30 Percival and Keary: Influence of Nitrogen Fertilization on Waterlogging Stresses Experiment 1. Determine the effect of N fertilizers on tree tolerance under prolonged waterlogging conditions; and Experiment 2. Determine the effect of N fertilizers in its ability to aid tree recovery from waterlogging damage. MATERIALS AND METHODS Plant Material The experiment used 5-year-old containerized stock of European beech (Fagus sylvatica L.) and English oak (Quercus robur L.), waterlogging-sensitive and -intermediate, respectively (Kreuzwieser et al. 2002). Trees were supplied bare root in No- vember 2003 and potted into 10 L (2.6 gal) pots remaining outdoors subject to natural environmental conditions during the 2004 growing season and watered as required. All pots were supplemented with a granular N:P:K (29:7:9) fertilizer (BOOST, The Doggett Corporation, Lebanon, NJ, U.S.) applied at a con- centration of 2.8 g (0.1 oz) per kilogram (2.2 lb) of growth media (loamy texture with 23% clay, 46% silt, 31% sand, 3.1% organic carbon, pH 6.6) to provide soil nutrients over the 2004 growing season. To ensure uniformity of stock for experimental purposes, trees were graded in March 2005 and only those conforming to the listed physical dimensions used: European beech (Fagus sylvatica L.) – height 1.2 m (3.96 ft) ± 0.060 m (0.20 ft), girth 2.8 cm (1.12 in) ± 0.14 cm (0.06 in). English oak (Quercus robur L.), height 1.5 m (4.95 ft) ± 0.075 m (0.25 ft), girth 3.1 cm (1.24 in) ± 0.18 cm (0.07 in). Both experiments began 4 May 2005 when all trees were in full leaf. All experiments were conducted outdoors subject to natural environmental conditions. During the experimental pe- riod, the mean minimum and maximum air temperatures were 7.5°C (45.5°F) and 22.7°C (72.9°F), respectively; daily relative humidity, sunshine hours, and rainfall were 69.8%, 7.01 hr, and 1.40 cm (0.6 in), respectively. All studies were conducted at Reading University, Shinfield experimental research site (51°43N, –1°08W). Waterlogging Treatments Experiment 1: Tolerance to Waterlogging Stress Waterlogging was performed by placing the 10 L (2.6 gal) inside larger 15 L (3.9 gal) plastic buckets and flooding with one of four solutions approximately 2.5 cm (1 in) above the growth media surface so that the shoots of the trees were still exposed to air. The solutions used were: 1. Tapwater; 2. Tapwater supplemented with 7.25 g (0.25 oz) N (supplied in the form of BOOST slow-release N fertilizer 29:7:9 N:P:K; The Doggett Corporation) per liter of water; 3. Tapwater supplemented with 14.5 g (0.51 oz) N per liter of water; and 4. Tapwater supplemented with 29 g (1.02 oz) N per liter of water. To achieve these N concentrations, BOOST was applied at 25 g (0.88 oz), 50 g (1.8 oz), and 100 g (3.5 oz) per liter (0.26 gal) of water (treatments 2, 3, and 4), respectively. Inspecting daily and watering when necessary maintained this depth. Trees growing in freely drained pots drenched with tap- water were used as controls. Trees were kept under waterlogged conditions for 18 days, a time period when adverse effects on tree physiology of similar species were visibly observed ©2008 International Society of Arboriculture (Kreuzwieser et al. 2002). At the end of the 18-day waterlogging period, six trees per treatment were destructively harvested. To study the influence of N fertilization on the regenerative poten- tial of the trees after waterlogging, excess water was decanted and six trees per treatment were left in freely drained growth media and watered when required for an additional 10 days, i.e., a total of 12 trees were used per treatment. Experiment 2: Recovery From Waterlogging Stress Waterlogging was performed by placing the 10 L (2.6 gal) inside larger 15 L (3.9 gal) plastic buckets and flooding with tapwater only as outlined in Expt. 1. At the cessation of the 18-day wa- terlogging period, a number of physiological measurements were made on leaf tissue as measures of tree vitality. Immediately after measurements, trees were fertilized with 0.00, 7.25 g (0.25 oz), 14.5 g (0.51 oz), and 29 g (1.02 oz) N per liter of water and placed outdoors subject to natural weather conditions. Recovery rates were measured at two weekly intervals over a 6-week pe- riod. Ten trees per treatment were used and trees grown in freely drained growth media not subject to 18 days waterlogging acted as controls. Tree Vitality Measurements For all measurements, only fully expanded 21- to 28-day-old leaves were measured to keep the physiological age of the leaves comparable. Because leaf chlorophyll fluorescence, Soil Plant Analysis Development (SPAD), and photosynthetic rates mea- surements are noninvasive and nondestructive, all three measure- ments were made on the same leaf. Leaves were tagged to ensure only measurements were taken from the same leaf. During the recovery and regeneration period if abscission or mortality of tagged leaves occurred during the investigation, the nearest ad- jacent leaf was tagged for future measurements. Only leaves present at the onset of the experiment were measured throughout this investigation to record damage to and regeneration of the leaf photosynthetic apparatus during and after waterlogging treatment, respectively. Chlorophyll Fluorescence To characterize tree vitality, chlorophyll fluorescence values were recorded on five leaves from the crown of each tree. Mea- surements at ambient temperature were performed with a Handy Plant Efficiency Analyzer (PEA) chlorophyll fluorometer (Han- satech Instruments, King’s Lynn, Norfolk, U.K.). During the measurement, the sample was shielded from ambient light by a clip system to reach a dark adapted state (30-min adaptation to the dark) and illuminated with 660 nm light from an LED source built into the fluorometer sensor. Continuous light excitation (at 2500 mol/m2/s–1) was provided by an array of six light- emitting diodes focused on the leaf surface to provide homoge- neous irradiation overa4mm (0.16 in) diameter leaf surface. A photosynthetic performance index (PI) based on an equation that combines the relationship of calculated relative number of reac- tion centers (RC) per light energy absorbed (ABS) and then multiplied by two expressions describing the yields of light trap- ping (po) and subsequent electron transport (0) were used to quantify any effects on leaf tissue (Clark et al. 1998): RC/ABS × po/(1-po) × 0/1 – 0) PI values have been shown to be a highly sensitive measure of leaf photosynthetic activities and provide an indirect measure of plant vitality (Clark et al. 2000; Percival and Fraser 2001). The Handy PEA automatically calculated PI values.
January 2008
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