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In this paper, a literature review about optical remote sensing (RS) of O(3) stress is presented. Studies on O(3)-induced effects on vegetation reflectance have been conducted since late '70s based on the analysis of optical RS data. Literature review reveals that traditional RS techniques were able to detect changes in leaf and canopy reflectance related to O(3)-induced stress when visible symptoms already occurred. Only recently, advanced RS techniques using hyperspectral sensors, demonstrated the feasibility of detecting the stress in its early phase by monitoring excess energy dissipation pathways such as chlorophyll fluorescence and non-photochemical quenching (NPQ). Steady-state fluorescence (Fs), measured by exploiting the Fraunhofer line depth principle and NPQ related xanthophyll-cycle, estimated through the photochemical reflectance index (PRI) responded to O(3) fumigation before visible symptoms occurred. This opens up new possibilities for the early detection of vegetation O(3) stress by means of hyperspectral RS.

Two C3 dicotyledonous crops and five C4 monocotyledons treated with three levels of nitrogen were used to evaluate quantitatively the relationship between the allocation of absorbed light energy in PSII and photosynthetic rates (P N) in a warm condition (25–26°C) at four to five levels [200, 400, 800, 1,200 (both C3 and C4) and 2,000 (C4 only) μmol m−2 s−1] of photosynthetic photon flux density (PPFD). For plants of the same type (C3 or C4), there was a linear positive correlation between the fraction of absorbed light energy that was utilized in PSII photochemistry (P) and P N, regardless of the broad range of their photosynthetic rates due to species-specific effect and/or nitrogen application; meanwhile, the fraction of absorbed light energy that was dissipated through non-photochemical quenching (D) showed a negative linear regression with P N for each level of PPFD. The intercept of regression lines between P and P N of C3 and C4 plants decreased, and that between D and P N increased with increasing PPFD. With P and D as the main components of energy dissipation and complementary to each other, the fraction of excess absorbed light energy (E) was unchanged by P N under the same level of PPFD. At the same level of P N, C4 plants had lower P and higher D than C3 plants, due to the fact that C4 plants with little or no photorespiration is considered a limited energy sink for electrons. Nevertheless there was a significant negative linear correlation between D and P when data from both C3 and C4 plants at varied PPFD levels was merged. The slope of regression lines between P and D was 0.85, indicating that in plants of both types, most of the unnecessary absorbed energy (ca. 85%) could dissipate through non-photochemical quenching, when P was inhibited by low P N due to species-specific effect and nitrogen limitation at all levels of illumination used in the experiment.

The light-use efficiency (LUE) of a mature Canadian Douglas-fir forest (DF49) was studied using high-resolution in situ temporal, spatial, and spectral measurements in conjunction with fluxes acquired from an instrumented tower. We examined the photochemical reflectance index (PRI), a spectral index responsive to high light conditions that alters reflectance at 531 nm, in combination with several alternative reference bands at 551, 570, and 488 nm. These indices were derived from directional reflectance spectra acquired by a hyperspectral radiometer system mounted on the DF49 tower, viewing the canopy through almost 360° rotations multiple times an hour daily throughout the 2006 growing season. From canopy structure information, three foliage sectors within the canopy were delineated according to instantaneous illumination conditions (sunlit, shaded, and mixed shaded–sunlit). On sunny days, the PRI indices for the sunlit foliage sector captured high light-induced stress responses, expressed as significant...

Life on Earth depends on photosynthesis. Photosynthetic systems evolved early in Earth history and have been stable for 2.5 billion years, providing prima facie evidence for the significance of pigments in plant functions. Photosynthetic pigments fill multiple roles from increasing the range of energy captured for photosynthesis to protective functions. Given the importance of pigments to leaf functioning, greater effort is needed to determine whether individual pigments can be identified and quantified in vivo using high fidelity spectroscopy. We review recent advances in detecting plant pigments at the leaf level and discuss successes and reasons why challenges remain for robust remote observation and quantification. New methods to identify and quantify individual pigments in the presence of overlapping absorption features would provide a major advance in understanding their biological functions, quantifying net carbon exchange, and identifying plant stresses.

We investigated the relationships of photosynthetic capacity (Pnsat, near light-saturated net photosynthetic rate measured at 1,200 μmol m−2 s−1 PPFD) to photosystem II efficiency (Fv/Fm) and to photochemical reflectance index [PRI = (R531 − R570)/(R531 + R570)] of Pinus taiwanensis Hay. needles at high (2,600 m a.s.l) and low-elevation (800 m a.s.l) sites through different seasons. Results indicate that at high-elevation site, Pnsat, Fv/Fm and PRI (both measured at predawn) paralleled in general with the air temperature. On the coolest measuring day with the minimum air temperature dropping to −2°C, Pnsat could decrease to ca. 15% of its highest value, which was measured in autumn. At low-elevation site, with the minimum air temperature of 10–12°C in cooler season and almost no seasonal variation of Fv/Fm, Pnsat dropped to ca. 65% of its highest value and PRI decreased ca. 0.02 in winter. Even though seasonal variation of Pnsat was affected by many factors, it was still closely related to PRI based on statistical analyses using data from both sites, through different seasons. On the contrary, seasonal variation of Fv/Fm of P. taiwanensis needles was influenced mainly by low temperature at high elevation. Therefore, the correlation of Pnsat − Fv/Fm was lower than that of Pnsat − PRI when data combined from both high- and low-elevation sites were analyzed. It is concluded that predawn PRI could be used as an indicator to estimate the seasonal potential of photosynthetic capacity of P. taiwanensis grown at low- and high-elevations of sub-tropical Taiwan.

The estimation of the carbon balance in ecosystems, regions, and the biosphere is currently one of the main concerns in the study of the ecology of global change. Current remote sensing methodologies for estimating gross primary productivity are not satisfactory because they rely too heavily on (i) the availability of climatic data, (ii) the definition of land-use cover, and (iii) the assumptions of the effects of these two factors on the radiation-use efficiency of vegetation (RUE). A new methodology is urgently needed that will actually assess RUE and overcome the problems associated with the capture of fluctuations in carbon absorption in space and over time. Remote sensing techniques such as the widely used reflectance vegetation indices (e.g. NDVI, EVI) allow green plant biomass and therefore plant photosynthetic capacity to be assessed. However, there are vegetation types, such as the Mediterranean forests, with a very low seasonality of these vegetation indices and a high seasonality of carbon uptake. In these cases it is important to detect how much of this capacity is actually realized, which is a much more challenging goal. The photochemical reflectance index (PRI) derived from freely available satellite information (MODIS sensor) presented for a 5-year analysis for a Mediterranean forest a positive relationship with the RUE. Thus, we show that it is possible to estimate RUE and GPP in real time and therefore actual carbon uptake of Mediterranean forests at ecosystem level using the PRI. This conceptual and technological advancement would avoid the need to rely on the sometimes unreliable maximum RUE.

The light use efficiency (LUE) of photosynthesis dynamically adapts to environmental factors, and this leads to complex spatio-temporal variations of photosynthesis on various scales from the leaf to the canopy level. These spatio-temporal pattern formations not only help to understand the regulatory properties of photosynthesis, but may also have a constructive role in maintaining stability in metabolic pathways and during development. Optical remote sensing techniques have the potential to detect physiological and biochemical changes in plant ecosystems, and non-invasive detection of changes in photosynthetic energy conversion may be of great potential for managing agricultural production in a future bio-based economy. Here we review the results from selected remote sensing projects for their potential to quantify LUE from the level of single leaves to the canopy scale. In a case study with soybean grown under elevated CO2 conditions at the SoyFACE facility, we tested the photochemical reflectance index (PRI) for its capacity to quantify higher photosynthetic efficiency. In this study the PRI failed to detect differences in photosynthetic light conversion, most likely because of the variable canopy structure of the soybean canopy. We thus conclude that at the current state of the art the PRI cannot serve as an easy remote sensing approach to detect changes in photosynthetic energy conversion in agriculture. As an alternative we present approaches that aim to quantify the fluorescence signal of chlorophyll and thus estimate photosynthetic efficiency. In a second case study, using avocado as a model species, an active laser induced fluorescence transient (LIFT) method was applied to deliver maps of different photosynthetic efficiency within the canopy. Cold-induced down-regulation of photosynthesis in the upper canopy was detected, so active fluorescence may prove its potential for non-invasive monitoring of crops. With a view to the future, we present a method for large scale managing of agricultural practices within the framework of the FLuorescence EXplorer (FLEX) mission, which proposed launching a satellite for the global monitoring of steady-state chlorophyll fluorescence in terrestrial vegetation. This mission was selected for inclusion in pre-phase A by the European Space Agency.

Cottonwoods (Populus spp.) are dioecious phreatophytes of hydrological and ecological importance in riparian woodlands throughout the Northern Hemisphere. In streamside zones of southern Alberta, groundwater and soil water typically decline between May and September. To understand how narrowleaf cottonwoods (Populus angustifolia James) are adapted to this seasonal decrease in water availability, we measured photosynthetic gas exchange, leaf reflectance, chlorophyll fluorescence and stable carbon isotope composition (delta(13)C) in trees growing in the Oldman River valley of southern Alberta during the 2006 growth season. Accompanying the seasonal recession in river flow, groundwater table depth (Z(gw)) declined by 1.6 m, but neither mean daily light-saturated net photosynthetic rate (A(max)) nor stomatal conductance (g(s)) was correlated with this change. Both A(max) and g(s) followed a parabolic seasonal pattern, with July 24 maxima of 15.8 micromol m(-2) s(-1) and 559 mmol m(-2) s(-1), respectively. The early summer rise in A(max) was related to an increase in the chlorophyll pool during leaf development. Peak A(max) coincided with the maximum quantum efficiency of Photosystem II (F(v)/F(m)), chlorophyll index (CI) and scaled photochemical reflectance index (sPRI), but occurred one month after maximum volumetric soil water (theta(v)) and minimum Z(gw). In late summer, A(max) decreased by 30-40% from maximum values, in weak correlation with theta(v) (r(2) = 0.50). Groundwater availability limited late-season water stress, so that there was little variation in mean daily transpiration (E). Decreasing leaf nitrogen (% dry mass), CI, F(v)/F(m) and normalized difference vegetation index (NDVI) were also consistent with leaf aging effects. There was a strong correlation between A(max) and g(s) (r(2) = 0.89), so that photosynthetic water-use efficiency (WUE; A(max)/E) decreased logarithmically with increasing vapor pressure deficit in both males (r(2) = 0.75) and females (r(2) = 0.95). The male:female ratio was unequal (2:1, chi(2) = 16.5, P < 0.001) at the study site, but we found no significant between-sex differences in photosynthetic gas exchange, leaf reflectance or chlorophyll fluorescence that might explain the unequal ratio. Females tended to display lower NDVI than males (P = 0.07), but mean WUE did not differ significantly between males and females (2.1 +/- 0.2 versus 2.5 +/- 0.2 mmol mol(-1)), and delta(13)C remained in the -28.8 to -29.3 per thousand range throughout the growth season, in both sexes. These results demonstrate changes in photosynthetic and water-use characteristics that collectively enable vigorous growth throughout the season, despite seasonal changes in water supply and demand.

We show that observed co-variations at sub-hourly time scales between the photochemical reflectance index (PRI) and canopy light use efficiency (LUE) over a Douglas-fir forest result directly from sub-hourly leaf reflectance changes in a 531 nm spectral window roughly 50 nm wide. We conclude then, that over a forest stand we are observing the direct effects of photosynthetic down-regulation on leaf-level reflectance at 531 nm. Key to our conclusion is our ability to simultaneously measure the LUE and reflectance of the Douglas-fir stand as a function of shadow fraction from the “hot spot” to the "dark spot"dark spot” and a new finding herein, based on radiative transfer theory, that the magnitude of a normalized reflectance difference index (NDRI) such as PRI can vary with shadow fraction only in case the reflectance of the shaded and sunlit leaves differ in at least one of the NDRI bands . Our spectrometer measurements over a nearly 6 month period show that at a forest stand scale, only two NDRIs (both containing a band near 570 nm) vary with shadow fraction and are correlated with LUE; an NDRI with a band centered at 531 nm roughly 50 nm wide, and another near 705 nm. Therefore, we are able to conclude that only these two bands' reflectance differ between the sunlit and the shaded elements of the canopy . Their reflectance changes on time scales of a few minutes or less. Our observations also show that the reflectance changes at 531 nm are more highly correlated with variations in canopy light use efficiency when only sunlit canopy elements are viewed (the hot spot), than when only shaded elements (the dark spot) are viewed. Taken together then, these results demonstrate that the observed sub-hourly changes in foliage reflectance at 531 nm and 705 nm can only result from corresponding variations in photosynthetic rates. The importance of our results are as follows: (1) We show that variations in PRI with LUE are a direct result of rapid changes in foliage reflectance at 531 nm resulting from photosynthetic down-regulation, and can be observed at forest scales. (2) Our findings also suggest a new sensor and methodology for the direct retrieval from space of changes in forest LUE by measuring PRI as a function of shadow fraction using a multi-angle spectrometer simultaneously retrieving both shadow fraction and PRI.

Monitoring of photosynthetic efficiency ( e ) over space and time is a critical component of climate change research as it is a major determinant of the amount of carbon accumulated by terrestrial ecosystems. While the past decade has seen progress in the remote estimation of e at the leaf, canopy and stand level using the photochemical reflectance index PRI (based on the normalized difference of reflectance at 531 and 570 nm), little is known about the temporal and spatial requirements for up-scaling PRI to landscape and global levels using satellite observations. One potential way to investigate these requirements is using automated tower-based remote sensing platforms, which observe stand level reflectance with high spatial, temporal, and spectral resolution. Prediction of e from PRI diurnally or over a full year requires observations of canopy reflectance over multiple view and sun-angles. As a result, these observations are subject to directional reflectance effects which can be interpreted in terms of the bidirectional reflectance distribution function (BRDF) using semi-empirical kernel driven models. These semi-empirical models use a combination of physically based BRDF shapes and empirical observations to standardize multi-angular observations to a common viewing and illumination geometry. Directional reflectance effects are thereby modeled as a linear superposition of mathematical kernels, representing the bi-direction variation in reflectance from isotropic, geometric, and volumetric scattering components of the vegetation canopy. However, because variations in plant physiological conditions can also introduce bidirectional reflectance variations, we introduce an approach to separate bidirectional effects arising purely from plant physiological status from other effects by stratifying PRI observations into categories based on environmental conditions for which the expected physiological variability is low. Within each of these PRI strata, the derived physically based BRDF shapes were used to standardize multi-angular PRI measurements to a common viewing and illumination geometry. The method significantly enhanced the relationship found between PRI and e (from r 2 = 0.38 for the directionally uncorrected case to r 2 = 0.82 for the directionally corrected case) from data measured continuously over the course of 1 year over an evergreen conifer forest using an automated platform. Results show that isotropic PRI scattering is highly correlated to changes in e , while geometric scattering can be related to canopy level shading. Instrumentation and approaches such as the one demonstrated in this study may be integrated into current efforts aiming at predicting e at global scales using satellite observations.

Direct estimation of photosynthetic light-use efficiency (LUE) from space would be of significant benefit to LUE-based models which use inputs from remote sensing to estimate terrestrial productivity. The Photochemical Reflectance Index (PRI) has shown promise in tracking LUE at the leaf- to small canopy levels, but its use at regional to global scales still remains a challenge. In this study, we used different formulations of PRI calculated from the MODIS ocean band centered at 531 nm and a set of alternative reference bands at 488, 551, and 678 nm to explore the relationship between PRI and LUE where LUE was measured at eight eddy covariance flux towers located in the boreal forest of Saskatchewan, Canada. The magnitude and variability of LUE was significantly lower at the times when useful MODIS ocean band images were available (i.e. around midday under clear-sky conditions) relative to the rest of the growing season. PRI678 (reference band at 678 nm) showed the strongest relationship (r2 = 0.70) with LUE90a (i.e. 90-minute mean LUE calculated using Absorbed Photosynthetically Active Radiation, APAR), but only when all sites were combined. Overall, the relationships between the MODIS PRIs and LUE90a were always stronger for observations closer to the backscatter direction and there were no significant differences in the strength of the correlations whether LUE was calculated based on incident PAR or on APAR. Predictions of ecosystem photosynthesis at the time of the MODIS overpasses were significantly improved by multiplying either PAR or APAR by MODIS PRI (r2 improved from 0.09 to 0.44 and 0.54 depending on the PRI formulation). We used our PRI-LUE model to create a regional LUE90a map for the three cover types covering 47,500 km2 around the flux sites. The MODIS PRI-derived LUE90a map appeared to capture more realistic spatial heterogeneity of LUE across the landscape compared to a daily LUE map derived using the look-up table in the MODIS GPP (MOD17) algorithm. While our LUE map is only a snapshot of minimum regional LUE90a values, with appropriate gap-filling methods it could be used to improve regional-scale monitoring of GPP. Moreover, the strong relationship between midday and daily LUE on clear days (r2 = 0.93) indicates that instantaneous MODIS observations of LUE90a could be used to estimate daily LUE. Finally, pixel shadow fraction from the 5-Scale geometric-optical model was closely related to both MODIS PRI and tower-derived LUE suggesting that differences in stand leaf area and in diffuse illumination among flux sites play a role in the relationship we observed between LUE and MODIS PRI.

View angle effects present in spectral vegetation indices can either be regarded as an added source of uncertainty for variable retrieval or as a source of additional information, enhancing the variable retrieval; however, the magnitude of these angular effects remains for most indices unknown or unquantified. We use the ESA-mission CHRIS-PROBA (Compact High Resolution Imaging Spectrometer onboard the Project for On-board Autonomy) providing spaceborne imaging spectrometer and multiangular data to assess the reflectance anisotropy of broadband as well as recently developed narrowband indices. Multiangular variability of Hemispherical Directional Reflectance Factor (HDRF) is a prime factor determining the indices´ angular response. Two contrasting structural vegetation types, pine forest and meadow, were selected to study the effect of reflectance anisotropy on the angular response. Calculated indices were standardized and statistically evaluated for their varying HDRF. Additionally we employ a coupled radiative transfer model (PROSPECT/FLIGHT) to quantify and substantiate the findings beyond an incidental case study. Nearly all tested indices manifested a prominent anisotropic behaviour. Apart from the conventional broadband greenness indices [e.g. Simple Ratio Index (SRI), Normalized Difference Vegetation Index (NDVI)], light use efficiency and leaf pigment indices [e.g. Structure Insensitive Pigment Index (SIPI), Photochemical Reflectance Index (PRI) and Anthocyanin Reflectance Index (ARI)] did express significant different angular responses depending on the vegetation type. Following the quantification of the impact, we conclude that the angular-dependent fraction of non-photosynthetic material is of critical importance shaping the angular signature of these VIs. This work highlights the influence of viewing geometry and surface reflectance anisotropy, particularly when using light use efficiency and leaf pigment indices.

This article explores the utility of spectral reflectance signals to assess changes in the photosynthetic efficiency (?PSII) of Sphagnum mosses exposed to reductions in water availability. Reflectance was measured in parallel to moisture content and chlorophyll fluorescence in five species of Sphagnum exposed to progressive drought. Decreases in moisture availability caused a significant reduction in ?PSII for all samples tested. An objective was to ascertain whether Sphagnum ?PSII was better correlated with (i) spectral indices directly related to photosynthetic processes through association with xanthophyll cycle pigmentation (the photochemical reflectance index; PRI), or (ii) indices indirectly correlated with photosynthetic activity but able to detect changes in canopy morphology (the normalized difference vegetation index; NDVI), chlorophyll stability (the structure insensitive pigment index; SIPI), or canopy moisture content (the floating water band index; fWBI). Strongest correlations were found between ?PSII and indices which were indirectly related to ?PSII. Both the SIPI and the NDVI exhibited linear correlations although these relationships were sometimes sample-specific. The fWBI was the index least affected by sample-specific relationships and showed a strong curvilinear correlation with ?PSII. Photosynthetic efficiency was correlated with the PRI but relationships were much weaker than for the other indices used, and in some cases negative. The NDVI and the fWBI show the most potential for monitoring Sphagnum photosynthetic activity at the ecosystem scale. The results from this study will help to monitor and understand the responses of these key species to hydrological disturbances.

We explored simple and useful spectral indices for estimating photosynthetic variables (radiation use efficiency and photosynthetic capacity) at a canopy scale based on seasonal measurements of hyperspectral reflectance, ecosystem CO2 flux, and plant and micrometeorological variables. An experimental study was conducted over the simple and homogenous ecosystem of an irrigated rice field. Photosynthetically active radiation absorbed by the canopy (APAR), the canopy absorptivity of APAR (fAPAR), net ecosystem exchange of CO2 (NEECO2) gross primary productivity (GPP), photosynthetic capacity at the saturating APAR (Pmax), and three parameters of radiation use efficiency (eN: NEECO2/APAR; eG: GPP/APAR; φ: quantum efficiency) were derived from the data set. Based on the statistical analysis of relationships between these ecophysiological variables and reflectance indicators such as normalized difference spectral indices (NDSI[i,j]) using all combinations of two wavelengths (i and j nm), we found several new indices that would were more effective than conventional spectral indices such as photochemical reflectance index (PRI) and normalized difference vegetation index (NDVI = NDSI[near-infrared, red]). eG was correlated well with NDSI[710, 410], NDSI[710, 520], and NDSI[530, 550] derived from nadir measurements. φ was best correlated with NDSI[450, 1330]. NDSI[550, 410] and NDSI[720, 420] had a consistent linear relationships with fAPAR throughout the growing season, whereas conventional indices such as NDVI showed very different relationships before and after heading. Off-nadir measurements were more closely related to the efficiency parameters than nadir measurements. Our results provide useful insights for assessing plant productivity and ecosystem CO2 exchange, using a wide range of available spectral data as well as useful information for designing future sensors for ecosystem observations.

View angle effects present in spectral vegetation indices can either be regarded as an added source of uncertainty for variable retrieval or as a source of additional information, enhancing the variable retrieval; however, the magnitude of these angular effects remains for most indices unknown or unquantified. We use the ESA-mission CHRIS-PROBA (Compact High Resolution Imaging Spectrometer onboard the Project for On-board Autonomy) providing spaceborne imaging spectrometer and multiangular data to assess the reflectance anisotropy of broadband as well as recently developed narrowband indices. Multiangular variability of Hemispherical Directional Reflectance Factor (HDRF) is a prime factor determining the indices´ angular response. Two contrasting structural vegetation types, pine forest and meadow, were selected to study the effect of reflectance anisotropy on the angular response. Calculated indices were standardized and statistically evaluated for their varying HDRF. Additionally we employ a coupled radiative transfer model (PROSPECT/FLIGHT) to quantify and substantiate the findings beyond an incidental case study. Nearly all tested indices manifested a prominent anisotropic behaviour. Apart from the conventional broadband greenness indices [e.g. Simple Ratio Index (SRI), Normalized Difference Vegetation Index (NDVI)], light use efficiency and leaf pigment indices [e.g. Structure Insensitive Pigment Index (SIPI), Photochemical Reflectance Index (PRI) and Anthocyanin Reflectance Index (ARI)] did express significant different angular responses depending on the vegetation type. Following the quantification of the impact, we conclude that the angular-dependent fraction of non-photosynthetic material is of critical importance shaping the angular signature of these VIs. This work highlights the influence of viewing geometry and surface reflectance anisotropy, particularly when using light use efficiency and leaf pigment indices.

Although the link between CO2 exchange and spectral reflectance is well established in laboratory conditions, limited research has been conducted in the field. To determine the applications of remote sensing in the estimation of the northern mixed grasslands as a carbon sink, the primary objective of this study was to evaluate several narrow-band vegetation indices and band depth analysis in the prediction of leaf CO2 exchange rates in a northern mixed grass prairie ecosystem. Spectral reflectance and CO2 exchange measurements were collected from 13 sites located in Grasslands National Park, Saskatchewan, Canada. Pearson's correlation found a significant relationship between the CO2 exchange rates and the Photochemical Reflectance Index (PRI). Linear regression showed that the PRI explained 46% of the variance seen in the leaf CO2 exchange rates. This is somewhat lower than previous experiments conducted in laboratory conditions; however, the current study was conducted in field conditions, where there are a number of different species in the field of view and background effects from soil, litter and dead materials, which are negligible in laboratory conditions.

The aim of this study was to evaluate the accuracy of the spectro-optical, photochemical reflectance index (PRI) for quantifying the disease index (DI) of yellow rust (Biotroph Puccinia striiformis) in wheat (Triticum aestivum L.), and its applicability in the detection of the disease using hyperspectral imagery. Over two successive seasons, canopy reflectance spectra and disease index (DI) were measured five times during the growth of wheat plants (3 varieties) infected with varying amounts of yellow rust. Airborne hyperspectral images of the field site were also acquired in the second season. The PRI exhibited a significant, negative, linear, relationship with DI in the first season (r 2 = 0.91, n = 64), which was insensitive to both variety and stage of crop development from Zadoks stage 3–9. Application of the PRI regression equation to measured spectral data in the second season yielded a coefficient of determination of r 2 = 0.97 (n = 80). Application of the same PRI regression equation to airborne hyperspectral imagery in the second season also yielded a coefficient of determination of DI of r 2 = 0.91 (n = 120). The results show clearly the potential of PRI for quantifying yellow rust levels in winter wheat, and as the basis for developing a proximal, or airborne/spaceborne imaging sensor of yellow rust in fields of winter wheat.

Attempts to estimate photosynthetic rate or gross primary productivity from remotely sensed absorbed solar radiation depend on knowledge of the light use efficiency (LUE). Early models assumed LUE to be constant, but now most researchers try to adjust it for variations in temperature and moisture stress. However, more exact methods are now required. Hyperspectral remote sensing offers the possibility of sensing the changes in the xanthophyll cycle, which is closely coupled to photosynthesis. Several studies have shown that an index (the photochemical reflectance index) based on the reflectance at 531 nm is strongly correlated with the LUE over hours, days and months. A second hyperspectral approach relies on the remote detection of fluorescence, which is a directly related to the efficiency of photosynthesis. We discuss the state of the art of the two approaches. Both have been demonstrated to be effective, but we specify seven conditions required before the methods can become operational.

Recent advances in spectral imaging technology have enabled the development of models that estimate various crop parameters from spectral imagery data. We developed partial least square (PLS) models to predict fruit yield of Satsuma mandarin using airborne hyperspectral imagery obtained several months before harvesting. Hyperspectral images in the 72 visible and near-infrared (NIR) wavelengths (from 407 to 898 nm) were acquired over a citrus orchard during the early growing seasons of 2003, 2004 and 2005. The canopy features of individual trees were identified using pixel-based average spectral reflectance values for all 72 wavelengths from the acquired images. The acquired canopy features were then used as prediction variables to develop yield prediction models. These were developed using three techniques: (1) normalized difference vegetation index (NDVI), simple ratio (SR) and photochemical reflectance index (PRI), (2) conventional multiple linear regression (MLR) models, and (3) PLS regression models. As we intended to predict yield several months before the harvesting season (generally late December), the conventional techniques (vegetation indices and MLR) did not predict well. In contrast, PLS models gave successful predictions for the three years. These results confirmed the hypothesized correlation between canopy features and citrus yield. The successful forecasting of yields several months or even one year ahead of the harvest season is expected to contribute to planning harvest schedules, generating prescription maps for dealing with fluctuations of yield in specific trees, control measures, and management practices.

The moderate-resolution spectroradiometer (MODIS) sensor has provided estimates of near-real-time gross and annual net primary production (GPP and NPP) since March 2000. We compare 2 years (2000 and 2001) of satellite-based calculations of GPP in the seasonally wet tropical rainforests of north Queensland, Australia, with estimates derived from an ecosystem process model, physiological principles predicting growth using satellite data (3-PGS), that was previously calibrated with ground-based measurements at 21 sites. This comparison provides an independent evaluation of the MOD-17 GPP product because the 3-PGS simulations account for spatial variation in soil properties (available soil water and fertility) and rely on different sources of meteorological data and derived estimates of the fraction of photosynthetically active radiation (fPAR). Results of the comparison show that estimates of averaged annual and monthly GPP from MOD-17 are slightly higher in magnitude than those from 3-PGS, although the two ...

Photosynthetic efficiency of higher plants dynamically adapts to changing light intensity and is greatly influenced by stress, such as water stress. We tested a new portable hyperspectral imaging system, the SOC-700, manufactured by Surface Optics, which produces 12-bit reflectance images between 440 nm and 880 nm with a 4 nm spectral resolution. We quantified the reflectance properties and photochemical reflectance index (PRI) during light adaptation of genetically modified Arabidopsis thaliana (L.) Heynh. plants lacking or over-expressing the PsbS protein, an essential component of the mechanism of non-photochemical dissipation. In a second experiment, PRI images of gradually water stressed leaves were compared to leaf-level measurements of reflectance using a second commercially available spectrometer, and chlorophyll fluorescence to detect dynamic, photosynthesis correlated changes in reflectance and PRI. In both experiments PRI measured with the SOC-700 changed, reflecting the biochemical adaptation of the photosynthetic apparatus to high light intensity (dynamic changes within minutes) and the gradual deactivation of photosynthesis during drying (changes within hours). The quantum efficiency of photosystem II (� F/Fm� ) and non-photochemical energy dissipation (NPQ) measured from chlorophyll fluorescence, were strongly correlated with PRI. Leaf area PRI values estimated from individual pixel spectra of the SOC-700 quantified photosynthetic efficiency more thoroughly than PRI values calculated from point measurements using the hand-held GER-1500. The applications, limitations, and potential of the SOC-700 for plant eco-physiology and remote sensing are also discussed.

The xanthophyll cycle is critical for protecting the photosynthetic apparatus from light-induced oxidative stress. A clear view of the xanthophyll cycle is thus important for understanding abiotic stresses that are closely related to plant growth and reproduction. The epoxidation state (EPS) is well correlated with the photosynthetic radiation use efficiency, and is widely used for assessing the xanthophyll cycle. The hyperspectral index, photochemical reflectance index (PRI), has been claimed to be closely related with the EPS, and offers instantaneous information of photosynthetic activity: its applications are, however, largely limited to herbaceous and coniferous species, and few studies have ever focussed on both sunlit and shaded leaves of deciduous plants. In the present study, we examined the possibility of applying PRI for tracing the xanthophyll cycle in a typical deciduous species (Fagus crenata Blume) as well as other species in a cold-temperate mountainous area. This is based on a series of experiments with only light stress and other inhibited treatments. Furthermore, searching for new hyperspectral indices has also been attempted based on both original and first derivative spectra. Results revealed that PRI had low correlations with the EPS of deciduous leaves, especially for sunlit leaves. As a comparison, the newly identified dD677, 803, a differential type of index using reflectance derivatives at 677 and 803 nm, had a much better performance. The robustness of the newly identified index has been confirmed from both inhibitor-treatments and an additional dataset from other deciduous species. The proposed index is hence applicable for tracing the xanthophyll cycle in deciduous species.

This study systematically analyzed the performance of multivariate hyperspectral vegetation indices of rice (Oryza sativa L.) in estimating the arsenic content in agricultural soils. Field canopy reflectance spectra was obtained in the jointing-booting growth stage of rice. Newly developed and published multivariate vegetation indices were initially calculated to estimate soil arsenic content. The well-performing vegetation indices were then selected using successive projections algorithm (SPA), and the SPA selected vegetation indices were adopted to calibrate a multiple linear regression model for estimating soil arsenic content. Results showed that a three-band vegetation index (R716-R568)/(R552-R568) performed best in the newly developed vegetation indices in estimating soil arsenic content. The photochemical reflectance index (PRI) and red edge position (REP) performed well in the published vegetation indices. Moreover, the linear combination of two vegetation indices ((R716-R568)/(R552-R568) and REP) selected using SPA improved the estimation of soil arsenic content. These results indicated that the newly developed three-band vegetation index (R716-R568)/(R552-R568) might be recommended as an indicator for estimating soil arsenic content in the study area. PRI and REP could be used as universal vegetation indices for monitoring soil arsenic contamination.

In remote sensing, the Photochemical Reflectance Index (PRI) provides insight into physiological processes occurring inside leaves in a plant stand. Developed by 1,2 , PRI evolved from laboratory reflectance measurements of individual leaves. Yet in a remotely sensed image, a pixel measurement may include light from both reflecting and transmitting leaves. We compared values of PRI based upon polarized reflectance and transmittance measurements of water and nutrient stressed leaves. Our results show the polarized leaf surface reflection should be removed when calculating PRI and that the leaf physiology information is in leaf interior reflectance, not leaf transmittance.

As part of the EnMAP preparation activities this study aims at estimating the uncertainty in the EnMAP L2A ground reflectance product using the simulated scene of Barrax, Spain. This dataset is generated using the EnMAP End-to-End Simulation tool, providing a realistic scene for a well-known test area. Focus is set on the influence of the expected radiometric calibration stability and the spectral calibration stability. Using a Monte-Carlo approach for uncertainty analysis, a larger number of realisations for the radiometric and spectral calibration are generated. Next, the ATCOR atmospheric correction is conducted for the test scene for each realisation. The subsequent analysis of the generated ground reflectance products is carried out independently for the radiometric and the spectral case. Findings are that the uncertainty in the L2A product is wavelength-dependent, and, due to the coupling with the estimation of atmospheric parameters, also spatially variable over the scene. To further illustrate the impact on subsequent data analysis, the influence on two vegetation indices is briefly analysed. Results show that the radiometric and spectral stability both have a high impact on the uncertainty of the narrow-band Photochemical Reflectance Index (PRI), and also the broad-band Normalized Difference Vegetation Index (NDVI) is affected.

The photochemical reflectance index (PRI) is a proxy for the activity of the photoprotective xanthophyll cycle and photosynthetic light use efficiency (LUE) in plants. Evergreen conifers downregulate photosynthesis in autumn in response to low temperature and shorter photoperiod, and the dynamic xanthophyll cycle-mediated non-photochemical quenching (NPQ) is replaced by sustained NPQ. We hypothesized that this shift in xanthophyll cycle-dependent energy partitioning during the autumn is the cause for variations in the PRI-LUE relationship. In order to test our hypothesis, we characterized energy partitioning and pigment composition during a simulated summer-autumn transition in a conifer and assessed the effects of temperature and photoperiod on the PRI-LUE relationship. We measured gas exchange, chlorophyll fluorescence and leaf reflectance during the photosynthetic downregulation in Pinus strobus L. seedlings exposed to low temperature/short photoperiod or elevated temperature/short photoperiod conditions. Shifts in energy partitioning during simulated autumn were observed when the pools of chlorophylls decreased and pools of photoprotective carotenoids increased. On a seasonal timescale, PRI was controlled by carotenoid pool sizes rather than xanthophyll cycle dynamics. Photochemical reflectance index variation under cold autumn conditions mainly reflected long-term pigment pool adjustments associated with sustained NPQ, which impaired the PRI-LUE relationship. Exposure to warm autumn conditions prevented the induction of sustained NPQ but still impaired the PRI-LUE relationship. We therefore conclude that alternative zeaxanthin-independent NPQ mechanisms, which remain undetected by the PRI, are present under both cold and warm autumn conditions, contributing to the discrepancy in the PRI-LUE relationship during autumn.

Photosynthetic light-use efficiency (LUE) has gained wide interest as an input to modeling forest gross primary productivity (GPP). The photochemical reflectance index (PRI) has been identified as a principle means to inform LUE-based models, using airborne and satellite-based observations of canopy reflectance. More recently, low-cost electronics have become available with the potential to provide for dense in situ time-series measurements of PRI. A recent design makes use of interference filters to record light transmission within narrow wavebands. Uncertainty remains as to the dynamic range of these sensors and performance under low light conditions, the placement of the reference band, and methodology for reflectance calibration. This paper presents a low-cost sensor design and is tested in a laboratory set-up, as well in the field. The results demonstrate an excellent performance against a calibration standard (R2 = 0.9999) and at low light conditions. Radiance measurements over vegetation demonstrate a reversible reduction in green reflectance that was, however, seen in both the reference and signal wavebands. Time-series field measurements of PRI in a Douglas-fir canopy showed a weak correlation with eddy-covariance-derived LUE and a significant decline in PRI over the season. Effects of light quality, bidirectional scattering effects, and possible sensor artifacts on PRI are discussed.

Vegetation indices such as photochemical reflectance index (PRI) have been applied in tracing various biophysical and biochemical parameters within plants. However, less attention has been focused on chlorophyll (Chl) a/b ratio in arid environment. Quantitative estimation of Chl a/b ratio through remote sensing may provide important information regarding carbon balance and stress response in desert plants. We investigated the potential of PRI to trace Chl a/b ratio in Haloxylon ammodendron trees at Gurbantunggut Desert, China. The results revealed a seasonal and vertical variation in correlations. PRI was negatively correlated with Chl a/b ratio in all datasets, with pronounced sensitivity to changes in light across the canopy where low coefficient of determination (R 2 = 0.30) was obtained in bottom compared to the middle and top canopy assimilating branches. Seasonally, PRI–Chl a/b ratio relationship exhibited a higher R 2 for May–June dataset (R 2 = 0.62, P

The FLuorescence EXplorer (FLEX) satellite mission, candidate of ESA's 8th Earth Explorer program, is explicitly optimized for detecting the sun-induced fluorescence emitted by plants. It will allow consistent measurements around the O2-B (687 nm) and O2-A (760 nm) bands, related to the red and far-red fluorescence emission peaks respectively, the photochemical reflectance index, and the structural-chemical state variables of the canopy. The sun-induced fluorescence signal, overlapped to the surface reflected radiance, can be accurately retrieved by employing the powerful spectral fitting technique. In this framework, a set of fluorescence retrieval algorithms optimized for FLEX are proposed in this study. Two main retrieval approaches were investigated: i) the optimization of the spectral fitting for retrieving fluorescence at the oxygen absorption bands; ii) the extension of the spectral fitting to a broader spectral window to retrieve the full fluorescence spectrum in the range from 670 to 780 nm. The accuracy of the retrieval algorithms is assessed by employing atmosphere-surface radiative transfer simulations obtained by coupling SCOPE and MODTRAN5 codes. The simulated dataset considers more realistic conditions because it includes directional effects, and the top-of-atmosphere radiance spectra are resampled to the current specifications of the FLuORescence Imaging Spectrometer (FLORIS) planned to serve as the primary instrument aboard FLEX. The retrieval accuracy obtained at the O2-A band is strongly affected by directional effects, and better performance is found in cases where directional effects are lower. However, the best performing algorithms tested provided similar performance, the RMSE (RRMSE) is 0.044 mW m− 2 sr− 1 nm− 1 (6.2%) at the O2-A band, 0.018 mW m− 2 sr− 1 nm− 1 (2.9%) at the O2-B band, and 6.225 mW m− 2 sr− 1 (6.4%) for the spectrally integrated fluorescence emission. The promising results achieved open new perspectives extending fluorescence studies not only in limited absorption bands, but its spectral behavior in relation to different plant species, photosynthetic rates and stress occurrences.

Imaging spectroscopy (IS) provides an efficient tool to assess vegetation status and functioning at ecologically relevant scales. Reliable extraction of vegetation information from spatial and spectral high resolution spectroscopy data requires accurate retrieval schemes to account for the complex radiative transfer in the coupled vegetation-atmosphere system. Particularly the coupling of the atmosphere and vegetation considering combined effects of anisotropy, absorption and scattering typically relies on many assumptions, rendering estimates of direct ( E dir ) and diffuse ( E dif ) surface irradiance error prone. This impacts the reliability of retrieved vegetation properties. In this study we discuss and quantify the retrieval sensitivity of vegetation information using high resolution IS data to inaccurate assumptions of direct and diffuse surface irradiance. We use observations and simulations and focus on the two vegetation indices normalized difference vegetation index (NDVI) and the photochemical reflectance index (PRI), and on sun-induced chlorophyll fluorescence (Fs). Our results indicate that, even if the irradiance field ( E ) is exactly known, reflectance based vegetation indices show an inherent variation of 9% (NDVI) and 12% (PRI) respectively. These variations are caused by complex interactions of surface irradiance and reflectance anisotropy. The emitted Fs signal was found to be almost unaffected by those variations, if the retrieval considers surface anisotropy. Further, estimation of vegetation properties is subject to large uncertainties if instantaneous E fields are unknown. In that case, they range up to 13% for the NDVI, up to 32% for the PRI, and up to 58% for Fs. We conclude that retrieval sensitivities of vegetation indices and Fs to illumination effects must be carefully considered in data interpretation and suggest using coupled surface-atmosphere models to exploit the full information content of IS data.

This study evaluates the potential of airborne remote sensing images to detect water stress in maize. Visible and near infrared CASI (Itres Research Ltd., Calgary, AL, Canada) and thermal AHS-160 (Sensytech Inc., Beverly, MA, USA) data were acquired at three different times during the day on a maize field (Zea mays L.) grown with three different irrigation treatments. An intensive field campaign was also conducted concurrently with image acquisition to measure leaf ecophysiological parameters and the leaf area index. The analysis of the field data showed that maize plants were experiencing moderate to severe water stress in rainfed plots and a weaker stress condition in the plots with a water deficit imposed between stem elongation and flowering. Vegetation indices including the normalized difference vegetation index (NDVI) and the photochemical reflectance index (PRI) computed from the CASI images, sun-induced chlorophyll fluorescence (F-760) and canopy temperature (Tc) showed different performances in describing the water stress during the day. During the morning overpass, NDVI was the index with the highest discriminant power due to the sensitivity of NDVI to maize canopy structure, affected by the water irrigation treatment. As the day progressed, processes related to heat dissipation through plant transpiration became more and more important and at midday Tc showed the best performances. Furthermore, Tc retrieved from the midday image was the only index able to distinguish all the three classes of water status. Finally, during the afternoon, PRI and F-760 showed the best performances. These results demonstrate the feasibility to detect water stress using thermal and optical airborne data, pointing out the importance of careful planning of the airborne surveys as a function of the specific aims of the study.

The emerging consistency of the relationship between biochemical, optical, and odorous signals emitted by plants and ecosystems offers promising prospects for continuous local and global monitoring of the energetic status of plants and ecosystems, and therefore of their processing of energy and matter.

Assessing photosynthesis rates at the ecosystem scale and over large regions is important for tracking the global carbon cycle and remote sensing has provided new and useful approaches for performing this assessment. The photochemical reflectance index (PRI) is a good estimator of short-term light-use efficiency (LUE) at the leaf scale; however, confounding factors appear at larger temporal and spatial scales. In this study, canopy-scale PRI variability was investigated for three species (Fagus sylvatica L., Quercus robur L. and Pinus sylvestris L.) growing under contrasting soil moisture conditions. Throughout the growing season, no significant differences in chlorophyll content and in violaxanthin, antheraxanthin and zeaxanthin were found between species or treatments. The daily PRI vs PAR (photosynthetically active radiation) relationships were determined using continuous measurements obtained at high frequency throughout the entire growing season, from early spring budburst to later autumn senescence, and were used to deconvolute the physiological PRI variability related to LUE variations due to phenological variability and related to temporal changes in the biochemical and structural canopy attributes. The PRI vs PAR relationship is used to show that the canopy-scale PRI measured at low radiation depends on the chlorophyll content of the canopy. The range of PRI variations at an intra-daily scale and the dynamics of the xanthophyll pool do not vary between days, which suggests that the PRI responds to a xanthophyll ratio. The PAR values at PRI saturation are mainly related to the canopy chlorophyll content during budburst and senescence and to the soil moisture content when the chlorophyll content is no longer a limiting factor. This parameter is significantly lower in the oak species that experience less stress from variations in soil moisture and is species dependant. These results provide new insights regarding the analysis and the meaning of PRI variability as a proxy for LUE at the canopy scale.

Solar induced chlorophyll fluorescence (SIF) from vegetation can now be obtained from satellites as well as ground-based field studies, at select wavelengths associated with atmospheric features. At the leaf level, full spectrum (650–800 nm) chlorophyll emissions (ChlF) can be measured using specialized instrumentation to support interpretation of these SIF observations. We found that ChlF spectra differ for leaf bottoms versus upper leaf surfaces, potentially affecting within-canopy radiative scattering. Our ChlF measurements for leaves of eight tree species (n≥125) obtained during fall 2013 senescence at the Duke Forest in North Carolina, USA and the 2014 growing season (n=72) at the USDA cornfield in Beltsville, MD, USA also demonstrate the benefit of acquiring measurements for both the Red and Far-Red emission peaks. The Red/Far-Red ChlF Ratio was strongly related to both the Photochemical Reflectance Index (PRI) for corn leaves (r≥0.76) and tree leaves (r≥0.89) and to a PSII Photosynthesis Efficiency parameter (r∼0.90).

This paper examines the relationship between spectral indices (the normalized difference vegetation index [NDVI] and photochemical reflectance index [PRI]) and the photosynthetic capacities based on chlorophyll fluorescence (Fv/Fm ) of moss‐, lichen‐ and cyanobacteria‐dominated biological soil crusts collected from the Gurbantonggut Desert, Xinjiang, China, based on a liquid–water hydration experiment. While no photosynthetic activity was detected for dry crusts, hydrated crusts showed significantly higher Fv/Fm values than dry crusts. A significant correlation between the PRI and Fv/Fm was found in moss‐ and lichen‐dominated crusts, and the determination coefficients were higher than those between the NDVI and Fv/Fm .

Exposure to high light caused a down-regulation of photosynthesis and net photo-oxidative damage in three shade-acclimated species (Alectryon excelsus, Corynocarpus laevigatus and Coprosma repens). Analysis of the relationships between the photochemical reflectance index (PRI), an optical measure of excess light dissipation mediated by xanthophyll cycle, and photosynthetic parameters revealed that although PRI remains a measure of down-regulation of photosynthesis when photodamage occurs, the relationship can be expected to vary with changes in growth habitat. Pigment analysis indicates that this is likely due to changes in the xanthophyll cycle pool and carotenoids/chlorophyll ratio. This result may have important implications for the interpretation of PRI measured under different growth light conditions.

Understanding and diagnosing plant responses to stress will benefit greatly from three-dimensional (3D) measurement and analysis of plant properties because plant responses are strongly related to their 3D structures. Light detection and ranging (lidar) has recently emerged as a powerful tool for direct 3D measurement of plant structure. Here the use of 3D lidar imaging to estimate plant properties such as canopy height, canopy structure, carbon stock, and species is demonstrated, and plant growth and shape responses are assessed by reviewing the development of lidar systems and their applications from the leaf level to canopy remote sensing. In addition, the recent creation of accurate 3D lidar images combined with natural colour, chlorophyll fluorescence, photochemical reflectance index, and leaf temperature images is demonstrated, thereby providing information on responses of pigments, photosynthesis, transpiration, stomatal opening, and shape to environmental stresses; these data can be integrated with 3D images of the plants using computer graphics techniques. Future lidar applications that provide more accurate dynamic estimation of various plant properties should improve our understanding of plant responses to stress and of interactions between plants and their environment. Moreover, combining 3D lidar with other passive and active imaging techniques will potentially improve the accuracy of airborne and satellite remote sensing, and make it possible to analyse 3D information on ecophysiological responses and levels of various substances in agricultural and ecological applications and in observations of the global biosphere.

Photosynthetic light response curves and reflectance spectra (380–2500 nm) were measured for soybean (Glycine max L. Merr.) leaves with a range of chlorophyll concentrations at various soil water contents. Regression lines for the relationship between the photosynthetic light use efficiency (LUEp) and photochemical reflectance index (PRI) under different soil water content θ almost all passed through a common point (PRI, LUEp) = (−0.04, 0), so that the LUEp could be expressed simply as LUEp = kAPRI using an adjusted PRI [APRI = (ρ531−ρ570)/(ρ531−ρ570)+0.04]. The effect of soil moisture was strong under dry conditions and gradually decreased with increasing θ. There was no effect of θ above 25% (v/v). The effect of θ on the APRI–LUEp relationship was expressed by a simple exponential function. These results should provide a new basis for applications in dynamic diagnosis of photosynthetic functioning of plant leaves and in the prediction of plant productivity. The change in the slope of LUE vs. APRI may pr...

There is growing interest in using the photochemical reflectance index (PRI) to estimate photosynthetic light‐use efficiency (LUE) in studies of terrestrial carbon exchange. However, evidence suggests that the PRI is correlated with LUE only while there is coordinated adjustment between PSII (photosystem II) photochemical efficiency (ΔF/Fm′) and the CO2 fixation rate. The range of environmental conditions under which this occurs has yet to be fully explored. In this study we examined for 13 species the response of ΔF/Fm′, LUE, and the PRI to short‐term exposure to elevated CO2 and low temperature. The PRI was linearly correlated with both ΔF/Fm′ and LUE under all conditions. Under elevated CO2, LUE was considerably enhanced without significantly affecting either ΔF/Fm′ or the PRI. This is consistent with current mechanistic understanding of the basis of the PRI/LUE relationship. In contrast, ΔF/Fm′, LUE, and the PRI were on average reduced proportionally by low temperature. This suggests simple multiplica...

A two‐year study was conducted in 2002 and 2003 at the University of Nevada, Las Vegas's center for urban water conservation to assess canopy spectral response of annual ryegrass (Lolium multiflorum Lam.) grown under various combinations of N and irrigation (based on leaching fraction: LF) treatments. Multispectral measurements were acquired using a ground‐based spectroradiometer (200–1100 nm) on a biweekly basis during the growing season (October–May) in 2002 and 2003. Multispectral parameters were correlated with soil–plant parameters and temporal variability was investigated. Results showed that the normalized difference vegetation index (NDVI), stress index (SI), photochemical reflectance index (PRI) and canopy reflectance at 693 nm, were highly correlated with tissue N concentration (TN), tissue moisture content (TM), TN×TM and canopy colour, as influenced by N and LF treatment combinations. Coefficients of determination ranged from 0.50 to 0.79 (P

Some form of the light use efficiency (LUE) model is used in most models of ecosystem carbon exchange based on remote sensing. The strong relationship between the normalized difference vegetation index (NDVI) and light absorbed by green vegetation make models based on LUE attractive in the remote sensing context. However, estimation of LUE has proven problematic since it varies with vegetation type and environmental conditions. Here we propose that LUE may in fact be correlated with vegetation greenness (measured either as NDVI at constant solar elevation angle, or a red edge chlorophyll index), making separate estimates of LUE unnecessary, at least for some vegetation types. To test this, we installed an automated tram system for measurement of spectral reflectance in the footprint of an eddy covariance flux system in the Southern California chaparral. This allowed us to match the spatial and temporal scales of the reflectance and flux measurements and thus to make direct comparisons over time scales ranging from minutes to years. The 3-year period of this study included both “normal” precipitation years and an extreme drought in 2002. In this sparse chaparral vegetation, diurnal and seasonal changes in solar angle resulted in large variation in NDVI independent of the actual quantity of green vegetation. In fact, one would come to entirely different conclusions about seasonal changes in vegetation greenness depending on whether NDVI at noon or NDVI at constant solar elevation angle were used. Although chaparral vegetation is generally considered “evergreen”, we found that the majority of the shrubs were actually semi-deciduous, leading to large seasonal changes in NDVI at constant solar elevation angle. LUE was correlated with both greenness indices at the seasonal timescale across all years. In contrast, the relationship between LUE and PRI was inconsistent. PRI was well correlated with LUE during the “normal” years but this relationship changed dramatically during the extreme drought. Contrary to expectations, none of the spectral reflectance indices showed consistent relationships with CO2 flux or LUE over the diurnal time-course, possibly because of confounding effects of sun angle and stand structure on reflectance. These results suggest that greenness indices can be used to directly estimate CO2 exchange at weekly timescales in this chaparral ecosystem, even in the face of changes in LUE. Greenness indices are unlikely to be as good predictors of CO2 exchange in dense evergreen vegetation as they were in the sparse, semi-deciduous chaparral. However, since relatively few ecosystems are entirely evergreen at large spatial scales or over long time spans due to disturbance, these relationships need to be examined across a wider range of vegetation types.

Three pines species, three evergreen broadleaf trees, one C(3) and two C(4) perennial grasses of subtropical Taiwan were studied to elucidate the correlation between photosystem II (PSII) efficiency and photochemical reflectance index (PRI = (R(531) - R(570))/(R(531) + R(570))). Measurements were made at two sites differing in altitude (800 and 2600 m) over several growing seasons. At high elevation, potential PSII efficiency, measured by chlorophyll fluorescence (the ratio of variable to maximal fluorescence; F(v)/F(m)) at predawn, decreased with decreasing air temperature and varied greatly among species. At the lowest air temperature (-3 degrees C) studied, variation in F(v)/F(m) among species ranged from 0.33 to 0.72. In contrast, at low elevation where air temperature was moderate, seasonal variation in F(v)/F(m) was small in all of the study species. When species, elevation and season data were pooled, despite the high variation in F(v)/F(m) among species, a good correlation between F(v)/F(m) and PRI was observed. When compared at the same value of PRI, F(v)/F(m) of evergreen trees was higher than that of perennial grasses; however, when the minimum temperature on the measurement day was below 0 degrees C, F(v)/F(m) was underestimated relative to PRI. We conclude that PRI could be used as a remote indicator of photosynthetic function when air temperature is above 0 degrees C.

In order to elucidate the effects of chilling-stress at night on photosystem 2 (PS2) efficiency under dim irradiance (DI), mango leaves were chilled to varied extent (8–3 °C) and for varied duration (0–12 h) in growth cabinets in the dark, and then exposed to DI (20 μmol m−2 s−1 PPFD) at each chilling-temperature for 1 h. Chilling in the dark had little effect on Fv/Fm of mango leaves. But both the extent and duration of chilling pre-treatments significantly affected Fv’/Fm’ when leaves were exposed to DI. This down-regulation of PS2 efficiency was closely related to xanthophyll de-epoxidation, assessed as photochemical reflectance index (PRI) and calculated from leaf spectral reflectance [(R531 − R570)/(R531 + R570)], and non-photochemical quenching (NPQ). The down-regulation of PS2 is a defence mechanism initiated at predawn in winter to alleviate the damage of PS2 by the sudden and strong irradiation at sunrise. Mango leaves, transferred suddenly from warm and dark room to DI and chilling showed a slight down-regulation of PS2 efficiency, in spite of an increased xanthophyll de-epoxidation. This might have been due to the unavailability of some cofactors required for NPQ.

The objectives of this study were to assess the potential of using spectral reflectance indices (SRI) as an indirect selection tool to differentiate spring wheat (Triticum aestivum L.) genotypes for grain yield under irrigated conditions. This paper demonstrates only the first step in using the SRI as indirect selection criteria by reporting genetic variation for SRI among genotypes, the effect of phenology and year on SRI and their interaction with genotypes, and the correlations between SRI and grain yield and yield components of wheat. Three field experiments—15 CIMMYT globally adapted genotypes (GHIST), 25 random F3–derived lines (RLs1), and 36 random F3–derived lines (RLs2)—were conducted under irrigated conditions at the CIMMYT research station in northwest Mexico in three different years. Five previously developed SRI (photochemical reflectance index [PRI], water index [WI], red normalized difference vegetation index [RNDVI], green normalized difference vegetation index [GNDVI], simple ratio [SR]) and two newly calculated SRI (normalized water index-1 [NWI-1] and normalized water index-2 [NWI-2]) were evaluated in the experiments. In general, genotypic variation for all the indices was significant. Near infrared radiation (NIR)–based indices (WI, NWI-1, NWI-2) gave the highest levels of association with grain yield during the 3 yr of the study. A clear trend for higher association between grain yield and the NIR-based indices was observed at heading and grainfilling than at booting. Overall, NIR-based indices were more consistent and differentiated grain yield more effectively compared to the other indices. The results demonstrated the potential of using SRI as a tool in breeding programs for selecting for increased genetic gains for yield.

We investigated seasonal changes in the photochemical reflectance index (PRI) and its relation to the diurnal profile of photosynthetic light use efficiency (LUE) in mature Japanese larch (Larix kaempferi Sarg.) forest throughout the growing season from June to October 2003. The daily mean value of needle PRI showed seasonal variation, strongly correlated with the chlorophyll concentration and carotenoid/chlorophyll ratio of the needles. During the green period from early June to late September, the hourly values of both PRI and LUE showed significant midday depression, and were positively correlated. In late October, however, because the PRI of yellowing needles tended to increase slightly at midday in contrast to the LUE, this correlation became negative. Even before autumn senescence, the sensitivity of PRI to LUE changed with the season. Correlation analysis indicated that the slope and intercept of the regression line of the PRI–LUE relationship increased during summer, with peaks in July and August,...

Massive anthropogenic changes in estuarine salinities, from manipulations of freshwater flows, are again occurring through governmental projects ‘‘correcting’’ past freshwater alterations. The downstream effects of increased freshwater on seagrass meadows, a major fisheries and ecosystem habitat, are not clear. Spectral responses to low salinities were quantitatively delimited for the important habitat seagrass Thalassia testudinum utilizing spectral reflectance measurements for the first time (non-invasive sampling). Over a range of salinities (32‐16 parts per thousand sea salts [ppt] for 24 h) and spectra (308‐1138 nm), Thalassia specimens showed statistically significant differences in spectral values (P , 0.05) between treatments at normal (32 ppt) and 50% reduced (16 ppt) seawater. Mature blades yellowed at low salinities. Reflectance changes at 525 nm and 650‐680 nm at low salinities suggested changes in xanthophylls and chlorophylls. Four indices were also used to characterize the reflectance spectra to delineate the effect of the salinity changes: (1) The normalized difference vegetation index (NDVI) for mature blades reduced at 16 ppt from that at 32 ppt. (2) The chlorophyll normalized difference index (Chl NDI) suggested chlorophyll content decreases in response to reduced salinity. (3) The structure independent pigment index (SIPI), higher in mature blades at 16 ppt than new blades, indicates a higher carotenoid : chlorophyll ratio in mature blades. (4) The photochemical reflectance index (PRI) suggested a lower photochemical efficiency at lower salinities. The main low-salinity effect on Thalassia physiology delineated herein is likely through changes in pigmentation (decreases in chlorophyll and changes in xanthophyll cycle epoxidation).

Reflectance measurements have been a long standing means of quantifying chlorophyll, and other chemical contents, within vegetation. Conventional metrics that are common include normalized difference vegetative index (NDVI) and photochemical reflectance index (PRI). However, to gain more information about the plant canopy and go beyond reflectance measurements, fluorescence measurements are also being pursued. While active laser- or illuminator-based methods exist, passive methods are often preferred due to their lower power requirements and their reduced system complexity and eye safety concerns. To this end, preliminary results for the development of a high throughput imaging Fraunhofer line discriminator (FLD) are presented. This system, which is based on a multiple-image Fourier transform spectrometer and a spatial heterodyne interferometer can, for the first time, leverage the snapshot advantage for high throughput FLD. In this paper, the concept of the system is presented and preliminary experimental calibration results are summarized.

Light use efficiency (LUE ) is an important parameter for GPP and and NPP estimation model, cause by the existing model method to estimate the actual LUE is always simple and rough, which may lead to serious bias by GPP and NPP. The photochemical reflectance index ( PRI ) has great potential for direct estimation the actual LUE. In this paper, wheat in different nitrogen treatments was designed in field trial during Wheat growing period, for obtain photosynthesis and reflective hyperspectral data, and then LUE and PRI was calculated in critical period of wheat growth. The results show that, at different growth stages under three different nitrogen conditions, LUE and PRI value were significantly increased with increasing nitrogen absorption; Last longer, more capable of absorbing nitrogen amount, the correlation between LUE and PRI was better，for example, the correlation coefficient is obviously larger in heading stage than elongation stage for same nitrogen treatment.