Arboriculture & Urban Forestry 42(1): January 2016 Arboriculture & Urban Forestry 2016. 42(1): 1–20 1 Cytokinin Phytohormonal Effects on Crown Structure Alisha M. Autio and Michael E. Day Abstract. This literature review explores the relevance of cytokinins to tree canopy form, integrating scientific research with current and potential applications to tree care methods. Current and most popular tree care methods call for growers to physically alter the shape of a tree by staking, pruning, and pinching, which can be time-consuming and expensive. Application of phytohormones (also known as plant growth regulators, PGRs) can provide ornamental growers and arborists with alternative methods to manip- ulate tree crown characteristics. Following a digest of the science of cytokinin activity, the review investigates the current and poten- tial uses of phytohormones as a cost-effective, alternative method of manipulating tree shape. It discusses how the different forms of cytokinin PGRs, acting alone and in concert with other PGRs, can be used, what they can be used for, methods of application, and timing of applications within the phenological cycles of trees. By integrating current basic and applied literature, the review seeks to summarize understanding of cytokinin regulation of crown structure, while exploring potential applications in the tree care industry. Key Words. Branching; Bud Formation; Crown Alteration; Cytokinin; Plant Growth Hormones; Plant Growth Regulators; Tree Physiology. A BRIEF DESCRIPTION OF CYTOKI- NINS AND THEIR ROLES IN PLANT DEVELOPMENT Cytokinins are one of the most important phyto- hormones in regulating development and growth in plants. Numerous endogenous cytokinins have been identified, and synthetic cytokinins are available on the market for exogenous application (Buban 2000; Wertheim 2000; NeSmith 2004; Carey 2008). Cytokinins play an essential role in plant develop- ment throughout the life cycle, ranging from the germination of seeds to fruit set (Sakakibara et al. 2006; Kyozuka 2007). Various cytokinins are syn- thesized in the plastids of plant tissues (including stems, leaf primordia, meristems, seeds, and espe- cially roots), regulate development in neighboring tissues, and, aſter transport in xylem and phloem, regulate whole-plant developmental patterns. Root- produced cytokinins coordinate shoot:root resource allocation and are important for communicating plant nitrogen status (Pons et al. 2001). Naturally occurring cytokinins are nitrogen-rich molecules that contain either aliphatic isoprenoid or aromatic side chains (Sakakibara 2006). When cytokinins trigger receptors in the cell membranes of target cells, they activate kinases cascades (molecular chains of phosphate transferring molecules) that regulate gene transcription in nuclei. Cytokinin- activated kinase cascades may interact with signals from other phytohormones or photoreceptors, such as phytochrome (Boonman et al. 2007; Taiz and Zeiger 2010). Cytokinins may be present in both active forms and as potential regulatory molecules in inactive conjugates with sugars, sugar phos- phates, and other molecules (Sakakibara 2006). At tissue and organ levels, cytokinin concentra- tions change throughout the year due to phenol- ogy and maturation state of plant tissues (Andres et al. 2002; Juvany et al. 2013). Activity is generally associated with regulating cell division. In repro- duction and embryo development, they regulate sex determination and pollination, seed dormancy, and germination (Schmulling 2002; Bishopp et al. 2006). Aſter germination, they enhance cell division in root/shoot meristems, regulate the formation of vascular elements, and regulate leaf develop- ment (Moore 1998; Carey 2008). In mature tissues, they regulate genes associated with chlorophyll ©2016 International Society of Arboriculture
January 2016
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