2 Autio and Day: Cytokinin Phytohormonal Effects on Crown Structure biosynthesis and nutrient partitioning, they open stomata, and they delay senescence. Important to plant crown formation, branch and meri- stem development are controlled by cytokinins (Roitsch and Ehneß 2000; Rasmussen et al. 2009). Cytokinin and other phytohormones are con- trolled by a system of checks and balances within the plant. Cytokinin synthesis is autonomously down-regulated as total concentrations of cytoki- nin increase in plant organs, and as inactivation by enzymes are stimulated by other phytohormones, particularly auxin. Developing plant shoot organs compete with one another for water and nutrients by producing negative-effect hormones, which either down-regulate production of phytohormones required by competing organs or inactivate available phytohormones. For example, the influence of apical bud dominance over lateral buds led to the discov- ery of auxin as the first identified phytohormone (Thimann and Skoog 1933), where the removal of apical bud and a primary leaf of bean plants (Phase- olus vulgaris) induced rapid expansion of secondary leaves by interruption of auxin produced in the shoot apex, leading to increased gibberellin con- tent (Humphries and Wheeler 1963). Unbeknownst to Thimann and Skoog, auxin and gibberellins phytohormonal checks and balances were at play. Basic research into plant physiological processes is usually conducted on “model plants,” such as Arabidopsis and Nicotiana, where the accumula- tion of vast data bases permit connecting the dots between genomic and biochemical activity and whole-plant physiology. As most physiological processes are highly conserved, models derived from basic research can be applied to other spe- cies with the caveat that physiognomy and envi- ronment may require their modification. While definitive understanding of the action of phyto- hormones in particular situations may require species-specific studies, the diverse body of lit- erature on phytohormones permits valuable gen- eralization of phytohormone effects and activity. CYTOKININ IN BRANCHING AND CROWN DEVELOPMENT Crown architecture is due to a complex interaction of phytohormones, most commonly auxin, gib- berellins, and ethylene, working in combination with cytokinins. Phytohormones are responsible ©2016 International Society of Arboriculture for regulating branch angle, the amount of branch- ing, shoot growth, and the activation of lateral buds (Cline and Dong 2002; Oates et al. 2004; Sansberro et al. 2006; Müller and Leyser 2011). In the shoot apical meristem, cytokinin is a major regulator of meristem characteristics and branch ini- tiation. Arabidopsis mutants that lacked cytokinin showed a severe reduction in shoot apical meri- stem size, indicating the importance of cytokinins in shoot development (Kyozuka 2007). Cytokinin controls the outcome of undifferentiated cells in shoot apical meristems (Kurakawa et al. 2007). In a developing shoot, there is a cell elongation zone of approximately 10 to 15 cm in length subtending the meristem. The elongation zone is an area of undif- ferentiated cells that begin to form into determined organs, as phytohormone balances shiſt in nearby tissues. In the apical meristems of tobacco plants (Nicotiana tabacum L.), cytokinins induce the bio- synthesis of auxin, which regulates the formation of bud primordia in the expanding shoot (Nakagawa et al. 2005; Cortleven and Valcke 2012). Bud dis- tance from an active growing meristem determines the amount of cytokinin, which is correlated with triggering lateral bud burst and further branch development. Therefore, the development of a new shoot meristem depends on whether the activated bud (and its associated undifferentiated cells) is associated with the internode that has not begun to elongate or one that has already partially completed the elongation process (Elfving and Visser 2006). The two principal regulatory pathways are apical dominance (when the terminal bud of a shoot or branch prevents activity of dormant lateral buds) and apical control (when the terminal bud inhibits the growth of branches from activated lateral buds). It has been long known that the apical meristems on dominant shoots inhibit bursting of lateral buds. The power of control of lateral bud bursting by shoot apices is the primary mechanism by which woody plants assume their typical crown form. Strong apical dominance leads to monopodial crowns, typical of many conifers, while weaker dominance results in highly branched sympodial-shaped crowns, typical of angiosperm trees and shrubs. The process of apical dominance is maintained by down-regulating cyto- kinin supply to lateral buds by the phytohormones auxin and abscisic acid (ABA) (Ward and Leyser 2004) from shoot apical meristems and develop-
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