Arboriculture & Urban Forestry 36(5): September 2010 217 Table 8. The influence of penconazole on growth of containerized Scots pine (Pinus sylvestris) at the end of a 12-week recovery period following the cessation of heat induced damage (50°C) for 10 minutes. Parameter Height (cm) Leaf Area (cm2 Root DW (g) Shoot DW (g) Total Plant DW Root:Shoot Ratio ) Nontriazole Treated control 127.6a 665.7a 11.8a 24.3a 36.1a 0.49a Penconazole (0.15g) 130.5a 700.4a 15.0ab 30.2ab 45.2ab 0.50a Penconazole (0.30g) 144.6a 803.5ab 15.8bc 34.5b 50.3b 0.46a All values mean of 10 trees. Lower case letters indicate significant differences in rows between means by LSD at (P < 0.05). DISCUSSION The fungitoxic effectiveness of penconazole against foliar patho- genic fungi such as apple scab and powdery mildew has been confirmed by several workers under laboratory and field condi- tions (Kenyon et al. 1997; Schnabel and Parisi 1997; Percival and Boyle 2005). Consequently, penconazole is fully approved for fo- liar pathogen control under UK pesticide legislation (Anonymous 2010). However, all triazoles, to include penconazole, have been shown to confer stress reducing properties in plants with recorded cases of less drought, freezing, and heat-related plant disorders following pre-treatment with these compounds (Gilley and Fletch- er 1997; Jaleel et al. 2007a; Manivannan et al. 2007; Percival and Noviss 2008). Results of this study support previous work. Foliar treatment of both Scots pine and evergreen oak with penconazole at 0.15 , 0.30, or 0.45 g a.i per liter of water resulted in significant- ly less heat induced damage to the leaf photosynthetic system as determined by chlorophyll fluorescence Fo and Fv/Fm emissions as a measure of stability of the chlorophyll a/b light-harvesting complex within photosystem II and leaf photochemical activity respectively. This result indicates pre-treatment with the triazole derivative penconazole would allow both tree species to tolerate a longer, high temperature episode compared to nonpenconazole- treated trees. Likewise, application of penconazole immediately after the cessation of 10 minutes 50°C heat stress improved re- covery rates of physiological adaptations associated with tree vitality to include chlorophyll fluorescence Fo and Fv/Fm emis- sions, total foliar chlorophylls, leaf photosynthetic rates, cellular membrane integrity and visual leaf necrosis so that by the end of a 12 week recovery period values in some instances were statisti- cally comparable to those of well watered non-heat stressed trees. Application of triazoles such as penconazole have been shown to induce synthesis of a range of stress protective en- zymes and metabolites within plants to include low-molecular mass antioxidants such as carotenoids (carotenes and xantho- phylls), reactive oxygen species (ROS), scavenging enzymes, such as superoxide dismutase, calatase, α-tocopherol, ascorbic acid, and compatible solutes such as proline and glycine beta- ine, which function as osmoprotectants for proteins (Fletcher et al. 2000; Apel and Hirt 2004; Still and Pill 2004; Fernandez et al. 2006; Jaleel et al. 2007b; Jaleel et al. 2008; Percival and Noviss 2008). High activities or naturally inherent concentra- tions of these stress protective compounds are regarded as key physiological characteristics conferring plant tolerance to envi- ronments where sub-optimal growing conditions prevail (e.g., drought, salinity, root de-oxygenation). As a result, penconazole- enhanced concentrations of these compounds would be important contributors to reducing heat damage to the leaf photosynthetic structure (Kraus and Fletcher 1994; Jaleel et al. 2006; Jaleel et al. 2008). Improved tolerance to, and recovery from heat there- fore may be explained in part, by penconazole-induced altera- tions to leaf anti-oxidant, metabolite and enzymatic activity. Chlorophyll fluorescence Fo and Fv/Fm emissions as a mea- sure of leaf chloroplast stability and photochemical efficiency provide an indirect measure of tree vitality (Clarke et al. 2000). Alterations to these values are sensitive indicators of damage to the leaf photosynthetic system caused by environmental stress (Percival and Sheriffs 2002). After heat exposure at fixed time periods, damage to the leaf photosynthetic system based on the stability of the chlorophyll a/b light-harvesting complex within photosystem II (Fo values) and leaf photochemical efficiency (Fv/Fm emissions) in penconazole-treated trees was significantly less than nonpenconazole-treated trees. The maintenance of rela- tively high fluorescence values in triazole treated plants under stress has been observed in previous studies (Pinhero and Fletch- er 1994) with higher values associated with reduced impairment of leaf photosynthetic activities (Percival et al. 2006). Robust- ness of the photosynthetic system is an important physiological trait associated with survivability under prolonged environmental stress conditions to ensure carbohydrate production via photo- synthesis necessary for the growth and repair of damaged tissue. Over a 12-week recover period following a 10-minute heat stress period, vitality of penconazole-treated trees as measured by chlorophyll fluorescence Fo emissions and leaf electrolyte leakage ranged from 20%–50% higher than nonpenconazole– treated trees. Limited studies have recorded the influence of tri- azoles as an aid toward heat-induced stress recovery. Leaf chlo- rophyll Fo and electrolyte leakage values were similar to those of well-watered (i.e., nonstressed trees) by week 12 post-recovery, indicating regeneration and full functioning of the leaf photo- synthetic system and cell membrane integrity. The importance of rapid recovery from stress has been shown elsewhere (Gilley and Fletcher 1997; Still and Pill 2004), with genotypes that re- bound to original or near original physiological levels most likely to survive and tolerate prolonged stress episodes compared to those that do not or are slower to recover (Aguilera et al. 1997; Bauerle and Dudley 2003). In this study, nonpenconazole-treat- ed control trees had the least capacity for recovery because leaf chlorophyll Fo and electrolyte leakage were 45%–55% lower at the cessation of the post-heat 12-week recovery period compared to well-watered trees. Improvements in the range of the previ- ously discussed physiological measurements may account for increased growth as measured by height, leaf area, shoot, root, and total plant dry weight in trees treated with penconazole at the end of a post-heat 12-week recovery period (Martin et al. 1987; ©2010 International Society of Arboriculture Penconazole Well-Watered (0.45g) 132.8a 792.4ab 18.7c 32.9b 51.6b 0.56a 149.3a 855.0b 17.8bc 33.9b 51.7b 0.53a
September 2010
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