Fakultäten der Universität Bonn

Understanding Stress-induced Dynamics of Solar-induced Chlorophyll Fluorescence with Leaf-level Hyperspectral Imaging

2026-05-08T11:00:25Z

Understanding Stress-induced Dynamics of Solar-induced Chlorophyll Fluorescence with Leaf-level Hyperspectral Imaging Peng, Huaiyue Proximal sensing of solar-induced chlorophyll fluorescence (SIF) is widely used as a non-invasive indicator of photosynthetic activity. Together with photochemical quenching (PQ) and non-photochemical quenching (NPQ), SIF represents a third pathway of energy dissipation in excited chlorophyll molecules and provides direct insight into the redox state of the photosystem, particularly under stress conditions, often with shorter response times than vegetation indices (VIs). Because SIF does not require artificial illumination, it can be applied across spatial scales from leaf to global satellite observations. However, two major challenges limit the direct interpretation of SIF in terms of photosynthetic efficiency: (1) accurate estimation of SIF yield due to confounding effects of absorbed PAR (APAR) and fluorescence escape probability, both influenced by illumination geometry, plant structural, and physiological properties; and (2) incomplete understanding of the relationships among SIF yield, NPQ, and PQ under stress. To address part of these challenges, this thesis first introduced a ground-based hyperspectral imaging system (HyScreen) for high-resolution retrieval of red and far-red SIF. The proposed measurement protocol and processing chain achieved high accuracy, with relative errors of 0.4–1.0% for red SIF and 0.2–0.5% for far-red SIF due to noise. Second, a controlled cold-stress experiment was conducted to evaluate the combined use of SIF and VIs as indicators of cold stress tolerance in Arabidopsis thaliana. Specifically, the study examined whether SIF and VIs can (1) detect cold stress duration, (2) distinguish genotypes with contrasting NPQ capacity, and (3) resolve differences in photoinhibition between young and mature leaves. The results show that (1) acute cold stress initially induced high SIF yield with high near-infrared (NIR) reflectance, followed by decreasing SIF, reduced NIR reflectance, and reduced red-edge normalized difference vegetation index (NDVIre) under photoinhibition. Low values of the effective quantum yield of PSII together with high NPQ indicated that NPQ dominated SIF quenching during cold stress, while changes in NIR reflectance and NDVIre indicated cold-induced alterations in leaf structure and reduced chlorophyll content, respectively. (2) Four genotypes differing in NPQ capacity were distinguishable by combining SIF yield, photochemical reflectance index (PRI), and NDVIre. Early SIF responses reflected the fast NPQ component qE, whereas the change and temporal profile of PRI captured both qZ and overall NPQ during prolonged stress. Importantly, higher qE or qZ capacity did not necessarily correspond to reduced photodamage, as chlorophyll degradation (indicated by reduced NDVIre) contributed strongly to photoinhibition. (3) Young leaves exhibited greater cold tolerance than mature leaves, as evidenced by higher Fv/Fm after cold exposure, characterized by higher qE capacity, retention of qZ activity, and better chlorophyll maintenance, as indicated by lower initial SIF yield, higher PRI, and higher NDVIre. In summary, HyScreen enables leaf-level SIF imaging while minimizing background effects and provides new opportunities to investigate spatial heterogeneity in photoprotective responses from leaf to plant scales. This study suggests that the combined use of SIF and VIs can capture NPQ dynamics, chlorophyll degradation, and leaf structural responses linked to cold stress tolerance across genotypes and leaf ages. This work also provides insights relevant to future validation activities for the ESA FLEX mission and to high-throughput screening of stress-tolerant plants in breeding programs.; Verständnis der stressinduzierten Dynamik der sonneninduzierten Chlorophyllfluoreszenz mittels blattbasierter hyperspektraler Bildgebung
Die bodennahe Fernerkundung der sonneninduzierten Chlorophyllfluoreszenz (SIF) wird in der Forschung häufig als nicht-invasiver Indikator für photosynthetische Aktivität eingesetzt. Zusammen mit dem photochemischen Quenching (PQ) und dem nicht-photochemischen Quenching (NPQ) stellt SIF einen dritten Weg der Energiedissipation angeregter Chlorophyllmoleküle dar und ermöglicht direkte Einblicke in die Physiologie der Photosysteme, insbesondere unter Stressbedingungen. Im Vergleich zu Vegetationsindizes (VIs) zeigt sich, dass SIF häufig eine schnellere Reaktion auf physiologische Veränderungen aufweist. Für SIF-Messungen ist keine künstliche Beleuchtung erforderlich, sodass sie über verschiedene Skalen hinweg angewendet werden können – vom Blatt bis zur globalen Satellitenbeobachtung. Die unmittelbare Interpretation der SIF in Bezug auf die photosynthetische Effizienz wird jedoch durch zwei zentrale Herausforderungen limitiert: Einerseits erschwert der Einfluss der absorbierten photosynthetisch aktiven Strahlung (APAR) sowie der Fluoreszenz-Austrittswahrscheinlichkeit die exakte Bestimmung der SIF-Ausbeute. Dabei sind die Beleuchtungsgeometrie sowie die strukturellen und physiologischen Eigenschaften der Pflanze zu berücksichtigen. Zweitens sind die Zusammenhänge zwischen SIF-Ausbeute, NPQ und PQ unter abiotischem Stress bislang unzureichend verstanden. Zur Bearbeitung dieser Fragestellungen wurde in dieser Arbeit zunächst ein bodengestütztes hyperspektrales Bildgebungssystem (HyScreen) zur hochauflösenden Erfassung von roter und fernroter SIF eingeführt. Das Messprotokoll und die Datenverarbeitung wiesen eine hohe Genauigkeit mit relativen Fehlern von 0,4–1,0 % für rote SIF und 0,2–0,5 % für fernrote SIF aufgrund von Rauschen auf. Im Anschluss wurde ein kontrolliertes Kältestressexperiment durchgeführt, um die kombinierte Verwendung von SIF und Vegetationsindizes als Indikatoren für die Kältestresstoleranz bei Arabidopsis thaliana zu bewerten. Im Rahmen der Untersuchung wurde analysiert, ob diese Parameter (1) die Dauer von Kältestress erfassen, (2) Genotypen mit divergierender NPQ-Kapazität differenzieren und (3) Diskrepanzen in der Photoinhibition zwischen jungen und alten Blättern auflösen können. Die Ergebnisse zeigen, dass (1) akuter Kältestress zunächst zu erhöhter SIF-Ausbeute und hoher Nahinfrarot-Reflexion (NIR) führte, gefolgt von abnehmender SIF, reduzierter NIR-Reflexion und vermindertem Red-Edge Normalized Difference Vegetation Index (NDVIre) unter Photoinhibition. Niedrige Werte der effektiven Quantenausbeute von PSII in Kombination mit hoher NPQ deuten darauf hin, dass NPQ die Abschwächung der SIF während des Kältestresses dominierte, während Änderungen der NIR-Reflexion und des NDVIre auf kälteinduzierte Veränderungen der Blattstruktur bzw. eine Abnahme des Chlorophyllgehalts hinwiesen. (2) Vier Genotypen mit unterschiedlicher NPQ-Kapazität konnten durch die kombinierte Analyse von SIF-Ausbeute, Photochemical Reflectance Index (PRI) und NDVIre unterschieden werden. Frühe SIF-Reaktionen spiegelten die schnelle NPQ-Komponente qE wider, während zeitliche Änderungen des PRI sowohl qZ als auch die gesamte NPQ-Dynamik während längerem Stress erfassten. Eine höhere qE- oder qZ-Kapazität ging dabei nicht zwingend mit geringerer Photodamage einher, da Chlorophyllabbau – angezeigt durch reduziertes NDVIre – wesentlich zur Photoinhibition beitrug. (3) Junge Blätter zeigten eine höhere Kältetoleranz als ältere Blätter, belegt durch höhere Fv/Fm-Werte nach Kälteeinwirkung und charakterisiert durch höhere qE-Kapazität, anhaltende qZ-Aktivität und bessere Erhaltung des Chlorophyllgehalts, angezeigt durch geringere anfängliche SIF-Ausbeute, höheren PRI und höheres NDVIre. Zusammenfassend ermöglicht HyScreen eine blattweise SIF-Bildgebung mit reduzierten Hintergrundeinflüssen und eröffnet neue Möglichkeiten zur Analyse räumlicher Heterogenität photoprotektiver Reaktionen vom Blatt- bis zum Pflanzenmaßstab. Die Ergebnisse legen nahe, dass die kombinierte Nutzung von SIF und Vegetationsindizes NPQ-Dynamiken, Chlorophyllabbau und strukturelle Blattveränderungen im Zusammenhang mit Kältetoleranz über verschiedene Genotypen und Blattalter hinweg erfassen kann und liefern zugleich wichtige Beiträge für zukünftige Validierungsaktivitäten der ESA-Mission FLEX sowie für die hochdurchsatzbasierte Selektion stresstoleranter Pflanzen in der Züchtung.

Effects of hydrolysable tannins and dry matter content on nitrogen metabolism in different ensiled forage species and feed intake behaviour of goats

2026-05-08T10:45:23Z

Effects of hydrolysable tannins and dry matter content on nitrogen metabolism in different ensiled forage species and feed intake behaviour of goats Hilgers, Bernd Wilhelm Ensiled grass and legume forages are key components of ruminant diets, providing locally available, resource-efficient sources of crude protein (CP), energy and fibre. However, inevitable degradation and conversion processes occur during the ensiling process, due to plant-derived and microbial enzyme activity, leading to proteolysis of CP. An extensive degradation of CP must be critically considered, as this process results in reduced feed intake and nitrogen use efficiency. The objective of the present thesis was to ascertain to which extent a treatment of forages with a hydrolysable tannin (HT) extract influenced fermentation and protein quality, as well as the feed choice behaviour of goats, under varying ensiling conditions. The initial focus was therefore on a comprehensive chemical characterisation of the prepared lucerne, Italian ryegrass and red clover silage treatments. It was demonstrated that increasing the dry matter (DM) content of plant material from 200 to 350 g/kg prior to ensiling significantly improved fermentation quality and gas production. Meanwhile, adding HT up to 40 g/kg DM was found to have a positive impact on protein quality due to a restricted accumulation of non-protein-nitrogen (NPN) compounds. The impact of the pre-ensiling treatments on the feed choice behaviour and short-term DM intake of goats was subsequently investigated in three independent feeding trials. The animals showed a significantly higher preference and feed consumption for treatments with a higher DM content, whereby this observation was particularly pronounced in legume silages. This was due to the enhanced fermentation quality of high DM silages, as the formation of volatile fermentation products such as acetic acid and ammonia-nitrogen (NH3-N), which were negatively associated with silage quality and short-term DM intake of goats, were considerably reduced due to field wilting. On the other hand, the application of HT prior to the ensiling only led to a higher feed intake in low DM silages. The final study involved a detailed examination of the formation of free amino acids (AA) and biogenic amines (BA) in all pre-ensiling treatments. The highest amounts of individual and total BA were consistently detected in low DM silages, with the highest concentrations in lucerne and lowest in red clover silages. In addition to increasing the DM content, HT treatment also significantly impaired proteolytic activity during the ensiling process, as the accumulation of free AA, ℽ-aminobutyric acid and BA was markedly reduced. Furthermore, multiple regression analysis clarified that NPN compounds, such as NH3-N and BA were valid indicators for the acceptance and DM intake of silages in feed choice behaviour trials. Overall, the findings indicate that both the fermentation and protein quality of ensiled forages and the subsequent feed intake can be systematically enhanced by pre-ensiling treatments, which offers potential for further research and practical use.

Near-Field Microscopy of Plasmonic Waveguides and Tight-Binding Lattices

2026-05-04T13:00:42Z

Near-Field Microscopy of Plasmonic Waveguides and Tight-Binding Lattices Schill, Hans-Joachim Electronic computing has dominated data processing for decades, but we are reaching a point where fundamental physical limitations motivate the exploration of alternative approaches. Photonic and plasmonic systems provide promising routes toward compact, high-speed, and low-energy information processing by enabling strong light–matter interactions at the nanoscale. In particular, plasmonic waveguides allow optical confinement below the diffraction limit, making them attractive building blocks for integrated nanophotonic circuits. Furthermore, many fundamental condensed matter or quantum effects can be investigated with coupled waveguide arrays. However, their characterization and control require advanced experimental techniques capable of resolving optical fields at subwavelength scales. This thesis investigates plasmonic waveguides with a focus on their fabrication, near-field characterization, and functional coupling schemes. After an introduction to surface plasmon polaritons and their excitation, the thesis describes the fabrication methods used to realize plasmonic nanostructures and discusses the associated challenges. As a key tool for nanoscale optical characterization, the principles and experimental implementation of scattering-type scanning near-field optical microscopy are then presented. These techniques are used to investigate three distinct plasmonic waveguide systems. First, metal strip waveguides with chiral couplers are studied, demonstrating spin-dependent directional excitation of surface plasmon polaritons based on the polarization of incident light. Second, dielectric-loaded plasmonic waveguides are investigated, including a detailed analysis of their electromagnetic field components. Building on this, coupled dielectric-loaded surface plasmon polariton waveguides are explored as a platform for studying coupling dynamics and analogies to solid-state systems. By implementing a plasmonic analogue of the Su–Schrieffer–Heeger model, topological edge states are experimentally realized and directly visualized using near-field microscopy.

Protostellar outflows across masses and environments

2026-05-04T12:30:43Z

Protostellar outflows across masses and environments Skretas, Iason-Michail Protostellar outflows mark one of the earliest, and most prominent signs of star formation, and have been detected in both low- and high-mass sources. Protostellar outflows are considered a key part of the process due to their ability to remove excess angular momentum from the protostar-disk system, which enables the accretion of material. They are typically observed via molecular transitions at radio wavelengths, but are also bright in shock excited transitions in the IR regime. Due to their close connection to the accretion process, understanding protostellar outflows is crucial in order to fully describe the formation of stars.
Throughout this thesis, I aimed to investigate protostellar outflows from sources across different environments, in order to investigate several of the open questions regarding their nature. A particular focus is placed on the impact of the large-scale environment onto the outflows, since this holds the potential to reveal whether the star formation process is influenced by its surroundings. In this work, I used observations from the NOEMA and IRAM 30m telescopes, as well as, observations from the JWST MIRI/MRS instrument.
In the first part of this thesis, I investigated the exceptional outflow of DR21 Main, which has been proposed to form part of a new type of protostellar outflows, known as explosive outflows. The extended bandwidth of the CASCADE observations enabled me to study this unique outflow at multiple different molecular transitions, and determine its morphological, kinematic, and energetic properties. Through comparisons with other explosive outflow candidates I aim to constrain the nature of the DR21 Main outflow thus placing further constraints in this different kind of protostellar outflows. The analysis revealed that the DR21 Main outflow, as seen in HCO+ J=1-0 is more akin a bipolar outflows than an explosive outflow. It's confirmed particularly high mass and energetic properties though still class it as one of the most extreme outflow cases known.
Subsequently, I studied the outflow activity along the entire DR21 filament, one of the most active, high-mass star-forming regions in our Galaxy. Using the HCO+ J=1-0, H13CO+ J=1-0, and SiO J=2-1 observations of the region, taken as part of the CASCADE project, I aimed to identify all protostellar outflows associated with dense molecular clumps along the DR21 ridge and estimate their physical and energetic properties. By comparing the properties of such a sample with the established correlations between outflow and source properties allowed me to investigate whether the extreme nature of the DR21 filament has any impact onto the outflows, and by extension the formation, of its sources. The results showed no clear connection between environment and outflow activity, with the sources in DR21 being indistinguishable to those of an extended literature sample. Notable exception is the location of the most prominent outflow sources at the intersections of filaments.
In the final part of the thesis, I take advantage of the unique capabilities of the JWST, to investigate the inner workings of protostellar outflows. Namely, I study the shock excited transitions of H2 along with various atomic and ionic transitions available in the MIRI range for a sample of 5 low-mass protostars in Ophiuchus. My aim with this analysis is to investigate the origin of this shock excitation, through comparisons of the observations with UV irradiated shock models. The analysis revealed the significant contribution of UV emission within these outflows. I found that the origin of this UV emission has to been from within the protostellar outflows themselves, and not from the external environment.
In this thesis, I analyzed outflows from sources across the entire mass regime, using observations in both the mm and IR regime. Throughout the multiple individual results of each project, My analysis showed that the properties of the large scale environment surrounding a forming protostar have little to no influence on the properties of its protostellar outflow. It becomes therefore apparent that the star formation process is primarily dictated from small scale processes, taking place within the star forming cores, and not impacted by the more extended environment.