398 Percival and AlBalushi: Paclobutrazol-induced Drought Tolerance in Containerized Oak Paclobutrazol has been shown to reduce the shoot growth of many tree species (Davis et al. 1985) and is commonly used on trees under utility lines (Watson 1996). According to Watson (2001), the application of PBZ as soil drench treat- ment at planting time may be able to stimulate root elongation and reduce water stress of trees after transplanting. A further study by Watson (2001) reported that after 10 weeks of drought stress, stem water potential of elms treated with PBZ was the same as the well-watered controls. Indeed, a number of workers conclude that PBZ can be used as a pretreatment to protect plants against drought and increase the percentage of plant survival after transplanting in semiarid climatic con- ditions (Asare-Boamah et al. 1986; Fletcher et al. 2000; Mar- shall et al. 2000). In most cases, PBZ-induced drought toler- ance has been associated with a decrease in transpiration, plant height, biomass, and leaf area and an increase in sto- matal resistance. Because applications of PBZ increased the stress tolerance of woody plants, it can be suggested that pretreatment appli- cation with PBZ would cause an increase in the stress toler- ance of other trees species widely planted into urban envi- ronments. Such a response may reduce outplanting losses with limited capital investment and only small adjustment to management procedure, the cost of which is negligible com- pared with the risk of tree death. The aims of this study are: • To determine the effectiveness and feasibility of PBZ applied as a root drench and foliar spray to improve the drought tolerance of two urban tree species, evergreen oak (Quercus ilex) and English oak (Q. robur); • To investigate the concentration conferring maximal re- sistance to and recovery from drought-induced stress; and • To evaluate PBZ-induced alterations to plant physiology that could contribute to drought tolerance. MATERIALS AND METHODS Plant Material Two plant species were selected for experimental purposes: evergreen oak (Quercus ilex) and English oak (Q. robur). Both species are commonly planted into UK urban environ- ments and provide forms with contrasting drought capabili- ties. Four-year-old cell-grown stock approximately 90 cm (36 in) high ± 8 cm (3.2 in) were obtained from a commercial supplier (Alba Trees, Lower Winton, East Lothian, UK). Six months before experiments, trees were potted into 4.5 L (1.17 gal) plastic pots filled with soil (loamy texture, 24% clay, 45% silt, 31% sand, 3.1% organic carbon, pH 6.2) supple- mented with the controlled-release nitrogen-based N:P:K (29: 7:9) fertilizer, Bartlett BOOST (The Doggett Corp., Lebanon, NJ) at a rate of 1 g/kg soil. After potting, trees remained outdoors subject to natural environmental conditions and wa- tered as required. In early April 2005, trees were moved to a ©2007 International Society of Arboriculture polythene tunnel to protect against possible spring frosts. As soon as the initial symptoms of bud burst were observed, i.e., leaf emergence (late April), trees were placed under glass- house conditions (22°C ± 2°C [71.6°F ± 35.6°F]) supple- mented with 400 W high-pressure sodium lamps providing a photoperiod of 16hr light/8hr dark and minimum 250 mol/ m−2/s−1 photosynthetically active radiation (PAR) at the tree crown until PBZ treatments began. Paclobutrazol Application Paclobutrazol treatments were applied when both tree species were in full leaf (2 May 2005). Foliar sprays of PBZ were applied until runoff using a handheld sprayer. As a root drench, PBZ was applied to the substrate surface until fully saturated, i.e., when liquid was seen emerging from the base of the pots. Drenching with distilled water served as the con- trol. Concentrations of PBZ applied were: 1) control (no ap- plication of PBZ); 2) 2 mL (0.06 fl oz) PBZ per liter water applied as spray until runoff; 3) 4 mL (0.12 fl oz) PBZ per liter water applied as spray until runoff; 4) 5 mL (0.15 fl oz) PBZ per 250 mL (7.5 fl oz) water applied as root drench to achieve a concentration of 0.25 g (0.01 oz) PBZ per 2.5 cm (1 in) girth, approximately 50 mL (1.5 fl oz) per pot; and 5) 10 mL (0.3 fl oz) PBZ per 250 mL (7.5 fl oz) water applied as root drench to achieve a concentration of 0.50 g (0.02 oz) PBZ per 2.5 cm (1 in) girth, approximately 50 mL (1.5 fl oz) per pot. Plants were left for 2 weeks under glasshouse conditions to permit absorption and uptake of PBZ. During this period, plants were watered as required, generally every 3 days. At week 2, after PBZ treatment (16 May 2005), measurements of plant physiology were made. Drought Treatment and Experimental Design Plants were subjected to drought by the cessation of watering for 3 weeks under glasshouse conditions. At week 3 (6 June 2005), a number of physiological measurements were made on leaf tissue as measures of tree vitality. Immediately after measurements, trees were watered as required and placed outdoors subject to natural weather conditions and recovery rates measured over the next 9 weeks at three weekly inter- vals (27 June, 18 July, 8 August). The experimental design used was a completely randomized block design in which pots were rerandomized on a weekly basis. Ten trees per treatment were used at 0.75 m (0.83 ft) spacings to prevent competition for light. Tree Vitality Measurements All tree vitality measurements were taken on leaf material present on the plant at the initiation of the experiment (ex- isting leaves). During recovery from drought, new leaf for- mation was observed at approximately weeks 6 to 7 on both control and PBZ-treated surviving plants. No tree vitality
November 2007
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