FLuorescence EXplorer (FLEX): an optimised payload to map vegetation photosynthesis from space

Although traditional remote sensing systems based on spectral reflectance can already provide estimates of the “potential” photosynthetic activity of terrestrial vegetation through the quantification of total canopy chlorophyll content or absorbed photosynthetic radiation, the determination of the “actual” photosynthetic activity of terrestrial vegetation requires information about how the absorbed light is used by plants, such as vegetation fluorescence, using very high spectral resolution spectroscopy in the range 650–800 nm. The Fluorescence Explorer (FLEX) mission, selected in November 2015 as the 8th Earth Explorer by the European Space Agency (ESA), carries the FLORIS spectrometer, with a spectral resolution of 0.3 nm and a spatial resolution of 300 m, with a swath of 150 km. The FLEX mission is designed to fly in tandem with the Copernicus Sentinel-3 satellite, in order to provide all the necessary information to disentangle emitted fluorescence from the background reflected radiance, and to allow proper interpretation of the fluorescence spatial and temporal changes in relation to photosynthesis dynamics, accounting for non-photochemical energy dissipation and canopy temperature effects.

The first European Space Agency (ESA) and NASA collaboration in an airborne campaign to support ESA’s FLuorescence EXplorer (FLEX) mission was conducted in North Carolina, USA during September–October 2013 (FLEX-US 2013) at the Parker Tract Loblolly Pine (LP) Plantation (Plymouth, NC, USA). This campaign combined two unique airborne instrument packages to obtain simultaneous observations of solar-induced fluorescence (SIF), LiDAR-based canopy structural information, visible through shortwave infrared (VSWIR) reflectance spectra, and surface temperature, to advance vegetation studies of carbon cycle dynamics and ecosystem health. We obtained statistically significant results for fluorescence, canopy temperature, and tower fluxes from data collected at four times of day over two consecutive autumn days across an age class chronosequence. Both the red fluorescence (F685) and far-red fluorescence (F740) radiances had highest values at mid-day, but their fluorescence yields exhibited different diurnal responses across LP age classes. The diurnal trends for F685 varied with forest canopy temperature difference (canopy minus air), having a stronger daily amplitude change for young vs. old canopies. The Photochemical Reflectance Index (PRI) was positively correlated with this temperature variable over the diurnal cycle. Tower measurements from mature loblolly stand showed the red/far-red fluorescence ratio was linearly related to canopy light use efficiency (LUE) over the diurnal cycle, but performed even better for the combined morning/afternoon (without midday) observations. This study demonstrates the importance of diurnal observations for interpretation of fluorescence dynamics, the need for red fluorescence to understand canopy physiological processes, and the benefits of combining fluorescence, reflectance, and structure information to clarify canopy function versus structure characteristics for a coniferous forest.

In the context of the development and implementation of the Fluorescence Explorer (FLEX) mission, selected in 2015 by the European Space Agency (ESA) as the 8th Earth Explorer, many recent developments in the field of vegetation chlorophyll fluorescence have taken place in the last years. These advances include new retrieval approaches, more accurate and robust, and a much better understanding of the variability of the signal, facilitating the interpretation of the dynamical changes and the usage of the fluorescence signal in Earth models and applications. Although chlorophyll fluorescence is related to the actual photosynthetic activity of the plants, the link between fluorescence and photosynthesis is not straightforward, and only advanced retrieval methods and usage of proper physical models allow a quantitative exploitation of the spatial, temporal and spectral variability of the fluorescence signal. This paper reviews such recent developments and describes achievements and perspectives from current ongoing research activities.

The study of water balance is considered here as a way to assess the performance of regional climate models and examine model uncertainty and as an approach to understanding regional hydrology, especially interactions between atmospheric and hydrological processes. We studied the atmospheric and terrestrial water balance over the Eastern Nile Basin (ENB) region using the weather research and forecasting (WRF) model. The model performance in simulating precipitation and surface air temperature is assessed by comparing the model output with the data from the Global Precipitation Climatology Center dataset for precipitation and from the University of Delaware for temperature. The results show that the simulated and observed values correlate well. In terms of water balance, the study region was found to be a sink for moisture, where the atmospheric convergence is negative during most of the time. Most of the precipitation originates from moisture fluxes from outside the domain, and the contribution of local evapotranspiration to precipitation is limited, with small values for the moisture recycling ratios year-round. The atmospheric moisture content does not show significant monthly or annual variation. The results indicate that the terrestrial water storage varies seasonally, with negative fluxes during most of the year, except June, July, and August, when most of the precipitation occurs.

In the present context, ‘model’ is expressed as a computer program that can be repeatedly run several times for computing several designed mathematical or statistical expressions (equations) governing crop growth-environment relations, given appropriate input data. The experiment station yields obtained under a rainfed situation without any nutrient deficiency mostly considered as the potential yields of rainfed crops. Actual yields are obtained by recording crop yields of farmers in the region under investigation and achievable yield is between actual and potential yield. Actual yields are compared with the potential yields to estimate yield gaps of crops for that area and others which have the same agro-ecology. Achievable yield is determined by factors like availability of moisture and nutrients, Precipitation and irrigation as input, Soil profile water holding characteristics, Plant water balance (transpiration, water uptake), Soil water balance (evaporation, infiltration, runoff, flow, drainage) and Nitrogen fertilizer applications as input, Soil nitrogen conditions, Plant nitrogen balance (uptake, fixation, mobilization), Soil nitrogen balance (mineralization, immobilization, nitrification, denitrification). Generally, modeling Achievable yield of farm depend on water and nutrient data of the area and Actual yield is determined by factors like weeds, insect pests, diseases and pollutants.

Irrigation scheduling methods using three threshold canopy temperatures (28 C, 30 C, and 32 C) were compared with soil water balance methods that replaced depleted soil moisture weekly (SWR) or at approximately two-week intervals (AUS) and a treatment that received only preplant irrigation (DRY). The largest quantity of water applied (rainfall plus irrigation) was to SWR (138 cm). The quantities for AUS (75 cm) and 28C (70 cm) treatments were similar, followed by 30C (46 cm) and 28 C (70 cm) treatments were similar, followed by 30C (46 cm), 32C (36 cm), and DRY (18 cm). Water used by these treatments was 133, 75, 78, 52, 46, and 29 cm, respectively. Volumetric soil profile moisture content of 0.28 in the SWR and AUS treatments at the end of the season showed little depletion while the 28 C, 30 C, and 32C treatments were lower at 0.24. Average canopy temperatures during the period DOY 187-274 when net radiation exceeded 200 w/m^ were 26.0, 26.8, 27.8, 24.1, 25.3, and 28.9 C, respectively, for 28 C, 30 C, 32 C, SWR, AUS, and the DRY treatments. Above 600 w/m^, the difference between canopy and air temperature increased in the 30 C, 32 C, and DRY treatments indicating water stress. Lint yield was positively related to water applied and water used, except for the SWR treatment. Rate of boll maturity was highest in the three threshold canopy temperature and DRY treatments. Cotton fiber length was decreased by water stress, micronaire value was reduced by low temperature, and fiber strength was reduced by either high water stress or low canopy temperature. Irrigation scheduled by threshold canopy temperature permits automated decision-making of both application time and amount.

Summary 1. The infrared (IR) heating technique used in terrestrial vegetation warming experiments has been continuously improved over the past two decades, but the temperature control methods applied thus far inadequately reflect natural conditions. 2. Current control methods base the IR radiation administered on measured canopy or air temperatures. However, these temperatures are influenced by stomatal responses of the vegetation, which depend on moisture conditions and therefore also on warming. In nature, drought‐stressed vegetation warms up more than well‐watered vegetation, leading to potential differences in heat stress. Current control methods preclude such differences from developing. 3. We propose an alternative approach to render temperature control independent of plant responses. Theoretical canopy temperatures associated with given (target) air temperatures are calculated, based on reference canopy conductance determined from controls and fluctuating meteorological conditions. The IR radiation needed to attain the theoretical temperature is subsequently applied. Actual canopy and air temperatures are free to deviate from that temperature, as measured temperatures no longer control the heaters’ radiation output. 4. Synthesis. We devised an alternative control method for IR heaters that is independent of plant responses to heating. This is especially critical in extreme event studies, where differences in plant water status, and therefore canopy temperatures, are likely to be exacerbated.

The temperature stress day (TSD) has been used as a remotely sensed indicator of plant water stress without a thorough knowledge of TSD behavior. The purpose of this study was to demonstrate a dependence of the TSD on net radiation (Rm), air temperature (Ta), and atmospheric water vapor pressure deficit (VPD), and to evaluate a replacement water status index. The dependence on these parameters was proven by the following methods: (i) theoretically, using the Penman‐Monteith resistance equation; (ii) empirically, using the non‐water‐stressed baseline equation relating canopy and air temperatures to VPD; and (iii) in a field experiment with alfalfa [Medicago sativa (L.)] on an Anthropic Torrifluvent soil. The effect of these parameters caused the field‐measured TSD to vary up to 7 °C, rendering its utility as a plant stress indicator questionable. A modification of the crop water stress index (CWSI), which accounts for Rn, Ta, and VPD, was proposed as the TSD replacement. The modification consisted of using a measured instead of estimated well‐watered canopy temperature (Tcl) in the canopy temperature ratio defined by the CWSI. This modification obviates the need for the canopy resistance value required to calculate the theoretically based CWSI. The modified CWSI responded to imposed irrigation regimes as indicated through yield and evapotranspiration comparisons. The modified CWSI appears to be a suitable replacement for the TSD by accounting for environmental dependence while maintaining the measurement simplicity of the TSD.

Hyperspectral reflectance is widely used for determining important properties of soil erosion. However, there have been few studies which focus on the influence of soil erosion intensity on the characteristics of hyperspectral reflectance, and such information would provide a new tool to improve quantitative understanding of soil erosion. In this study, 35 soil samples were collected from three regions with different erosion intensities in Changting County, a typical severely eroded county in the ferralic cambisol region of southern China, and classified into three groups according to different erosion controlling status. All the samples were scanned at wavelengths from 400 to 2,498 nm by an ASD Field Spec Portable Spectrometer, and the erosion intensity of each sample was calculated using the Revised Universal Soil Loss Equation. Multivariate stepwise linear regression was then employed to model the soil erosion intensity based on reflectance. The results suggested that the absorption peaks of each sample were in a similar wavelength range, while the absorption depth varied with different erosion status, and the reflectance of extremely eroded soil samples were the highest. During modelling of erosion intensity, the result was poor when all the samples were combined, but improved greatly at certain wavelength ranges when samples were classified into three groups based on different erosion controlling status. The extreme erosion group markedly outperformed the other two groups, in which the R 2 values between the actual and predicted erosion intensity were 0.67, 0.85 and 0.80 for each spectral type. The results indicated that hyperspectral reflectance is a promising method for accurately monitoring erosion intensity.

Evapotranspiration (ETc) partitioning and obtaining of FAO56 dual crop coefficient (Kc) for olive was carried out with the SIMDualKc software application for root zone and topsoil soil water balance based on the dual crop coefficients. A simplified two source-energy balance model (STSEB), based on daily remotely sensed soil and canopy thermal infrared data and retrieval of surface fluxes, also provided information on partitioning ETc for the olive orchard. Both models were calibrated and validated with ground-based, sap flow-derived transpiration rates, and their performance was compared in partitioning ETc for incomplete cover, intensive olive grown in orchards (≤300 trees ha−1). The SIMDualKc proved adequate in partitioning ETc. The STSEB model underestimated ETc mostly by inadequately simulating soil evaporation and its contribution to the total latent heat flux. Such results suggest difficulties in using information from the STSEB algorithm for assessing ETc and dual Kc crop coefficients of intensive olive orchards with incomplete ground cover.

Coloured and variegated leaves are common in shade-tolerant ornamentals. However, it remains unclear whether their photosynthetic performance is determined mainly by pigment abundance or by the organisation of chloroplasts and thylakoids. We tested this in three Epipremnum aureum phenotypes ('Neon', 'Golden' and 'Jade') that share a genetic background but contrast in leaf colour, chloroplast density and thylakoid membrane abundance. Plants were grown in a greenhouse and assessed by hyperspectral and thermal imaging, infrared gas exchange analysis, chlorophyll a fluorescence measurements, and structural, ultrastructural and biochemical analyses. Traits were integrated by principal component analysis, with the quantum yield of CO2 assimilation per absorbed photon (αCO2,abs) as the response variable. 'Neon' leaves had high specific leaf area and approximately 55% lower maximum Rubisco carboxylation (VcMAX) and electron transport capacity (JMAX) than 'Jade', as well as reduced chloroplast and thylakoid abundance and warmer canopies, despite carotenoid enrichment. JIP-test parameters and fluorescence light-response curves showed high absorption and dissipation per PSII reaction centre, elevated excitation pressure, modest non-photochemical quenching (NPQ), low αCO2,abs, small carbohydrate pools and low intrinsic water-use efficiency. 'Jade' leaves developed thick mesophyll with dense chloroplast populations, extensive thylakoid networks, highest NPQ, cool canopies and large carbohydrate reserves, whereas 'Golden' leaves combined thin laminae and intermediate chloroplast-thylakoid organisation with early light saturation of CO2 assimilation and the highest intrinsic water-use efficiency. Principal component analysis revealed a structural axis of chloroplast and thylakoid organisation that better predicted αCO2,abs, net carbon gain and canopy temperature than pigment abundance. In variegated E. aureum, 'photon economy' is therefore governed primarily by chloroplast and thylakoid membrane organisation and abundance rather than by carotenoid accumulation.

The evolution of the atmospheric and land surface states during extreme hydroclimate episodes over North America is investigated using the North American Regional Reanalysis (NARR), which additionally, and successfully, assimilates precipitation. The pentad-resolution portrayals of the atmospheric and terrestrial water balance over the U.S. Great Plains during the 1988 summer drought and the July 1993 floods are analyzed to provide insight into the operative mechanisms including regional circulation (e.g., the Great Plains low-level jet, or GPLLJ) and hydroclimate (e.g., precipitation, evaporation, soil moisture recharge, runoff). The submonthly (but supersynoptic time scale) fluctuations of the GPLLJ are found to be very influential, through related moisture transport and kinematic convergence (e.g., ∂υ/∂y), with the jet anomalies in the southern plains leading the northern precipitation and related moisture flux convergence, accounting for two-thirds of the dry and wet episode precipitation amplitude. The soil moisture influence on hydroclimate evolution is assessed to be marginal as evaporation anomalies are found to lag precipitation ones, a lead–lag not discernible at monthly resolution. The pentad analysis thus corroborates the authors’ earlier findings on the importance of transported moisture over local evaporation in Great Plains’ summer hydroclimate variability. The regional water budgets—atmospheric and terrestrial—are found to be substantially unbalanced, with the terrestrial imbalance being unacceptably large. Pentad analysis shows the atmospheric imbalance to arise from the sluggishness of the NARR evaporation, including its overestimation in wet periods. The larger terrestrial imbalance, on the other hand, has its origins in the striking unresponsiveness of the NARR’s runoff, which is underestimated in wet episodes. Finally, the influence of ENSO and North Atlantic Oscillation (NAO) variability on the GPLLJ is quantified during the wet episode, in view of the importance of moisture transports. It is shown that a significant portion (∼25%) of the GPLLJ anomaly (and downstream precipitation) is attributable to NAO and ENSO’s influence, and that this combined influence prolongs the wet episode beyond the period of the instigating GPLLJ.

An analysis of the atmospheric water balance over the Southern Hemisphere is conducted for the time period June 1980 through May 1984 that includes the 1982–1983 El Nino event. Notable spatial and temporal variations are evident in the moisture flux divergence fields, in the mean meridional transport of water vapor, and in the moisture content of portions of the tropical atmosphere that are attributable to the 1982–1983 ENSO event. Estimates of the net gain/loss of atmospheric water are provided for 10° latitude belts; although the magnitudes of the differences between evaporation and precipitation fall within the range of previous estimates, they are most probably underestimated. [Key words: atmospheric water balance, El Nino, hydrologic cycle, southern hemisphere, water vapor flux.]

The secret of high carbon fixation efficiency in Chlamydomonas and its potential to boost yields in land plants

Mono- and digalactosyldiacylglycerol (MGDG and DGDG, respectively) constitute the bulk of membrane lipids in plant chloroplasts. Mutant analyses in Arabidopsis have shown that these galactolipids are essential for chloroplast biogenesis and photoautotrophic growth. Moreover, these non-phosphorous lipids are proposed to participate in low-phosphate (Pi) adaptations. Under Pi-limited conditions, a drastic accumulation of DGDG occurs concomitantly with a large reduction in membrane phospholipids, suggesting that plants substitute DGDG for phospholipids during Pi starvation. Previously, we reported that among the three MGDG synthase genes (MGD1, MGD2 and MGD3), the type-B MGD2 and MGD3 are upregulated in parallel with DGDG synthase genes during Pi starvation. Here, we describe the identification and characterization of T-DNA insertional mutants of Arabidopsis type-B MGD genes. Under Pi-starved conditions, the mgd3-1 mutant showed a drastic reduction in DGDG accumulation, particularly in the root, indicating that MGD3 is the main isoform responsible for DGDG biosynthesis in Pi-starved roots. Moreover, in the roots of mgd2 mgd3 plants, Pi stress-induced accumulation of DGDG was almost fully abolished, showing that type-B MGD enzymes are essential for membrane lipid remodeling in Pi-starved roots. Reductions in fresh weight, root growth and photosynthetic performance were also observed in these mutants under Pi-starved conditions. These results demonstrate that Pi stress-induced membrane lipid remodeling is important in plant growth during Pi starvation. The widespread distribution of type-B MGD genes in land plants suggests that membrane lipid remodeling mediated by type-B MGD enzymes is a potent adaptation to Pi deficiency for land plants.