10 Autio and Day: Cytokinin Phytohormonal Effects on Crown Structure replacement for pinching in many crops with mixed results, depending on species and PGR product. Cy- tokinins have been shown to be somewhat effective in replacing the need for shearing in Christmas trees, such as Buxus sempervirens L. ‘Suffruticosa’, B. sem- pervirens ‘Vardar Valley’, and B. sinica var. insularis (Musselwhite et al. 2004), for pinching in Clematis spp. (Puglisi 2002), and holly (Oates et al. 2004), but not in Caladium spp. (Whipker et al. 2005). The value of cytokinins to promote branching in juvenile trees may be highly life-stage specific. The seedlings of many woody plants do not form lateral buds or branches in the earliest stages of development. As developmental states change rapidly during the early growth of tree seedlings (Day and Greenwood 2011), the effects of cytokinins may be highly life-stage and species specific and should be evaluated for indi- vidual species/developmental-state combinations. Cytokinins have been used to increase branch production on sweet cherry trees (Hrotkó et al. 1999; Neri et al. 2004; Magyar and Hrotkó 2005) and there is potential to use this effect on ornamen- tal plants and in urban landscapes (Carey 2008). Cytokinins could be co-applied with dikegulac sodium (Atrimmec®), or ethephon (Florel®) for syn- ergistic effects on branching. Dikegulac sodium or ethephon would inhibit the apical meristems and thus reduce apical dominance. In this procedure, cytokinins would stimulate adventitious bud and shoot formation and release dormant buds from quiescence. Cytokinins could also be used in con- junction with ethylene compounds to stimulate branching. The applications should not be made simultaneously, as cytokinin reduces sensitivity to ethylene, requiring that ethylene be applied first and followed by cytokinins a week later (Carey 2008). In addition, ethephon requires a low pH car- rier solution and cytokinins are absorbed best at neutral pH, suggesting they should not be mixed (Carey 2008). Nursery tree height can be regu- lated by timing bioregulator applications to obtain branches starting within a specific range of height above the bud union, as long as species-specific bud formation characteristics are taken into account. Producers interested in establishing branch meri- stems at a particular height on trees can adjust application programs based on the desired height of branching and the characteristics of the bud formation responses (Elfving and Visser 2006). ©2016 International Society of Arboriculture Improving Branching Angles As cytokinins stimulate branching, exogenous ap- plication may potentially limit upward growth by promoting competition for plant resources be- tween branches. Cytokinins have been observed to improve branch angles in some narrow-angled woody crops, such as Bradford pear (Pyrus callery- ana Decne.) (Wertheim 2000; Stern and Flaishman 2003; Costa et al. 2004). This effect could apply to other ornamental woody plants to improve branch angles during production and thus enhance retail quality and landscape aesthetics (Wilson 2000). In regulating branch angle, cytokinins may be used in mixes with other growth inhibitors, but no working combinations have yet been estab- lished. Products containing a mixture of cyto- kinins and gibberellins (Fascination®, Fresco®, Promalin) that are used to prevent leaf yellowing and bud burst in Easter lilies and tulips may prove useful in this respect. There may be a potential for growers to save money by substituting cytoki- nin PGRs for manual pinching (Mickelbart 2011). Photoreceptors, such as phytochrome and cryptochrome, and cytokinins interact through multiple signaling pathways to regulate pho- tosynthetic acclimation to light gradients. In this role, cytokinins regulate both anatomy and photosynthetic pigments to control the photo- synthetic capacity of plant crowns, including regulating branching (Boonman 2007). Exog- enously applied cytokinin or localized overpro- duction of cytokinin has been demonstrated to rescue the effects of partial shade treat- ments (Pons et al. 2001; Boonman et al. 2007). SUMMARY AND CONCLUSION Current research demonstrates roles for phyto- hormones, particularly cytokinins, for regulating branching, bud formation, and foliage patterns that affect crown shape in trees. Auxin and cytokinin work in conjunction to control bud formation. Many PGRs are made from synthetic phytohormones, with benzylaminopurine being the largest and most well-studied group of cytokinin-based PGRs. Newly developed information and technology in phytohormonal actions have great potential to improve techniques for regulating tree crown development and advancing the tree care and hor- ticulture industries. The goal of this study is to use
January 2016
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