34 Percival and Keary: Influence of Nitrogen Fertilization on Waterlogging Stresses values were made at the cessation of the 18-day waterlogging period only (Table 1). After 10 days regeneration with one ex- ception [significantly (P < 0.05) reduced leaf area], no signifi- cant effects were recorded compared with European beech trees grown in freely drained pots, although values were constantly lower (Table 3). Waterlogging in slow-release N solutions 14.5 g (0.51 oz) or greater N per liter (0.26 gal) of water significantly (P < 0.05) reduced the shoot:root ratio in the majority of cases (Table 3) indicating greater resource allocation in favor of roots over shoots in both tree species. Energy Fluxes per Leaf Cross-section of Photosystem II The effect of 18 days waterlogging on ABS, TRo, and ETo flux per CS of photosystem II is shown diagrammatically (Figures 1 and 2). At the cessation of the 18-day waterlogging period, light ABS, TRo, and ETo flux per CS of photosystem II were reduced by 10.1% and 5.3%, 20.0% and 361% and 49.2% and 90.1% compared with control values in English oak and European beech, respectively (Figures 1 and 2). Irrespective of species, least reductions in ABS, TRo, and ETo fluxes per CS of photo- system II were associated with N fertilization applied 14.5 g (0.51 oz) or greater N per liter (0.26 gal) of water to the water- logging solution. Dissipation (DIo) fluxes per CS of photosystem II were increased by 37.5% and 63.6% of the control values in English oak and European beech, respectively (Figures 1 and 2). English oak was identified as the most resistant to waterlogging- induced damage with lower percent reductions in light ABS, TRo, and ETo flux per CS of photosystem II compared with European beech indicating less damage to the leaf photosyn- thetic system as a result of 18 days waterlogging (Figures 1 and 2). Likewise, energy dissipation fluxes per CS of photosystem II were lower in English oak compared with European beech. Higher dissipation fluxes per CS of photosystem II indicate a switch from photochemical (light energy absorbed used to pro- vide the chemical energy for CO2 fixation) to nonphotochemical work (heat dissipation), a response indicative of increased res- piration resulting in impairment of the leaf photosynthetic sys- tem. Irrespective of species, least reductions in DIo fluxes per CS of photosystem II were associated with N fertilization ap- plied 14.5 g (0.51 oz) or greater N per liter (0.26 gal) of water to the waterlogging solution. In both English oak and European beech, increasing N content of the waterlogging solution lowered the number of reaction centers per CS of photosystem II that became inactivated. Higher inactivation of the reaction centers associated with non-N-supplemented English oak and European beech indicates an altered metabolic state from photochemical work to nonphotochemical energy dissipation. Experiment 2: Recovery From Waterlogging Stress The pattern of recovery over the next 6 weeks after 18 days waterlogging on chlorophyll fluorescence PI, photosynthetic rates, leaf chlorophyll content, and stomatal conductance is shown for English oak and European beech (Table 4). In addi- tion, the pattern of recovery is shown diagrammatically for both species with respect to leaf chlorophyll content (Figures 3 and 4). Irrespective of treatment, with or without N fertilization, all four parameters began to recover after waterlogging as quantified by quadratic regression analysis to compare the rate of recovery over the next 6 weeks (Table 4). In both English oak and Euro- pean beech, all N-fertilized trees were the most capable of re- covery as reflected by higher regression “b” values with respect to PI, photosynthetic rates, leaf chlorophyll content, and stomatal conductance (Table 4). Greatest recovery rates were recorded in both species fertilized with slow-release N 14.5 g (0.51 oz) or greater N per liter (0.26 gal) of water. Such responses in N- fertilized trees are associated with enhanced recovery of leaf photosynthetic integrity, improved photosynthetic efficiency, re- duced damage of the chlorophyll molecule, and enhanced sto- matal function compared with recovery rates of non-N-fertilized trees. Recovery as determined by chlorophyll fluorescence PI, photosynthetic rates, leaf chlorophyll content, and stomatal con- ductance rates of waterlogged trees treated with an N 14.5 g (0.51 oz) or greater per liter (0.26 gal) of water ranged from 30% to 50% higher than non-N-fertilized trees (Table 4). At the ces- sation of the 6-week recovery period, all values were comparable if not higher than controls (nonwaterlogged trees; Figures 3 and 4) indicating full functioning and regeneration of the leaf pho- tosynthetic apparatus. In all cases, non-N-fertilized had the least capacity for recovery after the cessation of 18 days waterlogging. Chlorophyll fluorescence PI, photosynthetic rates, leaf chloro- Table 4. Nitrogen (N) fertilization enhanced recovery of leaf chlorophyll fluorescence (PI), photosynthetic CO2 fixation (Pn), chlorophyll content, and stomatal conductance of containerized English oak (Quercus robur L.) and European beech (Fagus sylvatica L.) placed outdoors with time (6 weeks) based on quadratic regression analysis after 18 days waterlogging. Treatment PI English oak Control Tapwater y a+bT+cT2 r2 Pn y a+bT+ct2 r2 Chlorophyll content (SPAD) y a+bT+cT2 r2 Stomatal conductance y a+bT+cT2 y 4.80 + 0.10 + 0.00 0.96 y 5.57–0.34 + 0.08 0.93 y 48.11 + 2.70–0.46 0.95 y 0.55–0.04 + 0.01 0.98 y 0.75 + 0.41 + 0.03 0.89 y 1.11 + 0.26 + 0.04 0.78 y 27.16 + 2.38–0.08 0.81 y 0.16 + 0.01 + 0.00 0.96 7.25 g (0.25 oz) N y 0.73 + 0.52 + 0.03 0.89 y 1.24 + 0.27 + 0.06 0.81 y 26.76 + 3.78–0.20 0.78 y 0.16 + 0.05–0.00 0.98 14.5 g (0.51 oz) N y 1.01 + 1.82-0.19 0.85 y 1.26 + 0.58 + 0.03 0.77 y 26.82 + 4.01–0.08 0.88 y 0.15 + 0.11–0.01 0.91 29 g (1.02 oz) N y 0.95 + 1.70-0.14 0.90 y 1.08 + 1.81–0.16 0.76 y 27.73 + 2.78 + 0.18 0.91 y 0.17 + 0.06 + 0.00 0.90 European beech Control Tapwater y 4.33 + 0.02 + 0.01 0.95 y 5.69–0.08 + 0.00 0.90 y 49.66 + 1.83–0.28 0.92 y 0.53 + 0.02–0.00 0.93 y 0.79 + 0.30 + 0.04 0.82 y 1.28 + 0.14 + 0.06 0.81 y 18.50 + 1.50 + 0.51 0.76 y 0.13 + 0.04 + 0.00 0.84 7.25 g (0.25 oz) N y 0.79 + 0.82–0.04 0.88 y 1.35 + 0.67–0.03 0.80 y 18.52 + 2.75 + 0.38 0.81 y 0.15 + 0.04–0.01 0.87 14.5 g (0.51 oz) N y 0.85 + 1.93–0.19 0.78 y 1.27 + 1.33–0.11 0.69 y 17.35 + 3.93 + 0.44 0.65 y 0.12 + 0.07 + 0.01 0.80 29 g (1.02 oz) N y 0.79 + 1.74–0.16 0.75 y 1.11 + 1.88–0.18 0.71 y 18.25 + 6.13 + 0.19 0.81 y 0.13 + 0.07 + 0.01 0.79 PI and leaf chlorophyll content, values mean of mean of ten trees, five leaves per tree. Pn and stomatal conductance mean of ten trees, two leaves per tree. Regression: y, leaf PI, Pn, chlorophyll content, and stomatal conductance; a, leaf PI, Pn, chlorophyll content, and stomatal conductance (estimated calculated intercept); b, rate of leaf PI, Pn, chlorophyll content, and stomatal conductance with time (T); c, rate of change of leaf PI, Pn, chlorophyll content, and stomatal conductance. ©2008 International Society of Arboriculture
January 2008
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