212 Precival and Noviss: Penconazole Induced Heat Tolerance Arboriculture & Urban Forestry 2010. 36(5): 212–220 Penconazole Induced Heat Tolerance in Scots Pine (Pinus sylvestris) and Evergreen Oak (Quercus ilex) Glynn C. Percival and Kelly Noviss Abstract. The ability of penconazole, a triazole fungicide derivative, to protect against and ameliorate heat stress was studied in evergreen oak (Quer- cus ilex) and Scots pine (Pinus sylvestris). Under laboratory conditions, heat damage to the leaf photosynthetic system based on the stability of the chlorophyll a/b light-harvesting complex within photosystem II (chlorophyll fluorescence Fo responses) and leaf photochemical efficiency (chlorophyll fluorescence Fv/Fm emissions) of detached leaves was constantly less in penconazole treated trees. In both species, greatest protection of the leaf photosynthetic system to heat induced disorders was achieved by application of penconazole at a concentration of 30 g per liter of water compared to penconazole applied at a concentration of 0.15 or 0.45 g per liter of water. Subjecting containerized trees of both species to 10 minutes at 50°C significantly reduced tree vitality with respect to chlorophyll fluorescence Fo and Fv/Fm emissions, total foliar chlorophylls, leaf photosynthetic rates (Pn) and significantly increased damage to cellular membrane integrity as manifest by higher leaf electrolyte leakage and visual leaf necrosis between stressed and non-heat stressed well-watered trees. The influence of penconazole applied immediately after heat stress on the pattern of recovery over the following twelve weeks demonstrated penconazole treated trees were the most capable of recovery. With respect to chlorophyll fluorescence Fo and leaf electrolyte leakage values recovery rates of heat damaged trees treated with penconazole ranged from 20%–50% higher than non-triazole treated control trees. In all cases non-penconazole treated control trees had the least capacity for recovery. Regardless of species, height, leaf area, root, shoot, and total plant dry weight were, in virtually all instances, greater than non-penconazole treated controls. The tactical use of the triazole deriva- tive penconazole as an ameliorant against heat damage and recovery from heat stress in Scots pine and evergreen oak would be of benefit to improve tree recovery rates and growth. From a practical point of view penconazole at 30 g a.i. per liter of water is suggested based on the results of this study. Key Words. Chlorophylls; Chlorophyll Fluorescence; Electrolyte Leakage; Fungicides; Growth Inhibitor; Physiogenic Stress; Stress Enzymes. Higher temperatures in urban environments caused by the lack of transpirational cooling, heat convection, and long wave ra- diation from nonvegetative surfaces (e.g., buildings, roads) can be detrimental to the biology of trees (Kolb and Robberecht 1996; Ladjal et al. 2000). Under prolonged heat stress, high temperature injury is primarily manifest by leaf and wood des- iccation inducing water stress throughout the canopy (Sprugel et al. 1991). With prolonged desiccation, tree limbs and trunks can break and shed prematurely; a phenomenon known as sum- mer branch drop, which appears to be linked to internal wa- ter stress (Harris et al. 2004). Such a response is undesirable in highly populated urban areas. Photosynthesis is one of the most heat-sensitive processes in plant cells, leading to numer- ous changes within the structure and function of the photosyn- thetic apparatus (Georgieva et al. 2000). Within the photosyn- thetic system it has been recognized that photosystem II (PSII) is the most thermally liable component of the electron transport chain (Cajanek et al. 1998). Among partial reactions of PSII, the oxygen evolving complex is particularly heat sensitive (Georgieva et al. 2000). As higher rates of photosynthesis are recognized as a physiological characteristic conferring robust- ness during periods of stress, a reduction in leaf photosynthetic rates, or damage to the leaf photosynthetic system can reduce leaf photosynthetic productivity (Percival 2005). Consequent- ly, heat stress can limit the amount of carbohydrates available for growth and maintenance, and subsequently reduce nutri- ©2010 International Society of Arboriculture ent and water uptake resulting in leaf chlorosis, dieback, and plant death (Georgieva et al. 2000; Ladjal et al. 2000; Percival et al. 2006). Drought conditions recorded in 2003, 2004, and 2006 coupled with hose pipe bans in the South of England, and most climate models predicting hotter drier summers for the UK, have meant heat-related tree disorders may become even more prominent within urban landscapes (Percival et al. 2006). Improving hardiness against heat stress may ensure greater post-planting survival of newly-planted trees and increase lon- gevity of established ones. Consequently, finding exogenous chemicals that enhance tree stress tolerance may prove im- portant under prolonged heat conditions (Still and Pill 2004). Penconazole forms the active ingredient of a triazole fungi- cide commonly used within the horticultural, agricultural, and forestry industries for foliar pathogen control (Kenyon et al. 1997; Schnabel and Parisi 1997). Fungicidal properties are via inhibition of the C4-demethylase reactions in fungal sterol bio- synthesis (Allingham 2005). However, many triazole based fun- gicides such as penconazole have been shown to induce a suite of morphological and physiological adaptations that allow plants to tolerate a broad range of environmental stresses to include drought, herbicide, and elevated temperatures (Fletcher et al. 1986; Kraus and Fletcher 1994; Jaleel et al. 2006; Percival and Noviss 2008). Most studies have focused on the growth inhibitor paclobutrazol as a pre-stress treatment to protect plants against drought, herbicide and waterlogging (Asare-Boamah et al. 1986;
September 2010
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