188 Burger and Prager: Deep-Rooted Trees for Urban Environments Table 2. Mean liminal angle (LA) and individual root angle (IRA) for seed-propagated and cutting-propagated trees of Fraxinus and Zelkova.z Tree genotype 2Frax9 2Frax10 2Frax8 3Frax13 2Zelk4 2Zelk9 2Frax9 and 2Frax10 were shallow-rooted; 2Frax8 and 3Frax13 were deep-rooted. 2Zelk4 was shallow-rooted and 2Zelk9 was deep-rooted. Values followed by the same letter are not significantly different at P 0.01. z roots, whereas 15% of the cuttings from 2Zelk4 and 35% of the cuttings from 2Zelk9 formed adventitious roots. The genotypes of Fraxinus uhdei rooted relatively well, all showing rooting percentages above 60%. The differences in rooting potential shown here among closely related genotypes are common for woody perennials and provide additional evidence of the impor- tant role genetics plays in root morphology, physiology, and development. Vegetatively Propagated Trees: Fraxinus and Zelkova As seen in the seed-propagated populations, most of the roots from vegetatively propagated, field-grown Fraxinus and Zelkova clones exposed with the Air-Spade were heart or lateral roots. There were very few tap, sinker, or striker roots observed. For Fraxinus uhdei, trees 2Frax9 and 2Frax10 were chosen as shal- low-rooted genotypes and 2Frax8 and 3Frax13 were chosen as deep-rooted genotypes based on the mean slopes of their root systems. For 2Frax9, neither the LA nor IRA values for the seed-propagated parent and its cutting-propagated progeny population differed from one another (Table 2). For 2Frax10, the LA values for seed-propagated and cutting-propagated trees were not significantly different, but the values for IRA were (–44.5 to –25.0). The IRA for the cutting-propagated progeny was less negative (signifying a shallower root system) than its seed-propagated parent (Table 2). For 2Frax8 and 3Frax13, the deep-rooted genotypes, the cutting-propagated progeny had sig- nificantly higher values for LA and significantly less negative values for IRA indicating that they had shallower roots systems than their seed-propagated parents (Table 2). Similar results were found with the two Zelkova genotypes. Genotype 2Zelk4 was shallow-rooted, whereas 2Zelk9 was the deep-rooted geno- type. For both genotypes, the cutting-propagated progeny had significantly higher LA values and significantly less negative IRA values indicating that they had shallower roots systems than their seed-propagated parents (Table 2). For shallow-rooted Fraxinus and Zelkova genotypes, cutting-propagated progeny differed little, if at all, from their seed-propagated parents in LA (no differences for 2Frax9 and 2Frax10 and a difference of 6° for 2Zelk4), whereas for deep-rooted genotypes, cutting-propagated progeny always differed from their seed-propagated parents (12.1° for 2Frax8, 21.7° for 3Frax13, and 20° for 2Zelk9) by having shallower root systems (Table 2). The assessments of tree root architecture used in this study (slope, LA, and IRA) clearly support the ideas that: 1) there are genetic differences among species related to root architecture (Hamilton 1984a; Oyanagi et al. 1991; Rubio et al. 2003); and 2) ©2008 International Society of Arboriculture root systems develop and function by responding, within a ge- netically predetermined range, to their environment (Clausnitzer and Hopmans 1994; Rubio et al. 2003). The research site for this study was carefully chosen and tree management, especially ir- rigation, was performed so as to minimize influences of soil texture and depth and water availability that can influence root development (Perry 1982; Feldman 1988; Coutts and Nicoll 1991; Nakamoto 1994; Jourdan et al. 2000; Rubio et al. 2003). However, with the present information, the relative importance of inherent, genetic versus external, environmental factors can- not be pinpointed (Jourdan et al. 2000). Firn and Digby (1997) hypothesized that roots possessed a mechanism that allows them to attain a stable, gravitropic position (gravitropic set point [GSA]) and that the GSA can be developmentally changed and/ or regulated by environmental factors. This research is the first to make direct comparisons of root systems between seed-propagated trees and their cutting- propagated progeny and may indicate that the process of veg- etative propagation through adventitious root formation has pro- found influences over the inherent root architecture of the tree, perhaps by altering the roots’ responses to gravitropic influences. Others have noted that adventitious roots tend to be more hori- zontal than seminal roots (Kolesnikov 1930; Serebryakov 1962; Külla and Lõhmus 1999; Miller et al. 2003; Tsutumi et al. 2004). In this study in which relatively high concentrations of auxin were used to enhance the formation of adventitious roots, it is not possible to determine whether it was the auxin treatment, the de novo formation of adventitious roots from stem cells, or a com- bination of both that led to the decreased gravitropic response. In work with tea (Camellia sinensis [L.] Kuntze), Masataka et al. (1997) found that adventitious roots from cuttings grew more horizontally than seedling seminal roots. Their anatomic analy- ses showed that adventitious root cap cells contained fewer amy- loplasts than seminal roots and this was the reason for the dif- ferent gravitropic responses. If cutting-propagated trees do tend to have shallower root systems than individuals propagated by seeds, one may want to seriously consider not growing trees known to be propagated by stem cuttings in close proximity to urban infrastructure such as sidewalks, curbs, and streets. With a better understanding of the genetic and environmental influences on the gravitropic responses of roots of woody perennials, im- proved solutions to root system–urban infrastructure conflicts may be found. Acknowledgments. We acknowledge the help of P.A. Kiehl for her help in conducting the original tree survey, Z. Taylor for his help in con- structing the seedling tree root models, Rob Gross (DendroTech, Inc.) Root type Shallow Deep Shallow Deep Seed propagated 50.8 a 52.7 a 48.7 a 38.0 a 50.2 a 37.6 a LA (°)IRA (°) Cutting propagated 48.8 a Seed propagated –28.9 a 51.4 a 60.8 b 59.7 b 56.2 b 57.6 b –44.5 a –49.6 a –41.0 a –29.2 a –39.6 a Cutting propagated –26.6 a –25.0 b –20.9 b –17.0 b –21.3 b –14.6 b
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