Uli Schurr

New particle formation in forests inhibited by isoprene emissions

It has been suggested that volatile organic compounds (VOCs) are involved in organic aerosol formation, which in turn affects radiative forcing and climate. The most abundant VOCs emitted by terrestrial vegetation are isoprene and its derivatives, such as monoterpenes and sesquiterpenes. New particle formation in boreal regions is related to monoterpene emissions and causes an estimated negative radiative forcing of about -0.2 to -0.9 W m-2. The annual variation in aerosol growth rates during particle nucleation events correlates with the seasonality of monoterpene emissions of the local vegetation, with a maximum during summer. The frequency of nucleation events peaks, however, in spring and autumn. Here we present evidence from simulation experiments conducted in a plant chamber that isoprene can significantly inhibit new particle formation. The process leading to the observed decrease in particle number concentration is linked to the high reactivity of isoprene with the hydroxyl radical (OH). The suppression is stronger with higher concentrations of isoprene, but with little dependence on the specific VOC mixture emitted by trees. A parameterization of the observed suppression factor as a function of isoprene concentration suggests that the number of new particles produced depends on the OH concentration and VOCs involved in the production of new particles undergo three to four steps of oxidation by OH. Our measurements simulate conditions that are typical for forested regions and may explain the observed seasonality in the frequency of aerosol nucleation events, with a lower number of nucleation events during summer compared to autumn and spring. Biogenic emissions of isoprene are controlled by temperature and light, and if the relative isoprene abundance of biogenic VOC emissions increases in response to climate change or land use change, the new particle formation potential may decrease, thus damping the aerosol negative radiative forcing effect.

The art of growing plants for experimental purposes: a practical guide for the plant biologist

Every year thousands of experiments are conducted using plants grown under more-or-less controlled environmental conditions. The aim of many such experiments is to compare the phenotype of different species or genotypes in a specific environment, or to study plant performance under a range of suboptimal conditions. Our paper aims to bring together the minimum knowledge necessary for a plant biologist to set up such experiments and apply the environmental conditions that are appropriate to answer the questions of interest. We first focus on the basic choices that have to be made with regard to the experimental setup (e.g. where are the plants grown; what rooting medium; what pot size). Second, we present practical considerations concerning the number of plants that have to be analysed considering the variability in plant material and the required precision. Third, we discuss eight of the most important environmental factors for plant growth (light quantity, light quality, CO2, nutrients, air humidity, water, temperature and salinity); what critical issues should be taken into account to ensure proper growth conditions in controlled environments and which specific aspects need attention if plants are challenged with a certain a-biotic stress factor. Finally, we propose a simple checklist that could be used for tracking and reporting experimental conditions.

Growth of tobacco in short-day conditions leads to high starch, low sugars, altered diurnal changes in the Nia transcript and low nitrate reductase activity, and inhibition of amino acid synthesis

Abstract. Diurnal changes in carbohydrates and nitrate reductase (NR) activity were compared in tobacco (Nicotiana tabacum. L.cv. Gatersleben) plants growing in a long (18 h light/6 h dark) and a short (6 h light/18 h dark) day growth regime, or after short-term changes in the light regime. In long-day-grown plants, source leaves contained high levels of sugars throughout the light and dark periods. In short-day-grown plants, levels of sucrose and reducing sugars were very low at the end of the night and, although they rose during the light period, remained much lower than in long days and declined to very low levels again by the middle of the night. Starch accumulated more rapidly in short-day- than long-day-grown plants. Starch was completely re-mobilised during the night in short days, but not in long days. A single short day/long night cycle sufficed to stimulate starch accumulation during the following light period. In long-day-grown plants, the Nia transcript level was high at the end of the night, decreased during the day, and recovered gradually during the night. In short-day-grown plants, the Nia transcript level was relatively low at the end of the night, decreased to very low levels at the end of the light period, increased to a marked maximum in the middle of the night, and decreased during the last 5 h of the dark period. In long-day-grown plants, NR activity in source leaves rose by 2- to 3-fold in the first part of the light period and decreased in the second part of the light period. In short-day-grown plants, NR activity was low at the end of the night, and only increased slightly after illumination. Dark inactivation of source-leaf NR was partially reversed in long-day-grown plants, but not in short day-grown plants. In both growth regimes, mutants with one instead of four functional copies of the Nia gene had a 60% reduction in maximum NR activity in the source leaves, compared to wild-type plants. The diurnal changes in NR activity were almost completely suppressed in the mutants in long days, whereas the mutants showed similar or slightly larger diurnal changes than wild-type plants in short days. When short-day-grown plants were transferred to long-day conditions for 3 d, NR activity and the diurnal changes in NR activity resembled those in long-day-grown plants. Phloem export from source leaves of short-day-grown plants was partially inhibited by applying a cold-girdle for one light and dark cycle. The resulting increase in leaf sugar was accompanied by an marked increase in the Nia transcript level and a 2-fold increase in NR activity at the end of the dark period. When wild-type plants were subjected to a single short day/long night cycle of increasing severity, NR activity in source leaves at the end of the night decreased when the endogenous sugars declined below about 3 μmol hexose (g FW)−1. In sink leaves in short-day conditions, sugars were higher and the light-induced rise in NR activity was much larger than in source leaves on the same plants. The source leaves of wild-type plants in short-day conditions contained very high levels of nitrate, very low levels of glutamine, low levels of total amino acids, and lower protein and chlorophyll, compared to long-day-grown plants. Plants grown in short days had relatively high levels of glutamate and aspartate, and extremely low levels of most of the minor amino acids in their source leaves at the end of the night. Illumination led to a decrease in glutamate and an increase in the minor amino acids. A single short day/long night cycle led to an increase in glutamate, and a large decrease in the minor acids at the end of the dark period, and re-illumination led to a decrease in glutamate and an increase in the minor amino acids. It is proposed that sugar-mediated control of Nia expression and NR activity overrides regulation by nitrogenous compounds when sugars are in short supply, resulting in a severe inhibition of nitrate assimilation. It is also proposed that sugars exert a global control on amino acid metabolism. The importance of sugars for the regulation of nitrogen metabolism is strikingly illustrated by the finding that tobacco is carbon and nitrogen limited when it is grown in short-day conditions.

Study of Dynamical Processes with Tensor-Based Spatiotemporal Image Processing Techniques

Image sequence processing techniques are used to study exchange, growth, and transport processes and to tackle key questions in environmental physics and biology. These applications require high accuracy for the estimation of the motion field since the most interesting parameters of the dynamical processes studied are contained in first-order derivatives of the motion field or in dynamical changes of the moving objects. Therefore the performance and optimization of low-level motion estimators is discussed. A tensor method tuned with carefully optimized derivative filters yields reliable and dense displacement vector fields (DVF) with an accuracy of up to a few hundredth pixels/frame for real-world images. The accuracy of the tensor method is verified with computer-generated sequences and a calibrated image sequence. With the improvements in accuracy the motion estimation is now rather limited by imperfections in the CCD sensors, especially the spatial nonuniformity in the responsivity. With a simple two-point calibration, these effects can efficiently be suppressed. The application of the techniques to the analysis of plant growth, to ocean surface microturbulence in IR image sequences, and to sediment transport is demonstrated.

Root growth analysis in physiological coordinates

We present a method for botanical growth analysis of plant roots in physiological coordinates which is necessary for the evaluation of growth mechanisms in roots. The presented framework can be used on long image sequences up to several days. First the displacement vector field is estimated by the structure tensor method. Secondly the physiological coordinates of the root are determined by active contours fitted to the root boundary. Then the middle line as the object coordinate axis of the root is calculated. In the third step the displacement field is sampled and projected on the middle line. This yields an array of tangential displacements along the root which is used to calculate the spatially resolved expansion rate of the root along its length. The performance of the presented framework is demonstrated on both synthetic and real data.

Root Growth Measurements in Object Coordinates

We show a framework for growth analysis of plant roots in object coordinates which is one requirement for the botanical evaluation of growth mechanisms in roots. The method presented here is appliable on long image sequences up to several days, it has no limit for the sequence length. First we estimate the displacement vector field with the structure tensor method. Thereafter we determine the physiological coordinates of the root by active contours. The contours are first fitted on the root boundary and yield the data for the calculation of the middle line as the object coordinate axis of the root. In the third step the displacement field is sampled at the position of the middle line and projected onto it. The result is an array of tangential displacement vectors along the root which is used to compute the spatially resolved expansion rate of the root in physiological coordinates. Finally, the potential of the presented framework is demonstrated on synthetic and real data.

Analysis of influence of nutrient concentration on root growth dynamics by means of digital image sequence processing.

The influence of different nutrient concentrations (e.g. nitrate) on root growth dynamics of corn root was examined by digital image sequence analysis using a rhizotron set-up which enables quick changes in the composition of the root surrounding nutrient solution. Changes in the growth velocity of the root tip and the distribution of the relative elemental growth rates (REGR) along the growth zone before, during, and after increase of the nitrate concentration are obtained. Computer controlled moving stages were used to maintain the root tip in the imaging region during several hours.

Axial and lateral transport of calcium, magnesium and potassium in. Ricinus communis

Ion transport in the xylem was studied by stable isotope tracers in intact plants and detached organs of Ricinus communis. Kinetics of stabile isotope transport in the xylem after step-labelling in the nutrient solution were studied. Xylem sap was sampled from several sampling sites continuously and isotope fractions were quantified by SIMS and LAMMA. This experimental set-up combines the stable isotope tracer approach to analyse ion transport without changing the concentration in the external media with the sampling of xylem sap from intact transpiring plants by the root pressure chamber. The comparison of results from detached and intact plant experiments revealed significant differences in this respect.

Nutrient transport in the xylem of intact plants — dirunal variation and response to nutrient availability

Diurnal courses of nutrient transport in the xylem and their response to external availability of nutrients were studied. Stringently nutrient conditions were established in a novel pressure chamber. An aeroponic nutrient delivery system inside allows to sample xylem sap from intact plants under full control of the nutrient conditions at the root. Analysis of xylem transport under these highly defined conditions established that (1) diurnal variations in concentrations and fluxes in the xylem are dominated by plant internal processes. (2) Nitrate uptake and nitrate flux to the shoot are largely uncoupled. (3) In continuous light, diurnal variations of xylem sap concentrations vanish. Step changes in nitrate concentrations of the nutrient solution established that (4) the concomitant increase in nitrate concentration and flux in the xylem is delayed by 2–3 hours and is only transient.

3D-Modellierung von Planzenblättern mittels eines Depth-from-Motion-Verfahrens

In dieser Arbeit wird ein Verfahren zur Gewinnung von 3D-Modellen des Adergerusts von Pflanzenblattern fur die botanische Wachstumsanalyse vorgestellt. Dafur wird in einem Kamerasch wenk eine Bildsequenz des zu vermessenden Blattes aufgenommen. In einem Initialisierungsschritt wird im ersten Bild der Sequenz ein 2D-Splinemodell des Adergerusts und des Blattumrisses erzeugt. Dieses Modell wird durch die gesamte Sequenz verfolgt, wodurch eine zweite Ansicht des selben Modells entsteht. Da in diesem Modell trivialerweise korrespondierende Punkte der beiden Ansichten bekannt sind, ist das Korrespondenzproblem fur die 3D-Rekonstruktion gelost. Die Tiefe des Modells wird uber einfache geometrische Triangulation berechnet.