Arboriculture & Urban Forestry 42(1): January 2016 absorbed through the cuticle (Canal et al. 2000). Plants can absorb cytokinins through leaf or root epidermis and transport them to growing points. It is likely that younger tissues with thinner cuticle and epidermal layers absorb more cyto- kinins than older tissues (Carey 2008). Activity level depends on the form of synthetic cytokinin (Hartmann et al. 2001; Davies 2004b) and the amount applied to a particular site (Carey 2008). Currently, arborists use a mixture of chemi- cals and synthetic hormones to manipulate tree growth, shape, and increase branching, while avoiding unwanted damage to other plants, animals, and humans. Extrinsic regulation of development and form can be accomplished at any growth stage. Tree shape alteration can begin in embryos. Both Lyyra et al. (2006) and San- Jose (2001) found that a liquid BAP cytokinin pulse induced adventitious shoot formation in cotyledons of black willow (Salix nigra Marsh.) and sweet chestnut (Castanea sativa Mill.). At the seedling stage, spray treatments of cytokinin have been used to alter tree shape by hindering stem elongation and promoting bud formation in Douglas-fir adventitious seedlings (Lish- eng, et al. 2009). Cline et al. (2006) found that whole-tree (but not terminal leader) foliar BA applications promoted both bud formation and second flushing in six-year-old Douglas-fir seed- lings. During release from quiescence in mature trees, cytokinin levels increase, and branching is controlled by relative levels of cytokinin and auxin as the crown develops (Mazri 2013). Generally, cytokinin in stem tissue is required for activation of dormant lateral buds, and auxin transported basipetally from shoot apices interferes with this process through apical dominance. Branching agents work by interrupting apical dominance, which triggers lateral bud growth, which ‘fills in’ the plant. Apical dominance can be interrupted in several ways, including by pruning of apical meristems, to reduce auxin, or by applying exogenous cytokinins (Carey 2008). Christmas tree growers have also experimented with applying artificial cytoki- nins and increasing cytokinin to auxin ratios. The complexity of the branching pattern depends on the temporal and spatial develop- ment of branches. These characteristics, although 7 they are plastic in their response to environmen- tal cues and human intervention, are in large part genetically determined. Therefore, regu- lation of crown structure is determined by an interaction between the developmental pro- gram that specifies branching patterns in differ- ent plant species generating species-specific plant forms and natural and/or artificial influences. Terminal bud dominance and control can be manually manipulated by pruning. Growers and horticulturists commonly use pruning to shape the crown of commercial and urban trees, interrupting the basipetal flow of auxin. Arborists use a multi- tude of pruning techniques to achieve their goals of aesthetics, safety, and tree health. However, prun- ing is time-consuming and requires substantial skill and training to avoid unintended consequences. Pruning lateral branches will change stem taper and therefore affect wood quality and strength. Pruning and lopping of trees in ways that remove large amounts of foliage disrupts both auxins and cytokinins levels. As a conse- quence of poor pruning practice, large numbers of dormant buds, epicormic buds may develop rapidly. They are oſten poorly attached but grow quickly under proper environmental conditions. Because of their weak attachment, the branches are easily shed and present a significant hazard in urban settings (Moore 1998). Furthermore, overpruning of a tree will result in lower photo- synthetic and hormonal transport levels, which may directly decrease fruiting (Carey 2008). Plant growth regulators have outstanding potential for arboricultural manipulation of urban trees and avoiding some of the negative aspects of manual pruning. They are currently used in some contexts but, like pruning, require compe- tent and professional workers to avoid situations where direct application of hormones and PGRs result in unacceptable environmental risks. Cyto- kinins have impressive effects on plant growth and development, but there is an inherent dan- ger in their use (Moore 1998). Many PGRs can be harmful or toxic, and should be used with extreme care (Harrison et al. 2014). Documented and established protocols are available for best PGR practices, and as in any chemical released into the environment users must be thoroughly trained in best practices (as outlined in Table 2). ©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
