Growth Environment and Leaf Anatomy Affect Nondestructive Estimates of Chlorophyll and Nitrogen in Citrus sp. Leaves

Correlations between extractable leaf chlorophyll (Chl) concentration and portable, nondestructive leaf “greenness” meter readings imply that such estimates can be used as surrogate measurements of leaf nitrogen (N) status. However, few studies have actually found a direct relationship between Chl meter readings and leaf N. We evaluated the utility of two handheld transmittance-based Chl content meters (SPAD-502, Minolta Corp. and CCM-200, OptiSciences) and one refl ectance-based meter (Observer, Spectrum Technologies), in estimating Chl and N concentrations in intact leaves of several citrus cultivars. Total Chl determined analytically, correlated well with nondestructive Chl meter readings (r2: 0.72 to 0.97; P < 0.0001), but regression models differed among cultivars using the same meter and also among meters for a given cultivar. The relationships were generally more linear and stronger at low Chl concentrations (<0.5 mmol·m–2) than at higher Chl concentrations, refl ecting increased variability in Chl meter readings with increasing leaf Chl. Signifi cant relationships between Chl meter readings and measured leaf N concentrations were also found in all the cultivars tested (r2: 0.23 to 0.69; P < 0.01), but the data were more variable than those for Chl. Field-grown leaves were signifi cantly thicker and had higher Chl meter readings than greenhouse-grown leaves of similar Chl or N concentrations. The results suggest that nondestructive Chl content meters can overestimate Chl and N in thicker leaves and/or leaves with high Chl concentrations. A single prediction equation derived from a wide range of Chl or N concentrations could be applicable across the range of citrus cultivars when grown in the same environment. Potential limitations associated with leaf thickness as infl uenced by environmental factors may necessitate the development of more specifi c calibration equations. Nitrogen is the major nutrient element that most frequently limits the growth and productivity of nonleguminous plants (Below, 1995). Leaf N concentration can range from 2% to 6% of dry weight (Jones, 1998; Munson 1998) and is usually strongly correlated with photosynthetic capacity and yield (Evans, 1989). Dark green leaves, high photosynthetic activity, and vigorous vegetative growth are characteristics of plants with adequate N supply. Conversely, inadequate N supply can lead to chlorosis and stunted growth. Effi cient management of fertilizer N to achieve optimum productivity while preserving the quality of the environment is an important objective in modern agricultural systems. Theoretically, effective N management requires frequent plant and soil monitoring to ensure that neither too little nor too much fertilizer N is applied. The standard methods for determining plant N status (extraction and spectrophotometric determination) are destructive and time-consuming (Munson, 1998). Since leaf N content is often highly correlated with leaf chlorophyll (Chl) concentration (Syvertsen, 1987), noninvasive, portable Received for publication 3 Mar. 2003. Accepted for publication 8 Sept. 2004. This research was partially supported by grants from US/IS BARD and Spectrum Technologies Inc. and was funded by the Florida Agricultural Experiment Station. Approved for publication as Journal Series No. R-10091. Special thanks to Spectrum Technologies Inc. for loan of the Observer prototype and the Minolta SPAD-502 meters and to Dan Harkins of Opti-Sciences, Inc. for the loan of the CCM-200 prototype meter. The assistance of Jill Dunlop (CREC) is gratefully