# Application of Reflectance Indices for Remote Sensing of Plants and Revealing Actions of Stressors
## 4 Problems of Measurement and Analysis of Reflectance Indices
### Variability of Plants  
The relation between the plant reflectance and physiological, biochemical, and anatomical properties is the key problem for plant remote sensing. Three regions that are related to changes in the content and composition of pigments (400-700 nm), anatomy of mesophyll cells in leaves and canopy structure (700-1300 nm), and water content in plants (1300-2500 nm) can be shown within the reflectance spectra of plants [7; 160]. The spatial heterogeneity of plants in the ecosystem, which can be caused by interspecific properties and characteristics for individual species, is a source of the variability in the reflectance measurements in all the spectral regions. Time-dependent fluctuations in environmental conditions are another source of this variability. The variability causes errors of reflectance measurements and increases the noise of the measured signal. The photosynthetic pigments influence the reflectance of the visible light because they are strongly related to the light absorption in this spectral range [7]. This light absorption by pigments is related to photosynthesis [81], and its stress changes include, e.g., the light dissipation by anthocyanins [130] and xanthophylls [12], which play a protective role for photosynthetic machinery. As a result, the displacement in the spectral peaks of the light absorption by pigments (and, thereby, in the reflectance spectra) shows disturbances in the balance between the processes of photosynthetic energy consumption and processes of photosynthetic stress changes [7]. These processes can be the basis for the spatial heterogeneity in the plant reflectance and its time-dependent fluctuations related to photosynthetic pigments. The changes in reflectance are caused by both short-term (seconds and minutes) and long-term (days, months, and seasons) processes. Shifts in the content of photosynthetic pigments are related to the long-term variability of plants. It is known that the composition of photosynthetic pigments is very variable for different species [150; 161]; this composition changes regarding the individual development of the plant [162; 163; 164], and it can be strongly dependent on local environmental conditions [161]. Particularly, the mineral nutrition is an important factor influencing concentrations of pigments in plants; e.g., changes in the nitrogen nutrition can influence concentrations of photosynthetic pigments and the content of RUBISCO, which is the key photosynthetic enzyme [7; 165; 166; 167; 168]. Thus, heterogeneity of the pigment composition can be the long-term factor modifying the efficiency of the plant remote sensing. It should be noted that mathematical models of radiation transfer (e.g., PROSAIL [150; 169]) are effectively used tools for the analysis of relations between the heterogeneity in the content of photosynthetic pigments and heterogeneity in RIs. On the other hand, the environmental conditions can induce short-term changes in the pigment light absorption, which can contribute increasing noise of signals through the stimulation of the time-dependent fluctuation in the reflectance and spatial heterogeneity in RIs. For example, the light fluctuations cause changes in the photosynthetic activity and increase the NPQ [21], which is the important photosynthetic protective mechanism [170]. The light intensity and spectrum profile can also influence the regulatory processes for chlorophyll [171], carotenoid [172], and anthocyanin [173] synthesis. Additionally, the spatial heterogeneity and time-dependent fluctuations in the environmental conditions can influence the characteristics of photosynthetic pigments through the plant signaling systems [174; 175]; these influences can also induce variability of reflectance spectra. Particularly, the pigment modifications under stress conditions can be induced by changes in the pH and Ca\({}^{2+}\) concentrations or ROS generation.