Carbon Balance Model of a Growing Apple Shoot: I. Development of the Model

A computer model simulating the C balance of a growing ‘Jonamac’ apple ( Malus domestica Borkh.) shoot was constructed to estimate the time of first net carbohydrate export from the shoot. The model was based on measurements of net photosynthesis and dark respiration rates and estimates of the dry weight in the different components of the shoot. Under the prevailing weather of 1981, the model indicates that a shoot growing to a final length of 50 cm became a net exporter of carbohydrates 19 days after budbreak, a time corresponding to a shoot 4 cm long with 10 unfolded leaves. Assuming the same early growth rates, a shoot with a final length of 2 cm starts exporting at 15 days after budbreak. The total export of carbohydrates remains higher from short shoots than long shoots until 36 days after budbreak, indicating that short shoots supply greater amounts of carbohydrates to the rest of the plant during this early period. The model estimates the total import of carbohydrates from reserves of about 165 mg for the long shoot and 80 mg for the short shoot. In each instance, these reserves only accounted for about 20% of the total carbohydrates used by the shoot up to that point. The remainder was supplied by current photosynthates.