# Application of Reflectance Indices for Remote Sensing of Plants and Revealing Actions of Stressors
## 3 Main Reflectance Indices
### Pigment Indices  
The reflectance in the region of visible light is strongly related to the content of photosynthetic pigments. These plant pigments play an important role in many physiological processes, including the absorption of light energy and its consumption [118], photoprotection [119; 120], photoreception [121; 122], and others. Changes in the concentrations of pigments can also be used for the estimation of forming stress responses in plants. Chlorophylls are the main photosynthetic pigments and have complicated spectra of light absorption. Their spectra include blue and red maximums, which differ for chlorophylls of different types. The absorption maximums of chlorophyll a and b, which are the main chlorophylls in higher plants, are 438 and 668 nm and 463 and 650 nm, respectively [123]. The region around 700 nm (\"red edge\") is interesting because it corresponds to the border between the spectral range with the high light absorption by chlorophylls (wavelengths are less than 700 nm) and the high light reflectance (wavelengths are more than 700 nm) [81]. As a result, this slope is sensitive to changes in the chlorophyll concentration. The other slope (\"green edge\"), which is placed at about 500-550 nm wavelengths, is also sensitive to chlorophyll concentration. The NIR region around 750-900 nm has low light absorption by chlorophylls and is insensitive to changes in chlorophyll concentrations; this region is widely used as the reference reflectance in the pigment reflectance indices [81; 82]. It should be noted that the absorption at the red edge is mostly caused by chlorophyll a; in contrast, the green edge is related to concentrations of chlorophyll a and b and, additionally, carotenoids [81]. Combinations of reflectance at these wavelengths can be used for the precise estimation of the concentration of chlorophylls. The recent meta-analysis [124] shows that RIs, which are the most effective for the estimation of the concentrations of total chlorophyll and chlorophyll a, should be calculated on the basis of combinations of reflectance at 550-560 nm (the green) and at 680-750 nm (the red edge); RIs that are the most effective for the estimation of the concentration of chlorophyll b should be calculated on the basis of combinations of reflectance at 630-660 nm (the red), at 670-710 nm (the red edge), and at 800-810 nm (NIR). It should be noted that these spectral regions include both measuring (sensitive) wavelengths and reference ones. The main reflectance indices that are sensitive to chlorophyll contents in plants are summarized in Part 3 of Table 1. There are the Chappelle index (\(\frac{R_{250}}{R_{700}}\)) [88], Datt index (\(\frac{R_{660}}{R_{700}R_{850}}\)) [89], and index proposed by [PERSON] et al. (2003) (\(\frac{R_{750-800}}{R_{660-750}}\) -1) [90], which can be more sensitive to concentrations of chlorophylls in plants than the VIS (e.g., more sensitive than the NDVI [90]) and allow the accurate estimation of the chlorophyll content in a wide range of concentrations [81; 90; 125; 126]. Additionally, the estimation of the ratio of carotenoid concentration to chlorophyll one (Car/Chl) can be important for the diagnosis of stress changes in plants. It is known that the degradation of chlorophylls under the action of stressors or at senescence is faster than the degradation of carotenoids [12; 13]. The different rates of the degradation of pigments cause non-proportional changes in the blue (absorption of the blue light is related to both carotenoids and chlorophylls) and red (absorption of the red light is related to chlorophylls) spectral regions [12]. The correlation with Car/Chl can be observed for the simple ratio pigment index (SRPI), normalized difference pigment index (NDPI), structure intensive pigment index (SIPI), normalized phaeophytinization index (NPQI), and plant senescence reflectance index (PSRI) [12; 91; 92].