Arboriculture & Urban Forestry 42(1): January 2016 Sun/Shade Foliar Adaptation and Chlorophyll Synthesis Boonman et al. (2007; 2009) linked sun-shade attributes with cytokinin activity, transpira- tion rates and chlorophyll b concentrations. Low transpiration rates are found in shade environ- ments, areas with high humidity, and in the tops of crowns of old trees (presumably, due to xylem length and complexity). Low transpiration rates correlate with low cytokinin levels because of the lowered delivery of cytokinin through translo- cation. Boonman et al. concluded that pigment content is influenced by the interaction of light environment throughout its effect on the tran- spiration stream and cytokinin concentrations. Therefore, the addition of cytokinin can enhance shade effects in canopies as it down-regulates chlo- rophyll synthesis. Cytokinins have been linked to sun-shade foliar responses in herbaceous species. Carabelli et al. (2007) reported that cytokinins are associated with suppression of leaf bud development in Arabidopsis. Decreased concentra- tions of cytokinin in shade foliage resulted from lower transpiration rates and decreased water flow. CYTOKININS AS TOOLS IN ARBORI- CULTURE AND HORTICULTURE Industries that could benefit from hormonal tech- nology include the tree care industry, woody plant horticulture, and fruit care industries. Syn- thetic plant growth regulators (PGRs) are usually structural analogues of hormones (Moore 1998). Cytokinins are based on nitrogen-carbon rings and naturally occur in both isoprenoid and aro- matic forms. Forms vary in their side chains, which impart receptor specificity in target cells (Sakakibara 2006). Analogous manufactured chemicals that mimic the structures and functions of natural phytohormones can also have signifi- cant effects on plant growth and development. The best-known hormonal cytokinin found in plants is zeatin, and the most common forms of synthetic cytokinin are its analogs based on ben- zyladenine, or BA (Moore 1998). One cytokinin product registered for use on ornamental plants is Configure®; a benzyladenine-based PGR. It can be applied as a foliar spray or a substrate drench at concentrations from 1 to 3000 mg·L-1 (Carey 2008; Table 1; Table 2). A few others, such as 5 Topolin® (Buban 2000), 9-Benzyladenine (Canal et al. 2000), Triacanthine, 6-Chloroprine, and 4PU (Zhang and Hasenstein 1999) have resulted in larger fruit sizes and increased return bloom the following year, a movement from inactive to active forms of BA, an increase in lateral bud- break, and a decrease in terminal budbreak as seen in experimental studies (Table 1; Table 2). In addition, many other chemicals are known to have partial cytokinin activity (Carey 2008). Table 1. Cytokinin formulations and trade names. See text for further information on group components. Cytokinin group BA Common names 2iP, BA, BA 0.1%, BAP, BA 0.2, BA 0.5, Verdan, BA + TIBA®, BA + Alar®, BA + TIBA® + Alar, 6-CP, Cytex®, BA + IBA®, GA + BA + IBA, ProShear®, and other BA/GA mixes BA + Kinetin BA + Kinetin®—also contains GA and auxin or IBA BPA BPA, Accel, SD8339® Kinetin BA + GA Kinetin® 2–500 ppm, Kinetin (270 ppm) with or with- out GA3 90, GA + Kinetin 100 ppm + 100 ppm BA or BA + GA, Promalin®, PBA, Thidiazuron®, Fascination®, Lanolin® Many attempts have been made over the years to experimentally integrate cytokinin products into horticultural practices focused on lateral branch stimulation in young nursery trees (Elfving and Visser 2005). Benzyladenine+GA4,7 was first reg- istered in the late 1970s for agricultural crops, and pyranylbenzyladenine PBA was registered in the late 1970s for use on ornamentals. From the 1970s until the 1990s, a product called Pro-shear® (2% BA) was used on white pine trees (Pinus strobus L. in Christmas tree plantations) to increase branch development by increasing lateral bud set in the year of application. However, it is no longer produced (Baker 2001). Some products are not labeled for use in every state or country due to variations in state regulation laws and oſten by marketing decisions by manufacturers. In many cases, cost–benefit analysis may discourage efforts at registration compliance in multiple jurisdictions. Cytokinins-based PGRs are commonly used in a 1:1 ratio with Gibberel- lin 4,7 in a product called Promalin® (Carey 2008). Exogenous cytokinins are usually applied to plants via foliar sprays or drenches and are ©2016 International Society of Arboriculture
January 2016
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