Arboriculture & Urban Forestry 33(6): November 2007 Arboriculture & Urban Forestry 2007. 33(6):397–409. 397 Paclobutrazol-induced Drought Tolerance in Containerized English and Evergreen Oak Glynn C. Percival and Ali Mohammed Salim AlBalushi Abstract. The influence of paclobutrazol (PBZ) applied as a foliar spray and root drench on drought tolerance and recovery from drought, of containerized English and evergreen oak was investigated. PBZ treatment induced a suite of physiological adaptations that would allow both species to tolerate drought; more specifically, increased total leaf content of carotenoids (lutein:-carotene:neoxanthin:-carotene) and xanthophylls (zeaxanthin:antheraxanthin:violaxanthin), chlorophylls, pro- line, superoxide dismutase, and catalase. In addition, PBZ strengthened leaf membrane integrity and increased leaf photosynthetic efficiency and light-induced CO2 fixation before and at the cessation of the drought treatment. Irrespective of species, recovery rates of droughted trees treated with PBZ were 20% to 50% higher than non-PBZ-treated trees. In all cases, control trees (non-PBZ-treated) had the least capacity for recovery. Application of some of the PBZ treatments induced overregulation of newly emerged leaves. Results of this investigation indicate applications of the growth inhibitor PBZ either as a foliar spray or root drench induce a suite of physiological adaptations that confer a useful degree of drought tolerance and aid in the recovery from drought-induced damage. It is suggested that PBZ-induced protection of both English and evergreen oak from damage caused by drought stress is mediated by increased antioxidant enzyme and pigment activities. Key Words. Carotenoids; chlorophyll fluorescence; chlorophylls; electrolyte leakage; growth inhibitor; proline; Q. Ilex; Quercus robur; stress enzymes; transplanting. The establishment of trees after planting in urban environ- ments is difficult because of the harsh environmental condi- tions that exist. These include soil compaction caused by road surfacing, machinery, and pedestrians (Craul 1994); air and soil pollutants (oxides of S and N, SO2, polychlorinated bi- phenyls); heavy metals (Zn, Cu, Cd); atmospheric particu- lates; ozone; ethylene; deicing salts; and water stress (drought, waterlogging). Such stresses contribute to tree de- cline through reduced translocation of photosynthates to roots, impaired stomatal function, premature leaf senescence, clogged lenticels, branch dieback, stem/trunk lesions, leaf scorch, tip burn of conifers, and eventual tree death (Percival et al. 2006). During the first few years of establishment, water is vital to trees for survival. Plant cells are killed or injured by des- iccation resulting from moisture lost by transpiration through leaves as the stomata that facilitate photosynthetic gaseous exchange also allow water vapor to escape (Mooney 1982). According to Watson (2001), “maintaining favorable water status is crucial for successful establishment of newly planted trees in the landscape.” Because trees are grown in per- fect conditions in nurseries, when they are planted outside into harsher urban environments, it is difficult for a tree to adapt. Consequently, the percentage of survival after out- planting has often been low. Improving hardiness against drought before outplanting may ensure greater postplant- ing survival of newly planted trees. This in turn will re- duce labor and replacement cost. Hence, finding exogenous chemicals that improve a plant’s stress tolerance may save trees during severe drought conditions (Still and Pill 2004). Paclobutrazol (PBZ) is a triazole-based plant growth regu- lator developed by Imperial Chemical Industries, now known as Zeneca and sold under the trade name “Cultar,” for use on fruit trees and “Bonsi” for use on ornamentals in the United Kingdom (Fletcher et al. 1986). “Cultar” is formulated in Europe as a suspension concentrate that can be applied as a spray or soil drench treatment. The retardant activity is not accompanied by phytotoxicity or scorch, even when applied at higher rates (Dalziel and Lawrence 1984). The principal mode of action of PBZ is through inhibition of gibberellin biosynthesis (Dalziel and Lawrence 1984). PBZ is readily taken up through the roots, stems, and leaves but is trans- ported almost exclusively in the xylem to its site of action, the subapical meristem, where it has a persistent effect (Anon, 1984). ©2007 International Society of Arboriculture
November 2007
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