[Nitrogen content testing and diagnosing of cucumber leaves based on multispectral imagines]

Using CCD camera and special filters, the growth parameters of cucumber plants, the nitrogen content and the area of leaves, were investigated in an experimental greenhouse. In order to make nutrient stress to the plants, different nitrogen levels were prepared. The basic nitrogen content was 0.067 kg x L(-1) and four different levels of nitrogen contents were made to be 2, 1, 0.5, and 0.2 N, respectively. The genetic and water-segment methods were used to separate IR and R2 images from the background. It was found that the result of water-segment is better. It has clearer boundary, less noise and closer result to the original image. After the reflectance information of cucumber leaves was obtained, the vegetation indexes, RVI, NDVI and GNDVI, were calculated and then the correlation coefficients between each vegetation index and nitrogen content or leave area were analyzed. The result shows that there is an obvious linear correlation between NDVI and nitrogen content of leaves or leave area and the R2 are 0.8209 and 0.7017, respectively. The high correlations were also observed between GNDVI and nitrogen content of leaves or leave area, and the R2 are 0.7625 and 0.6762, respectively. The reason is that the reflectance of IR is mainly affected by reflectivity and the canopy structure of cucumber leaves. As biomass and area of leaves increase with the nitrogen content, the reflectivity of leaves becomes stronger. And the gap among cells of high nitrogen content leaves is large. Cell wall has more water, which has a strong effect on the reflectivity of NIR At visible wavelength, the reflectance of cucumber leaves decreases as nitrogen content increases since the chlorophyll content increases as nitrogen content increases. The trend of correlation between RVI and nitrogen content disagreed with that of the correlation between RVI and leave area. There is an obvious linear correlation between RVI and leave area, and the R2 is 0.8577. However, the correlations between RVI and nitrogen content exhibit a nonlinear relationship, and R2 is only 0.5988. It is because as cucumbers grow older, the reflectance of canopy increases at visible wavelength but decreases at near infrared wavelength. The experimental result proves that CCD camera and special filters can be used as a good method for diagnosing nitrogen content of cucumber plants.