Carbon partitioning between apple fruits: short- and long-term response to availability of photosynthate

Source-sink interactions play an important role in determining both crop yield and the distribution of individual fruit sizes. While there exists a qualitative description of these interactions, there is little or no causal understanding of the interactions. Short-term changes in partitioning of recently fixed photosynthate between alternative apple fruitlets were followed using in vivo measurement of photosynthate movement. The data indicate that apple fruitlets import photosynthate at a rate matching their utilization capacity, so that increases in availability of photosynthate cause only a small immediate increase in import. Over several hours, further and much larger increases occur, suggesting that an increase in enzymatic activity is induced by the increased availability. Short-term reduction in photosynthate supply to a fruitlet resulted in an incapacity of the fruitlet to return to its initial import rate, indicative of the reduction in the activity of enzymes.