Dynamic root growth in response to depth-varying soil moisture availability:a rhizobox study

Plant roots are highly adaptable, but their adaptability is not included in crop and land surface models. They rely on a simplified representation of root growth, which is independent of soil moisture availability. Data of subsurface processes and interactions, needed for model setup and validation, are scarce. Here we investigated soil-moisture-driven root growth. To this end, we installed subsurface drip lines and small soil moisture sensors (0.2 L measurement volume) inside rhizoboxes (length × width × height of 45 × 7.5 × 45 cm). The development of the vertical soil moisture and root growth profiles is tracked with a high spatial and temporal resolution. The results confirm that root growth is predominantly driven by vertical soil moisture distribution, while influencing soil moisture at the same time. Besides support for the functional relationship between the soil moisture and the root density growth rate, the experiments also suggest that the extension of the maximum rooting depth will stop if the soil moisture at the root tip drops below a threshold value. We show that even a parsimonious one-dimensional water balance model, driven by the water input flux (irrigation), can be convincingly improved by implementing root growth driven by soil moisture availability

A histochemical study of root nodule development

In cooperation with soil bacteria of the genera Rhizobium , Bradyrhizobium or Azorhizobium , many members of the legume family are able to form specialized organs on their roots, called root nodules. The bacteria, wrapped up inside a plant membrane, are accomodated in large parenchymatic cells located centrally in these root nodules. For this, they reward their host by converting atmospheric nitrogen into a form usable for the plant. The central infected tissue of the nodule is surrounded by a peripheral tissue provided with vascular bundles through which metabolites are exchanged with the other parts of the plant.In the interaction with the bacteria, the host plant expresses specific genes that are not transcribed at a detectable level in other parts of the plant. The products of several of these genes are made during the formation of the nodule and are named early nodulins.The present study aims at elucidating the role of these early nodulins in the formation and infection of the root nodules. For that purpose, we set out to combine the molecular approach of studying gene expression with the microscopical approach of studying the structural development of the nodule.To provide a background to these studies, chapter 11 summarizes existing knowledge about nodule development from an anatomical/cytological point of view, supplemented with data on already described nodulins and with brief excursions into physiological phenomena relevant to the rest of our study.In chapter III and IV, nodulin gene expression is analysed in common vetch ( Vicia sativa ) nodules elicited by a panel of bacterial strains with various defined genetic changes. Such nodules were blocked at different stages in the development of the central tissue depending on the bacterium involved; the precise stage at which the blockade occurred was determined by light- and electron-microscopical observations. In that way, insight could be gained in the diverse genetic information supplied by the bacterium for nodule development to proceed through the successive developmental stages and the induction of the appropriate nodulin genes going with it. Furthermore, the start of the expression of individual nodulin genes, for instance the early nodulin Nps-40', could be related to certain stages of central tissue development. In the case of the leghemoglobin genes, such a correlation between nodulin gene expression and specific developmental stages could be confirmed by the direct localization of the leghemoglobin proteins in pea ( Pisum sativum ) nodule sections comprising different consecutive developmental stages, by immunolabeling.Such direct approach of studying nodulin gene expression in nodule sections was further pursued in the chapters V, VI, VII and VIII. In chapters V, VI and VII early nodulin gene transcripts for which sequenced cDNA clones had become available were localized by in situ hybridization: in chapter V, ENOD2 in soybean ( Glycine max ) and pea nodules, respectively; in chapter VI, ENOD2 in alfalfa ( Medicago sativa ) nodules; and in chapter VII, PsENOD12 in pea. In chapter VIII an attempt to localize the Nps-40' protein by immunolabeling in pea nodules is described. By these in situ localization methods, different temporal and spatial patterns of gene expression for each early nodulin were determined. Speculations about the functions of the individual nodulins are made based upon the gene expression patterns and the amino acid sequences of the nodulins as deduced from the nucleotide sequence of the corresponding cDNA clones.In addition, in chapter VI, in situ localization of MsENOD2 transcripts was performed on alfalfa nodules induced by certain engineered bacterial strains or by auxin transport inhibitors. Such nodules do not have bacteria in their central tissue and also differ in other structural details from effective nodules, but nevertheless were shown to exhibit a tissue- specific expression pattern of the MsENOD2 gene similar to effective nodules. In chapter VII the results of further experiments are reported pertaining to the influence of the bacterium on nodulin gene expression, particularly the involvement of bacterial factors and the bacterial nod genes in the induction of the expression of the PsENOD12 genes.Finally, chapter IX summarizes the results of the in situ localization of early nodulin gene products. In the light of these results, the significance of our histochemical approach to elucidating the role of nodulins in root nodule development is discussed

Gate-tunable band structure of the LaAlO3_3-SrTiO3_3 interface

The 2-dimensional electron system at the interface between LaAlO$_{3}$ and SrTiO$_{3}$ has several unique properties that can be tuned by an externally applied gate voltage. In this work, we show that this gate-tunability extends to the effective band structure of the system. We combine a magnetotransport study on top-gated Hall bars with self-consistent Schr\"odinger-Poisson calculations and observe a Lifshitz transition at a density of $2.9\times10^{13}$ cm$^{-2}$. Above the transition, the carrier density of one of the conducting bands decreases with increasing gate voltage. This surprising decrease is accurately reproduced in the calculations if electronic correlations are included. These results provide a clear, intuitive picture of the physics governing the electronic structure at complex oxide interfaces.Comment: 14 pages, 4 figure

De relatie tussen stikstofopname en gewasreflectie bij vijf buitenbloemen

Bij buitenbloemen is er een grote variatie aan gewassen. Het is daarom niet mogelijk om voor ieder gewas afzonderlijk bemestingsonderzoek uit te voeren. Een N -opnamecurve is een belangrijk hulpmiddel bij het ontwikkelen van bijmestsystemen. Onderzoek is gedaan naar de mogelijkheid om met non-destructieve middelen, o.a. refelectiemetingen, iets te zeggen over de vorm van de N-opnamecurve bij een vijftal gewassen, die representatief zijn voor een groot deel van het sortimen

Magnetically induced chessboard pattern in the conductance of a Kondo quantum dot

We quantitatively describe the main features of the magnetically induced conductance modulation of a Kondo quantum dot -- or chessboard pattern -- in terms of a constant-interaction double quantum dot model. We show that the analogy with a double dot holds down to remarkably low magnetic fields. The analysis is extended by full 3D spin density functional calculations. Introducing an effective Kondo coupling parameter, the chessboard pattern is self-consistently computed as a function of magnetic field and electron number, which enables us to quantitatively explain our experimental data.Comment: 4 pages, 3 color figure

Enhancing crop resilience to combined abiotic and biotic stress through the dissection of physiological and molecular crosstalk

Plants growing in their natural habitats are often challenged simultaneously by multiple stress factors, both abiotic and biotic. Research has so far been limited to responses to individual stresses, and understanding of adaptation to combinatorial stress is limited, but indicative of non-additive interactions. Omics data analysis and functional characterization of individual genes has revealed a convergence of signaling pathways for abiotic and biotic stress adaptation. Taking into account that most data originate from imposition of individual stress factors, this review summarizes these findings in a physiological context, following the pathogenesis timeline and highlighting potential differential interactions occurring between abiotic and biotic stress signaling across the different cellular compartments and at the whole plant level. Potential effects of abiotic stress on resistance components such as extracellular receptor proteins, R-genes and systemic acquired resistance will be elaborated, as well as crosstalk at the levels of hormone, reactive oxygen species, and redox signaling. Breeding targets and strategies are proposed focusing on either manipulation and deployment of individual common regulators such as transcription factors or pyramiding of non- (negatively) interacting components such as R-genes with abiotic stress resistance genes. We propose that dissection of broad spectrum stress tolerance conferred by priming chemicals may provide an insight on stress cross regulation and additional candidate genes for improving crop performance under combined stress. Validation of the proposed strategies in lab and field experiments is a first step toward the goal of achieving tolerance to combinatorial stress in crops

Josephson supercurrent in a topological insulator without a bulk shunt

A Josephson supercurrent has been induced into the three-dimensional topological insulator Bi1.5Sb0.5Te1.7Se1.3. We show that the transport in Bi1.5Sb0.5Te1.7Se1.3 exfoliated flakes is dominated by surface states and that the bulk conductivity can be neglected at the temperatures where we study the proximity induced superconductivity. We prepared Josephson junctions with widths in the order of 40 nm and lengths in the order of 50 to 80 nm on several Bi1.5Sb0.5Te1.7Se1.3 flakes and measured down to 30 mK. The Fraunhofer patterns unequivocally reveal that the supercurrent is a Josephson supercurrent. The measured critical currents are reproducibly observed on different devices and upon multiple cooldowns, and the critical current dependence on temperature as well as magnetic field can be well explained by diffusive transport models and geometric effects