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 this cases, the maximum difference in reflectance within the 400 - 850 nm wavelength range between control and stressed states occurred as a reflectance increase at wavelength 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 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.

/pdf/leaf-optical-properties-in-higher-plants-linking-spectral-cgj8iuers0.pdf

Leaf optical properties in higher plants: linking spectral characteristics to stress and chlorophyll concentration

Ratios of leaf reflectances that were measured within narrow wavebands (2nm) 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 695 nm divided by reflectance at 670 nm (R695/R670), were affected by some but not all stress agents. However, R695/R420, R605/R760, R695/R760 and R710/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 695 nm divided by reflectance at 420 nm or 760 nm.

Ratios of leaf reflectances in narrow wavebands as indicators of plant stress

Remote sensing of solar-induced chlorophyll fluorescence: Review of methods and applications

Leaf spectral reflectances were measured to determine whether leaf reflectance responses to plant stress may differ according to the agent of stress and species. As a result of decreased 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.

Responses of leaf spectral reflectance to plant stress.

https://www.dora.lib4ri.ch/eawag/islandora/object/eawag%3A19601/datastream/PDF2/Mohammed-2019-Remote_sensing_of_solar-induced_chlorophyll-%28accepted_version%29.pdf

Remote sensing of solar-induced chlorophyll fluorescence (SIF) in vegetation: 50 years of progress.

Detection of solar-excited chlorophyll a fluorescence and leaf photosynthetic capacity using a fraunhofer line radiometer

Response of Leaf Spectral Reflectance in Loblolly Pine to Increased Atmospheric Ozone and Precipitation Acidity

The Conservation Reserve Program (CRP) has resulted in substantial acreage planted to trees in the southeastern United States. Some CRP sites have experienced excessive pine seedling mortality, and multiple plantings on some sites have failed to achieve adequate stocking. This work attempted to identify factors or complexes of agents that may account for this pine seedling mortality. Application of the highest labeled rate of sulfometuron (even on soil with a pH as high as 6.5) decreased loblolly pine (Pinus taeda L.) mortality on CRP sites. Herbicide residuals from prior agronomic crops do not appear to explain loblolly pine mortality on prior agricultural sites. Carbofuran decreased mortality of loblolly pine seedlings and reduced the frequency of damage by root-feeding insects, such as white grubs and white-fringe beetle larvae. Root-feeding insects; fungi, such as Fusarium spp. and Macrophomina spp.; and nematodes were present on most CRP sites sampled and may be causing or exacerbating pine mortality.

Factors associated with loblolly pine mortality on former agricultural sites in the Conservation Reserve Program

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Detection of solar-excited chlorophyll a fluorescence and leaf photosynthetic capacity using a fraunhofer line radiometer

Response of Leaf Spectral Reflectance in Loblolly Pine to Increased Atmospheric Ozone and Precipitation Acidity

Factors associated with loblolly pine mortality on former agricultural sites in the Conservation Reserve Program