Submitted to: Australian Journal of Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/4/2001
Publication Date: 11/5/2001
Citation: Zhou, R., Sicher Jr, R.C., Quebedeaux, B. 2001. Diurnal changes of carbohydrates metabolism in apple mature leaves. Australian Journal of Plant Physiology. 28:1143-1150. Interpretive Summary: Many economically important fruit tree species, including apple, pear, apricot and peach, synthesize sorbitol, a unique sugar alcohol, as the principal product of photosynthesis. Understanding the role of sorbitol in fruit tree species is necessary in order to develop insights into factors that determine fruit growth and overall yields. In the current study, the activity of aldose 6-phosphate reductase (A6PR) was measured. This important enzyme manufactures sorbitol from specific sugars in apple source leaves. The results showed that A6PR activity was greater in leaves sampled in the light than in the dark and that the light and dark forms of the enzyme were stable during partial purification. By transferring apple saplings to continuous light or darkness, it was possible to show that variations of A6PR activity in apple leaves were the result of a diurnal rhythm. These results comprise an important contribution toward understanding how sorbitol synthesis is integrated with the metabolism of sugars, the initial products of photosynthesis in apple leaves. The above findings should benefit horticultural scientists, plant biochemists, molecular biologists and possibly crop modelers.
Technical Abstract: Net photosynthesis, individual carbohydrate concentrations, and the activities of some enzymes involved in a carbohydrate biosynthesis were monitored in mature apple leaves at regular intervals throughout a natural day night cycle. Sorbitol was the major carbohydrate in mature apple leaves. Its concentration increased gradually after dawn and reached its highest level of 18.0 mg per gram fresh weight (FW) and then declined to its lowest level of 9.6 mg per gram FW at the end of the dark period. Starch also showed a similar diurnal pattern with a lower maximum of 4.6 mg per gram FW and a minimum of 1.8 mg per gram FW. 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 appeared at 10:00 h with 37.2 (mol/hour/ gram FW, which was about 30% higher than that in the night. The diurnal alteration of A6PR activity was also found under controlled environment conditions. Treatments of 14-hour light/10-hour dark photoperiod, continuous light and continuous darkness did not change the pattern of diurnal variation in A6PR activity. Significant correlation between A6PR activity, sorbitol concentration and a photosynthetic rate suggested that the processes of photosynthesis and carbohydrate biosynthesis are coordinated.