Arboriculture & Urban Forestry 42(1): January 2016 ing foliage, and by strigolactones, a form of steroid produced in roots (Hayward et al. 2009). Auxin pro- duced in the shoot apex travels basipetally through xylem parenchyma and stimulates the production of cytokinin dehydrogenase, which inactivates cytoki- nins diffusing toward lateral buds. ABA appears to have a role in suppressing lateral bud development in woody plants that acts independently of auxin (Frewen et al. 2000). The mechanism associated with strigolactones, is less well understood, but their presence also down-regulates cytokinin activity. Both auxin and ABA may also down-regulate cyto- kinin synthesis in root plastids (Frewen et al. 2000; Tanaka et al. 2006). These phytohormonal mecha- nisms maintain dominant shoot status by sustaining its larger sink for water, nutrients, and carbohydrates. Branch buds are initiated in the shoot apex (apical meristem and developing foliage), and their flushing activity requires activation by cytokinin. The effect of the shoot apex on lateral bud activity diminishes as distance between apical and lateral buds lengthen. As the influence of negative-effect phytohormones decreases and cytokinin and resource availability to lateral buds increase with distance from the apical meristem, lateral buds are released from suppres- sion (Rasmussen et al. 2009). When lateral buds are released from suppression, their endogenous cytokinin levels increase, high nutrient levels are associated with high rates of cell division, and new branches form (Humphries and Wheeler 1963). Auxin synthesized in the shoot apex has also been identified as the principle regulator of api- cal control (Cline 1991; Wilson 2000). Both apical dominance and apical control are regulated by phy- tohormones through their effects on vascular tissue development and by altering sink strength for car- bohydrates and nutrients. In apical dominance, connections between the vascular system and dor- mant buds are prevented. In apical control, cambial stimulation by phytohormones from the shoot apex results in selective enhancement of vascular tissue supplying the shoot apex (Cline and Harrington 2006). What both developmental mechanisms have in common is the reduction or elimination of cytokinin availability to lateral buds and branches. In apical dominance, cytokinins are actively deac- tivated. While in apical control, the transpiration streams (with its root-produced cytokinins) are diverted to the shoot apex. Selective loading mecha- 3 nisms for cytokinins are believed to be active in both xylem (i.e., transpiration streams) and phloem, regulating transport to sinks for cytokinin in other plant tissues (Sakakibara et al. 2006). In turn, cyto- kinins affect growth metabolism, enzyme activity, and the biosynthesis of other phytohormones through regulation of gene activity (Sakakibara et al. 2006). Cytokinin effects are largely limited to aboveground via manipulation of sink strength. Branch density and angle are affected by both exogenous and endogenous cytokinin and its interaction with auxin. When the terminal bud is clipped (auxin supply is interrupted), many of these dormant buds develop, and the branch den- sity increases as apical control is lost, resulting in a ‘full’ tree. Once the apical meristem is excised, auxin levels in the stem decrease, repression of cytokinin biosynthesis is released, and cytokinins levels increase to promote lateral bud growth. Aſter a lateral bud is activated, it assumes control as the apical meristem of its subtending shoot. An endog- enous supply of auxin is synthesized in the new apical meristem and represses cytokinin availability to lateral buds on its branch (Tanaka et al. 2006). Cytokinins generally induce branching, but the concentrations and effects vary widely among spe- cies and by interactions with other phytohormones (Carey 2008). In addition to interactions with auxin, gibberellins and cytokinins are mutually antagonistic. Cytokinins inhibit gibberellin forma- tion, and gibberellin inhibits cytokinins responses (Weiss and Ori 2007). Abscisic acid inhibits seed germination, stimulates ethylene production, and causes stomata to close (Kong et al. 2009). Cytoki- nin inhibits the activity of ABA on all of these pro- cesses (Carey 2008; Kong et al. 2009), maintaining more rapid growth and, in many cases, preventing phase change of vegetative meristems to floral meri- stems and prolonging juvenile maturation states. Additional factors that affect cytokinin activity in branching are the level of the particular cytoki- nin (Carey 2008), the sensitivity/responsiveness of the tissue to cytokinin, and for exogenous appli- cations, the location of cytokinin application (Bangerth et al. 2000). High levels of cytokinins in branch shoots stimulate bud growth and lat- eral stem formation (Bangerth et al. 2000; Carey 2008). A particular tissue may or may not be com- petent to recognize the presence of cytokinins ©2016 International Society of Arboriculture
January 2016
| Title Name |
Pages |
Delete |
Url |
| Empty |
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success. You will be contacted by Washington Gas with follow-up information regarding your request.
This process might take longer please wait
