Diurnal changes in carbohydrate metabolism in mature apple leaves

Net photosynthesis, individual carbohydrate concentrations, and the activit ies of some enzymes involved in carbohydrate biosynthesis were monitored in mature apple leaves at regular intervals throughout a natural day/night cy cle. Sorbitol was the major carbohydrate in mature apple leaves. Its concen tration increased gradually after dawn and reached its highest level of 18. 0 mg g(-1) fresh weight, and then declined to its lowest level of 9.6 mg g( -1) at the end of the dark period. Starch also showed a similar diurnal pat tern with a lower maximum of 4.6 mg g(-1) fresh weight and a minimum of 1.8 mg g(-1) fresh weight. Aldose-6-phosphate reductase (A6PR) activity showed significant diurnal fluctuations, whereas sucrose-phosphate synthase (SPS) activity, under both saturating and limiting substrate conditions, showed no variations. The highest A6PR activity under natural light conditions app eared at 1000 h with 37.2 mu mol h(-1) g(-1) fresh weight, which was about 30% higher than that in the night. The diurnal alteration of A6PR activity was also found under controlled environmental conditions in growth chambers . Treatments of 14/10 h light/dark photoperiod, continuous light and contin uous darkness, did not change the pattern of diurnal variation in A6PR acti vity. Significant correlation between A6PR activity, sorbitol concentration and photosynthetic rate suggested that the processes of photosynthesis and carbohydrate biosynthesis are coordinated.