# Application of Reflectance Indices for Remote Sensing of Plants and Revealing Actions of Stressors
## 1 Introduction  
Agriculture is one of the most important fields of human activity because it is the basis for food security and industry. However, plant cultivation is a long-term process and requires various resources, including energy, water, fertilizer, etc. The consumption of the products of plant cultivation increases with the increasing population of Earth. Plant cultivation can be limited for several reasons: (i) the variability of environmental conditions, including changes in temperature, precipitation, illumination, and others [1, 2, 3]; (ii) the local human activity contributing to pollution, unregulated fertilization, disturbances of the water regime in the soil, etc. [4]; (iii) the global climate changes that cause the expansion of droughty (arid) regions, soil salimization, etc. [5]. Important targets of action of these stressors in plants are energetic processes (namely photosynthesis and respiration), mineral exchange, hormone synthesis, stress programs in plant cells, and others [3]. The short-term influence of the stressors can increase the plant survivability; in contrast, their prolonged action exhausts plants [3, 6]. Thus, the changes in environmental conditions become harmful for agricultural plants, which are strategically significant for regional development. The control of the plant cultivation under changeable environmental conditions becomes an important problem, which requires the development of the methods of plantremote sensing [7; 8]. Plant remote sensing is an important technology of a digital agriculture that provides the short-term and long-term monitoring of plants [9]. This sensing is mainly based on optical methods that are non-invasive, relatively simple, and high-throughput. Moreover, the important advantage of the optical methods is their safety for the environment. It is important that the modern optical methods can indicate plant physiological changes and damage that cannot be detected by a simple visual inspection. Particularly, measurements of reflected light at specific wavelengths and calculation of reflectance indices (RIs) are widely used methods of the estimation of the development, productivity, and forming stress changes in plants [9]. There are several advantages of the application of the reflectance indices. First, the calculation of RIs usually requires measurements of the reflected light at only two to three wavelengths [10]; i.e., simple technical solutions can be used for these measurements. Second, RIs can be related to specific physiological processes [11] and biochemical compositions [12; 13] in plants. This means that RIs can be used for the remote and non-invasive estimation of the details of plant development or stress changes [7; 14]. Third, the imaging of RIs can have different temporal and spatial resolutions; this property provides additional tools for plant remote sensing [15; 16]. Particularly, the spatial scale is the basis for the choice between the investigation of the single plant in detail and the complex analysis of the whole canopy of plants [9; 17]. The analysis of the plant level contributes to reveal specific characteristics of single plant organisms, e.g., the forming physiological changes in leaves caused by the actions of stressors, including drought, salinity, excess light, non-optimal temperatures [11; 18; 19], and others. In the future, this analysis can be the basis for the recognition of the type of stressor that acts on the plant because the spatial distribution of RIs in the plant organism can be related to this type [1; 16]. However, the spatial distribution of RIs in the canopy can also be heterogeneous [9; 16]. This variability in the canopy optical properties can be related to large-scale differences in biomass, structures of leaves and canopy, pigment concentration, and others [16; 20]. These differences can be caused by spatial heterogeneity in mineral nutrition [20], light intensity [16; 21], precipitation [1], and other environmental factors.