Development of system for monitoring chlorophyll content of plant population using reflectance spectroscopy

Abstract: To monitor the changes of group chlorophyll for field plants, in this paper, a real-time monitoring system was developed based on Vis/NIR (visible and near-infrared) reflected spectroscopy using a nondestructive method. Reflectance at 700 nm was found to be a very sensitive indicator of chlorophyll concentration, but reflectance of wavelengths at a near infrared band was not. Calculating the reflectance properties of wavelengths at 700 nm and 830 nm can represent chlorophyll values in green leaves. To obtain physiological information of a plant from remote and different fields, a wireless sensor network (WSN) was set up using a ZigBee protocol in the system, in which the terminal device consisted of an optical unit, C8051F020 micro controller unit, WSN node, and a temperature and humidity sensor. The terminal device could obtain the signals of an incident and reflected light from the plant canopy at 700 and 830 nm which were transmitted to the coordinator. Then the coordinator sent the signals to the user center to be calculated and stored. Also, the data of temperature and humidity was sent to the user monitoring center at the same time. The performance of the system was investigated in a tall fescue lawn, and a total of 124 samples were measured in the experiments. They were divided into a training set and a test set. Thirty-six samples were selected as the training set to establish the quantitative analytical model for the experiment. The results showed a good correlation (the coefficient of determination R2 was 0.919 and the standard deviation was 9.26958) between the group chlorophyll index determined by the standard instrument (FieldScout CM-1000 handheld chlorophyll meter) and the absorbance values at the two wavelengths. Based on the model established using a linear prediction equation, we calculated the group chlorophyll value for the test set samples. Promising results were obtained for the predicted values of group chlorophyll for the test samples, with R2 =0.913 for the test set. The coefficient of variation (CV) of the system for showing the stability was less than 1.82%. We found that the changing solar altitude did not affect system measurement during our experiments. The results suggested that this system could be used for accurate measurement and real-time monitoring of the group chlorophyll in a natural light condition, which would provide a theoretical and practical basis for the automatic nondestructive testing technology of precision agriculture.