Diagnosis of nitrogen status in Chinese cabbage (Brassica rapa chinensis) using the ratio of amide II to amide I in leaves based on mid-infrared photoacoustic spectroscopy

In order to quickly diagnose the nitrogen status of Chinese cabbage (Brassica rapa chinensis), Fourier transform mid-infrared photoacoustic spectroscopy, a nondestructive and rapid approach, was used. Two clear stable peaks, in the range 1680–1650 cm−1 associated with amide I and in the range of 1530–1520 cm−1 associated with amide II, were observed and responded positively to nitrogen treatments. The characteristic bands of amide I and amide II were also accurately separated by curve-fitting via deconvolution using a Gauss expansion function. The ratio of the photoacoustic intensities of amide II to amide I was used for correlation analysis with nitrogen treatment, and the correlation coefficients (R2, n = 59) at seedling and growth stage were 0.9769 (P ≤ 5%) and 0.9884 (P ≤ 5%), respectively. The ratio of the amide II and amide I peaks was found to be an indicator of nitrogen input during the seedling stage such that a ratio of less than 0.9 indicated a low nitrogen input, a ratio between 0.9 and 1.0 indicated a moderate nitrogen input, and a ratio of more than 1.0 implied an excessive N input.