66 Percival and Fraser: Sugars to Improve Root Growth and Transplant Success USE OF SUGARS TO IMPROVE ROOT GROWTH AND INCREASE TRANSPLANT SUCCESS OF BIRCH (BETULA PENDULA ROTH.) By Glynn C. Percival1 and Gillian A. Fraser2 Abstract. Two field trials undertaken in 1999 and 2003 investi- gated the influence of a range of sugars applied as a root drench at 25, 50, and 70 g/L (3.4, 6.8, and 10.3 oz/gal) of water on root and shoot growth, chlorophyll fluorescence, photosynthetic rates, and leaf carotenoid and chlorophyll concentrations of birch (Betula pendula Roth.). Irrespective of concentration and year, the sugars galactose and rhamnose had no significant effects on tree growth or leaf photosynthetic properties. Application of the sugar maltose increased shoot and root dry weight in the 1999 trial but had no effect in the 2003 trial. Sucrose, fructose, and glucose increased shoot and root dry weight in both 1999 and 2003 trials; however, growth responses were influenced by the concentration of sugar applied. In many cases, sugar application increased the number of new roots formed by week 6 but had no significant effects on the length of existing roots or shoot growth. By week 24, increases in both root and shoot growth were recorded. Sugar feeding at 25 g/L (3.4 oz/gal) of water had no significant effect on leaf chlorophyll fluorescence, photosynthetic rates, or carotenoid and chlorophyll concentrations; however, sugar feeding at 50 and 75 g/L (6.8 and 10.3 oz/gal) of water reduced these values by week 6. At the cessation of the experiment, maximal increase in root and shoot growth was associated with a root drench of sucrose at a concentra- tion of 70 g/L (10.3 oz/gal) of water in both 1999 and 2003 trials. Lower mortality rates recorded in sugar-treated trees indicate applications of sugars would aid in the survival of young birch trees following transplanting. Key Words. Carbohydrates; resource allocation; gene expres- sion; transplant shock; chlorophyll fluorescence; photosynthesis; chlorophyll; carotenoid; plant vitality. A major determinant of the performance of a field-grown, transplanted tree is the root:shoot ratio. The transplanting process reduces the root system, which is not paralleled by a reduction in the shoot system. This results in severe water stress because the root system is now of insufficient size to support the tree crown (Haase and Rose 1993). Even when accepted nursery practices are followed, less than 5% of the actual absorbing root system may be moved with the tree (Watson and Himelick 1982). This extreme state of imbal- ance between the root system and the crown results in an extended period of stress often described as “transplant shock.” Consequently, high mortality rates (30% to 50%) are common the first year after planting, with “transplant shock” identified as a major criterion for failure (Johnston ©2005 International Society of Arboriculture and Rushton 1999). In the United Kingdom, where approxi- mately £300 (US$450) million is spent annually on tree plantings, even a 5% loss rate results in high financial loss. Although a number of factors have been associated with transplant shock, it is now widely believed that survival of newly planted trees is largely dependent on rapid extension of roots that absorb water to replenish transpirational water loss and thus reduce water stress (Gilbertson and Bradshaw 1990; Watson and Himelick 1997). Ideally a cheap, nontoxic, and environmentally friendly compound that can be applied to badly damaged or severely pruned root systems as a dip, soil amendment, and/or foliar spray that increases root vigor (i.e., new root regeneration and elongation of existing roots to rapidly restore the root:shoot ratio) is required. Until recently, the control of plant growth and develop- ment was believed to be coordinated by a range of plant growth regulators, such as auxins that stimulate root growth and cytokinins that stimulate vegetative growth (Percival and Gerritsen 1998). Recent evidence has, however, shown that in plants, sugars such as sucrose, glucose, and fructose function not only as substrates for growth but affect sugar- sensing systems that initiate changes in gene expression and subsequent plant growth (Koch 1996). Sugar depletion, for example, upregulates genes for photosynthesis, carbon remobilization, and export, resulting in vegetative or shoot growth. In contrast, incubation of root systems in sugar solutions (i.e., sucrose or glucose) leads to the repression of photosynthetic genes, decreased rates of net photosynthe- sis, and carbon remobilization in favor of enhanced root development (Koch 1996; Martin et al. 1997). Further, supplementing wheat root systems with sugars (i.e., sucrose, glucose, or fructose) significantly increases lateral root branching and root formation compared with controls (Bingham and Stevenson 1993; Bingham et al. 1997, 1998). This raises the possibility that transplant shock may be reduced by treating transplants during or immediately after with sugars. Objectives of this investigation were to (1) evaluate the influence a factorial combination of six different sugars (galactose, rhamnose, sucrose, glucose, fructose, and maltose) and three concentrations (25, 50, and 70 g/L [3.4, 6.8, and 10.3 oz/gal] of water) on root and shoot growth, chlorophyll fluorescence, photosynthesis, and leaf chloro-
March 2005
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