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Terrestrial primary productivity and carbon cycle impacts of droughts are commonly quantified using vapour pressure deficit (VPD) data and remotely sensed greenness, without accounting for soil moisture. However, soil moisture limitation is known to strongly affect plant physiology. Here, we investigate light use efficiency, the ratio of gross primary productivity (GPP) to absorbed light. We derive its fractional reduction due to soil moisture (fLUE), separated from VPD and greenness changes, using artificial neural networks trained on eddy covariance data, multiple soil moisture datasets and remotely sensed greenness. This reveals substantial impacts of soil moisture alone that reduce GPP by up to 40% at sites located in sub-humid, semi-arid or arid regions. For sites in relatively moist climates, we find, paradoxically, a muted fLUE response to drying soil, but reduced fLUE under wet conditions. fLUE identifies substantial drought impacts that are not captured when relying solely on VPD and greenness changes and, when seasonally recurring, are missed by traditional, anomaly-based drought indices. Counter to common assumptions, fLUE reductions are largest in drought-deciduous vegetation, including grasslands. Our results highlight the necessity to account for soil moisture limitation in terrestrial primary productivity data products, especially for drought-related assessments., (© 2018 The Authors. New Phytologist © 2018 New Phytologist Trust.)

Leaf quantity (i.e., canopy leaf area index, LAI), quality (i.e., per-area photosynthetic capacity), and longevity all influence the photosynthetic seasonality of tropical evergreen forests. However, these components of tropical leaf phenology are poorly represented in most terrestrial biosphere models (TBMs). Here, we explored alternative options for the representation of leaf phenology effects in TBMs that employ the Farquahar, von Caemmerer & Berry (FvCB) representation of CO 2 assimilation. We developed a two-fraction leaf (sun and shade), two-layer canopy (upper and lower) photosynthesis model to evaluate different modeling approaches and assessed three components of phenological variations (i.e., leaf quantity, quality, and within-canopy variation in leaf longevity). Our model was driven by the prescribed seasonality of leaf quantity and quality derived from ground-based measurements within an Amazonian evergreen forest. Modeled photosynthetic seasonality was not sensitive to leaf quantity, but was highly sensitive to leaf quality and its vertical distribution within the canopy, with markedly more sensitivity to upper canopy leaf quality. This is because light absorption in tropical canopies is near maximal for the entire year, implying that seasonal changes in LAI have little impact on total canopy light absorption; and because leaf quality has a greater effect on photosynthesis of sunlit leaves than light limited, shade leaves and sunlit foliage are more abundant in the upper canopy. Our two-fraction leaf, two-layer canopy model, which accounted for all three phenological components, was able to simulate photosynthetic seasonality, explaining ~90% of the average seasonal variation in eddy covariance-derived CO 2 assimilation. This work identifies a parsimonious approach for representing tropical evergreen forest photosynthetic seasonality in TBMs that utilize the FvCB model of CO 2 assimilation and highlights the importance of incorporating more realistic phenological mechanisms in models that seek to improve the projection of future carbon dynamics in tropical evergreen forests., (© 2017 John Wiley & Sons Ltd.)

Light use efficiency (LUE) is one of the important parameters on calculating terrestrial gross primary productivity (GPP) and net primary productivity (NPP). Based on 5-year (2006-2010) carbon flux and climatic variable data of a mixed plantation in north China, the seasonal and interannual variation of LUE was investigated and the biophysical controls were examined. Our results show that LUE had a distinct seasonal course, and peaked in the vigorous growing season with a value of 0.92-1.27 g C MJ -1 . During the period of 2006-2010, annual mean LUE ranged between 0.54 and 0.62 g C MJ -1 , and it was linearly correlated with annual GPP. In the growing season, LUE was significantly linked with the water availability variables (including monthly mean vapor pressure deficit (VPD), precipitation, evaporative fraction (EF), and the ratio of precipitation to evapotranspiration (P/ET)) and canopy conductance (g c ). However, EF was a better estimator of LUE compared with other biophysical variables. LUE decreased with an increase of the clearness index (CI), indicating that LUE was higher under cloudy sky conditions than that under sunny sky conditions in the mixed plantation.

A nitrogen management experiment with three nitrogen levels (0, 120, and 180 kg·hm -2 , namely N 0 , N 120 , N 180 ) and three nitrogen allocation modes (N A : base fertilizer 100%; N B : base fertilizer 70% + seedling fertilizer 30%; N C : base fertilizer 60% + jointing fertilizer 40%) was conducted at four sites (Chongqing, Renshou, Guanghan and Xichang) during two consecutive years, the SPAD value, canopy photosynthetic rate (CAP), photosynthetically active radia-tion (PAR) interception efficiency and grain yield were determined, and the nitrogen use efficiency and PAR use efficiency were calculated. The results showed that the SPAD of upper-most three leaves, CAP, PAR interception efficiency and grain yield were promoted with increasing nitrogen fertilizer, but nitrogen fertilizer use efficiency, productivity efficiency, uptake efficiency and use efficiency were decreased. The promoting effects of nitrogen fertilizer postponing were different among nitrogen levels, with the highest SPAD in N 180 treatment and the highest CAP in N 120 treatment. The light use efficiency of different nitrogen fertilization patterns differed among four sites. Furthermore, nitrogenous fertilizer postponing significantly increased nitrogen agricultural fertilizer use efficiency, productivity efficiency, uptake efficiency and apparent nitrogen recovery efficiency, but declined nitrogen use efficiency, and the performance of N C was better than N B. Among different sites, Guanghan had the highest SPAD, CAP, PAR interception efficiency and grain yield, Xichang had higher SPAD and nitrogen use efficiency, lower CAP and PAR use efficiency, Chongqing and Renshou had the lowest SPAD, light use efficiency, nitrogen use efficiency and grain yield. Biomass had significant positive relationships with grain yield, CPA, SPAD, and PAR interception efficiency. Therefore, the increase of nitrogen fertilizer could promote yield at all sites, and nitrogenous fertilizer postponing could further optimize grain yield component and improve nitrogen and light use effi-ciency. But the effects depended on the years and sites, thus a target nitrogen management mode should be site-specifically made.

We estimated the light use efficiency ( LUE ) via vegetation canopy chlorophyll content ( CCC canopy ) based on in situ measurements of spectral reflectance, biophysical characteristics, ecosystem CO 2 fluxes and micrometeorological factors over a maize canopy in Northeast China. The results showed that among the common chlorophyll-related vegetation indices (VIs), CCC canopy had the most obviously exponential relationships with the red edge position (REP) ( R 2  = .97, p  <   .001) and normalized difference vegetation index (NDVI) ( R 2  = .91, p  <   .001). In a comparison of the indicating performances of NDVI, ratio vegetation index (RVI), wide dynamic range vegetation index (WDRVI), and 2-band enhanced vegetation index (EVI2) when estimating CCC canopy using all of the possible combinations of two separate wavelengths in the range 400-1300 nm, EVI2 [1214, 1259] and EVI2 [726, 1248] were better indicators, with R 2 values of .92 and .90 ( p  <   .001). Remotely monitoring LUE through estimating CCC canopy derived from field spectrometry data provided accurate prediction of midday gross primary productivity ( GPP ) in a rainfed maize agro-ecosystem ( R 2  = .95, p  <   .001). This study provides a new paradigm for monitoring vegetation GPP based on the combination of LUE models with plant physiological properties.

We investigated the radiative energy budgets of a heterogeneous photosynthetic coral reef sediment and a compact uniform cyanobacterial biofilm on top of coastal sediment. By combining electrochemical, thermocouple and fiber-optic microsensor measurements of O 2 , temperature and light, we could calculate the proportion of the absorbed light energy that was either dissipated as heat or conserved by photosynthesis. We show, across a range of different incident light regimes, that such radiative energy budgets are highly dominated by heat dissipation constituting up to 99.5% of the absorbed light energy. Highest photosynthetic energy conservation efficiency was found in the coral sediment under low light conditions and amounted to 18.1% of the absorbed light energy. Additionally, the effect of light directionality, i.e., diffuse or collimated light, on energy conversion efficiency was tested on the two surface-associated systems. The effects of light directionality on the radiative energy budgets of these phototrophic communities were not unanimous but, resulted in local spatial differences in heat-transfer, gross photosynthesis, and light distribution. The light acclimation index, E k , i.e., the irradiance at the onset of saturation of photosynthesis, was >2 times higher in the coral sediment compared to the biofilm and changed the pattern of photosynthetic energy conservation under light-limiting conditions. At moderate to high incident irradiances, the photosynthetic conservation of absorbed energy was highest in collimated light; a tendency that changed in the biofilm under sub-saturating incident irradiances, where higher photosynthetic efficiencies were observed under diffuse light. The aim was to investigate how the physical structure and light propagation affected energy budgets and light utilization efficiencies in loosely organized vs. compact phototrophic sediment under diffuse and collimated light. Our results suggest that the optical properties and the structural organization of phytoelements are important traits affecting the photosynthetic efficiency of biofilms and sediments.

Although quantifying the massive exchange of carbon that takes place over the Amazon Basin remains a challenge, progress is being made as the remote sensing community moves from using traditional, reflectance-based vegetation indices, such as the Normalized Difference Vegetation Index (NDVI), to the more functional Photochemical Reflectance Index (PRI). This new index, together with satellite-derived estimates of canopy light interception and Sun-Induced Fluorescence (SIF), provide improved estimates of Gross Primary Production (GPP). This paper traces the development of these new approaches, compares the results of their analyses from multiple years of data acquired across the Amazon Basin and suggests further improvements in instrument design, data acquisition and processing. We demonstrated that our estimates of PRI are in generally good agreement with eddy-flux tower measurements of photosynthetic light use efficiency (ε) at four sites in the Amazon Basin: r 2 values ranged from 0.37 to 0.51 for northern flux sites and to 0.78 for southern flux sites. This is a significant advance over previous approaches seeking to establish a link between global-scale photosynthetic activity and remotely-sensed data. When combined with measurements of Sun-Induced Fluorescence (SIF), PRI provides realistic estimates of seasonal variation in photosynthesis over the Amazon that relate well to the wet and dry seasons. We anticipate that our findings will steer the development of improved approaches to estimate photosynthetic activity over the tropics., Competing Interests: Conflicts of Interest The authors declare no conflict of interest.

Light wavelengths preferentially absorbed by chlorophyll (chl) often display steep absorption gradients. This over-saturates photosynthesis in upper chloroplasts and deprives lower chloroplasts of blue and red light. Reducing chl content could create a more even leaf light distribution and thereby increase leaf light-use efficiency and overall canopy photosynthesis. This was tested on soybean cultivar 'Clark' (WT) and a near-isogenic chl b deficient mutant, Y11y11, grown in controlled environment chambers and in the field. Light attenuation was quantified using a novel approach involving light sheet microscopy. Leaf adaxial and abaxial surfaces were illuminated separately with blue, red, and green wavelengths, and chl fluorescence was detected orthogonally to the illumination plane. Relative fluorescence was significantly greater in deeper layers of the Y11y11 mesophyll than in WT, with the greatest differences in blue, then red, and finally green light when illuminated from the adaxial surface. Modeled relative photosynthesis based on chlorophyll profiles and Beer's Law predicted less steep gradients in mutant relative photosynthesis rates compared to WT. Although photosynthetic light-use efficiency was greater in the field-grown mutant with ~50% lower chl, light-use efficiency was lower in the mutant when grown in chambers where chl was ~80% reduced. This difference is probably due to pleiotropic effects of the mutation that accompany very severe reductions in chlorophyll and may warrant further testing in other low-chl lines., (© The Author 2016. Published by Oxford University Press on behalf of the Society for Experimental Biology.)

In the Forestry Experimental Station of Shandong Agricultural University, we determined the photosynthetic physiological and biochemical indexes including photosynthetic parameters, chlorophyll fluorescence, and chlorophyll content of one-year-old poplar seedlings under different doses (0.17, 0.33, 0.67, 1.00, 1.33 and 1.67 g·kg -1 ) of Chaetomium globosmn ND35 fungus fertilizer treatment to study the effects of fungus fertilizer on photosynthesis of poplar in replanted soil. The results showed that with the increase of fungus fertilizer dose, the chlorophyll content of poplar lea-ves (Chl) showed an increasing trend, while the physiological indexes such as electron transport rate (ETR), net photosynthetic rate (P n ), quantum efficiency (Φ), nitrate reductase (NR) activity and root vigor showed an increasing first and then decreasing trend. Meanwhile, the heat dissipation that depended on xanthophyll cycle decreased and non-photochemical quenching (NPQ) increased first and then decreased. When the dose of C. globosmn ND35 fungus fertilizer was 0.67 g·kg -1 or 1.00 g·kg -1 , excess light energy of photosynthetic apparatus was reduced, and photosynthetic apparatus distributed more light energy to the direction of photochemical reactions, which improved the energy use efficiency. Applying an appropriate amount (0.67-1.00 g·kg -1 ) of C. globosmn ND35 fungus fertilizer could improve root physiological activity and light use ability of poplar leaves, which was conducive to the improvement of operating states of photosynthetic apparatus, and then increased the photosynthetic efficiency of the poplar leaves.

The high energy consumption of a plant factory is the biggest issue in its rapid expansion, especially for lighting electricity, which has been solved to a large extent by light-emitting diodes (LED). However, the remarkable potential for further energy savings remains to be further investigated. In this study, an optical system applied just below the LED was designed. The effects of the system on the growth and photosynthesis of butterhead lettuce (Lactuca sativa var. capitata) were examined, and the performance of the optical improvement in energy savings was evaluated by comparison with the traditional LED illumination mode. The irradiation patterns used were LED with zoom lenses (Z-LED) and conventional non-lenses LED (C-LED). The seedlings in both treatments were exposed to the same light environment over the entire growth period. The improvement saved over half of the light source electricity, while prominently lowering the temperature. Influenced by this, the rate of photosynthesis sharply decreased, causing reductions in plant yield and nitrate content, while having no negative effects on morphological parameters and photosynthetic pigment contents. Nevertheless, the much higher light use efficiency of Z-LEDs makes this system a better approach to illumination in a plant factory with artificial lighting.

Understanding plant photosynthesis, or Gross Primary Production (GPP), is a crucial aspect of quantifying the terrestrial carbon cycle. Remote sensing approaches, in particular multi-angular spectroscopy, have proven successful for studying relationships between canopy-reflectance and plant-physiology processes, thus providing a mechanism to scale up. However, many different instrumentation designs exist and few cross-comparisons have been undertaken. This paper discusses the design evolution of the Automated Multiangular SPectro-radiometer for Estimation of Canopy reflectance (AMSPEC) series of instruments. Specifically, we assess the performance of the PP-Systems Unispec-DC and Ocean Optics JAZ-COMBO spectro-radiometers installed on an updated, tower-based AMSPEC-III system. We demonstrate the interoperability of these spectro-radiometers, and the results obtained suggest that JAZ-COMBO can successfully be used to substitute more expensive measurement units for detecting and investigating photosynthesis and canopy spectra. We demonstrate close correlations between JAZ-COMBO and Unispec-DC measured canopy radiance (0.75 ≤ R² ≤ 0.85) and solar irradiance (0.95 ≤ R² ≤ 0.96) over a three month time span. We also demonstrate close agreement between the bi-directional distribution functions obtained from each instrument. We conclude that cost effective alternatives may allow a network of AMSPEC-III systems to simultaneously monitor various vegetation types in different ecosystems. This will allow to scale and improve our understanding of the interactions between vegetation physiology and spectral characteristics, calibrate broad-scale observations to stand-level measurements, and ultimately lead to improved understanding of changing vegetation spectral features from satellite.

Population growth and globally increasing standards of living have put a significant strain on the energy-food-water nexus. Limited water availability particularly affects agriculture, as it accounts for over 70% of global freshwater withdrawals (Aquastat). This study outlines the fundamental nature of plant water consumption and suggests a >50% reduction in renewable freshwater demand is possible by engineering more reflective crops. Furthermore, the decreased radiative forcing resulting from the greater reflectivity of crops would be equivalent to removing 10-50 ppm CO2 from the atmosphere. Recent advances in engineering optical devices and a greater understanding of the mechanisms of biological reflectance suggest such a strategy may now be viable. Here we outline the challenges involved in such an effort and suggest three potential approaches that could enable its implementation. While the local benefits may be straightforward, determining the global externalities will require careful modelling efforts and gradually scaled field trials., (Published 2015. This article is a U.S. Government work and is in the public domain in the USA.)

Vegetation productivity metrics such as gross primary production (GPP) at the canopy scale are greatly affected by the efficiency of using absorbed radiation for photosynthesis, or light use efficiency (LUE). Thus, close investigation of the relationships between canopy GPP and photosynthetically active radiation absorbed by vegetation is the basis for quantification of LUE. We used multiyear observations over irrigated and rainfed contrasting C3 (soybean) and C4 (maize) crops having different physiology, leaf structure, and canopy architecture to establish the relationships between canopy GPP and radiation absorbed by vegetation and quantify LUE. Although multiple LUE definitions are reported in the literature, we used a definition of efficiency of light use by photosynthetically active "green" vegetation (LUE(green)) based on radiation absorbed by "green" photosynthetically active vegetation on a daily basis. We quantified, irreversible slowly changing seasonal (constitutive) and rapidly day-to-day changing (facultative) LUE(green), as well as sensitivity of LUE(green) to the magnitude of incident radiation and drought events. Large (2-3-fold) variation of daily LUE(green) over the course of a growing season that is governed by crop physiological and phenological status was observed. The day-to-day variations of LUE(green) oscillated with magnitude 10-15% around the seasonal LUE(green) trend and appeared to be closely related to day-to-day variations of magnitude and composition of incident radiation. Our results show the high variability of LUE(green) between C3 and C4 crop species (1.43 g C/MJ vs. 2.24 g C/MJ, respectively), as well as within single crop species (i.e., maize or soybean). This implies that assuming LUE(green) as a constant value in GPP models is not warranted for the crops studied, and brings unpredictable uncertainties of remote GPP estimation, which should be accounted for in LUE models. The uncertainty of GPP estimation due to facultative and constitutive changes in LUE(green) can be considered as a critical component of the total error budget in the context of remotely sensed based estimations of GPP. The quantitative framework of LUE(green) estimation presented here offers a way of characterizing LUE(green) in plants that can be used to assess their phenological and physiological status and vulnerability to drought under current and future climatic conditions and is essential for calibration and validation of globally applied LUE algorithms., (Copyright © 2015 Elsevier GmbH. All rights reserved.)

In order to predict the potential impacts of global change, it is important to understand the impact of increasing global atmospheric [CO2] on the growth and yield of crop plants. The objectives of this study were to determine the interaction of N fertilization rates and atmospheric [CO2] on radiation interception and radiation-use efficiency of rice (Oryza sativa L. cv. IR72) grown undertropical field conditions. Rice plants were grown inside open top chambers in a lowland rice field at the International Rice Research Institute in the Philippines at ambient (about 350 Mumol mol-1) or elevated (about 600 Mumol mol-1 during the 1993 wet season and 700 Mumol mol-1 during the 1994 dry season) in combination with three levels of applied N (0, 50 or 100 kg N ha-1 in the wet season; 0, 90 or 200 kg N ha-1 in the dry season). Light interception was not directly affected by [CO2], but elevated [CO2] indirectly increased light interception through increasing total absorbed N. Plant N requirement for radiation interception was similar for rice grown under ambient [CO2] or elevated [CO2] treatments. The conversion efficiency of intercepted radiation to dry matter, radiation-use efficiency (RUE), was about 35% greater at elevated [CO2] than at ambient [CO2]. The relationship between leaf N and RUE was curvilinear. At ambient [CO2], RUE was fairly stable across levels of leaf N, but leaf N less than about 2.5% resulted in lower RUEfor plants grown with elevated [CO2] than for plant grownat ambient [CO2]. Decreased leaf N with increased [CO2], therefore decreased RUE of rice plants grown at elevated [CO2]. When predicting responses of rice to elevated [CO2], RUE should be adjusted with a decrease in leaf N. [ABSTRACT FROM AUTHOR]

This article develops a new carbon exchange diagnostic model [i.e. Southampton CARbon Flux (SCARF) model] for estimating daily gross primary productivity (GPP). The model exploits the maximum quantum yields of two key photosynthetic pathways (i.e. C3 and C4 ) to estimate the conversion of absorbed photosynthetically active radiation into GPP. Furthermore, this is the first model to use only the fraction of photosynthetically active radiation absorbed by photosynthetic elements of the canopy (i.e. FAPARps ) rather than total canopy, to predict GPP. The GPP predicted by the SCARF model was comparable to in situ GPP measurements (R(2)  > 0.7) in most of the evaluated biomes. Overall, the SCARF model predicted high GPP in regions dominated by forests and croplands, and low GPP in shrublands and dry-grasslands across USA and Europe. The spatial distribution of GPP from the SCARF model over Europe and conterminous USA was comparable to those from the MOD17 GPP product except in regions dominated by croplands. The SCARF model GPP predictions were positively correlated (R(2)  > 0.5) to climatic and biophysical input variables indicating its sensitivity to factors controlling vegetation productivity. The new model has three advantages, first, it prescribes only two quantum yield terms rather than species specific light use efficiency terms; second, it uses only the fraction of PAR absorbed by photosynthetic elements of the canopy (FAPARps ) hence capturing the actual PAR used in photosynthesis; and third, it does not need a detailed land cover map that is a major source of uncertainty in most remote sensing based GPP models. The Sentinel satellites planned for launch in 2014 by the European Space Agency have adequate spectral channels to derive FAPARps at relatively high spatial resolution (20 m). This provides a unique opportunity to produce global GPP operationally using the Southampton CARbon Flux (SCARF) model at high spatial resolution., (© 2013 John Wiley & Sons Ltd.)

The quantitative simulation of gross primary production (GPP) at various spatial and temporal scales has been a major challenge in quantifying the global carbon cycle. We developed a light use efficiency (LUE) daily GPP model from eddy covariance (EC) measurements. The model, called EC-LUE, is driven by only four variables: normalized difference vegetation index (NDVI), photosynthetically active radiation (PAR), air temperature, and the Bowen ratio of sensible to latent heat flux (used to calculate moisture stress). The EC-LUE model relies on two assumptions: First, that the fraction of absorbed PAR (fPAR) is a linear function of NDVI; Second, that the realized light use efficiency, calculated from a biome-independent invariant potential LUE, is controlled by air temperature or soil moisture, whichever is most limiting. The EC-LUE model was calibrated and validated using 24,349 daily GPP estimates derived from 28 eddy covariance flux towers from the AmeriFlux and EuroFlux networks, covering a variety of forests, grasslands and savannas. The model explained 85% and 77% of the observed variations of daily GPP for all the calibration and validation sites, respectively. A comparison with GPP calculated from the Moderate Resolution Imaging Spectroradiometer (MODIS) indicated that the EC-LUE model predicted GPP that better matched tower data across these sites. The realized LUE was predominantly controlled by moisture conditions throughout the growing season, and controlled by temperature only at the beginning and end of the growing season. The EC-LUE model is an alternative approach that makes it possible to map daily GPP over large areas because (1) the potential LUE is invariant across various land cover types and (2) all driving forces of the model can be derived from remote sensing data or existing climate observation networks., Earth and Planetary Sciences