406 Percival and AlBalushi: Paclobutrazol-induced Drought Tolerance in Containerized Oak Stem electrolyte leakage indicates tissue damage resulting in the loss of cell membrane semipermeability (Cameron and Dixon 1997). The principle of electrolyte leakage is that the movement of cell contents to and from cells is controlled mainly by the structural proteins present at points along the lipid bilayer of the cell membrane. When healthy plant tissue is immersed in ion-free water, there is a slight leakage into the surrounding water, which can be detected and quantified using a conductivity meter. If the cell membrane is ruptured or the transmembrane protein pumps impaired, the cell con- tents leak at a greater rate. Indeed, levels of damage to plant tissue caused by drought have been investigated using elec- trolyte leakage (Martin et al. 1987; McKay 1992). In all cases, PBZ-treated plants maintained a lower degree of elec- trolyte leakage, i.e., higher degree of membrane integrity, than the respective controls at the cessation of 3 weeks of droughts. In addition to reduced damage at the cessation of 3 weeks of drought, recovery rates of PBZ-treated plants ranged from 20% to 50% higher than non-PBZ-treated trees. Few inves- tigations have recorded the potential of PBZ to aid in the recovery of urban trees from drought damage. Indeed, most physiological parameters recorded in this investigation as measures of tree vitality (PI, leaf necrosis, leaf chlorophyll content, electrolyte leakage) were comparable with original levels at day 0 by week 4 to 6 postrecovery indicating regen- eration and full functioning of the leaf photosynthetic struc- ture and chlorophyll molecule. Gilley and Fletcher (1997) observed that almost no decrease in chlorophyll concentration in wheat treated with PBZ compared with a loss of 28% of total chlorophyll after 3 hr at 50°C (122°F) heat stress in the non-PBZ-treated plants. Recently, in a study by Still and Pill (2004) they showed that ‘Marglobe’ tomato plants treated with PBZ either by soaked seeds or sprayed plants have more stress tolerance compared with nontreated plants. They also reported that treated ‘Marglobe’ tomato plants had lower loss of total chlorophyll during the 10-day poststress recovery period compared with nontreated controls. The importance of rapid recovery from stress has been shown elsewhere. Geno- types that rebound to original or near original physiological levels most likely survive and tolerate drought episodes com- pared with those that do not or are slower to recover (Ag- uilera et al. 1997; Bauerle and Dudley 2003). In all cases, control trees (non-PBZ-treated) had the least capacity for re- covery in which PI, leaf necrosis, leaf chlorophyll content, and electrolyte leakage were still 15% to 25% lower at the cessation of the experiment than original basal levels re- corded at day 0. Overregulation of growth as manifest by stunted crinkled foliage is a major disadvantage when using growth inhibitors such as PBZ. In many cases, overregulatory effects on growth can last 2 to 3 years. In this experiment, three of the four ©2007 International Society of Arboriculture treatments, although increasing the drought tolerance and re- covery of both English and evergreen oak, resulted in a sta- tistically significantly smaller trees in terms of height and mean leaf size. Such an effect is undesirable when planting trees into amenity environments where aesthetics are a major factor regarding tree selection. In conclusion, water stress after planting is generally rec- ognized as a major factor resulting in death of newly trans- planted trees (Gilbertson and Bradshaw 1990). 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 high degree of drought tolerance and aid in the recovery from drought-induced dam- age. It is suggested that PBZ-induced protection of both En- glish and evergreen oak from damage caused by drought stress is mediated by increased antioxidant enzyme and pig- ment activities. LITERATURE CITED Aguilera, C., C.M. Stirling, and S.P. Long. 1997. Genotypic variation within Zea mays for susceptibility to and rate of recovery from chill-induced photoinhibition of photosyn- thesis. Physiologia Plantarum 106:429–436. Ain-Lhout, F., M. Zunzunegui, M.C. Diaz Barradas, R. Ti- rado, A. Clavijo, and F. Garcia Novo. 2001. Comparison of proline accumulation in two Mediterranean shrubs sub- jected to natural and experimental water deficit. Plant and Soil 230:175–183. Anon. 1984. Paclobutrazol Plant Growth Regulator for Fruit. I.C.I. Technical Data Sheet. Apel, K., and H. Hirt. 2004. Reactive oxygen species: Me- tabolism, oxidative stress, and signal transduction. Annual Review of Plant Biology 55:373–399. Asare-Boamah, N.K., G. Hofstra, R.A. Fletcher, and E.B. Dumbroff. 1986. Triadimefon protect bean plants from water stress through its effect on abscisic acid. Plant & Cell Physiology 27:383–390. Bañón, S., A. González, E.A. Cano, J.A. Franco, and J.A. Fernández. 2002. Growth, development and colour re- sponse of potted Dianthus caryophyllus cv Mondriaan to paclobutrazol treatment. Scientia Horticulturae 94: 371–377. Bauerle, W.L., and J.B. Dudley. 2003. Genotypic variability in photosynthesis, water use, and light absorption among red and freeman maple cultivars in response to drought stress. Journal of the American Society of Horticultural Science 128:337–342. Cameron, R.W.F., and G.R. Dixon. 1997. Air temperature, humidity and rooting volume affecting freezing injury to Rhododendron and other perennials. The Journal of Hor- ticultural Science & Biotechnology 72:553–562.
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