Deriving leaf chlorophyll content of green-leafy vegetables from hyperspectral reflectance

Different nitrogen (N) treatments of four common green-leafy vegetable varieties with different leaf color: lettuce (Lactuca sativa L. var. crispa L.) with yellow green leaves, pakchoi (Brassica chinensis L.) var. aijiaohuang in Chinese (AJH) with middle green leaves, spinach (Spinacia oleracea L.) with green leaves and pakchoi (B. chinensis   L.) var. shanghaiqing in Chinese (SHQ) with dark green leaves, were carried out to achieve a wide range of chlorophyll content. The relationship of vegetable leaf hyperspectral response to its chlorophyll content was examined in this study. Almost all reported successful leaf chlorophyll indices in the literature were evaluated for their ability to predict the chlorophyll content in vegetable leaves. Some new indices based on the first derivative curve were also developed, and compared with the chlorophyll indices published. The results showed that most of the indices showed a strong relation with leaf chlorophyll content. In general, modified indices with the blue or near red edge wavelength performed better than their simple counterpart without modification, ratio indices performed a little better than normalized indices when chlorophyll expressed on area basis and reversed when chlorophyll expressed on fresh weight basis. A normalized derivative difference ratio (BND: (D722−D700)/(D722+D700)(D722−D700)/(D722+D700) calibrated by Maire et al. [Maire, G., Francois, C., Dufrene, E., 2004. Towards universal broad leaf chlorophyll indices using PROSPECT simulated database and hyperspectral reflectance measurements. Remote Sensing of Environment 89 (1), 1–28]) gave the best results among all published indices in this study (RMSE=22.1 mg m−2), then the mSR-like indices with the RMSE between 22.6 and 23.0 mg m−2. The new indices EBAR (ratio of the area of red and blue, ∑ dRE/∑ dB), EBFN (normalized difference of the amplitude of red and blue, (dRE−dB)/(dRE+dB)) and EBAN (normalized difference of the area of red and blue, (∑ dRE−∑ dB)/(∑ dRE+∑ dB)) calculated with the derivatives also showed a good performance with the RMSE of 23.3, 24.15 and 24.33 mg m−2, respectively. The study suggests that spectral reflectance measurements hold promise for the assessment of chlorophyll content at the leaf level for green-leafy vegetables. Further investigation is needed to evaluate the effectiveness of such techniques on other vegetable varieties or at the canopy level.