Arboriculture & Urban Forestry 32(2): March 2006 47 bath (Fisher Isotemp, Indiana, PA) at 80°C (176°F) for 1 hr to induce cell rupture. The vials were again placed on the Innova 2100 platform shaker for 20 hr at 21°C (70°F), and final conductivity was measured for each vial. The percent- age of electrolyte leakage for each bud was calculated as: (initial conductivity/final conductivity) × 100. Visual Discoloration Following NaCl treatments, buds not used in the electrical conductivity test were placed in 15 cm (6 in) Petri dishes containing filter paper, moistened with deionized water, sealed with parafilm, and incubated in dark coolers at 21°C (70°F) for 10 to 12 days. One bud per node was then re- moved, cut longitudinally, and examined under a dissecting microscope for tissue discoloration (six buds per species/ NaCl treatment). Buds were rated on outer tissue discolor- ation, inner (primordial) tissue discoloration, and fungal growth. Outer tissue of scaled buds was defined as the outer, lignified bud scales and immediately interior nonlignified bud tissue. Outer tissue for naked buds included the tomentose surface covering of primordial leaves and the outer edge of leaf primordia. Inner tissue was defined as any bud tissue located interior to outer bud tissue. Observations on discol- oration were rated on a scale of 1 to 5 [1 green/yellow tissue, 2detection of light-colored brown tissue (<10% of bud surface area), 3 light-colored brown tissue (>10% of bud surface area), 4 medium-brown tissue (100% of bud surface area), 5 dark-brown to black tissue (100% of bud surface area)]. Fungal growth was rated on a scale of 1 to 3 [1no fungal growth, 2detection of fungal growth (5% to 10% of bud surface area), 3 notable fungal growth (>10% of bud surface area)]. Three or four nodal sections from each species were cut longitudinally and examined for tissue discoloration. Although discoloration and fungal growth sometimes occurred on the nodal ends, little discol- oration occurred near the bud attachment point, suggesting that minimal NaCl uptake had occurred through xylem and phloem tissues. The experimental design within each sampling date was a completely randomized design with a factorial arrangement of treatments. Individual buds were the experimental units. Data were subjected to analysis of variance (ANOVA) pro- cedures and regression analysis (SAS Institute Inc., Cary, NC) (Table 1). Percentage data were also analyzed after arc- sin transformation; results were similar to the untransformed data, so mean results presented are for untransformed data. Mean separations were performed by pairwise t-test compari- sons at P 0.05. Significant treatment effects for electrolyte leakage data identified by ANOVA procedures were subject to a log transformation to achieve a linear regression model for estimated NaCl concentrations corresponding to 50% electrolyte leakage (Figure 2). RESULTS AND DISCUSSION Electrolyte Leakage Measurements of electrolyte leakage from dead and injured cells provided a quantitative estimate of total bud NaCl in- jury. Electrolyte leakage from buds generally increased when NaCl concentrations increased and was dependent on the time of year (dormancy state) and the species (Figure 3). Sodium ions from NaCl displace Ca2+ ions in the plasma membrane of cells, possibly accounting for the increased electrolyte leakage (Taiz and Zeiger 1998). December Collection Daily maximum temperatures preceding the December col- lection ranged from −1°C to 16°C (30°F to 61°F) (Figure 1). The buds were fully dormant (under endodormancy, Lang et al. 1987); no budbreak occurred in any of the cut branch samples. Average electrolyte leakage ranged from 28% in buds treated with deionized water to 68% in buds treated with 64,000 mg/L (64,000 ppm) NaCl (Figure 3A). Buds of Norway maple, littleleaf linden, and wayfaringtree viburnum had a significant quadratic response to NaCl (P 0.0001). A log transformation of significant electrolyte leakage data was conducted to achieve a linear regression model that estimated NaCl concentrations corresponding to 50% electrolyte leak- age (Figure 2). Tissues having electrolyte leakage measure- ments greater than 50% are generally considered dead (Shirazi and Fuchigami 1993). However, because the relation between electrolyte leakage and bud survival could differ among species and tissue type (inner bud tissue or outer bud tissue), NaCl concentrations at 50% electrolyte leakage were regarded as severely injurious and potentially lethal but may not represent a lethal dose (LD). Table 1. Analysis of variance (ANOVA) for electrolyte leakage and inner and outer tissue discoloration for buds collected in December, January, and March. Electrolyte leakage 4 Dec. 2001 Species NaCl Species × NaCl 0.001 30 Jan. 2002 12 March 2002 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 0.002 0.881 0.051 Inner tissue discoloration 4 Dec. 2001 30 Jan. 2002 12 March 2002 <0.0001 <0.0001 <0.0001 0.001 <0.0001 <0.0001 0.445 <0.0001 Outer tissue discoloration 4 Dec. 2001 0.101 30 Jan. 2002 12 March 2002 <0.0001 <0.0001 <0.0001 0.002 <0.0001 <0.0001 0.009 <0.0001 ©2006 International Society of Arboriculture
March 2006
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