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 (Malu s domestica Borhk. cv. Gala) leaves and to determine whether photoperi od altered photosynthate partitioning among carbohydrates. Sorbitol ac counted for > 80% of newly fixed C-14-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 ph otoperiod increased from 1 to 10 h, sorbitol, glucose, fructose, and s tarch 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 ra tios increased from 2 in the 1-h photoperiod to 5 in the 10-h photoper iod. The relative partitioning of C-14 into sorbitol increased, wherea s the partitioning into sucrose, glucose, and fructose decreased as ph otoperiod increased from 1 to 10 h. Our results suggest that longer ph otoperiods favoured sorbitol over sucrose accumulation while shorter p hotoperiods favoured sucrose over sorbitol synthesis. The observed cha nges in sorbitol and sucrose concentrations with changes in photoperio d may result from different rates of synthesis and export.