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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. Fluorescence emissions from vegetation occur in five primary regions of the spectrum, namely; ultraviolet (UV), blue, green, red, and far-red (FR). Many investigators have demonstrated relationships between these fluorescence intensities and ratios of these intensities to various forms of plant stress. The observed fluorescence from plant constituents varies with concentration and location within the leaf due to the interactions of diffused fluorescence with the optical properties (i.e. absorption and transmission characteristics) of neighboring compounds. Recently there has been considerable debate as to the extent UV excitation sources penetrate the leaf and to what regions of the leaf can the majority of these in vivo fluorescence emissions be attributed. The deeper a compound is located within the leaf the lower the probability that fluorescence emissions will be received from this compound due to decreases in the quanta of excitation energy and increases in the probability that the fluorescence emission will be reabsorbed. These studies demonstrated that a portion of the fluorescence excitation radiation at 280 nm (4.5 W/m2 at the leafs surface) was transmitted through both field grown corn (Zea mays L.) and soybean (Glycine max Merr.). Furthermore, UV transmittance increased toward longer wavelengths leading to an increased quanta UV light exciting a higher percentage of compounds located throughout the mesophyll and bundle sheath layers of the leaf. Significant amounts of fluorescence were observed in the green and far-red bands at the abaxial (bottom) surface of the leaf with adaxial (top) surface excitation, while fluorescence emissions in the UV, blue, and red bands were to a large extend reabsorbed. Leaf transmittance is relatively high in the green and far-red regions of the spectrum giving rise to these emissions at the bottom surface. In addition, both UV and blue fluorescence emissions were observed from the leaf epidermis and quantified to 15% of the blue band fluorescence and up to 30% of the UV band fluorescence emanating from the intact leaf.
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