# Application of Reflectance Indices for Remote Sensing of Plants and Revealing Actions of Stressors
## 3 Main Reflectance Indices
### Photochemical Reflectance Index and Its Modifications  
In our review, we considered the photochemical reflectance index (PRI) and its modifications (Part 4 of the Table 1) as separate reflectance indices that are distinguished from other pigment indices. It is shown that the PRI can depend on the photosynthetic pigment concentrations and ratio between the concentrations of carotenoids and chlorophylls [2; 131]; i.e., the PRI and its modifications can be used as pigment indices that are sensitive to slow changes in concentrations of pigments [11; 132]. However, changes in the PRI can also be caused by short-term processes (minutes and even seconds), which are strongly associated with photosynthetic changes [133; 134]. There are several mechanisms of short-term changes in the PRI. The PRI is related to an activity of the xanthophyll cycle, which is an important mechanism of fast photosynthetic adaptation to the action of stressors (the non-photochemical quenching of chlorophyll fluorescence, NPQ) [11; 54]. Typical PRI is based on the reflected light at 531 nm (the measuring wavelength); the 531 nm wavelength is the center of the green shift in reflectance due to the de-epoxidation of violaxanthin to zeaxanthin via anteraxanthin [133]. This de-epoxidation, which is an important photosynthetic defense mechanism under excess intensity of illumination and action of other stressors, is induced by the acidification of the chloroplast lumen [11; 54]. The reflected light at 570 nm is usually used as a reference wavelength for PRI calculation because the reflectance at 570 nm changes weakly [54; 95; 96]. These pH-induced changes in the xanthophyll cycle are relatively quick: their characteristic time is about ten minutes [133]. Additionally, fast and superfast changes in the PRI are also shown in experiments. Fast changes in the PRI can be observed for several seconds [134] and can be caused by the light-induced shrinkage of chloroplasts; this shrinkage is also related to the acidification of lumen in chloroplasts [96; 135]. Superfast changes in the PRI can be formed for hundreds of milliseconds and, probably, are related to the electrochromic shift in the light absorption by carotenoids with 515 to 520 nm maximum [17]; this shift is induced by the activation of the electron-transport chain of chloroplasts and forming electrical gradient across the thylakoid membrane [136]. As a result of these complex mechanisms of changes in the PRI, a modification of the measuring wavelength (531 nm) at the PRI calculation can influence the sensitivity of this index to photosynthetic parameters. Modified PRIs calculated on the basis of 515 and 525 nm measuring wavelengths mainly show changes in the xanthophyll cycle; in contrast, modified PRIs calculated on the basis of 545 and 555 nm measuring wavelengths mainly show changes in the shrinkage of chloroplasts [96; 97]. This means that using these different measuring wavelengths for the PRI calculation seems to be promising for plant remote sensing and can be used for increasing the accuracy of the investigations of characteristics of fast photosynthetic changes. There are several meta-analyses [11; 54; 137] that show that the PRI (mainly the typical PRI) can be strongly related to the photosynthetic radiation-use efficiency (RUE), quantum yield of photosystem II, NPQ, and pigment composition. However, there are several factors that can strongly modify the values of correlations between the PRI and photosynthetic parameters (the leaf or canopy levels of measurements, the sunlight or artificial light at these measurements, different intensities of action of stressors on plants, and other factors) [54; 137]. The efficiency of the PRI using the estimation of the photosynthetic parameters in plants can be increased through the application of light-induced changes in this index [17; 138; 139; 140; 97]: the correlations of these changes with RUE, NPQ, and quantum yield of photosystems I and II are stronger than the correlations of the absolute values of PRI with these photosynthetic parameters.