Surface Conductance and Water Balance of Developing Apple (Malus pumila Mill.) Fruits

Studies of the relative magnitude of the various components of the water balance of developing apple fruits are described. Water entering the fruit can be used in growth, evaporation or, subsequently, a reverse flow to other tissues. Estimates of evaporation were obtained from the weight loss by detached fruit hanging in their natural positions in the orchard. Evaporation rates could be estimated with reasonable precision using the maximum daily vapour pressure deficit × fruit surface conductance to water vapour. A method is described for the measurement of fruit surface conductance in the laboratory or the field. Although the conductance declined markedly during the season from ca. 1.5 × 10−3 m s−1 in early May to less than 1.0 × 10−4 m s−1 in September, the rate of water loss per fruit only declined slightly over this period. There were significant varietal differences in surface conductance, with Bramley apples having the lowest conductances of those studied, and Egremont Russet the highest. Fruit diameter gauges were used to provide continuous records of diurnal fluctuations in size of attached fruit. These records were used to estimate diurnal growth and shrinkage. The calculated volume shrinkage each day averaged 31% of net growth in early August and was largely accounted for by evaporation, with relatively little water flowing out of the fruit to other tissues.