Arboriculture & Urban Forestry 34(2): March 2008 Arboriculture & Urban Forestry 2008. 34(2):89–100. 89 The Potential of a Chlorophyll Content SPAD Meter to Quantify Nutrient Stress in Foliar Tissue of Sycamore (Acer pseudoplatanus), English Oak (Quercus robur), and European Beech (Fagus sylvatica) Glynn C. Percival, Ian P. Keary, and Kelly Noviss Abstract. The chlorophyll content (or SPAD meter) is a simple, portable diagnostic tool that measures the greenness or relative chlorophyll content of leaves. Compared with the traditional destructive methods of chlorophyll extraction, the use of this equipment saves time, space, and resources. The objective of this study was to establish a correlation between the leaf photo- synthetic pigment content (chlorophylls, carotenoids) extracted in aqueous acetone, total leaf nitrogen (N) content, and chlorophyll fluorescence Fv/Fm values with the SPAD-502 readings in sycamore (Acer pseudoplatanus), beech (Fagus sylvatica), and English oak (Quercus robur) leaves displaying visual symptoms of N deficiency. In addition, this study aimed to determine a critical foliar N content below which a reduction in photosynthetic efficiency occurs. Irrespective of species, high correlations were recorded between SPAD readings, total leaf chlorophyll and carotenoid content, foliar N content, and leaf photosynthetic efficiency as measured by chlorophyll fluorescence Fv/Fm values; however, a poor correlation between SPAD values and total chlorophyll: carotenoid ratios was obtained. In the case of Acer pseudoplatanus, Fagus sylvatica, and Quercus robur, SPAD readings lower than 25 indicated impairment of leaf photosynthetic process that in turn were correlated with a foliar N content less than 1.5%, a value associated with a critical N deficiency. Results of this study indicate that the chlorophyll content SPAD-502 m potentially offers a useful nondestructive, handheld system to aid in the evaluation of tree health. However, users should be aware of the limitations of this system. Consistency in sample collection and seasonal timing may necessitate species and cultivar calibration equations to correlate SPAD values with reductions in tree vitality. Key Words. Carotenoids; chlorophyll fluorescence; chlorophylls; light transmittance; nitrogen fertilization; stress detection; tree evaluation. Urban environments present an array of environmental factors hostile to the biology of trees (soil deoxygenation, compaction, aerial pollution, and deicing salts). These stresses limit the amount of carbohydrates available for growth and reduce nutrient uptake resulting in leaf chlorosis and necrosis (Jimenez et al. 1997; Mohammed et al. 1997; Maki and Colombo 2001). These symptoms become manifest as leaf yellowing that are visible indicators arborists interpret to assess tree vitality (Percival 2004). Visual observations can be very subjective be- cause they are based on human knowledge and interpretation that can significantly differ between individuals (Percival 2004). Consequently, health evaluations can be markedly different be- tween assessors. Field diagnostic tools are now required to ob- jectively evaluate stress disorders in trees as a basis for manage- ment decisions on cultural practice and for proactive monitoring in urban treescapes (Loh et al. 2002). Such diagnostic instru- ments and methodologies should ideally detect stress before the visible symptoms of plant deterioration become manifest and therefore make possible effective remedial intervention (Percival and Fraser 2001). Both biotic and abiotic stress factors affect the content and efficiency of leaf photosynthetic pigments and/or their reciprocal ratio (Bacci et al. 1998). For example, the assessment of leaf photosynthetic pigments is an important indicator of senescence because breakdown of leaf chlorophyll is associated with envi- ronmental stress (Brown et al. 1991). Likewise, the variation in total chlorophyll/carotenoids ratio has been used as a useful in- dicator of stress in plants because a rapid increase in total leaf carotenoid content is a recognized plant stress response (Hendry and Price 1993). Exact knowledge of foliar chlorophyll concen- trations, i.e., “greenness,” consequently may provide a robust and accurate estimation of tree vitality. Traditional methods of extracting chlorophylls from leaves using chemical solvents such as acetone, dimethylsulfoxide, and methanol require laboratory conditions, are time-consuming, and involve destructive leaf analysis (Lichtenthaler and Wellburn 1983). The chlorophyll content meter (or SPAD meter) is a commer- cially available portable piece of equipment that is used to mea- sure greenness based on optical responses when a leaf is exposed to light that in turn is used to estimate foliar chlorophyll con- centrations (Kariya et al. 1982). The meter makes instantaneous and nondestructive readings on a plant based on the quantifica- tion of light intensity (peak wavelength: approximately 650 nm: red light-emitting diode [LED]) absorbed by the tissue sample. A second peak (peak wavelength: approximately 940 nm: infrared LED) is emitted simultaneous with red LED to compensate for thickness of the leaf (Hoel 1998). Past research has shown a close link between leaf chlorophyll concentration and leaf nitrogen (N) content in agricultural crops such as rice, maize, and wheat because the majority of leaf N is contained within the chlorophyll molecules (Peterson et al. 1993). Consequently, chlorophyll content meters are widely used ©2008 International Society of Arboriculture
March 2008
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