218 Percival: Use of Chlorophyll Fluorescence to Identify Oak Stress capacity, and photochemical efficiency (Percival et al. 2003). For the purposes of this investigation, no distin- guishable characteristics were observed with which to identify salt damage as the cause of tree decline using the fluorescence parameters Fo, Fv/Fm, and OJIP curves. Photosynthesis is one of the most heat-sensitive processes in plant cells, leading to numerous changes of the structure and function of the photosynthetic apparatus (Georgieva et al. 2000). Within the photosynthetic system, it has been recognized that PSII is the most thermally labile component of the electron transport chain (Cajanek et al. 1998). Among partial reactions of PSII, the oxygen-evolving complex is particularly heat sensitive (Georgieva et al. 2000). In response to heat, the greatest reductions in Fv/Fm and increases in Fo were recorded at day 1, indicating maximal damage to the leaf photosynthetic system (Figures 2b and 3b). Reductions in Fv/Fm at day 1 post-treatment indicated species heat tolerance in the order evergreen oak > English oak > red oak (Figure 3b). From day 1 onward, a steady recovery of the leaf photosynthetic system was recorded in all species until day 18. In the case of red and evergreen oak, Fv/Fm and Fo values at day 12 were comparable to those at day 0, indicating full recovery of the leaf photosynthetic system from heat damage. In the case of English oak, values were not comparable until day 14, indicating that a marginally longer time period is necessary for recovery from heat damage (Figures 2b and 3b). In response to heat, a new K peak situated at about 200 μs (i.e., OKJIP) was observed in all three oak species (Figure 5). For reasons of clarity, effects on the OJIP curves for each oak species are shown at days 0, 1, and 8 (Figure 5). Previous research has shown that formation of the K peak is linked to inhibition of the PSII oxygen-evolving complex (Lazar et al. 1997). Consequently, observation of the K peak in response to heat damage has been used as a diagnostic measure by which to quantify plants for horticul- tural and agricultural purposes based on their tolerance to elevated temperatures (Guisse et al. 1995; Lazar et al. 1997, 1999; Lazar and Ilik 1997). Higher temperatures in urban environments caused by the lack of transpirational cooling and increased heat convection and long-wave radiation from nonvegetative surfaces such as buildings and roads can be a problem (Montague et al. 1998). Under prolonged heat stress, especially during a hot, dry summer, tree limbs and trunks can break and fall—a phenomenon known as summer branch drop (Harris 1992). Such a response is undesirable in highly populated areas. Results of this investigation indicate that alterations in the OJIP curve to an OKJIP curve can be detected using chlorophyll fluorescence analysis and used as a means of identifying tree decline due to elevated temperatures. This ©2005 International Society of Arboriculture information would allow for remedial action prior to a tree becoming a danger to pedestrians and traffic. The alteration from an OJIP curve to an OKJIP curve appears to be a heat stress–specific response, as such alterations have not been recorded elsewhere in plants subjected to environmental stresses encountered in urban landscapes, such as elevated ozone, CO2 , heavy metals, salt, light, and water (Meinander et al. 1996; Lazar et al. 1997; Lu and Zhang 1998; Pospisil et al. 1998). Spray application with the herbicides Alpha Atrazine 50 its binding site, effectively blocking the electron transport chain between QA_ curve), causing the maximum fluorescence from O–J that was observed at week 5 of post-herbicide treatment (Guisse et al. 1995; Hall and Rao 1999) (Figures 6 and 7). Even at week 13 post-treatment, alterations to the OJIP curve (i.e., and QB (i.e., the I–P part of the OJIP one- or two-electron carrier—are involved in this electron transfer process. The active ingredients in Diuron and Alpha Atrazine 50 SC (DCMU and atrazine, respectively) act by displacing the secondary electron acceptor QB ), a one-electron carrier, and plastoquinone B (QB from and Diuron caused maximal damage to the leaf photosyn- thetic system between weeks 3 and 5, as manifest by the greatest reductions in Fv/FM and increases in Fo values (Figures 2c, 3c, 2d, and 3d). From week 5 onward, a steady recovery of the leaf photosynthetic system was recorded in all species until the cessation of the experiment (week 21). In the case of English and evergreen oak, Fv/Fm and Fo values from week 13 were comparable to those at day 0, indicating full recovery of the leaf photosynthetic system from herbicide damage (Figures 2c, 3c, 2d, and 3d). In the case of red oak, values were not comparable with day 0 even at the cessation of the experiment, indicating impair- ment of the leaf photosynthetic system at week 21 post- herbicide application (Figures 2c, 3c, 2d, and 3d). This response and reductions in Fv/Fm at week 5 following treatments showed species herbicide tolerance in the order evergreen oak > English oak > red oak (Figures 3c and 3d). In response to herbicide application, a marked effect on the OJIP curve was observed. Irrespective of species, a maximum fluorescence intensity at about 2 ms around the same time that the J-step appears was observed (i.e., the OJIP curve becomes an O–J curve) (Figures 6 and 7). For reasons of clarity, effects on the OJIP curves for each oak species are shown at week 0, 5, and 13 (Figures 6 and 7). Diuron and Alpha Atrazine 50 SC are soil residual herbicides with foliar-acting activity and are recommended for control of annual dicotyledons and annual grasses and perennials in amenity situations. In thylakoids of plant chloroplasts, a pool of plastoquinone molecules found in photosystem II are involved in electron transfer between photosystems I and II. Two plastoquinones—plastoquinone A (QA ), a
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