32 Percival and Keary: Influence of Nitrogen Fertilization on Waterlogging Stresses K2HPO4, 0.5% (v/v) TRITON x-100, pH 7.8 at 4°C (39.2°F). After centrifugation [45 min, 16,000 × g, 4°C (39.2°F)], the clear supernatants were separated from low-molecular peptides by passing over a Sephadex G-25 column (Nap-5 Pharmacia; Wa- ters HPLC Solutions, Centennial Park, Elstree, Hertfordshire, U.K.); 50 L of the eluted extract was analyzed in the protein assay based on a reaction of biuret reagent with proteins and a determination of the products at 562 nm by colorimetry. Foliar Nitrogen Content Leaf samples (six leaves per tree) were thoroughly washed and dried in a convection oven at 85°C (185°F) for 48 hr before grinding through a 0.5 mm (0.02 in) cyclone mill (Retsch, Middlesborough, U.K.); 0.5 g (0.02 oz) samples were placed into 150 mL (4.5 fl oz) volumetric flasks and digested in 20 mL (0.6 fl oz) of 7:1 nitric/perchloric acid. After cooling, the solutions were brought to volume with deionized water and analyzed by inductively coupled plasma-emission spectroscopy elemental analysis. Nutrient values were expressed as grams per kilogram dry weight. Dry Weights and Leaf Area At the conclusion of the experiments, trees were destructively harvested. Leaf, shoot, and root dry weight were recorded after oven-drying at 85°C (185°F) for 48 hr. Leaf areas were quanti- fied using a Delta-T area meter (Cambridge, England, U.K.). Experimental Design and Statistical Analysis For both Expts. 1 and 2, the design used was a completely randomized block design in which pots were rerandomized on a weekly basis. Trees were placed at 1 m (3.3 ft) spacing to pre- vent competition for light. Data for each species was analyzed independently by analysis of variance using the Genstat V (Com- mittee of the Statistics Department, Rothamsted Experimental Station, Harpenden, Hertfordshire, U.K.) program for Windows after checks for normality and equal variance. When significant differences occurred, the means were separated by least signifi- cant difference test ( 0.05 significance level). In Expt. 2, because measurements over time were obtained from the same plant, the relationship between chlorophyll fluorescence and SPAD readings over time after N fertilization treatments was quantified using repeated measures analysis (quadratic regres- sion). RESULTS Irrespective of treatment, no tree mortalities were recorded throughout this investigation. Experiment 1: Tolerance to Waterlogging Stress Regardless of species, at the cessation of an 18-day waterlogging period in the absence of N fertilizers, chlorophyll fluorescence PI, photosynthetic rates, leaf chlorophyll content, stomatal con- ductance, leaf and root protein concentration, and foliar N con- tent values were significantly lower (P < 0.05) than trees grown in freely drained pots, i.e., controls (Table 1). The effects of 18-day waterlogging on growth measurements taken at the ces- sation of the waterlogging period and after a 10-day regeneration period did not significantly differ (Student’s t test). Conse- quently, values were pooled for statistical purposes (Table 3). In all cases, growth as measured by leaf area, shoot, root, and total plant dry weight of both test species was significantly lower (P < 0.05; Table 3) in trees waterlogged with tapwater compared with freely drained controls. Effects on the shoot:root ratio dif- fered between species. English oak waterlogged with tapwater significantly enhanced the shoot:root ratio (Table 3); however, no significant effect on the shoot:root ratio was recorded in European beech (Table 3). Waterlogging in a slow-release N solution at 7.25 g (0.25 oz) N per liter (0.26 gal) of water induced similar effects as those recorded by waterlogging in tapwater only, i.e., a significant reduction (P < 0.05) in the majority of tree vitality and growth parameters at the cessation of the 18-day waterlogging period and after a 10-day regeneration phase. Ex- ceptions to this include chlorophyll fluorescence PI, leaf chlo- rophyll content, and foliar N content in English oak at the ces- Table 1. The effects of 18 days waterlogging with and without nitrogen (N) fertilization on chlorophyll fluorescence (PI), photosynthetic CO2 fixation (Pn), leaf chlorophyll content, stomatal conductance leaf and root protein concentration, and foliar N content of English oak (Quercus robur L.) and European beech (Fagus sylvatica L.).z PI Pn English oak Control Tapwater 7.25 g (0.25 oz) N 14.5 g (0.51 oz) N 29 g (1.02 oz) N LSD European beech Control Tapwater 7.25 g (0.25 oz) N 14.5 g (0.51 oz) N 29 g (1.02 oz) N LSD Measurements were made immediately at the cessation of the waterlogging period. Trees were situated outdoors subject to natural climatic conditions. FWfresh leaf weight. PI and leaf chlorophyll content, values mean of mean of six trees, five leaves per tree. Pn and stomatal conductance values mean of mean of six trees, two leaves per tree. Leaf, root protein, and foliar N content mean of six trees, two leaves per tree were used and leaves pooled to provide a mean value per tree. z *Significantly different from controls according to least significant difference (LSD) at P < 0.05. ns not significantly different from control value. ©2008 International Society of Arboriculture 4.3 1.0* 3.3ns 3.9ns 4.0ns 1.55 4.2 0.7* 1.4* 2.8* 3.1ns 1.18 5.7 1.0* 2.9* 4.5ns 4.2ns 2.28 5.8 1.5* 2.0* 2.4* 3.0* 1.56 Chlorophyll content (SPAD) 48.6 28.3* 34.6ns 42.5ns 44.1ns 16.55 48.9 18.7* 21.8* 30.2* 27.6* 14.75 Stomatal conductance (g) 0.48 0.18* 0.30* 0.38ns 0.39ns 0.161 0.53 0.13* 0.19* 0.27* 0.22* 0.129 Leaf protein concentration (mg/g FW) 55.0 24.3* 33.7* 43.1ns 47.8ns 18.90 33.4 18.7* 20.5* 26.5ns 27.2ns 10.69 Root protein concentration (mg/g FW) 22.1 10.3* 13.8* 17.5ns 16.8ns 7.54 14.8 4.3* 5.8* 8.9* 9.3* 3.78 3.17 1.81* 2.25ns 2.78ns 2.90ns 1.15 3.00 1.23* 1.40* 2.59ns 2.38ns 0.900 Foliar N content (%)
January 2008
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