400 Percival and AlBalushi: Paclobutrazol-induced Drought Tolerance in Containerized Oak tions) equilibrated with 50 mM Na2CO3 buffer (pH 10.2) to remove low-molecular-weight components and exchange the buffer. The assay was performed using 0.1mMdiethylenetri- aminepentaacetic acid in the reaction mixture and the subse- quent increase in absorbance at 550 nm followed on a spec- trophotometer (PU8800 Pye Unicam) equipped with a kinet- ics module for 6 min at 24°C (75.2°F). Catalase Activity A total of 0.3 g (0.11 oz) of foliar tissue (generally five leaves per plant) were homogenized in 5 mL (0.15 fl oz) of 0.2 M Tris-Cl− (buffer pH 7.8) containing 0.13 mM EDTA and 80 M soluble polyvinylpyrrolidone with a Polytron homog- enizer (Glen Mills, Clifton, NJ) for 30 sec on ice and centri- fuged at 3000 × g for 10 min. Catalase activity was deter- mined by following the consumption of hydrogen peroxide (extinction coefficient 39.4 mm−1/cm−1) at 240 nm for 2 min. The reaction mixture contained 2 mL of 100 mM Na2HPO4/ NaH2PO4 buffer (pH 6.5) and 50 L of plant extract and the reaction was initiated by adding 10 L of 30% (w/v) hydro- gen peroxide. Proline Leaf samples (1 g [0.035 oz]) were extracted with 3% sul- phosalicylic acid. Extracts (2 mL [0.06 fl oz]) were held for 1 hr in boiling water by adding 2 mL (0.06 fl oz) ninhydrin and 2 mL (0.06 fl oz) glacial acetic acid after which cold toluene (4 mL [0.12 fl oz]) was added. Proline content was measured by a spectrophotometer (Shimadzu UV 1601; Shi- madzu UK, Milton Keynes, Buckinghamshire, England) at 520 nm and calculated as g/g dry leaf weight against stan- dard proline. Plant Height and Leaf Mean Leaf Area At the cessation of the experiment, trees were destructively harvested and height recorded by measuring the distance from the tip of the leading apical shoot to the soil surface. Mean leaf size was calculated by quantifying total leaf area per tree using a Delta-T leaf area meter (Delta-T Devices, Cambridge, UK) and dividing by the total number of leaves per tree. Statistical Analysis Data for each species was analyzed independently by one- way analysis of variance using the Genstat V program (Com- mittee of the Statistics Department, Rothamsted Experimen- tal Station, Harpenden, Hertfordshire, UK) for Windows. Be- fore the analysis, data were examined for normality and homogeneity of variance (Levene 1960) and data were trans- formed [log (y + 0.5)] when necessary. When significant differences occurred, the means were separated by least sig- nificant difference test (P < 0.05). Survival recorded as per- centage and visual leaf necrosis were analyzed after appro- priate statistical transformation (arcsin[x0.5]). Because mea- surements over time were obtained from the same plant, the ©2007 International Society of Arboriculture relationship between chlorophyll fluorescence and SPAD readings over time after PBZ treatments was quantified using repeated measures analysis (quadratic regression). RESULTS Basal Alterations to Plant Physiology 2 Weeks After Paclobutrazol Application Irrespective of concentration applied and mode of application (foliar spray or root drench), PBZ induced a number of significant (P < 0.05) changes in whole plant physiology of both English and evergreen oak compared with non- PBZ-treated controls (Tables 1 and 2). Leaf membrane integ- rity as assessed by leakiness of leaf tissue was reduced by 14% to 34%, whereas PI and Pn was increased by 3 to 17 and 3% to 18%, respectively, in both test species (Tables 1 and 2). In all cases, PBZ-treated plants had a higher leaf content of total carotenoids and chlorophylls with levels increased by 14 to 25 and 12% to 29%, respectively, compared with non- PBZ-treated controls. Similar percentage increases were re- corded in the leaf content of carotenoids (lutein:- carotene:neoxanthin:-carotene) and xanthophylls (zeaxan- thin:antheraxanthin:violaxanthin), although no effects were found on their ratios (Tables 1 and 2). No PBZ-induced ef- fects were found on the ratio of chlorophyll a/b in comparison with control plants. Irrespective of species, PBZ-treated plants had 4% to 20%, 8% to 31%, and 9% to 37% more proline, superoxide dismutase, and catalase, respectively, than controls. No symptoms of phytotoxicity were observed at week 2 after PBZ application, i.e., no leaf necrosis ob- served on foliage of evergreen oak and leaf necrosis values ranging from 0 to 0.4 in (0 to 0.16 cm) English oak (Tables 1 and 2). Chlorophyll Fluorescence, Light-induced Photosynthetic CO2 Fixation, Membrane Integrity, Leaf Chlorophyll Content, and Necrosis at Week 3 Postdrought Treatment Irrespective of concentration or mode of application (foliar spray, root drench), in all cases, PBZ protected both English and evergreen oak from injury resulting from drought as manifest by reduced mortality in PBZ-treated trees compared with non-PBZ-treated controls (Tables 3 and 4). Such a re- sponse was particularly pronounced in English oak in which all PBZ-treated trees survived a 3-week drought period com- pared with controls in which a significantly lower 20% sur- vival rate was recorded (Table 3). In the case of English oak, PI values as a measure of leaf photosynthetic efficiency, Pn, and leaf chlorophyll content were all significantly (P < 0.05) higher than non-PBZ-treated controls (Table 3). A similar response was recorded in evergreen oak in which in all cases, PI, Pn, and leaf chlorophyll content were always higher than non-PBZ-treated controls, if not significantly so in every in stance. Such a response indicates reduced damage to the leaf
November 2007
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