Arboriculture & Urban Forestry 36(5): September 2010 Fletcher et al. 2000; Marshall et al. 2000). Paclobutrazol-induced reductions in plant growth are well documented (Watson 2001). Pa- clobutrazol blocks gibberellic acid biosynthesis, more specifically the pathway at the ent-kaureno oxidation stage. Two to three-year growth over-regulation has been associated with PBZ applications that may be undesirable for landscape planting where aesthetics are a major factor regarding tree selection (Fletcher et al. 2000; Percival 2008). Such long-term over-regulatory growth responses are not associated with penconazole. The influence of penconazole on enhancing the heat tolerance of woody plants has received limited investigation. Consequently, the aims of this study were: * To determine the effectiveness of penconazole applied as a foliar spray at a range of concentrations on improving the fo- liar heat tolerance of evergreen oak (Quercus ilex) and Scots pine (Pinus sylvestris) under laboratory conditions using detached leaf material. * To determine the ability of penconazole to aid whole tree recovery from heat-induced damage. MATERIALS AND METHODS Plant Material Evergreen oak and Scots pine were selected for experimental pur- poses because of their importance in the amenity and forestry industry respectively. Four-year-old cell grown stock ca. 103.5 ± 15.4 cm and 110.0 ± 11.3 cm high were used (Alba Trees, Lower Winton, East Lothian, Scotland). Six months prior to experiments trees were potted into 10 liter plastic pots filled with loam soil (24% clay, 45% silt, 31% sand, 3.1% organic carbon, pH 6.2), supplemented with the controlled release N:P:K (29:7:9) fertil- izer (Bartlett BOOST, The Doggett Corporation, Lebanon, New Jersey, U.S.) at 1 g per kg of soil. Following potting, trees re- mained outdoors subject to natural environmental conditions and watered as required. Studies were conducted at Reading Univer- sity, Earley Gate research site glasshouses (51°43N, -1°08W). Triazole Application Foliar sprays of penconazole (Trade name Topas, United Agri- Products, Alconbury Weston, Huntingdon, UK) were applied until runoff (ca. 50 ml per tree) using a hand-held sprayer. The influence of concentration was determined by spraying 10 trees at 0.15, 0.30, or 0.45 g active ingredient per liter of wa- ter. Ten trees were sprayed once at each concentration. Non- triazole-treated trees acted as controls. Plants were left for two weeks under glasshouse conditions (22 ± 2°C), supple- mented with 400W SON/T high pressure sodium lamps (TLC Electrical, St Philips, Bristol, UK), providing a 16 hour pho- toperiod and minimum 250 mmol m-2 s-1 photosynthetically ac- tive radiation at the tree crown to promote absorption and up- take of each triazole derivative. During this period, plants were watered until container runoff generally every three days. Foliar Tolerance to Heat Stress Under Laboratory Conditions At week 2 after spraying, five fully expanded nonsenescing leaves per tree were excised at the base of the petiole using a razor blade and placed abaxial surface down, in a petri dish on moist Watmans filter paper sealed with a thin polythene film permeable to air but 213 not water prior to placing in darkness in a Merck Environmental Growth Chamber for 5, 10, 15, and 20 minutes at a temperature of 50°C. All leaf material was prepared within two hours of collection. Recovery of Containerized Trees from Heat Stress Heat stress was induced by placing containerized stock in a Merck Environmental Growth Chamber for 10 minutes at a tem- perature of 50°C. At the cessation of the heat period a number of physiological measurements were made on leaf tissue to measure tree vitality. Immediately following these measurements, trees were sprayed until run-off with penconazole at 0.15, 0.30, or 0.45 g active ingredient per liter of water. Ten trees were sprayed once at each concentration. Nontriazole treated trees acted as controls. Trees were then placed outdoors subject to natural weather con- ditions. Recovery rates were measured at three weekly intervals (June 26, July 17, August 7, August 28) over a 12-week period, and growth recorded at week 12 (August 28). A well-watered group of non-heat stressed trees were used for comparative evaluation of tree vitality and growth under outdoor conditions. Tree Vitality Measurements All vitality measurements were taken on leaf material present on the plant at the initiation of the experiment (existing leaves). Dur- ing recovery from heat induced damage new leaf formation was observed at ca. weeks 6–7 on both nonpenconazole-treated con- trol and penconazole-treated trees. No tree vitality measurements were taken on any newly formed leaf tissue (i.e., new leaves not present at the time of triazole application). If abscission or mortality of tagged leaves occurred the nearest adjacent leaf was tagged for future measurements. At each sampling date, five leaves per tree selected throughout the crown were used for chlorophyll fluores- cence and chlorophyll content measurements. Leaves were then tagged to ensure the same leaf was measured throughout the study. Chlorophyll Fluorescence Chlorophyll fluorescence was used as a measure of damage to the leaf photosynthetic system. Leaves were adapted to darkness for 30 minutes by attaching light exclusion clips to the leaf surface and chlorophyll fluorescence was measured using a HandyPEA portable fluorescence spectrometer (Hansatech Instruments Ltd, King’s Lynn, UK). Measurements were recorded up to 1 second with a data acquisition rate of 10ms for the first two milliseconds and of one millisecond thereafter. The fluorescence responses were induced by a red (peak at 650nm) light of 1500 mmol m-2 s-1 PAR intensity provided by an array of six light emitting diodes. The ratio of variable (Fv = Fm-Fo) to maximal (Fm) fluorescence (i.e., Fv/Fm where Fo = minimal fluorescence), of dark-adapted leaves was used to quantify the detrimental effects of heat on leaf tissue. Fv/Fm is considered a quantitative measure of the maxi- mal or potential photochemical efficiency or optimal quantum yield of photosystem II (Willits and Peet 2001). Likewise Fv/Fm values are the most popular index used as a measure of plant vi- tality and early diagnostic of stress (Maxwell and Johnson 2001). Alterations with Fo values in leaf tissue are associated with dissociation of the light-harvesting chlorophyll a/b complexes from the reaction centre complex of photosystem II and/or al- terations to the xanthophyll cycle-dependent nonradiative energy dissipation process (Hong and Xu 1999; Yamane et al. 2000). ©2010 International Society of Arboriculture
September 2010
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