114 Watson: Effects of PBZ Treatment on White Oaks The Effect of Paclobutrazol Treatment on Starch Content, Mycorrhizal Colonization, and Fine Root Density of White Oaks (Quercus alba L.) Gary W. Watson Abstract. Mature white oaks (Quercus alba L.) averaging 61 cm (24.4 in) dbh and in a moderate state of decline were treated with 0.8 g active ingredient (a.i.)/cm (2 g/in) paclobutrazol (PBZ) on 11 April 1995, and again with 1.2 g a.i./cm (3 g/in) on 8 October 1998, as a basal drench. PBZ produced no reduction in leaf size or twig growth of the white oaks at any time during the 7 years of the study. Rapidly growing plants may be more effectively growth-regulated by PBZ. Fine root density, starch content of the woody roots, and percentage of mycorrhizal root tips were unaffected by PBZ treatment. This is the first direct evidence that mycorrhizae are not reduced by the fungicidal properties of PBZ. The root zone of these trees was mulched but laterally restricted. Root density and mycorrhizae may have already been high under these condi- tions, limiting the ability of PBZ to improve them further. Key Words. Carbohydrates; plant growth regulator; Quercus alba; tree growth regulator. Gibberellin-inhibiting tree-growth regulators such as pa- clobutrazol (PBZ) have been shown to reduce shoot elonga- tion, leaf expansion, and stem diameter growth of many tree species (Keever et al. 1990; Burch et al. 1996). More re- cently, PBZ has been shown to increase root growth of trees in certain landscape situations (Watson 1996, 2004; Watson and Himelick 2004). PBZ has promise as a tool for improving the health and vitality of urban trees. Factors such as tree health and site quality may play a role in increased root development by PBZ. PBZ treatment did result in a significant increase in fine root density on chlorotic trees compared to untreated controls, but not on trees with normal foliage color (Watson and Himelick 2004). Fine root density of the healthy green trees may have been adequate, with little room for improvement. Poor soil conditions may limit response as well. Root density of chlorotic trees was improved by PBZ treatment in minimally disturbed golf course soils, but not along streets where soils are highly dis- turbed by the development process (Watson and Himelick 2004). Improvement of fine root density on white oak and pin oak also occurred in minimally disturbed soils (Watson 1996). Too much crown growth regulation can reduce root devel- opment rather than enhance it. PBZ increased root extension growth of transplanted Green Column black maples (Acer nigrum ‘Green Column’) in the first year after treatment, prior to the onset of aboveground growth regulation, but not in the second year, when leaf and twig growth reduction was evident (Watson 2004). High twig and leaf regulation of elm seedlings leads to decreased root growth (Watson 2001). Arborists have expressed concerns that mycorrhizae could be reduced by PBZ treatment because PBZ is closely related ©2006 International Society of Arboriculture to triazole fungicides and exhibits fungicidal properties (Jacobs and Berg 2000). The current commercial formula- tions of PBZ available for urban forestry applications are applied as a basal soil injection or drench. Only a few roots at the base of the tree are in direct contact with the PBZ. PBZ is absorbed by roots and translocated in the xylem only, to- ward the branch tips, with little or no phloem mobility (Cou- ture 1982). If PBZ is not transported to roots, it should have no fungicidal effect on mycorrhizal fungi, but this has never been confirmed. Carbohydrate content of various plant tissues can be in- creased by PBZ (Wood 1984; Wieland and Wample 1985; Wang et al. 1986). Steffens et al. (1963) reported a 132% increase in total carbohydrates in fibrous roots of apple seed- lings. Because mycorrhizal fungi receive carbohydrates from the plant as one of the major benefits of the symbiotic rela- tionship, increased carbohydrates in the plant could result in increased mycorrhizae. The objective of this study was to measure changes in fine root development, starch content, and mycorrhizal coloniza- tion of mature white oaks resulting from PBZ treatment in an urban landscape. MATERIALS AND METHODS Eighteen mature white oaks (Quercus alba L.) were selected on the grounds of the Morton Arboretum, Lisle, Illinois, U.S., near roads and buildings, similar to typical suburban settings. They were 42 to 76 cm (16.8 to 30.4 in) dbh [61 cm (24.4 in) average] located within 500 m (1,650 ft) of each other. All were in a moderate state of decline as judged by the amount of dieback, thinness, and/or chlorosis in the crown. Nearly all
May 2006
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