Diurnal fruit shrinkage in apple as affected by leaf water potential and vapour pressure deficit of the air

In a series of experiments with potted apple trees cultivar ‘Cox's Orange Pippin’, daily shrinkage of fruits was measured over a wide range of vapour pressure deficits of the air. The trees were either well watered or the water supply was reduced to induce drought. Wrapping of fruits in aluminium-foil to prevent evaporation did not noticeably reduce shrinkage, but weight-loss data of detached fruits hanging in the tree under the same conditions showed that evaporation accounted for 20–35% of the fruit shrinkage. Shrinkage was much more closely related to leaf water potential than to the vapour pressure deficit of the air. Reverse flow of water to other parts of the tree was the main factor determining the degree of fruit shrinkage during the day.