PUBLICATION 422-025 Physiology of Pruning Fruit Trees Richard P. Marini, Extension Specialist, Horticulture; Virginia Tech Introduction ways plants can grow. Woody plants are pruned to maintain a desired size and shape and to promote a certain type of growth. Primary growth is the increase in length of shoots Ornamental plants are pruned to improve the aes- and roots, and is responsible for increases in can- thetic quality of the plant, but fruit trees are pruned opy height and width. to improve fruit quality by encouraging an appro- Secondary growth is the increase in thickness of priate balance between vegetative (wood) and stems and roots. reproductive (fruiting) growth. Annual pruning of Both types of growth require cell division followed fruit trees always reduces yield, but enhances fruit by cell enlargement and differentiation. quality. Pruning increases fruit size because excess flower buds are removed and pruning encourages Plant Growth the growth of new shoots with high-quality flower Meristems are regions of cell division and there buds. Pruning improves light penetration into the are two types of plant meristems. An apical mer- canopy, and light is required for flower-bud devel- istem is located at the tip of every shoot and root opment, fruit set and growth, and red color devel- (Figure 1). As cells divide in these apical mer- opment. Pruning also makes the canopy more open istems, the shoots and roots elongate as cells are and improves pest control by allowing better spray piled one on another. Behind the region of cell penetration into the tree; air movement throughout division is a region of cell differentiation, where the canopy is increased, which improves drying cells enlarge and differentiate into various tissues. conditions and reduces severity of many diseases. In the axil of each leaf is a small apical meristem This publication describes why plants respond to called an axillary meristem that forms an axillary pruning and other forms of plant manipulation used bud, which usually remains dormant until well to train trees. This information applies to all plants, after the subtending leaf is fully developed. An but application to fruit trees is emphasized. axillary bud may remain dormant or develop into Pruning fruit trees is somewhat of an art based on a lateral branch or a flower. an understanding of plant physiology and develop- There are two distinct layers of meristematic tissue ment. In other words, if we understand how plants within the stem or root responsible for secondary grow and how they will respond to different types growth, the vascular cambium and the cork cam- of plant manipulations, we can alter vegetative bium (Figure 2). The vascular cambium is a cyl- growth and fruiting to obtain trees and fruit with inder of specialized cells, usually five to ten cells desirable characteristics. thick, running the length of the plant, including the A basic understanding of certain aspects of plant roots, and is responsible for the radial growth of physiology is a prerequisite to understanding prun- plant parts. Phloem cells are produced to the out- ing. Unlike animals, plants continue to increase side of the cambium and xylem cells are produced in size throughout their lives. There are only two to the inside of the cambium. www.ext.vt.edu Produced by Communications and Marketing, College of Agriculture and Life Sciences, Virginia Polytechnic Institute and State University, 2009 Virginia Cooperative Extension programs and employment are open to all, regardless of race, color, national origin, sex, religion, age, disability, political beliefs, sexual orientation, or marital or family status. An equal opportunity/affirmative action employer. Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. RIck D. Rudd, Interim Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg; Alma C. Hobbs, Administrator, 1890 Extension Program, Virginia State, Petersburg. vascular phellogen Leaf cambium primordia Apical meristem bumps phloem xylem periderm (bark) axillary Figure 2. Longitudinal cross-section of a tree trunk shows buds the vascular and cork cambiums. Figure 1. Longitudinal section of shoot tip shows an apical a vegetative bud is sliced longitudinally during meristem, successively older leaf primordia, and axillary the winter and viewed under magnification, the bud primordia. apical meristem at the tip, leaf primordial (devel- oping leaves), axillary meristems, developing axil- Downward transport of sugars, nutrients, and hor- lary buds, and procambial tissue (tissue that will mones from the top of the tree to the roots occurs develop into the cambium) are all visible. in the phloem tissue. Xylem cells are tube shaped, become hollow and die to form a pipe-like sys- Buds on fruit trees usually have about seven tem through which water, hormones and mineral leaves and initial shoot elongation in the spring nutrients move from the roots to the top of the tree. results from cell expansion. During late June and Most of the radial growth of woody plants is due July some of the shoot apices will flatten out and to activity of the vascular cambium, but a small develop into flower buds. Flower buds are actu- amount results from activity in another lateral ally modified shoots and the various flower tissues meristem, the cork cambium, located outside the (petals, stigmas, anthers, etc.) are actually modi- vascular cambium. The cork cambium (phellogen) fied leaves. Although the process of switching together with the cork cells, constitute the perid- from vegetative to reproductive buds is not fully erm: a protective layer of suberized dead cork cells understood, hormones that can be influenced by forming the bark. Suberization is the impregnation environmental factors, stresses, and plant nutrition of cell walls of cork tissue with a fatty substance control the process. called suberin. Each season new layers of cells are There are several things we can do to influence produced and appear as growth rings when viewed whether or not a bud becomes a flower bud or in cross-section. Over time, the xylem cells at the remains vegetative. In general, factors that favor center of the trunk or limb are crushed and become rapid growth, such as high nitrogen levels in the nonfunctional as transport pipes, but they do pro- shoot tissues, inhibit the development of flower vide structural support to hold the plant upright. buds. Applying growth-promoting plant growth While grafting it is important to line up the cam- regulators such as gibberellins usually inhibits biums of the scion and the rootstock to ensure a flower-bud induction, whereas ethylene may pro- successful graft union. mote flower-bud development. Mild stresses such as shoot bending and water stress may also pro- Buds mote flower-bud development. Buds are important to the vegetative and reproduc- Producing annual crops of high-quality fruit tive growth of trees. Fruit tree training and, to a lesser requires a balance between reproductive and veg- extent, pruning primarily involves bud manipula- etative growth. Fruit producers use various tech- tion. Buds are actually undeveloped shoots. When niques, including pruning, branch bending, and 2 plant growth-regulator sprays, to manipulate tree Classification by arrangement on the stem growth and flowering. Often these techniques The bud arrangement influences the arrangement affect bud dormancy, so knowledge of buds and of a fruit tree’s branches and thus the tree’s shape bud dormancy is essential if we are to understand and how easy it is to manage. A node is the joint on how pruning influences tree growth. It is also a stem where a leaf is or was attached (Figure 3). important to be able to identify the different types Axillary buds are located in the axis above where a of buds on a tree, especially to distinguish between leaf is attached to the stem. In apples there is usu- flower and vegetative buds. ally only one leaf per node, whereas three leaves Buds may be classified as to location, contents, or often arise from a node on peach shoots. When a activity. leaf falls in the autumn, a leaf scar remains just below the axillary bud (Figure 3). Buds are oppo- Classification by content Several types of buds site when there are two at the same node but on commonly develop on fruit trees. Vegetative opposite sides of the stem. Forsythia is an example buds only develop into leafy vegetative shoots. of a plant with opposite buds. Buds are alternate Flower buds produce only flowers. Stone fruit when there is only one from each node and no one trees (peach, nectarine, apricot, plum, and cherry) bud is on the same side of the stem as the one next produce vegetative buds and flower buds. Apple above or below it. Deciduous fruit trees have buds and pear trees produce vegetative and mixed that spiral along a shoot (Figure 4). The spiraling buds. Both leafy shoots and flowers emerge from three-dimensional arrangement of leaves around a mixed buds. stem is known as Phyllotaxy and is expressed as a fraction, where the numerator is the number of Classification by location turns to get to a leaf directly above another and the Terminal buds are located at the tip of a shoot. denominator is the number of buds passed. On stone fruit trees terminal buds are vegetative buds. Terminal buds on apple and pear trees are usually vegetative; however, some varieties such Terminal bud as ‘Rome Beauty’ produce mixed buds terminally Node and are referred to as “tip bearers” or “terminal Internode bearers.” Most mixed buds on apple and pear Node Lateral bud trees are formed terminally on short, less than six- inch, shoots that terminate with a rosette of leaves. Leaf scar These short shoots are called “spurs.” Lateral buds form in the axils of leaves and are often referred Flower bud scar to as “lateral buds” or “axillary buds.” On stone- Lenticel fruit trees, lateral buds may be either vegetative Terminal bud scar or flower.