Studies on the regulation of carbohydrate metabolism in mature apple leaves

The photosynthetic end products of the Rosaceae plants are sorbitol, sucrose and starch. The regulation of carbohydrate biosynthesis in mature leaves of apple plants was studied in several aspects.; Aldose-6-phosphate reductase (A6PR) was purified to apparent homogeneity from apple leaves. Mg2+, Ca2+ and Mn 2+ were found to activate A6PR activity in lower concentrations of glucose-6-phosphate and sorbitol-6-phosphate. The Km value was reduced from 12.2 to 2.8 mM for glucose-6-phosphate and from 3.1 to 1.3 mM for sorbitol-6-phosphate in the presence of 2.5 mM Mg2+. Zn2+ and Cu 2+ were shown to be potent inactivators of A6PR. These observations suggest that these divalent ions may have a significant physiological impact on sorbitol biosynthesis in apple leaves.; Partially purified apple leaf sucrose-phosphate synthase (SPS) showed hyperbolic saturation kinetics for both fructose-6-phosphate (Km = 0.36 mM) and uridine 5′-diphosphoglucose (Km = 6.49 mM). In apple, glucose-6-phosphate is an activator in vitro while inorganic phosphate was found to be a less effective inhibitor. Sorbitol-6-phosphate was found to be a competitive inhibitor for apple SPS with a Ki of 1.83 mM. These results suggest that sorbitol synthesis may regulate sucrose synthesis in vivo in apple leaves.; The regulation of carbohydrate metabolism and photosynthesis in mature apple leaves was investigated with whole plant source-sink manipulations. A dramatic decrease in photosynthetic rates was observed with shoot girdling treatments, but this inhibition was mainly attributed to stomatal closure. On the other hand, partial defoliation treatments resulted in a significant increase in photosynthetic capacity of the remaining apple source leaves. The activity of A6PR and SPS increased in response to higher sink/source ratio to facilitate the synthesis of transport compounds. ADPGlucose pyrophosphatase activity was stimulated to allocate more carbon into starch when carbon export was reduced with shoot girdling treatments.; Sorbitol was found to be a major temporary storage product that regulates the diurnal carbon export from apple leaves. A6PR activity exhibited a similar diurnal oscillation as sorbitol concentration in mature apple leaves, while SPS activity did not change during the day and night. A6PR is a main factor controlling carbon allocation in apple leaves. Significant correlation between A6PR activity, sorbitol concentration and photosynthetic rates suggested that the processes of photosynthesis and carbohydrate biosynthesis are highly coordinated in apple.