92 tion about tree growth and physiologic plasticity in response to changing environments (Larcher 1995; Richardson et al. 2002). The amount of solar radiation absorbed by a leaf is largely a function of the foliar concentrations of photosynthetic pigments. Low concentrations of chlorophyll can therefore directly limit photosynthetic potential and hence primary production (Filella et al. 1995). A SPAD meter is ideally suited for the arborist re- quiring minimal training in its use and no detailed scientific background knowledge. Analysis is rapid (1 to 2 sec per reading) allowing for many trees to be evaluated in a single day. Impor- tantly, measurements are nondestructive and noninvasive allow- ing for periodic repetitive sampling (Loh et al. 2002; Richardson et al. 2002) SPAD versus Total Carotenoids Quadratic polynomial (Acer pseudoplatanus) and logarithmic re- gression (Fagus sylvatica, Quercus robur) models were adequate in explaining the relationships between SPAD and total leaf carotenoids with R2 of 0.85, 0.84, and 0.82, respectively (Figure 2). Results show that an indirect carotenoid quantification can be obtained for SPAD values up to 50 using the SPAD-502 despite the 650 nm quantifying system that is the wavelength relevant to chlorophyll absorption (Torres Netto et al. 2005). These infer- ences can be obtained as a result of the direct relationship be- tween the total chlorophyll and carotenoid concentration within leaves. In higher plants, carotenoids generally consist of 7% to 9% of total leaf photosynthetic pigments, consistent with values obtained in this study for all three tree species (Hall and Rao 1999; Lawlor 2001). The importance of a nondestructive means of quantifying carotenoid concentrations lies in the fact that an initial plant stress response is stomatal closure to conserve tran- spirational water loss. Such a response can be highly detrimental to the leaf photosynthetic system as a result of the prevention of light energy conversion into photochemical energy caused by low CO2 concentrations within the leaf tissue in turn resulting in the production of high-energy reactive oxygen species (ROS) such as superoxide and singlet oxygen (Lawlor 2001). Buildup of ROS results in oxidization damage to leaf membranes, i.e., chlorophyll bleaching and cellular membrane destruction. To minimize the effects of oxidative stress, plants have evolved an antioxidant system consisting of carotenoids that function as protective photooxidative pigments responsible for the quench- ing of these ROS (Kraus and Fletcher 1994). Because an increase in total leaf carotenoid content is a widely recognized plant stress response (Peñuelas and Filella 1998), quantification of total leaf content can provide indicators of plant responsiveness to stresses frequently encountered in urban and landscape environments (Strauss-Debenedetti and Bazzaz 1991; Hendry and Price 1993; Vieira 1996). SPAD versus Total Chlorophyll:Carotenoid Ratio A poor relationship between the total leaf chlorophyll:carotenoid ratio was shown for all three species. Goodness of fit R2 values of 0.49 (Acer pseudoplatanus), 0.54 (Fagus sylvatica), and 0.13 (Quercus robur) were recorded using quadratic polynomial re- gression models (Figure 3). Contrary to this, the ratio between chlorophyll and carotenoids has been shown to be a sensitive marker distinguishing natural senescence, senescence resulting from environmental stresses (Buckland et al. 1991), drought, and photooxidative damage in plants (Seel et al. 1992; Hendry and Price 1993). The poor relationship recorded in this study may ©2008 International Society of Arboriculture Percival et al.: Quantifying Nutrient Stress in Foliar Tissue relate to the type of stress imposed. An adequate N supply is essential for the formation of chloroplast and carotenoid struc- ture (Peoples et al. 1980). When N availability is low, both the leaf chlorophyll and carotenoid content are reduced (Doncheva et al. 2001). However, analysis of the ultrastructure of leaves indicated a more marked influence of N deficiency on leaf chlo- rophyll content compared with carotenoid content, which may account for this poor correlation (Peñuelas and Filella 1998; Doncheva et al. 2001). Results of this investigation therefore indicate that leaf chlorophyll:carotenoid ratios do not provide as robust a system of identifying stress disorders in trees caused by N deficiency compared with other plant vitality systems such as chlorophyll fluorescence Fv/Fm values used in this investigation. SPAD versus Chlorophyll Fluorescence (Fv/Fm) Leaf chlorophyll content is often well correlated with leaf pho- tosynthetic rates (Evans 1983; Seeman et al. 1987). Correlations between SPAD values and Fv/Fm values as measures of photo- system II efficiency are limited. According to the quadratic fitted model, the maximum quantum efficiency of the photosystem II, indicated by the Fv/Fm ratio, started to fall at around a SPAD value of 25 (Figure 4) irrespective of species. Exceedingly high goodness of fit values (R2 greater than 0.94) were associated with these models. The Fv/Fm ratio is positively correlated to the PSII quantum yield and an indirect measurement of plant physi- ologic status (Kitajima and Butler 1975; Maxwell and Johnson 2001) for which values of 0.8 ± 0.05 correspond to highly effi- cient use of the excitation energy in photochemical processes (Björkman and Demmig 1987; Mohammed et al. 1995; Percival 2005). Consequently, results of this investigation indicate that SPAD readings around 25 appear to be the start of PSII impair- ment caused by N deficiency in the species used in this study. Past research by Percival (2004) and Maki and Colombo (2001) indicated Fv/Fm values of 0.6 below which trees were affected in terms of reduced survival, height growth, and foliar necrosis. Fv/Fm ratios of 0.6 were associated with SPAD values between 15 and 20 in this study. However, this study was conducted on three tree species at one stage during the growing season. Con- sequently, although preliminary results are promising, further work is needed to assess the applicability of SPAD values versus reductions in photosynthetic efficiency as measured by chloro- phyll fluorescence for other ornamental tree species. An example of the dangers of extrapolating SPAD values from one plant species to another can be gauged by reference to work by Torres Netto et al. (2005), who concluded SPAD values less than 40 were correlated with reductions in photosynthetic efficiency as measured by Fv/Fm values, not 25 as reported in this study. Such a response indicates individual regression models need to be developed for differing species and cultivar. SPAD versus Leaf Nitrogen Content Nitrogen is one of the most important factors in plant growth physiology. It is related to leaf photosynthetic rate (Evans 1989), dark respiration (Anten et al. 2000), quantum yield (Hirose and Werger 1987), leaf extension (Gastal et al. 1992), mesophyll size (Körner 1989), leaf aging (Escudero and Mediavilla 2003), leaf lifespan (Hikosaka and Hirose 2000), and leaf chlorophyll con- centration (Pons et al. 1994). Optimal correlations between total leaf N concentrations and SPAD values were obtained using quadratic (Acer pseudoplatanus, Quercus robur) and linear (Fa- gus sylvatica) regression models with higher SPAD values re-
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