Journal of Arboriculture 31(5): September 2005 215 THE USE OF CHLOROPHYLL FLUORESCENCE TO IDENTIFY CHEMICAL AND ENVIRONMENTAL STRESS IN LEAF TISSUE OF THREE OAK ( ) SPECIES By Glynn C. Percival Abstract. During their lifetime, urban trees are susceptible to a range of environmental and chemical stresses that can result in tree decline. Chlorophyll fluorescence has been used as a nondestructive and noninvasive means of quantifying damage to the leaf photosynthetic system of deciduous and evergreen trees. Aims of this investigation were to determine whether there were unique chlorophyll fluorescence profiles for different chemical (salt, herbicide) and environmental (heat) stresses in leaf tissue of three oak (Quercus) species. Results demonstrate that alterations in the OJIP curve as a measure of electron transport within the leaf plastoquinone pool of photosystem II could be used to identify tree decline due to herbicide and heat but not to salt damage. The benefits of this system to make rapid, stress-specific diagnosis in the field for professionals involved in urban tree management are discussed. Key Words. English oak; evergreen oak; heat; herbicide; leaf tissue damage; red oak; salinity; stress detection; Quercus ilex; Quercus robur; Quercus rubra. Urban landscapes present an environment detrimental to the biology of trees due to soil deoxygenation, compaction, air pollution, and de-icing salt stresses. These stresses limit the amount of carbohydrates available for growth and mainte- nance and reduce nutrient and water uptake, resulting in leaf chlorosis, branch dieback, and, ultimately, death. Initial symptoms of tree decline are generally manifest as leaves yellowing, which is a visible indicator that arborists interpret to assess tree vitality (Percival 2004). Appropriate remedial action at this stage may alleviate the problem prior to physical deterioration of tree structure. Physiological tests such as shoot and root electrolyte leakage, root growth potential, leaf starch concentration, leaf chlorosis, and photosynthesis have been successfully used to identify low vigor or damaged plants; however, none of these systems can identify the causative stress or stresses responsible for the visible signs of tree decline (Percival 2004; Richardson et al. 2004). Consequently, identifying the actual cause of tree decline (environmental stress, chemical damage, inappropriate soil conditions, etc.) can be costly, time consuming, and labor intensive. Chlorophyll fluorescence has been used to provide a rapid, nondestructive diagnostic method for detecting and quantifying damage to the leaf photosynthetic apparatus in ornamental and coniferous trees in response to environ- mental stress (Palta 1992; Sestak and Stiffel 1997; Percival 2004). The technique measures changes in chlorophyll a fluorescence due to altered photosystem II (PSII) activity, caused directly or indirectly by stress. Most commercially available fluorimeters commonly measure the minimal, maximal, and variable chlorophyll fluorescence (Fo, Fm, and Fv, respectively, where Fv = Fm – Fo). Ratios of the parameters Fv/Fm, Fo, and Fv/Fo provide estimates of various aspects of leaf photosynthetic and photochemical activities (Hong and Xu 1999; Yamane et al. 2000; Percival and Fraser 2002). In addition, many fluorescence systems offer the flexibility to store intermedi- ate data points at specific times within the 1-s fluorescence induction. For example, when a healthy leaf is suddenly illuminated after a period of darkness, a time-dependent fluorescence emission kinetic is observed, known as the Kautsky effect. Two parts to this induction kinetic can be distinguished: (1) the fast fluorescence rise to the maximum fluorescence, which is completed in 100 to 500 ms, and (2) the slow fluorescence decrease to the steady-state level. The analysis of the intermediate data points of the fast fluorescence rise forms the basis of what is termed the OJIP curve. In brief, all higher plants that contain chlorophyll as the main photosynthetic pigment (trees, shrubs, crops, grasses, etc.) exhibit a rise of chlorophyll fluorescence the first second after illuminating a dark-adapted sample that follows a universal OJIP curve (Figure 1*). Within leaf tissue of all higher plants, a pool of special- ized molecules known as plastoquinones exist that are involved in photosynthetic electron transfer between photosystems I and II. The two plastoquinone molecules responsible for electron transport are plastoquinone A (QA a one-electron carrier, and plastoquinone B (QB ), a one- or two-electron carrier. The initial chlorophyll fluorescence at O level reflects the minimal fluorescence yield when all *Figures for this article appear on pages 222–227. ©2005 International Society of Arboriculture ), QUERCUS
September 2005
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