Albert Porcar-Castell

Linking chlorophyll a fluorescence to photosynthesis for remote sensing applications: mechanisms and challenges

Chlorophyll a fluorescence (ChlF) has been used for decades to study the organization, functioning, and physiology of photosynthesis at the leaf and subcellular levels. ChlF is now measurable from remote sensing platforms. This provides a new optical means to track photosynthesis and gross primary productivity of terrestrial ecosystems. Importantly, the spatiotemporal and methodological context of the new applications is dramatically different compared with most of the available ChlF literature, which raises a number of important considerations. Although we have a good mechanistic understanding of the processes that control the ChlF signal over the short term, the seasonal link between ChlF and photosynthesis remains obscure. Additionally, while the current understanding of in vivo ChlF is based on pulse amplitude-modulated (PAM) measurements, remote sensing applications are based on the measurement of the passive solar-induced chlorophyll fluorescence (SIF), which entails important differences and new challenges that remain to be solved. In this review we introduce and revisit the physical, physiological, and methodological factors that control the leaf-level ChlF signal in the context of the new remote sensing applications. Specifically, we present the basis of photosynthetic acclimation and its optical signals, we introduce the physical and physiological basis of ChlF from the molecular to the leaf level and beyond, and we introduce and compare PAM and SIF methodology. Finally, we evaluate and identify the challenges that still remain to be answered in order to consolidate our mechanistic understanding of the remotely sensed SIF signal.

PRI assessment of long-term changes in carotenoids/chlorophyll ratio and short-term changes in de-epoxidation state of the xanthophyll cycle.

The Photochemical Reflectance Index (PRI), an index based on leaf reflectance at 531 nm, has been found suitable for tracking variations in photosynthetic activity from leaf to ecosystem levels. This suitability has been attributed to PRI correlation with xanthophyll interconversion and photosynthetic radiation-use efficiency. However, other pigments and factors may be involved in such relationships. We studied the relationship between PRI and xanthophylls and other carotenoids in saplings of two widely distributed evergreen species (Scots pine and Holm oak) submitted to experimentally changing light conditions in a field experiment. PRI was strongly correlated with the de-epoxidation state of xanthophylls (DEPS, an expression of the relative concentration of the three xanthophyll cycle pigments), but also with carotenoids/chlorophyll ratio and β-carotene/chlorophyll ratio in both species. However, following momentary decreases in light due to clouds, PRI changed following the DEPS changes, while the carotenoids/chlorophyll ratio remained constant. The results show that PRI was able to reveal short-term changes in de-epoxidation state, i.e. the signal of xanthophyll interconversion, but it also tracked long-term changes in carotenoids/chlorophyll. Carotenoids other than xanthophylls, e.g. β-carotene, are also related to photoprotective processes, thus also making PRI effective as a measure of changes in photosynthetic light-use efficiency in response to stress on a long-term level.

Physiology of the seasonal relationship between the photochemical reflectance index and photosynthetic light use efficiency

The photochemical reflectance index (PRI) is regarded as a promising proxy to track the dynamics of photosynthetic light use efficiency (LUE) via remote sensing. The implementation of this approach requires the relationship between PRI and LUE to scale not only in space but also in time. The short-term relationship between PRI and LUE is well known and is based on the regulative process of non-photochemical quenching (NPQ), but at the seasonal timescale the mechanisms behind the relationship remain unclear. We examined to what extent sustained forms of NPQ, photoinhibition of reaction centres, seasonal changes in leaf pigment concentrations, or adjustments in the capacity of alternative energy sinks affect the seasonal relationship between PRI and LUE during the year in needles of boreal Scots pine. PRI and NPQ were highly correlated during most of the year but decoupled in early spring when the foliage was deeply downregulated. This phenomenon was attributed to differences in the physiological mechanisms controlling the seasonal dynamics of PRI and NPQ. Seasonal adjustments in the pool size of the xanthophyll cycle pigments, on a chlorophyll basis, controlled the dynamics of PRI, whereas the xanthophyll de-epoxidation status and other xanthophyll-independent mechanisms controlled the dynamics of NPQ at the seasonal timescale. We conclude that the PRI leads to an underestimation of NPQ, and consequently overestimation of LUE, under conditions of severe stress in overwintering Scots pine, and most likely also in species experiencing severe drought. This severe stress-induced decoupling may challenge the implementation of the PRI approach.

Thermal energy dissipation and xanthophyll cycles beyond the Arabidopsis model

Thermal dissipation of excitation energy is a fundamental photoprotection mechanism in plants. Thermal energy dissipation is frequently estimated using the quenching of the chlorophyll fluorescence signal, termed non-photochemical quenching. Over the last two decades, great progress has been made in the understanding of the mechanism of thermal energy dissipation through the use of a few model plants, mainly Arabidopsis. Nonetheless, an emerging number of studies suggest that this model represents only one strategy among several different solutions for the environmental adjustment of thermal energy dissipation that have evolved among photosynthetic organisms in the course of evolution. In this review, a detailed analysis of three examples highlights the need to use models other than Arabidopsis: first, overwintering evergreens that develop a sustained form of thermal energy dissipation; second, desiccation tolerant plants that induce rapid thermal energy dissipation; and third, understorey plants in which a complementary lutein epoxide cycle modulates thermal energy dissipation. The three examples have in common a shift from a photosynthetically efficient state to a dissipative conformation, a strategy widely distributed among stress-tolerant evergreen perennials. Likewise, they show a distinct operation of the xanthophyll cycle. Expanding the list of model species beyond Arabidopsis will enhance our knowledge of these mechanisms and increase the synergy of the current studies now dispersed over a wide number of species.

A high-resolution portrait of the annual dynamics of photochemical and non-photochemical quenching in needles of Pinus sylvestris

Partitioning of excitation energy between photochemical quenching (PQ) and non-photochemical quenching (NPQ) processes is constantly adjusted in the leaf in order to preserve the photosynthetic energy balance. Adjustments in PQ and NPQ often result from a combination of different temporal components that can be simplified into reversible and sustained components. While reversible PQ and NPQ are relatively well understood, the controls behind the sustained components of PQ and NPQ, or the interaction between sustained and reversible NPQ, remain elusive. In this study, I used a full year of high-resolution chlorophyll fluorescence (ChlF) data obtained with a Monitoring-PAM fluorometer (Walz, Effeltrich, Germany) in needles of boreal Pinus sylvestris in situ to quantitatively analyse the dynamics and interaction between temporal components of NPQ and PQ and their control by the environment. To enable the estimation of sustained and reversible components of PQ and NPQ, a number of key ChlF parameters were reviewed and adapted to the analysis of long-term monitoring data. Overall, NPQ was drastically enhanced during winter via the accumulation of sustained NPQ in a process regulated by air temperature. Reversible NPQ retained some functionality even at temperatures well below zero and was not inhibited by the presence of sustained NPQ per se but by low temperatures alone. This suggests that temporal NPQ components co-operate in an additive rather than complementary fashion, conferring additional flexibility to the photoprotective role of NPQ. Finally, the potential of the sustained photochemical quenching parameter (qL(s) ) to track photoinhibition in situ was discussed.

A new monitoring PAM fluorometer (MONI-PAM) to study the short- and long-term acclimation of photosystem II in field conditions

We present and evaluate the performance of a new field monitoring PAM fluorometer (MONI-PAM) which is intended for short- and long-term monitoring of the acclimation of photosystem II (PSII). The instrument measures chlorophyll fluorescence, photosynthetic photon flux density (PPFD), and temperature in the field, and monitors exactly the same leaf area over prolonged periods of time, facilitating the estimation of both rapidly reversible and sustained non-photochemical quenching (NPQ). The MONI-PAM performance is evaluated in the lab and under natural conditions in a Scots pine canopy during spring recovery of photosynthesis. The instrument provides a new tool to study in detail the acclimation of PSII to the environment under natural field conditions.

OCO-2 advances photosynthesis observation from space via solar-induced chlorophyll fluorescence

INTRODUCTION Reliable estimation of gross primary production (GPP) from landscape to global scales is pivotal to a wide range of ecological research areas, such as carbon-climate feedbacks, and agricultural applications, such as crop yield and drought monitoring. However, measuring GPP at these scales remains a major challenge. Solar-induced chlorophyll fluorescence (SIF) is a signal emitted directly from the core of photosynthetic machinery. SIF integrates complex plant physiological functions in vivo to reflect photosynthetic dynamics in real time. The advent of satellite SIF observation promises a new era in global photosynthesis research. The Orbiting Carbon Observatory-2 (OCO-2) SIF product is a serendipitous but critically complementary by-product of OCO-2’s primary mission target—atmospheric column CO2 (XCO2). OCO-2 SIF removes some important roadblocks that prevent wide and in-depth applications of satellite SIF data sets and offers new opportunities for studying the SIF-GPP relationship and vegetation functional gradients at different spatiotemporal scales. RATIONALE Compared with earlier satellite missions with SIF capability, the OCO-2 SIF product has substantially improved spatial resolution, data acquisition, and retrieval precision. These improvements allow satellite SIF data to be validated, for the first time, directly against ground and airborne measurements and also used to investigate the SIF-GPP relationship and terrestrial ecosystem functional dynamics with considerably better spatiotemporal credibility. RESULTS Coordinated airborne measurements of SIF with the Chlorophyll Fluorescence Imaging Spectrometer (CFIS) were used to validate OCO-2 retrievals. The validation shows close agreement between OCO-2 and CFIS SIF, with a regression slope of 1.02 and R2 of 0.71. Landscape gradients in SIF emission, corresponding to differences in vegetation types, were clearly delineated by OCO-2, a capability that was lacking in previous satellite missions. The SIF-GPP relationships at eddy covariance flux sites in the vicinity of OCO-2 orbital tracks were found to be more consistent across biomes than previously suggested. Finally, empirical orthogonal function (EOF) analyses on OCO-2 SIF and available GPP products show highly consistent spatiotemporal correspondence in their leading EOF modes across the globe, suggesting that SIF and GPP are governed by similar dynamics and controlled by similar environmental and biological conditions. CONCLUSION OCO-2 represents a major advance in satellite SIF remote sensing. Our analyses suggest that SIF is a powerful proxy for GPP at multiple spatiotemporal scales and that high-quality satellite SIF is of central importance to studying terrestrial ecosystems and the carbon cycle. Although the possibility of a universal SIF-GPP relationship across different biome types cannot be dismissed, in-depth process-based studies are needed to unravel the true nature of covariations between SIF and GPP. Of critical importance in such efforts are the potential coordinated dynamics between the light-use efficiencies of CO2 assimilation and fluorescence emission in response to changes in climate and vegetation characteristics. Eventual synergistic uses of SIF with atmospheric CO2 enabled by OCO-2 will lead to more reliable estimates of terrestrial carbon sources and sinks—when, where, why, and how carbon is exchanged between land and atmosphere—as well as a deeper understanding of carbon-climate feedbacks. The marked ecological gradients depicted by OCO-2’s high-resolution SIF measurements along a transect of temperate deciduous forests, crops, and urban area from Indiana to suburban Chicago, Illinois. Quantifying gross primary production (GPP) remains a major challenge in global carbon cycle research. Spaceborne monitoring of solar-induced chlorophyll fluorescence (SIF), an integrative photosynthetic signal of molecular origin, can assist in terrestrial GPP monitoring. However, the extent to which SIF tracks spatiotemporal variations in GPP remains unresolved. Orbiting Carbon Observatory-2 (OCO-2)’s SIF data acquisition and fine spatial resolution permit direct validation against ground and airborne observations. Empirical orthogonal function analysis shows consistent spatiotemporal correspondence between OCO-2 SIF and GPP globally. A linear SIF-GPP relationship is also obtained at eddy-flux sites covering diverse biomes, setting the stage for future investigations of the robustness of such a relationship across more biomes. Our findings support the central importance of high-quality satellite SIF for studying terrestrial carbon cycle dynamics.

A temperature-controlled spectrometer system for continuous and unattended measurements of canopy spectral radiance and reflectance

This paper describes the development of a fully automated system for collecting high-resolution spectral data over a forested footprint. The system comprises a pair of off-the-shelf spectrometers in a custom-built thermal enclosure with a fixed off-nadir downward (target)-pointing fibre and upward-pointing fibre for irradiance measurement. Both instruments sample simultaneously via custom-written and user-controlled software during all weathers and sky conditions. The system is mounted on a 25 m eddy covariance scaffolding tower, approximately 7 m from a Scots pine forest canopy. The system was installed at the University of Helsinki’s SMEAR-II Field Station in Hyytiälä in March 2010 and has been operating continuously through a joint programme between the Universities of Edinburgh and Helsinki. The system was designed to capture diurnal and seasonal variation in vegetation light-use efficiency and fluorescence through the capture and analysis of well-defined narrow spectral features, but its implementation would permit the extraction of further optical signals linked to vegetation biophysical variables, and provide a continuous data stream with which to validate satellite data products including vegetation indices such as the photochemical reflectance index (PRI) as well as spectral indicators of solar induced fluorescence.

Field and controlled environment measurements show strong seasonal acclimation in photosynthesis and respiration potential in boreal Scots pine

Understanding the seasonality of photosynthesis in boreal evergreen trees and its control by the environment requires separation of the instantaneous and slow responses, as well as the dynamics of light reactions, carbon reactions, and respiration. We determined the seasonality of photosynthetic light response and respiration parameters of Scots pine (Pinus sylvestris L.) in the field in southern Finland and in controlled laboratory conditions. CO2 exchange and chlorophyll fluorescence were measured in the field using a continuously operated automated chamber setup and fluorescence monitoring systems. We also carried out monthly measurements of photosynthetic light, CO2 and temperature responses in standard conditions with a portable IRGA and fluorometer instrument. The field and response measurements indicated strong seasonal variability in the state of the photosynthetic machinery with a deep downregulation during winter. Despite the downregulation, the photosynthetic machinery retained a significant capacity during winter, which was not visible in the field measurements. Light-saturated photosynthesis (Psat) and the initial slope of the photosynthetic light response (α) obtained in standard conditions were up to 20% of their respective summertime values. Respiration also showed seasonal acclimation with peak values of respiration in standard temperature in spring and decline in autumn. Spring recovery of all photosynthetic parameters could be predicted with temperature history. On the other hand, the operating quantum yield of photosystem II and the initial slope of photosynthetic light response stayed almost at the summertime level until late autumn while at the same time Psat decreased following the prevailing temperature. Comparison of photosynthetic parameters with the environmental drivers suggests that light and minimum temperature are also decisive factors in the seasonal acclimation of photosynthesis in boreal evergreen trees.

Dynamics of the energy flow through photosystem II under changing light conditions: a model approach

Several biochemical models of photosynthesis exist that consider the effects of the dynamic adjustment of enzymatic and stomatal processes on carbon assimilation under fluctuating light. However, the rate of electron transport through the light reactions is commonly modelled by means of an empirical equation, parameterised with data obtained at the steady state. A steady-state approach cannot capture the dynamic nature of the adjustment of the light reactions under fluctuating light. Here we present a dynamic model approach for photosystem II that considers the adjustments in the regulative non-photochemical processes. The model is initially derived to account for changes occurring at the seconds-to-minutes time-scale under field conditions, and is parameterised and tested with chlorophyll fluorescence data. Results derived from this model show good agreement with experimentally obtained photochemical and non-photochemical quantum yields, providing evidence for the effect that the dark reactions exert in the adjustment of the energy flows at the light reactions. Finally, we compare the traditional steady-state approach with our dynamic approach and find that the steady-state approach produces an underestimation of the modelled electron transport rate (ETR) under rapidly fluctuating light (1 s or less), whereas it produces overestimations under slower fluctuations of light (5 s or more).