216 Percival: Use of Chlorophyll Fluorescence to Identify Oak Stress plastoquinone (QA ) molecules are in the oxidized associated with the primary photochemical reactions of photosystem II. The intermediate step I and the final step P reflect the existence of fast- and slow- reducing PQ centers. Consequently, the OJIP curve represents a measure of the plastoquinone pool within leaf tissue. Importantly, the OJIP curve changes its shape according to differing environmental stresses such as light intensity, temperature, drought, chemi- cals, or heavy metals and has been used to provide a stress- specific diagnosis (Govindjee 1995; Haldimann and Strasser 1999; Popovic et al. 2003). Work by many researchers (Guisse et al. 1995; Cajanek et al. 1998; Haldimann and Strasser 1999; Georgieva et al. 2000), however, suffers from a number of limitations. In most instances, young seedling material of crop species has been the test species of choice, which may not reflect the leaf physiology and subsequent fluorescence responses of tree material acclimated to the harsh atmospheric conditions, such as elevated ozone or car exhaust pollutants, that can exist within a urban landscape. In addition, assessments of fluorescence have been conducted mainly under laboratory and glasshouse conditions, which do not reflect conditions under field or landscape environments. Finally, fluorescence responses in most cases were recorded with a few hours following stress treatments and do not provide long-term information as to how long fluorescence effects remain altered by environmental stresses (Srivastava et al. 1998; Lazar et al. 1999; Hong and Xu 1999; Yamane et al. 2000). Aims of this investigation were to determine whether molecules are in the reduced state. Steps J and I occur between the O and P steps. The O to J phase results from the reduction of QA state. The end, or P level, corresponds to the state in which all QA to QA– and is interpretation of a range of chlorophyll fluorescence re- sponses could be used to distinguish between environmental (heat) and chemical (salt, herbicide) stress in foliar tissue of three oak (Quercus) species and determine the length of time that alterations to fluorescence responses were recorded before tree recovery. For the purposes of this investigation, stresses were applied to larger, cell-grown stock approxi- mately 3 to 4 years old, rather than to seedling material grown under field, not glasshouse or laboratory, conditions. MATERIALS AND METHODS Plant Material and Experimental Design The experiment used cell-grown stock of evergreen (Quercus ilex L.), English (Q. robur L.), and red oak (Q. rubra L.) obtained from a commercial supplier (Table 1). Trees were planted directly into the ground by hand in early November 2001 and left to establish throughout the 2002 growing season to overcome any potential effects of transplant shock. Five trees of each oak species were also potted into 4.5 L (1.2 ©2005 International Society of Arboriculture Table 1. Physical characteristics of evergreen (Quercus ilex L.), English (Q. robur L.), and red oak (Q. rubra L.) after grading to ensure uniformity of stock for experimental purposes. Attribute Height (cm) Girth (cm) Evergreen oak English oak Red oak 40.0 (1.30)* Height:girth ratio (cm/cm) 17.4 (0.66) Shoot dry weight (g) Root dry weight (g) Shoot:root ratio (g/g) Root area (cm2 2.3 (0.11) 9.2 (0.41) ) 8.2 (0.48) 1.12 (0.10) 33.1 (1.71) *Values are mean and standard errors for 12 trees. gal) pots with trial site soil and then planted into the ground to facilitate removal with minimal disturbance when trees were subjected to heat stress under controlled conditions at a later date. The trial site consisted of a 2.5 ha (6.25 ac) block located at the University of Reading Shinfield Experimental Site, Berkshire, UK. Planting distances were 1 × 1 m (3.3 × 3.3 ft). No watering was applied during the trial; however, on 10 May 2002, a broadcast application of granular N:P:K (29:7:9) fertilizer (Bartlett BOOST, The Doggett Corpora- tion, Lebanon, NJ) was applied at 40 g/m2 (0.13 oz/ft2 ). The soil was a sandy loam containing 4% to 6% organic matter; pH of 6.2; available P, K, Mg, Na, and Ca were 52, 659.1, 175.2, 49.4, and 2,188 mg/L (0.007, 0.9, 0.02, 0.006, and 0.3 oz/gal), respectively, at the time treatments com- menced (May 2003). Weeds were controlled chemically using glyphosate (Roundup, Green-Tech, Sweethills Park, Nun Monkton, York, UK) from the day of planting in November 2001 to the cessation of the experiment in August 2003. The experimental design was a randomized complete block with five single-tree replications. All treatments commenced under field conditions in early May 2003, when all oak species were in full leaf. To keep the physi- ological age of the leaves comparable throughout the experiment, measurements of chlorophyll fluorescence were made only on fully expanded, 21- to 28-day-old leaves. In all cases, five trees per treatment for each oak species were used for experimental purposes, with chlorophyll fluores- cence measurements taken from six leaves (two from the top of the crown, two in the center, and two at the base) per tree. Leaves were tagged to ensure that the same leaves were measured throughout the experimental period. Treatments Prior to salt and herbicide sprays, polythene screens 1 m (3.3 ft) high were erected around each tree to prevent dispersal of sprays and possible cross contact with other trees. The base of the tree was covered with a 0.5 m × 0.5 m (1.65 × 1.65 ft) polythene mulch to prevent percolation of salt or herbicide into the soil. 35.6 (1.51) 33.4 (1.55) 1.33 (0.09) 1.16 (0.09) 27.8 (1.50) 28.8 (1.54) 1.39 (0.09) 1.06 (0.08) 3.98 (0.25) 2.88 (0.15) 0.35 (0.04) 0.37 (0.03) 34.21 (1.64) 25.21 (1.77)
September 2005
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