Application of Reflectance Indices for Remote Sensing of Plants and Revealing Actions of Stressors

Abstract

Environmental conditions are very changeable; fluctuations in temperature, precipitation, illumination intensity, and other factors can decrease a plant productivity and crop. The remote sensing of plants under these conditions is the basis for the protection of plants and increases their survivability. This problem can be solved through measurements of plant reflectance and calculation of reflectance indices. Reflectance indices are related to the vegetation biomass, specific physiological processes, and biochemical compositions in plants; the indices can be used for both short-term and long-term plant monitoring. In our review, we considered the applications of reflectance indices in plant remote sensing. In Optical Methods and Platforms of Remote Sensing of Plants, we briefly discussed multi- and hyperspectral imaging, including descriptions of multispectral and hyperspectral cameras with different principles and their efficiency for the remote sensing of plants. In Main Reflectance Indices, we described the main reflectance indices, including vegetation, water, and pigment reflectance indices, as well as the photochemical reflectance index and its modifications. We focused on the relationships of leaf reflectance and reflectance indices to plant biomass, development, and physiological and biochemical characteristics. In Problems of Measurement and Analysis of Reflectance Indices, we discussed the methods of the correction of the reflectance indices that can be used for decreasing the influence of environmental conditions (mainly illumination, air, and soil) and plant characteristics (orientation of leaves, their thickness, and others) on their measurements and the analysis of the plant remote sensing. Additionally, the variability of plants was also considered as an important factor that influences the results of measurement and analysis.

Modified Photochemical Reflectance Indices as New Tool for Revealing Influence of Drought and Heat on Pea and Wheat Plants

“…In this case, resulting changes in PRI(531,570) can be strongly affected by moderate changes in experimental conditions. This explanation is in good accordance with results showing participation of transitions in the xanthophyll cycle [ 35 , 38 , 40 , 45 , 46 , 47 ], modifications of light scattering [ 49 , 50 ], and shifts in concentrations of carotenoids and chlorophylls [ 51 , 52 , 53 ] in changes in PRI(531,570). Additionally, this hypothesis can be supported by different changes in PRI(531,570) in similar experiments; e.g., we did not show significant changes in this index under the action of water shortage on pea plants in work [ 50 ], but showed a significant increase of PRI(531,570) in similar experiments (action of water shortage on pea plants) in work [ 44 ].…”

“…Development of methods of remote sensing to measure photosynthetic changes caused by action of stressors is an important scientific task contributing to both fundamental and applied photosynthetic investigations. Measurements of spectral characteristics of plant reflectance including multi- and hyperspectral imaging is the perspective method of remote sensing of photosynthetic processes [ 35 , 37 , 42 , 45 , 46 , 69 ] including their changes under action of stressors.…”

“…Considering this point, early revealing the photosynthetic responses can be important for protection of plants under actions of stressors and prediction of plant productivity. Measurements of plant reflectance at different spectral bands and calculation of reflectance indices (RIs) are important methods of this revealing [ 35 ]. Particularly, there are RIs (e.g., the normalized difference vegetation index (NDVI), normalized pigment chlorophyll ratio index (NPCI), MERIS terrestrial chlorophyll index (MTCI) and others [ 36 , 37 ]) that can be strongly correlated with photosynthetic parameters in plants.…”

“…Equation (1) can be used for PRI(531,570) calculation [ 42 , 45 , 46 , 47 ]: where R 531 and R 570 are the reflectance at 531 nm (the measuring wavelength) and 570 nm (the reference wavelength), respectively. It is traditionally considered [ 35 , 38 , 40 , 45 , 46 , 47 , 48 ] that fast changes in R 531 are mainly related to transitions in the xanthophyll cycle; however, induction of the light scattering in the green–yellow spectral range, which is probably related to chloroplast shrinkage [ 41 ], can also participate in development of these PRI(531,570) changes [ 35 , 40 , 49 , 50 ]. Slow changes in PRI(531,570) are caused by changes in ratio between concentrations of carotenoids and chlorophylls and changes in the total pool size of xanthophylls [ 51 , 52 , 53 ].…”

“…The dependence value of PRI(531,570) on the de-epoxidation of violaxanthin to zeaxanthin [ 35 , 38 , 40 , 45 , 46 , 47 ] is the basis of relations between PRI(531,570) and photosynthetic parameters because the xanthophyll cycle is an important mechanism of NPQ [ 54 , 55 ] depended on acidification of the lumen of chloroplasts. The chloroplast shrinkage [ 41 , 42 ] (and the light scattering in the green–yellow spectral range [ 56 , 57 , 58 ]) is also stimulated by this acidification and can support the relation between the photochemical reflectance index and photosynthetic parameters.…”