Abstract: Leaf chlorophyll and nitrogen concentrations of cotton (Gossypium hirsutum L.) are important indicators of plant N status. They can provide valuable insights into the physiological performance of leaves. The objectives of this study were to determine the relationships between chlorophyll, nitrogen, SPAD readings and leaf spectral and color parameters in cotton. Spectral and color parameters for the non-destructive estimation of chlorophyll, nitrogen contents and SPAD readings were obtained by using digital camera (Olympus C-5060) and imaging spectrometer (MSI200) so a wide range of them was established in cotton. The dataset was separated into two parts using for calibration (n=100) and validation (n=60), respectively. Therefore, a systematic analysis was undertaken on quantitative relationships of chlorophyll, nitrogen, SPAD readings to major spectral indices, such as the ratio index (RI), normalized difference index (ND) and difference index (DI), composed of any two wavelengths with original reflectance and color parameters. The results indicated that the maximum sensitivity of reflectance to variation in chlorophyll, nitrogen contents and SPAD readings was found in the far-red wavelength region at 710 nm and in the red wavelength region (R) for two sensors, respectively. Furthermore, spectral indices could improve the prediction ability obviously, and difference indices (DI and R-B) of different sensors composed of blue and red wavelengths gave a better prediction performance. The models to retrieve chlorophyll, nitrogen contents and SPAD readings using DI were the most feasible models with the maximum determination coefficients (R²) and the minimum RMSE, especially, DI (R₄₄₀, R₇₁₀), DI (R₄₄₀, R₇₁₀), DI (R₄₂₀, R₇₁₀), DI (R₄₂₀, R₇₂₀) and DI (R₄₉₀, R₇₁₀) were the optimum indices for the models of chlorophyll a+b, chlorophyll a, chlorophyll b and N, and SPAD readings, respectively. R-B was the optimum index of digital camera but its prediction performances were lower than these of DI. Additional, b* (CIE 1976 L*a*b* color model) and S (HSI color model) were the optimum color parameters, and the prediction ability of b* was lower than that of DI. However, the prediction performance of S was relative weak with the highest RRMSE and the lowest R². Thus, measurements of leaf reflectance and color in visible range by using digital camera and imaging spectrometer may provide a real-time and accurate means of estimating leaf chlorophyll and nitrogen contents and monitoring of cotton plant nitrogen status and N fertilizer management in the field.

Key words: Cotton leaves, Spectral index, Color parameter, Chlorophyll, Nitrogen, SPAD readings