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 . The growth of the fruit 25-30 days after petal fall is mostly due to cell expansion. This cell expansion is highly dependent upon an adequate supply of moisture which provides weight and diameter increases of the fruit, and one of the first responses to water deficit by an apple tree is that fruit growth slows down. As a matter of fact, fruit circumference growth is used as a sensitive and practical measurement of water stress in the tree.