Proximal Imaging of Changes in Photochemical Reflectance Index in Leaves Based on Using Pulses of Green-Yellow Light

Abstract

Plants are affected by numerous environmental factors that influence their physiological processes and productivity. Early revealing of their action based on measuring spectra of reflected light and calculating reflectance indices is an important stage in the protection of agricultural plants. Photochemical reflectance index (PRI) is a widely used parameter related to photosynthetic changes in plants under action of stressors. We developed a new system for proximal imaging of PRI based on using short pulses of measuring light detected simultaneously in green (530 nm) and yellow (570 nm) spectral bands. The system has several advances compared to those reported in literature. Active light illumination and subtraction of the ambient light allow for PRI measurements without periodic calibrations. Short duration of measuring pulses (18 ms) minimizes their influence on plants. Measurements in two spectral bands operated by separate cameras with aligned fields of visualization allow one to exclude mechanically switchable parts like filter wheels thus minimizing acquisition time and increasing durability of the setup. Absolute values of PRI and light-induced changes in PRI (ΔPRI) in pea leaves and changes of these parameters under action of light with different intensities, water shortage, and heating have been investigated using the developed setup. Changes in ΔPRI are shown to be more robust than the changes in the absolute value of PRI which is in a good agreement with our previous studies. Values of PRI and, especially, ΔPRI are strongly linearly related to the energy-dependent component of the non-photochemical quenching and can be potentially used for estimation of this component. Additionally, we demonstrate that the developed system can also measure fast changes in PRI (hundreds of milliseconds and seconds) under leaf illumination by the pulsed green-yellow measuring light. Thus, the developed system of proximal PRI imaging can be used for PRI measurements (including fast changes in PRI) and estimation of stressors-induced photosynthetic changes.

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

“…In many handheld devices (see, e.g., the devices by Photon System Instruments, Czech), the positions of the leaf, light source, and light detector are fixed; this means that the measurements of the spectral characteristics of the reflected light can be very accurate. It should be noted that artificial illumination or a combination of the artificial illumination and sunlight can be used with these handheld systems [52] (e.g., halogen lamps are optimal for hyperspectral measurements [44] or light-emitting diodes for multispectral measurements [17]); in contrast, most of the flying mobile platforms for plant remote sensing cannot support the artificial illumination of objects (only sunlight is used because the distances between the camera and objects are too large). However, the velocity and spatial and temporal resolution of these measurements (Figure 4) are strongly limited by the speed of manual measurements; any automatization of the trajectory of the measurements is absent [33].…”

“…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].…”

“…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 [17,97,[138][139][140]: 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. Finally, it should be noted that the measurements of PRI can be successfully performed on the basis of different variants of the mobile platforms, including satellites (e.g., MODIS and GOME) [54]; in the last case, the PRI can be used for the estimation of GPP on ecosystem spatial scales.…”

“…Changeability of illumination parameters Using of white and black reflectance standard [22,141] Using short pulses of measuring light [17,39] Orientation of leaves and parameters of their surface Spectra correction using mathematical models of PROCOSIN and PROSPECT [142] Measuring at multitude angles. [143] Combination of 3D plant model and hyperspectral images [144,145] The atmosphere and soil influence on plant reflectance…”

“…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.…”