8 Autio and Day: Cytokinin Phytohormonal Effects on Crown Structure Although cytokinins contribute to many plant processes, they are not yet widely used in horti- culture outside of a few specialized areas, such as tissue culture, apple/cherry production, and Christmas tree farms. In the apple industry, when applied to flowering trees, they promote bloom thinning, which results in higher fruit production in future years (Buban 2000; Baker 2001). When sprayed onto young fruit, cytokinins improve the shape of fruit and increase size. They are used on young apple trees to promote lateral bud break and increase radial growth on branches (Wilson 2000). They are also used on pear trees and nut trees for Promalin similar reasons (Andres show species et al specific 2002). Cytokinin PGRs are also used on table grapes to increase the size of fruits (Cruz-Castillo et al. 1999). The bioregulators Cyclanilide® (CYC) and (PR) differ- ences in their effects of induced branch height in apple and sweet cherry trees in nurseries (Elf- ving and Visser 2005; Elfving and Visser 2006). Application of the cytokinin 6-benzyladenine (BA) with or without gibberellic acid isomers GA4 and GA7 (GA4 and GA7) also improves branch formation in apple and sweet cherry trees (Elfving and Visser 2006). Cytokinins gen- erally increase branch size and the number of branches of apple trees, which in turn increases fruit yield. Cytokinins are promising as fruit thinners because they are not harmful to ben- eficial insects (as carbaryl thinners are) and also promote branching and flowering the following year (Elfving and Visser 2006; Mickelbart 2011). Both CYC and PR act by altering auxin to cyto- kinin ratios. CYC is an auxin transport and action inhibitor (Pederson et al. 1997), and PR contains the cytokinin growth promoter 6-benzyladenine (Sachs and Thimann 1967). When Elfving and Visser (2006) applied both to separate tree plots of nursery sweet cherry trees, they found that although the mode of action of CYC and PR prod- ucts are different, their effects on branching and tree structure seem to be dictated more by tree spe- cies than product. CYC induced new lateral shoots starting above the bud union, and lower than spontaneously formed branches that developed on untreated trees. PR did not affect the final height of the lowest induced branch. CYC produced branches starting at a progressively lower height above the ©2016 International Society of Arboriculture bud union on the central leader as the product concentration increased. In contrast, PR did not affect the final height of the lowest induced branch. In another study, a cytokinin treatment of 0.5 mM BA was applied five to six weeks following two weeks of cytokinin foliar spray treatments of the terminal spring-flushing leader shoots, which resulted in a small increase in the total number of lateral buds and a large increase in current bud outgrowth (Cline et al. 2006). A second terminal bud flushing occurred in only 28.4% of the con- trol seedlings in the five experiments compared with 81.5% of the BA-treated seedlings, indi- cating that BA substantially enhanced second flushing of the terminal buds. Aſter testing BA applications in different concentrations, both as drops and sprays, 0.5mM BA foliar spray was found to be optimal under greenhouse conditions. Cytokinins are currently registered for use on apples (Accel®, Promalin) and sweet cherries (Pro- malin) as fruit thinners, to improve ‘typiness’, and to induce lateral branching and budbreak (Buban 2000; Western Plant Growth Regula- tor Society 2000; Baker 2001; Yu et al. 2008). Application Considerations The most commonly applied cytokinin, BA, is only slightly soluble in water (Hrotkó et al. 1999; Magyar and Hrotkó 2005). However, other highly active cytokinins, such as 6-{N-[2-(N-Methoxy- N-methylamino) ethyl]amino} purine are water soluble (Maruyama et al. 1993; Pons et al. 2001; Oates et al. 2004; Sugiura 2004), but are not readily available as commercial preparations. Benzylad- enine mixtures therefore require an organic sol- vent, such as isopropanol (BA soluble up to 3960 mg·L-1 ), chloroform (soluble up to 288 mg·L-1 ), HCL, KOH, DMSO, propylene glycol, ethoxyl- ated tallowamines, ammonium hydrazide, sodium hydrazide, methanol or ethanol, and topical ap- plications usually employ a surfactant, such as Tween®. Once in solution, further dilutions can be accomplished with water or a water-alcohol solvent. Mirdehghan and Rahemi (2004) reported that BA can also be dissolved in hot water and then cooled to a supersaturated solution. Common application concentrations range from 0.1 to 10 mg·L-1 (BA), which correspond to typical concentrations used in tissue culture media (Table 2). Researchers who
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
