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Green vegetation when excited by specific wavelengths of light dissipates a portion of the absorbed energy as light emissions in the form of fluorescence in several broad areas of the spectrum. Currently, leaf level fluoresence emissions have been broken down into five primary regions, namely; ultraviolet (UV), blue, green, red, and near-infrared (NIR). The optimal excitation wavelengths for each of these bands was verified for healthy soybean leaves through the use of the EEM. Intact vegetation when excited at 280 nm emits substantial fluorescence in two bands; the first centered near 335 nm, and the second centered near 440 nm. UV band fluorescence from vegetation treated with varying levels of nitrogen decreases relative to the blur fluorescence as a function of total protein concentration. These studies indicate that in vivo UV band fluorescence can be utilized as a non-destructive tool to remotely sense variations in protein concentration due to nitrogen fertilization level. It has been well established that this fluorescence emission originates from proteins contain aromatic amino acids. The majority of plant proteins contain these amino acids and as a result have the potential to fluorescence in the region of the spectrum discussed here. Pure ribulose 1,5-bisphosphate carboxylase in aqueous solution exhibited intense UV fluorescence characteristics with excitation and emission distributions similar to those of intact vegetation. Due to its high concentration we believe this protein contributes to the UV band fluorescence emanating from the intact leaf. The red and NIR fluorescence emissions can be excited within the broad wavelength region from 250 to 675 nm with excitation maxima at 430 nm, 470 nm, 600 nm, and 660 nm. The ratio of red to NIR fluorescence excitation spectra produces a ratio spectrum which exhibits striking similarities to the action spectrum of photosynthesis. The relative differences between these two emission bands depend on the wavelength of excitation. Moreover, by comparing the ratio spectrum of a healthy versus nitrogen deficient leaf, one finds areas of crossover where trends can be completely reversed by changing excitation wavelength. As a result, the success of studies involving the measurement of chlorophyll a fluorescence depend greatly on the appropriate selection of excitation wavelength. Fluorescence sensing systems based on the above emission bands are being proposed or developed for ground based mobile vans, helicopters, and small aircraft. The goals of these efforts were to better define the origins of fluorescence and to improve our understanding of these light emissions in relationship to the physiological status of the plant.
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