Henk Van As | Wageningen University and Research Centre (original) (raw)

Papers by Henk Van As

Spatially Resolved Magnetic Resonance

Abstract The effect of osmotic stress on plant water status and apical plant growth has been stud... more Abstract The effect of osmotic stress on plant water status and apical plant growth has been studied in six week old maize plants in well defined and controlled climate conditions using quantitative low field NMR imaging. Simultaneous with the NMR measurements several other techniques including water uptake measurements and photosynthetic activity measurements were used. Quantitative single parameter images of spin density and T2 were used to follow the water status and to discriminate between the different tissue types. ...

Water Science and Technology, 2020

Despite aerobic granular sludge wastewater treatment plants operating around the world, our under... more Despite aerobic granular sludge wastewater treatment plants operating around the world, our understanding of internal granule structure and its relation to treatment efficiency remains limited. This can be attributed in part to the drawbacks of time-consuming, labor-intensive, and invasive microscopy protocols which effectively restrict samples sizes and may introduce artefacts. Time-domain nuclear magnetic resonance (NMR) allows non-invasive measurements which describe internal structural features of opaque, complex materials like biofilms. NMR was used to image aerobic granules collected from five full-scale wastewater treatment plants in the Netherlands and United States, as well as laboratory granules and control beads. T1 and T2 relaxation-weighted images reveal heterogeneous structures that include high- and low-density biofilm regions, water-like voids, and solid-like inclusions. Channels larger than approximately 50 μm and connected to the bulk fluid were not visible. Both c...

Journal of Visualized Experiments, 2021

This protocol describes a signal-to-noise ratio (SNR) calibration and sample preparation method f... more This protocol describes a signal-to-noise ratio (SNR) calibration and sample preparation method for solenoidal microcoils combined with biological samples, designed for high-resolution magnetic resonance imaging (MRI), also referred to as MR microscopy (MRM). It may be used at pre-clinical MRI spectrometers, demonstrated on Medicago truncatula root samples. Microcoils increase sensitivity by matching the size of the RF resonator to the size of the sample of interest, thereby enabling higher image resolutions in a given data acquisition time. Due to the relatively simple design, solenoidal microcoils are straightforward and cheap to construct and can be easily adapted to the sample requirements. Systematically, we explain how to calibrate new or home-built microcoils, using a reference solution. The calibration steps include: pulse power determination using a nutation curve; estimation of RFfield homogeneity; and calculating a volume-normalized signal-to-noise ratio (SNR) using standard pulse sequences. Important steps in sample preparation for small biological samples are discussed, as well as possible mitigating factors such as magnetic susceptibility differences. The applications of an optimized solenoid coil are demonstrated by high-resolution (13 x 13 x 13 μm 3 , 2.2 pL) 3D imaging of a root sample.

Soft matter, Jan 25, 2016

The shear flow of microfibrillated cellulose dispersions is still not wholly understood as a cons... more The shear flow of microfibrillated cellulose dispersions is still not wholly understood as a consequence of their multi-length-scale heterogeneity. We added carboxymethyl cellulose, a charged polymer, that makes cellulose microfibril dispersions more homogeneous at the submicron and macro scales. We then compared the yielding and flow behavior of these dispersions to that of typical thixotropic yield-stress fluids. Despite the apparent homogeneity of the dispersions, their flow velocity profiles in cone-plate geometry, as measured by rheo-MRI velocimetry, differ strongly from those observed for typical thixotropic model systems: the viscosity across the gap is not uniform, despite a flat stress field across the gap. We describe these velocity profiles with a nonlocal model, and attribute the non-locality to persistent micron-scale structural heterogeneity.

The diffusive properties of mesophilic methanogenic granular sludge have been studied using diffu... more The diffusive properties of mesophilic methanogenic granular sludge have been studied using diffusion analysis by relaxation time separated pulsed field gradient nuclear magnetic resonance (DARTS PFG NMR) spectroscopy. NMR measurements were performed at 22°C with 10 ml granular sludge at a magnetic field strength of 0.5 T (20 MHz resonance frequency for protons). Spin-spin relaxation (T2) time measurements indicate that three 1H populations can be distinguished in methanogenic granular sludge beds, corresponding to water in three different environments. The T2 relaxation time measurements clearly differentiate the extragranular water (T2 ≈ 1000 ms) from the water present in the granular matrix (T2 = 40-100 ms) and bacterial cell associated water (T2 = 10-15 ms). Self-diffusion coefficient measurements at 22°C of the different 1H-water populations as the tracer show that methanogenic granular sludge does not contain one unique diffusion coefficient. The observed distribution of self-diffusion coefficients varies between 1.1 × 10−9 m2/s (bacterial cell associated water) and 2.1 × 10−9 m2/s (matrix associated water).

Journal of Membrane Science, 2013

Shrubs and trees with secondary phloem tissue produced by successive cambia mainly occur in habit... more Shrubs and trees with secondary phloem tissue produced by successive cambia mainly occur in habitats characterized by a periodical or continuous lack of water availability. The amount of this secondary phloem tissue in stems of Avicennia trees rises with increasing soil water salinity and decreasing inundation frequency. Hence, increased water storage in secondary phloem tissue produced by successive cambia was put forward to be advantageous in harsh environmental conditions. It was however never tested whether the secondary phloem cells over the entire stem of woody species showing this wood anatomical feature are indeed water-filled as expected. In this preliminary and pioneering study, we use magnetic resonance imaging (MRI) to visualize the stem water content of three species with successive cambia, the mangroves Avicennia marina and A. officinalis and the non-mangrove Bougainvillea spectabilis. Measurements were conducted in living plants. We tested the hypothesis that not only the outermost phloem tissue has high water content but also the secondary phloem tissues over the entire stem from the bark inward to the pith, herewith serving as water storage sites. We can conclude that all secondary phloem tissue of both Bougainvillea and Avicennia has high water contents. This aligns with the contribution of secondary phloem tissue produced by successive cambia to ecological success in conditions of physiological drought. Further study should however be done to understand the mechanisms through which this secondary phloem tissue contributes to the water household of plants in conditions of water shortage.

Scientific Reports, 2020

Interactions between plants and the soil’s microbial & fungal flora are crucial for the health of... more Interactions between plants and the soil’s microbial & fungal flora are crucial for the health of soil ecosystems and food production. Microbe-plant interactions are difficult to investigate in situ due to their intertwined relationship involving morphology and metabolism. Here, we describe an approach to overcome this challenge by elucidating morphology and the metabolic profile of Medicago truncatula root nodules using Magnetic Resonance (MR) Microscopy, at the highest magnetic field strength (22.3 T) currently available for imaging. A home-built solenoid RF coil with an inner diameter of 1.5 mm was used to study individual root nodules. A 3D imaging sequence with an isotropic resolution of (7 μm)3 was able to resolve individual cells, and distinguish between cells infected with rhizobia and uninfected cells. Furthermore, we studied the metabolic profile of cells in different sections of the root nodule using localised MR spectroscopy and showed that several metabolites, including...

Procedia Food Science, 2011

The internal moisture distribution that arise in food products during drying, is a key factor for... more The internal moisture distribution that arise in food products during drying, is a key factor for the retention of quality attributes. To reveal the course of moisture content in a product, internal moisture profiles in broccoli florets are measured by MRI imaging during drying experiments with controlled air flow and temperature. The 3D images concern a matrix size of 64×64×64 elements. Signal intensity is converted to product moisture content with a linear relationship, while taking a minimum detectable moisture content of 0.3 kg water/ kg dry matter into account. Moisture content as a function of time is presented for a 2D cross sectional area in the middle of a broccoli sample. The average moisture contents for the cross sectional area obtained from the MRI imaging are compared with spatial model simulations for the moisture distribution. In that model the effective diffusion coefficient is based on the Free Volume Theory. This theory has the advantage that the changed mobility of water in the product during drying is taken into account and the theory also predicts the moisture transport in the porous broccoli floret. Key parameters for the Free Volume Theory are estimated by fitting to the experimental MRI results and the effective diffusion coefficient is given as a function of the product water content.

Magnetic Resonance Imaging, 1996

Nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) have been applied to visual... more Nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) have been applied to visualize physiological phenomena in plants and agricultural crops. Imaging sequences that result in contrast of a combination of parameters (e.g., proton density, T,, T2, T2*) cannot be used for a correct and unique interpretation of the results. In this study multiecho imaging together with monoexponential T2 decay fitting was applied to determine reliable proton density and T2 distributions over a mushroom. This was done at three magnetic field strengths (9.4, 4.7, and 0.47 T) because susceptibility inhomogeneities were suspected to intluence the T2 relaxation times negatively, and because the htlluences of susceptibility inhomogeneities increase with a rise in magnetic field strength. Electron microscopy was used to understand the different Tz's for the various tissue types in mushrooms. Large influences of the tissue ultrastructure on the observed T2 relaxation times were found and explained. Based on the results, it is concluded that imaging mushrooms at low fields (around or below 0.47 T) and short echo times has strong advantages over its high-field counterpart, especially with respect to quantitative imaging of the water balance of mushrooms. These conclusions indicate general validity whenever NMR imaging contrast is influenced by susceptibility inhomogeneities.

Magnetic Resonance Imaging, 1992

NMR and MRI have been applied to transport processes, that is, net flow and diffusion/perfusion, ... more NMR and MRI have been applied to transport processes, that is, net flow and diffusion/perfusion, of water in whole plants, cells, and porous materials. By choosing proper time windows and pulse sequences, magnetic resonance imaging can be made selective for each of the two transport processes. For porous media and plant cells the evolution of the spatial distribution of excited spins has been determined by q-space imaging, using a 20 MHz pulsed 1H NMR imager. The results of these experiments are explained by including spin-relaxation and exchange at boundaries. A 10 MHz portable 1H NMR spectrometer is described, particularly suitable to study the response of net flow in plants and canopies to changing external conditions.

Applied and Environmental Microbiology, 2001

The metabolic properties and ultrastructure of mesophilic aggregates from a full-scale expanded g... more The metabolic properties and ultrastructure of mesophilic aggregates from a full-scale expanded granular sludge bed reactor treating brewery wastewater are described. The aggregates had a very high methanogenic activity on acetate (17.19 mmol of CH 4 /g of volatile suspended solids [VSS]·day or 1.1 g of CH 4 chemical oxygen demand/g of VSS·day). Fluorescent in situ hybridization using 16S rRNA probes of crushed granules showed that 70 and 30% of the cells belonged to the archaebacterial and eubacterial domains, respectively. The spherical aggregates were black but contained numerous whitish spots on their surfaces. Cross-sectioning these aggregates revealed that the white spots appeared to be white clusters embedded in a black matrix. The white clusters were found to develop simultaneously with the increase in diameter. Energy-dispersed X-ray analysis and back-scattered electron microscopy showed that the whitish clusters contained mainly organic matter and no inorganic calcium prec...

Angewandte Chemie International Edition, 1998

ABSTRACT

Scientific reports, Jan 17, 2017

Chloroplast thylakoid membranes contain virtually all components of the energy-converting photosy... more Chloroplast thylakoid membranes contain virtually all components of the energy-converting photosynthetic machinery. Their energized state, driving ATP synthesis, is enabled by the bilayer organization of the membrane. However, their most abundant lipid species is a non-bilayer-forming lipid, monogalactosyl-diacylglycerol; the role of lipid polymorphism in these membranes is poorly understood. Earlier 31P-NMR experiments revealed the coexistence of a bilayer and a non-bilayer, isotropic lipid phase in spinach thylakoids. Packing of lipid molecules, tested by fluorescence spectroscopy of the lipophilic dye, merocyanine-540 (MC540), also displayed heterogeneity. Now, our 31P-NMR experiments on spinach thylakoids uncover the presence of a bilayer and three non-bilayer lipid phases; time-resolved fluorescence spectroscopy of MC540 also reveals the presence of multiple lipidic environments. It is also shown by 31P-NMR that: (i) some lipid phases are sensitive to the osmolarity and ionic s...

Modern Magnetic Resonance, 2017

This chapter deals with the principles and the applications of magnetic resonance imaging (MRI) f... more This chapter deals with the principles and the applications of magnetic resonance imaging (MRI) for assessment of the distribution and of the amount of intercellular gas-filled spaces and major metabolites in fruit and vegetable tissues. Combining this information with measurements of water characteristics could enable the use of MRI in an integrative approach to plant characterization. In MRI, the presence of gas-filled intercellular spaces in plant tissues impacts the NMR relaxation behavior of water molecules because gas and water have different magnetic susceptibilities. This phenomenon can be exploited for the noninvasive detection of certain physiological disorders in fruit and vegetable tissues or for quantification of the spatial distribution of apparent microporosity. On the other hand, the amount and the distribution of major metabolites (sugars, starch, lipids, etc.) can be accessed by MRI using approaches based on differences in relaxation times or on chemical shift between water and metabolites protons. Here we provide an overview of the theoretical aspects of MRI methods and a description of different approaches. The imaging protocols for specific applications for both air space and metabolite imaging are discussed with respect to their application to fruits and vegetables.

Food Hydrocolloids, 2016

The addition of polysaccharides to proteins during gel formation can alter the mechanical and tex... more The addition of polysaccharides to proteins during gel formation can alter the mechanical and textural properties of the resultant gels. However, the effect of addition of different polymers on mechanical properties of whey protein (WP) gels including their ability to elastically store energy, often measured in terms of the recoverable energy (RE), or dissipate energy, has not been fully reported. In this paper heatinduced WP gels containing high (HM) or low (LM) methylated negatively charged pectin or the neutral pullulan were prepared to study how the addition of polysaccharides to WP affects the mechanical properties of the formed gels. These gels were subjected to uniaxial compression and mechanical properties, including RE, were evaluated. The addition of pullulan to WP did not enhance the RE, whereas an increase in LM pectin resulted in higher RE. For gels containing HM pectin, the presence of the polymer induced an initial decrease of the RE. Nevertheless, RE increased with further increase in pectin concentration. These findings indicate that the addition of polysaccharides to whey proteins during gel formation results in changes in the RE but to different extent for different polymers. The results from this study suggest that the addition of polysaccharides to WP can be used as a tool to modulate the ability of whey protein gels to elastically store energy upon mechanical deformation.

The Journal of Physical Chemistry A, 2011