Vegetative growth and water. If there is no adequate stored soil moisture early in the season, a drought at this time will result in reduced shoot length and leaf size. However, if there is sufficient soil moisture early in the season followed by a late season drought, shoot growth may be as good in a non-irrigated tree as in an irrigated one because shoot growth is completed within six weeks after growth began. When less water is available to carry the nutrients from the soil particles into the tree, nutrient deficiencies are aggravated. Uptake of water soluble nutrients such as nitrogen, boron, magnesium or potassium is most affected.
Reproductive growth is usually more sensitive to water stress than vegetative growth.
Because flower bud initiation and differentiation is a photosynthate requiring process, water stress occurring during mid-summer and fall may result in decreased cropping the following year. In Washington state, severely drought stressed trees failed to bloom, and if they bloomed, the flowers had many abnormalities. Fruit set is also sensitive to water stress. Under adequate moisture conditions, apple fruit growth occurs at an almost linear rate during the entire season. During the first 3 to 5 weeks after bloom fruit increases in size due to cell division, with some cell expansion beginning at this time .