Determination of nitrogen and potassium content in greenhouse tomato leaves using a new spectro-goniophotometer

We assessed the feasibility of determining the nitrogen (N) and potassium (K) content of fresh, greenhouse-grown tomato leaves by using a new polarisation reflectance spectrum spectro-goniophotometer system developed by our research group and coupled with appropriate multivariate calibration methods. The main factors that affect the polarised reflectance characteristics of tomato leaves are discussed, including incident zenith angle, azimuth, detection zenith angle, and polariser angle. Orthogonal experiments and range analyses were performed to verify the optimum angle combination from the polarised reflectance parameters. Optimum angle combination experiments were then conducted to fine-tune the optimal parameters, which resulted in the following conditions: incident zenith angle, 60°; viewing zenith angle, 45°; polariser on light source, 0°; polariser on detector, 45°; and azimuth, 180°. On this basis, 122 fresh leaves of greenhouse-grown tomato were used to establish models of N and K content. Results showed that the performance of the iPLS-GA model under incident zenith angle 60° was superior to that of the other models. The optimal model for N was achieved with R = 0.9418 and root mean square error of prediction (RMSEP) = 0.519 in the prediction set; the optimal model for K was achieved with R = 0.8645 and RMSEP = 0.700 in the prediction set. The results show that it is feasible to measure the nutrient content of fresh, greenhouse-grown tomato leaves by polarisation reflectance spectroscopy with an appropriate multivariate calibration model under angle selection. This method allows for in-depth study of plant nutrient status and rapid detection at the single-leaf scale and has theoretical and practical significance.