mature apple ( Malus domestica Borhk. cv. Gala) leaves and to determine whether photoperiod altered photosynthate partitioning among carbohydrates. Sorbitol accounted for > 80% of newly fixed 14C-labelled carbohydrate during the light period. Forty-seven percent of sorbitol that had accumulated at the end of the light period disappeared during the dark period. As photoperiod increased from 1 to 10 h, sorbitol, glucose, fructose, and starch concentrations in mature apple leaves increased, but were stable from 10 to 14 h. Sucrose concentration, on the other hand, decreased slightly as photoperiod increased. As a result, sorbitol to sucrose ratios increased from 2 in the 1-h photoperiod to 5 in the 10-h photoperiod. The relative partitioning of 14C into sorbitol increased, whereas the partitioning into sucrose, glucose, and fructose decreased as photoperiod increased from 1 to 10 h. Our results suggest that longer photoperiods favoured sorbitol over sucrose accumulation while shorter photoperiods favoured sucrose over sorbitol synthesis. The observed changes in sorbitol and sucrose concentrations with changes in photoperiod may result from different rates of synthesis and export.