Transpiration, the loss of water from the leaf tissue into the atmosphere, has two important functions in the plant. First, the cells in the leaves are cooled because water is being evaporated. This maintains the cells at an optimum temperature for metabolic processes such as photosynthesis to take place. If transpiration is limited due to water stress in the plant, photosynthesis will be limited and reductions in plant growth, fruit quality and yield will result. Secondly, since water is moving from the roots to the different parts of the plant, this movement facilitates the passive movement of important minerals and dissolved chemicals throughout the plant.
Nutrient deficiencies are likely to be present under water stress situations.
During the winter months, apple trees use very little water. Water absorption from the soil by roots increases as leaf surface of the plant increases in the spring, and most of the water comes from the upper soil layer which is warmer and has greater feeder-root penetration. The 'potential water' available to the plant depends on the root volume and the soil water holding capacity. The roots of deciduous trees such as apples are usually extensive and have been found at depths of 6 feet. In sandy soil, roots have been found to extend laterally 2 to 3 times the branch spread and 1.5 times in loam or clay soils.