Journal of Arboriculture 31(5): September 2005 217 Salt Treatments. The salinity treatment consisted of 60 g/L (8 oz/gal) sodium chloride distilled water solution (6% NaCl solution) applied as a spray to the foliage of trees until runoff. Herbicide Treatments. Foliar sprays of the herbicides atrazine (trade name Alpha Atrazine 50 SC) and DCMU (3- (3,4-dichlorophenyl, trade name Diuron) were applied until runoff using a hand-sprayer at a concentration of 200 μM. Heat Treatment. Heat damage to leaf foliar tissue was achieved by gently lifting five trees per species previously potted into 4.5 (1.2 gal) containers in 2001 from the ground and placing whole trees in darkness in a Merck environmental growth chamber for 5 min at a temperature of 50°C (122°F). Trees were then returned to the trial site and gently replanted into their original planting hole. Chlorophyll Fluorescence. Fluorescence values were obtained by placing leaves in darkness for 30 min by attaching light-exclusion clips to the leaf surface of whole trees; chloro- phyll fluorescence was measured using a HandyPEA portable fluorescence spectrometer (Hansatech Instruments Ltd., King’s Lynn, UK). Measurements were recorded up to 1 s with a data acquisition rate of 10 μs for the first 2 ms and of 1 ms thereaf- ter. The fluorescence responses were induced by a red (peak at 660 nm) light of 1,500 μmol m2 /s photosynthetically active radiation (PAR) intensity provided by an array of six red light– emitting diodes. Fluorescence values recorded were as follows: 1. Minimal fluorescence, or Fo, as a measure of the stability of the light-harvesting complex (Yamada et al. 1996). 2. The ratio of variable (Fv = Fm – Fo) to maximal (Fm) fluorescence (Fv/Fm, which represents the maximum quantum yield of photosystem II), which in turn is highly correlated with the quantum yield of net photosynthesis (Demmig and Björkman 1987; Bolhar- Nordenkampf et al. 1989; Adams et al. 1995). 3. OJIP curves to monitor effects on plastoquinone electron acceptor reactions, plastoquinone pool size, and heterogeneity of photosystem II (shown on a logarithmic scale to allow visualization of the complete fluorescence transient) (Kruger et al. 1997; Srivastava et al. 1998; Lawlor 2001). All values were automati- cally calculated and graphed by the HandyPEA. Statistical Analysis Treatment effects on chlorophyll fluorescence were deter- mined by one-way analyses of variance (ANOVA) following checks for normality and equal variance distributions. Data for each species were analyzed independently. Differences between treatment means were separated by the least significant difference (LSD) at the 95% confidence level (P > 0.05) using the Genstat V program. RESULTS AND DISCUSSION Fv/Fm measurements from 0.78 to 0.85 are associated with healthy, nonstressed deciduous and evergreen trees (Demming and Björkman 1987; Maki and Columbo 2001; Percival 2004). Fv/Fm values of all species tested were within this range at day 0, indicating that all trees tested in this investigation were healthy at the onset of the experiment and not suffering from any prior stresses (Figures 3a–3d). Although the magnitude of the stress response and recovery from stress differed among species, effects on chlorophyll fluorescence followed similar trends in re- sponse to salt, heat, and herbicide damage; namely, Fv/Fm significantly decreased (P < 0.05) and Fo values signifi- cantly (P < 0.05) increased (Figures 2a–3d). Alterations to these fluorescence expressions represent damage to the photosystem II reaction centers and reduced photosynthetic capacity and photochemical efficiency (Yamada et al. 1996; Willits and Peet 2001) and concomitant damage or impair- ment of the leaf photosynthetic system and reduced photosynthetic CO2 fixation (Meinander et al. 1996; Lazar et al. 1997; Lu and Zhang 1998; Pospisil et al. 1998; Percival and Henderson 2003). Salt damage to leaf tissue of woody plants includes disrupted stomata, collapsed cell walls, disorganized or disintegrated protoplasts, coarsely granular cytoplasm, disintegrated chloroplasts and nuclei, and disorganized phloem (Dobson 1991). In this investigation, maximal damage to the leaf photosynthetic system (greatest reduc- tions in Fv/Fm and increases in Fo) were recorded between weeks 3 and 5 (Figures 2a and 3a). Reductions in Fv/Fm at week 5 post-treatment indicated species salt tolerance in the order evergreen oak > red oak > English oak. From week 5 onward, a steady recovery of the leaf photosynthetic system was recorded in all species until the cessation of the experiment (week 14). In the case of red and evergreen oak, Fv/Fm and Fo values at week 10 were comparable to those at day 0, indicating full recovery of the leaf photosynthetic system from salt damage. In the case of English oak, values were not comparable until week 14, indicating a longer time period is necessary for recovery from salt damage (Figures 2a and 3a). Results of this investigation show that application of salt stress decreased the area of the OJIP curve but did not markedly change the shape of the curve affecting both the O–I and I–P rises equally (Figure 4). For reasons of clarity, effects on the OJIP curves for each oak species are shown at day 0 and weeks 5 and 10. Interpretation of this response means salinity reduced electron transfer to the plasto- quinone pool and plastoquinone pool size. Ultimately, these detrimental effects are manifest by reduced photosynthetic metabolism (Koves Pechy et al. 1998), photosynthetic ©2005 International Society of Arboriculture
September 2005
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