Effects of water stress on apple trees
Stomata activity. Stomata are pores on the leaf epidermis through which gas exchange (carbon dioxide, oxygen and water vapor ) takes place. Since these gases are involved in respiration, photosynthesis, and transpiration, the opening and closing of the stomata regulate these processes in the plant. Stomata are open less wide and for a shorter duration on trees (a) in dry soils vs. those in a moist soil and (b) on plants during high temperatures and low relative humidity vs. those on a cool humid day. When dryness is severe, stomata may not open at all to prevent water loss from the leaves. When stomata are less active or closed due to drought, both photosynthesis and transpiration are reduced as much as 40% before the leaves show any wilting and over 90% at wilting. When the stomata are closed food and energy loss by respiration are increased.
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. Trunk diameter may be reduced by a mid-or late-summer drought.
Nutrient supply. 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.
Fruiting. 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. In a study conducted in England on Cox's Orange Pippin, fruit set was reduced 65% for trees receiving no rain or irrigation from March to June.
Fruit growth. 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. Water stress during any time of the season ymay impair the ability of the fruit to increase in size.
Fruit cracking. Fruit cracking may result after water is supplied by rain or by irrigation following an extended dry period.
Postharvest drop. Apple drop just before harvest is common on trees affected by water stress. The effectiveness of "stop-drop" sprays such as NAA is reduced. The use of ReTain® is not recommended for water-stressed trees.
Yield and quality. Any tree subjected to water stress can be expected to have reduced yield and poorer fruit quality. Table 4 shows the effect of apple quality for varying periods of drought before harvest.