Actively growing shoots produce the growth regulator auxin, which moves downward with gravity (toward the Earth's center). As it moves downward in the shoot, the auxin actively inhibits the development of lateral buds and shoots. The process directs nutrients, growth regulators, photosynthates, and other resources to the actively growing shoot tip at the expense of other shoots and buds. This phenomenon, called apical dominance, allows actively growing shoot tips to dominate growth and thereby influence the number and length of lateral shoots as well as the angle (including crotch angles) at which these lateral shoots develop. Strong apical dominance favors vegetative shoot growth at the expense of flower bud production. Degree of dominance varies among species and among cultivars within species.
342 CONCISE ENCYCLOPEDIA OF TEMPERATE TREE FRUIT SHOOT ORIENTATION EFFECT ON APICAL DOMINANCE
Apical dominance is strongest in the actively growing terminal buds of vertical shoots and limbs. Thus, limb orientation has a dramatic effect on apical dominance and, thereby, the pattern of vegetative and reproductive growth. A vertical shoot would tend to be the most vegetative. As limb orientation shifts toward horizontal, terminal shoot growth decreases while number and length of lateral shoots increase. Generally, when limbs or shoots are oriented at 30 to 60 degrees from vertical, vegetative shoot growth in the terminal area is reduced while the number and length of lateral shoots farther away from the terminal are increased. As such, more moderate limb or shoot orientations have the potential to provide a balance between terminal and lateral shoot development as well as promote the development of flower buds. When limbs or shoots become at or below horizontal, however, the influence of apical dominance is lost. As a result, the lateral shoots that are normally influenced by apical dominance can develop unchecked into vigorous, upright water sprouts. These water sprouts then become independent vertical shoots with strong apical dominance.
Balancing vegetative growth and flower bud production is the basis for the limb spreading or positioning that is commonly practiced in tree fruit production. Response varies with cultivar, rootstock, tree age, degree of spreading, and time of spreading. If both cultivar and rootstock are precocious (prone to early and heavy fruiting), extreme limb spreading may result in excessive flower bud production and insufficient shoot growth, whereas, with more vigorous cultivars, wide limb spreading can result in severe loss of flowers due to production of water sprouts. Time of spreading or positioning also affects response. Spreading in late season after growth has terminated will have little effect on vegetative growth during that season. Conversely, extreme spreading in the dormant season before growth begins can result in weak terminal shoot growth and excessive water sprout development. Cultivars vary in response to limb orientations. For example, spur-type 'Delicious' trees are prone to water sprout development when scaffolds are oriented more than 60 degrees from vertical, whereas well-branched cultivars such as 'Golden Delicious' are not.
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