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Selection of Optimum Reflectance Ratios for Estimating Leaf Nitrogen and Chlorophyll Concentrations of Field-Grown Cotton

^a Dep. of Plant and Soil Sci., Box 9555, Mississippi State Univ., Mississippi State, MS 39762
^b USDA-ARS, Crop Sci. Res. Lab., P.O. Box 5367, Mississippi State, MS 39762

View larger version (31K): [in a new window]   Fig. 1. Changes in N concentrations of cotton uppermost, fully expanded mainstem leaves over the growing seasons in 2001 and 2002 as affected by N and mepiquat chloride (MC). Data are means ± standard error of three replications. Two arrows from left to right indicate the first square (FS) and the first flower (FF) stages, respectively.

View larger version (36K): [in a new window]   Fig. 2. Changes in chlorophyll concentrations of cotton uppermost, fully expanded mainstem leaves over the growing seasons in 2001 and 2002 as affected by N and mepiquat chloride (MC). Data are means ± standard error of three replications. Vertical arrows from left to right indicate the first square (FS) and the first flower (FF) stages, respectively.

View larger version (28K): [in a new window]   Fig. 3. Cotton leaf (A) hyperspectral reflectance of the four N treatments in the N study, (B) reflectance differences of treatments from the control (168 kg N ha–1), and (C) reflectance sensitivity to N fertilizer rate. Data are means of five (2001) or eight (2002) measuring dates. Each time, measurements were made on 15 individual leaves from three replications of each treatment.

View larger version (38K): [in a new window]   Fig. 4. Effects of the plant growth regulator mepiquat chloride (MC) on cotton (A) leaf reflectance difference and (B) reflectance sensitivity based on the control without MC application. Data are means of five measuring dates. Each time, measurements were taken on 15 individual leaves from three replications of each treatment.

View larger version (20K): [in a new window]   Fig. 5. Coefficients of determination (r2) vs. wavelengths for the relationships between cotton leaf N concentration and (A) leaf reflectance at all wavelengths (400 to 2500 nm) and leaf reflectance ratios of (B) R517/Ri and (C) R701/Ri for the 2001 N study. The r2 values were based on a linear model, and data were pooled across the four N treatments, five sampling dates, and three replications (n = 60). Wavelengths with the greatest r2 values are presented in the figure. Leaf N concentrations were expressed on both dry weight (DW) and leaf area basis.

View larger version (33K): [in a new window]   Fig. 6. Linear regression of cotton leaf N concentrations with the values of a specific reflectance ratio (R517/R413) that had the greatest r2 value with leaf N. Leaf N concentrations were expressed on both (left) a leaf dry weight basis and (right) a leaf area basis. Data were pooled across the studies, treatments, replications, and sampling dates (n = 120 in 2001 and n = 156 in 2002). MC, mepiquat chloride.

View larger version (28K): [in a new window]   Fig. 7. Linear regression of cotton leaf chlorophyll with corresponding reflectance values at wavelengths of 551 (R551) and 708 (R708) nm and two reflectance ratios (R551/R915 and R708/R915) that had the greatest r2 values with chlorophyll. Data were pooled across studies, treatments, replications, and sampling dates (n = 76 in 2001, n = 156 in 2002). Chlorophyll in 2001 was measured using a SPAD-502 chlorophyll meter. MC, mepiquat chloride.