Optimal fluorescence excitation wavelengths for detection of stress in vegetation

Abstract

Considerable efforts are being directed by many research groups toward the development of instrumentation to excite and remotely sense fluorescence of vegetation at the field and forest canopy levels. Green vegetation when exposed to appropriate wavelengths of light, dissipates a portion of the absorbed energy as light emissions (fluorescence) in several broad areas of the spectrum. Currently, leaf level fluorescence emissions have been broken down into five primary regions of the spectrum, namely; ultraviolet (UV), blue, green, red, and near-infrared (NIR). Optimal excitation wavelengths for the UV, blue, and green fluorescent areas are bands centered at 284 nm, 340 nm, and 380 nm respectively. The red and NIR fluorescent emissions can be excited within the broad wavelength region from 250 to 675 nm with excitation peaks at 430 nm, 470 nm, 600 nm, and 660 nm. The ratio of red to NIR fluorescence excitation spectra produced a ratio spectrum. The relative differences between these two emission bands are not constant and depend on the wavelength of excitation. Fluorescence sensing systems based on the above emission bands are being developed for ground based mobile vans, helicopters, and small aircraft. The goals of these efforts were to determine the optimal regions of the spectrum to excite vegetation in order to differentiate plant stress condition based on fluorescence emission bands.