278 Al-Habsi and Percival: Sucrose-Induced Tolerance to and Recovery from Deicing Salt Damage tolerance to the electron transport inhibiting herbicide atra- zine in seedling material by maintaining high levels of leaf chlorophylls, photoxidative protecting carotenoid pigments, and photosynthetic efficiency (Sulmon et al. 2004). Work by Vartapetian (1993) demonstrated a positive effect of supple- menting sugars for the survival of plants under anaerobic conditions. Protection of the leaf photosynthetic system has been shown to be important for the survival of plants under harsh environmental conditions. If the leaf photosynthetic apparatus remains unimpaired, then plants are able to produce the essential carbohydrates required for growth and repair of damaged tissue after the cessation of stress. If the leaf pho- tosynthetic system is badly damaged, then the carbohydrates required for repair cannot be synthesized (Levitt 1980; Kitao et al. 1998; Arntz et al. 2000). These studies raise the possibility that the salt tolerance of trees may be enhanced by applying sugars at or around the root zone potentially offering a simple system of reducing salt-induced tree losses in urban landscapes. Such research also has a number of practical advantages to professionals involved in urban tree management because sugars are water- soluble, nontoxic, environmentally safe, and inexpensive to purchase. As a prerequisite to larger tree studies, the objectives of this investigation, using small containerized trees, were to determine the effectiveness and feasibility of sugar (sucrose) to improve tolerance to and recovery from deicing salt (NaCl) damage. MATERIALS AND METHODS The experiment used 4 year old cell grown stock of Quercus robur L. (English oak) and Ilex aquifolium L. (holly) obtained from a commercial supplier. Six months before experiments (early November 2002), trees were potted into 4.5 L (1.17 gal) plastic pots filled with sterilized, i.e., heat and pressure- treated (1 hr at 121°C [249.8°F] and 0.103 MPa) growing medium (loamy texture, 26% clay, 44% silt, 30% sand, pH 6.5) supplemented with the controlled-release nitrogen-based fertilizer Bartlett BOOST (N:P:K 24:7:7; The Doggett Corporation, Lebanon, NJ) at a rate of 1 g/kg compost. After potting, trees remained outdoors on a free-draining gravel surface subject to natural environmental conditions and wa- tered as required. In early April, trees were moved to a poly- thene tunnel to protect against possible spring frosts and placed outdoors once the possibility of frosts had passed (30 April 2003). Sugar and salt treatments began in late May when both species were in full leaf. All physiological mea- surements taken throughout this investigation were made on leaf material present on the plant at the initiation of the ex- periment (existing leaves). Leaves were also tagged to ensure only the same leaf was measured repeatedly. Although new leaf formation was observed in both species between weeks 6 and 7 (experiment 2), no measurements of newly formed leaf ©2006 International Society of Arboriculture tissue were made. Weeds were chemically controlled on the gravel surface using glyphosate before planting and by hand during the trial. Experiment 1: Prevention of Salt-Induced Damage by Prior Application of Sucrose Root drenches of sucrose, obtained from a local supermarket, at a concentration of either 25 g (0.9 oz) or 50 g (1.8 oz) sugar per liter of water were applied. Each tree received 0.5 L (0.13 gal) of sugar solution, a volume deemed sufficient to fully saturate the soil as solution was observed emerging from drainage holes. Seventy-two hours after sugar application, root drenches (0.5 L [0.13 gal] per pot) of NaCl were applied at a concentration of either 30 g (1.1 oz) or 60 g (2.1 oz) NaCl per liter of water. Containerized trees were left outdoors until day 15. During this period, no watering or fertilizers were applied. At day 15, a range of physiological measurements were made on leaf tissue as measures of tree vitality. Experiment 2: Effects of Sucrose on Recovery From Salt Damage Half-liter root drenches of NaCl (30 g [1.1 oz] or 60 g [2.1 oz] NaCl per liter of water) were applied to each containerized tree. Trees remained outdoors until day 14 when symptoms of NaCl toxicity could be visibly observed (leaves yellowing, marginal necrosis, wilting). At this stage, a number of physi- ological measurements were made on leaf tissue as measures of tree vitality. Immediately after measurements, 0.5 L (0.13 gal) root drenches of sucrose at a concentration of either 25 g (0.9 oz) or 50 g (1.8 oz) sugar per liter of water were applied. Sucrose treatments were reapplied at weeks 2, 4, and 6 after NaCl application. At weeks 1, 3, 5, and 7 after NaCl treat- ment, trees were watered as required. Experimental Design The experimental design used was a completely randomized block design (CRBD) in which pots were rerandomized on a weekly basis. Six trees per treatment were used at 1 m (3.3 ft) spacings to prevent competition for light. Physiological Measurements Leaves were adapted to darkness for 30 min by attaching light exclusion clips to the leaf surface and chlorophyll fluores- cence was measured using a HandyPEA portable fluores- cence spectrometer (Hansatech Instruments Ltd., King’s Lynn, U.K.). Measurements were recorded up to 1 sec with a data acquisition rate of 10 s for the first 2 ms and of 1 ms thereafter. The fluorescence responses were induced by a red (peak at 650 nm) light of 1,500 mol m−2 s−1 photosyntheti- cally active radiation (PAR) intensity provided by an array of six light-emitting diodes. A performance index (PI) based on an equation that combines the relationship of calculated rela-
November 2006
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