Nitrogen deficiency is a common but readily managed constraint to grain yield. A quick and non-destructive detection of crop N status using remote sensing could be a means to increased N use efficiency. Research was conducted in a greenhouse in 2006 at the University of Nebraska-Lincoln to establish the relationship of spectral reflectance with N status in leaves of grain sorghum, to develop indices for interpretation of the results and to predict chlorophyll content. Nitrogen stress decreased chlorophyll meter reading and leaf N content, but increased leaf and canopy reflectance. The SPAD values were significantly increased by both water and N stress. Reciprocal reflectance in the green range (549 to 560 nm), and red edge range (710 to 718 nm) wavelength of the spectrum were good indicators of N stress. The best fit regression between leaf chlorophyll content and the indices in the green and red edge wavebands were linear with an R2 of 0.76 to 0.79. A model calibrated using these wavelengths minus reciprocal reflectance of NIR (750 nm), predicted leaf chlorophyll content with root mean square error (RMSE) ranging between 52 and 56 mg m-2, and reduced the intercept of the model from 312 to 35 mg m-2 in the green range and 486 to 21 mg m-2 in the red edge. Future studies will be conducted to evaluate the effectiveness of the indices at the canopy level of grain sorghum.
Key words: Chlorophyll, grain sorghum, nitrogen, red edge, reflectance, SPAD.