A number of studies have linked responses in leaf spectral reflectance, transmittance, or absorptance to physiological stress. A variety of stressors including dehydration, flooding, freezing, ozone, herbicides, competition, disease, insects, and deficiencies in ectomycorrhizal development and N fertilization have been imposed on species ranging from grasses to conifers and deciduous trees. In all cases, the maximum difference in reflectance within the 400–850 nm wavelength range between control and stressed states occurred as a reflectance increase at wavelengths near 700 nm. In studies that included transmittance and absorptance as well as reflectance, maximum differences occurred as increases and decreases, respectively, near 700 nm. This common optical response to stress could be simulated closely by varying the chlorophyll concentration of model leaves (fiberglass filter pads) and by the natural variability in leaf chlorophyll concentrations in senescent leaves of five species. The optical response to stress near 700 nm, as well as corresponding changes in reflectance that occur in the green–yellow spectrum, can be explained by the general tendency of stress to reduce leaf chlorophyll concentration.
Leafspectral reflectances were measured to determine whether leafreflectance responses to plant stress may differ according to the agent ofstress and species. As a result ofdecreased absorption by pigments, reflectance at visible wavelengths increased consistently in stressed leaves for eight stress agents and among six vascular plant species. Visible reflectance was most sensitive to stress in the 535-640-nm and 685-700-nm wavelength ranges. A sensitivity minimum occurred consistently near 670 nm. Infrared reflectance was comparatively unresponsive to stress, but increased at 1,400-2,500 nm with severe leaf dehydration and the accompanying decreased absorption by water. Thus, visible rather than infrared reflectance was the most reliable indicator of plant stress. Visible reflectance responses to stress were spectrally similar among agents of stress and species.
Ratios of leaf reflectances that were measured within narrow wavebands (2 nm) were evaluated as indicators of plant stress. Wavebands used in ratio computation were based on earlier studies that determined the wavelength regions in which reflectance was most affected by 8 stress agents among 6 plant species. Several ratios, such as reflectance at 695nm divided by reflectance at 670nm (R695/R670), were affected by some but not all stress agents. However, R695/R420, R605/R760, R695/R760 and R71 O/R760 were significantly greater (p';;;0'05) in stressed compared with non-stressed leaves for all stress agents. The ratios that most strongly indicated plant stress were reflectance at 695nm divided by reflectance at 420 nm or 760nm.
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