Functional characterisation and cell specificity of BvSUT1, the transporter that loads sucrose into the phloem of sugar beet (Beta vulgaris L.) source leaves.

Sugar beet (Beta vulgaris L.) is one of the most important sugar-producing plants worldwide and provides about one third of the sugar consumed by humans. Here we report on molecular characterisation of the BvSUT1 gene and on the functional characterisation of the encoded transporter. In contrast to the recently identified tonoplast-localised sucrose transporter BvTST2.1 from sugar beet taproots, which evolved within the monosaccharide transporter (MST) superfamily, BvSUT1 represents a classical sucrose transporter and is a typical member of the disaccharide transporter (DST) superfamily. Transgenic Arabidopsis plants expressing the ß-GLUCURONIDASE (GUS) reporter gene under control of the BvSUT1-promoter showed GUS histochemical staining of their phloem; an anti-BvSUT1-antiserum identified the BvSUT1 transporter specifically in phloem companion cells. After expression of BvSUT1 cDNA in bakers' yeasts (Saccharomyces cerevisiae) uptake characteristics of the BvSUT1 protein were studied. Moreover, the sugar beet transporter was characterised as a proton-coupled sucrose symporter in Xenopus laevis oocytes. Our findings indicate that BvSUT1 is the sucrose transporter that is responsible for loading of sucrose into the phloem of sugar beet source leaves delivering sucrose to the storage tissue in sugar beet taproot sinks.

On the coarsening dynamics of a granular lattice gas.

We investigated experimentally and theoretically the dynamics of a driven granular gas on a square lattice and discovered two characteristic regimes: Initially, given the dissipative nature of the collisions, particles move erratically through the system and start to gather on selected sites called traps. Later on, the formation of those traps leads to a strong decrease of the grain mobility and slows down dramatically the dynamics of the entire system. We realize detailed measurements linking a trap's stability to the global evolution of the system and propose a model reproducing the entire dynamics of the system. Our work emphasizes the complexity of coarsening dynamics of dilute granular systems.

Bernal random loose packing through freeze-thaw cycling.

We study the effect of freeze-thaw cycling on the packing fraction of equal spheres immersed in water. The water located between the grains experiences a dilatation during freezing and a contraction during melting. After several cycles, the packing fraction converges to a particular value η(∞)=0.595 independently of its initial value η(0). This behavior is well reproduced by numerical simulations. Moreover, the numerical results allow one to analyze the packing structural configuration. With a Voronoï partition analysis, we show that the piles are fully random during the whole process and are characterized by two parameters: the average Voronoï volume µ(v) (related to the packing fraction η) and the standard deviation σ(v) of Voronoï volumes. The freeze-thaw driving modify the volume standard deviation σ(v) to converge to a particular disordered state with a packing fraction corresponding to the random loose packing fraction η(BRLP) obtained by Bernal during his pioneering experimental work. Therefore, freeze-thaw cycling is found to be a soft and spatially homogeneous driving method for disordered granular materials.

Granular transport in driven granular gas.

We numerically and theoretically investigate the behavior of a granular gas driven by asymmetric plates. The injection of energy in the dissipative system differs from one side to the opposite one. We prove that the dynamical clustering which is expected for such a system is affected by the asymmetry. As a consequence, the cluster position can be fully controlled. This property could lead to various applications in the handling of granular materials in low-gravity environment. Moreover, the dynamical cluster is characterized by natural oscillations which are also captured by a model. These oscillations are mainly related to the cluster size, thus providing an original way to probe the clustering behavior.

Clustering and segregation in driven granular fluids.

In microgravity, the successive inelastic collisions in a granular gas can lead to a dynamical clustering of the particles. This transition depends on the filling fraction of the system, the restitution of the used materials and on the size of the particles. We report simulations of driven bi-disperse gas made of small and large spheres. The size as well as the mass difference imply a strong modification in the kinematic chain of collisions and therefore alter significantly the formation of a cluster. Moreover, the different dynamical behaviors can also lead to a demixing of the system, adding a few small particles in a gas of large ones can lead to a partial clustering of the taller type. We realized a detailed phase diagram recovering the encountered regimes and developed a theoretical model predicting the possibility of dynamical clustering in binary systems.

Dynamics of a grain-filled ball on a vibrating plate.

We study experimentally how the bouncing dynamics of a hollow ball on a vibrating plate is modified when it is partially filled with liquid or grains. Whereas empty and liquid-filled balls display a dominant chaotic dynamics, a ball with grains exhibits a rich variety of stationary states, determined by the grain size and filling volume. In the collisional regime, i.e., when the energy injected to the system is mainly dissipated by interparticle collisions, an unexpected period-1 orbit appears independently of the vibration conditions, over a wide range. This is a self-regulated state driven by the formation and collapse of a granular gas within the ball during one cycle. In the frictional regime (dissipation dominated by friction), the grains move collectively and generate different patterns and steady modes: oscillons, waves, period doubling, etc. From a phase diagram and a geometrical analysis, we deduce that these modes are the result of a coupling (synchronization) between the vibrating plate frequency and the trajectory followed by the particles inside the cavity.

How dynamical clustering triggers Maxwell's demon in microgravity.

In microgravity, the gathering of granular material can be achieved by a dynamical clustering whose existence depends on the geometry of the cell that contains the particles and the energy that is injected into the system. By compartmentalizing the cell in several subcells of smaller volume, local clustering is triggered and the so formed dense regions act as stable traps. In this paper, molecular dynamics simulations were performed in order to reproduce the phenomenon and to analyze the formation and the stability of such traps. Depending on the total number N of particles present in the whole system, several clustering modes are encountered and a corresponding bifurcation diagram is presented. Moreover, an iterative model based on the measured particle flux F as well as a theoretical model giving the asymptotical steady states are used to validate our results. The obtained results are promising and can provide ways to manipulate grains in microgravity.

Strong interlocking of nonconvex particles in random packings.

We present a numerical study of random packings made of nonconvex grains. These particles are built by the agglomeration of overlapping spheres in order to control their sphericity φ. The contact number C is found to be much larger than the coordination number Z, providing a significant difference with convex grains. The packing properties are found to be highly dependent on the morphological parameters of the grains : packing fractions as low as 0.3 have been reached. More importantly, the way nonconvex grains develop multiple contacts, i.e., interlocking, is found to be a relevant effect in such packings. Interlocking provides more stability to loose packings.

How does an ice block assembly melt?

The melting of an assembly of ice blocks contained in a vertical cylinder and under an unidirectional load was investigated. The total volume occupied by the ice blocks and the volume of ice were simultaneously measured which allowed one to determine the volume fraction of the ice in the cylinder. While the ice volume continuously decreases, sudden breakdowns of the total volume were observed. Large reorganizations of the whole assembly occur. However, the maximal volume fraction found just after a large reorganization decreased with time. In addition, the modifications of the pile structure were investigated using an x-ray tomography imaging before and after one collapse. As the packing is better ordered along the walls, we suggest that the motion of the piston is governed by the layer of ice blocks located along the container wall. This layer was modeled by a two-dimensional assembly of disks. The model supports the idea that the geometrical frustrations explain the dynamics of the successive reorganization due to the shrinkage of the grains. Finally, numerical simulations allow one to conclude that the dynamics of the melting of the ice blocks is governed (i) by the confinement effect which induces defects in the packing and (ii) by the low friction between the ice blocks.

How relative humidity affects random packing experiments.

The influence of relative humidity (RH) on the extremely slow compaction dynamics of a granular assembly has been experimentally investigated. Millimeter-sized glass beads are considered. Compaction curves are fitted by stretched exponentials with characteristic time τ and exponent δ, which are seen to be deeply affected by the moisture content. A kinetic model, taking into account both triboelectric and capillary effects, is in excellent agreement with our results. It confirms the existence of an optimal condition at a relative humidity ≈45% for minimizing cohesive interactions between glass beads. The exponent δ is seen to depend strongly on the diffusive character of grains and voids inside the packing: diffusion for cohesiveless particles and subdiffusion when cohesion plays a role. As a consequence, the RH represents a relevant parameter that should be reported for every experimental work on a slowly driven dense random packing.

Influence of a reduced gravity on the volume fraction of a monolayer of spherical grains.

Centrifuge force is used to study granular materials in low gravity conditions. We consider a monolayer of noncohesive spherical grains placed on a plate. Reduced gravity conditions can be simulated in the plane by tilting or by rotating the plate. We compare both approaches experimentally. The volume fraction is found to increase with the apparent gravity and saturates. A model based on the exponential distribution of the Voronoi cell areas has been built and is in excellent agreement with the experimental data by extrapolating the fits of the data. Moreover, numerical simulations exhibit that more arches can be maintained at low apparent gravities than at high.

Phase transitions in vibrated granular systems in microgravity.

We numerically investigated various dynamical behaviors of a vibrated granular gas in microgravity. Using the parameters of an earlier Mini-Texus 5 experiment, three-dimensional simulations, based on molecular dynamics, efficiently reproduce experimental results. Using Kolmogorov-Smirnov tests, four dynamical regimes have been distinguished: gaseous state, partial clustering, complete clustering, and bouncing aggregates. Different grain radii and densities have been considered in order to describe a complete (r,η)-phase diagram. The latter exhibits rich features such as phase transitions and triple points. Our work emphasizes the complexity of diluted granular systems and opens fundamental perspectives.

Compaction dynamics of wet granular assemblies.

The extremely slow compaction dynamics of wet granular assemblies is studied experimentally. The cohesion, due to capillary bridges between neighboring grains, is tuned using different liquids having specific surface tension values. The compaction dynamics of a cohesive packing obeys an inverse logarithmic law, like most dry random packings. However, the characteristic relaxation time τ grows strongly with cohesion. A model, based on free volume kinetic equations and the presence of a capillary energy barrier, is able to reproduce quantitatively the experimental curves.

The influence of grain shape, friction and cohesion on granular compaction dynamics.

This article is a review of our recent and new experimental works on granular compaction. The effects of various microscopic parameters on the compaction dynamics are addressed, in particular the influence of the grain shape, the friction and the cohesion between the grains. Two dimensional and three dimensional systems are analysed. And the role of dimensionality will be emphasized. Theoretical and numerical investigations provide additional informations about that phenomenon. Indeed numerical models permit us to study the influence of some parameters not easily accessible experimentally. Our results show that the above mentioned parameters have a deep impact on the compaction dynamics. Anisotropic grains lead to two different compaction regimes separated by a "burst" of the packing fraction. Friction is observed to modify how the grains are arranged in the pile. This is confirmed by numerical simulations. Cohesive forces between particles inhibit compaction and lead to extremely low values of the packing fraction.

Reversing the Brazil nut effect.

We propose a lattice model for studying the Brazil Nut Effect (BNE), i.e. the phase segregation occurring when a granular material is vertically shaked. The model considers the tap intensity and the mobility mu of the grains as the main physical parameters. Different mobilities for different grain species lead to segregation (BNE) patterns, reverse segregation (RBNE) patterns, "sandwhich" layered structures or vertical domains. A phase diagram (decompaction chi, mobility difference between both species Delta mu) is obtained in which the different phases are emphasized. In a narrow region of the diagram, different phases coexist. It is shown that the BNE segregation could be reversed by increasing the tap intensity or the characteristics of the grains. Numerical results are compared with earlier experimental works.

Effect of friction in a toy model of granular compaction.

A toy model of granular compaction which includes some resistance due to granular arches is proposed. In this model, the solid/solid friction of contacting grains is a key parameter and a slipping threshold omega(c) is defined. Realistic compaction behaviors have been obtained. Two regimes separated by a critical point omega*(c) of the slipping threshold have been emphasized: (i) a slow compaction with lots of paralyzed regions and (ii) an inverse logarithmic dynamics with a power-law scaling of grain mobility. Below the critical point omega*(c) , the physical properties of this frozen system become independent of omega(c) . Above the critical point omega*(c) --i.e., for low friction values--the packing properties behave as described by the classical Janssen theory for silos.

High CO2-mediated down-regulation of photosynthetic gene transcripts is caused by accelerated leaf senescence rather than sugar accumulation.

The influence of elevated atmospheric CO2 on transcript levels of photosynthetic genes was investigated in leaves of Nicotiana tabacum cv. SamsunNN and cv. Wisconsin38 plants. Plants were grown under ambient (400 ppm) and elevated (800/1,000 ppm) atmospheric CO2, and transcript levels were determined in leaves of different age. Down-regulation of photosynthetic gene transcripts was apparent in senesing leaves only. A correlation between transcript levels and leaf contents of soluble sugars could not be found. To investigate whether a shift in leaf ontogeny would be involved in the regulation of photosynthetic genes transgenic tobacco plants expressing either the gus or ipt gene under control of the senescence-specific SAG-12 promoter [Gan, S. and Amasino, R.M. (1995) Science 270, 1986-1988] were included in our studies. As expected SAG-12-driven GUS activity increased with leaf age. This increase of GUS activity was stimulated by elevated atmospheric CO2, accompanied by a loss of chlorophyll and the down-regulation of photosynthetic genes, verifying that high CO2 accelerates leaf ontogeny. Senescence as well as down-regulation of photosynthetic genes could be delayed by ipt expression. Levels of soluble sugars were indistinguishable from wild type or even slightly elevated in ipt transgenic plants. Therefore, sugar accumulation as a cause for down-regulation of photosynthetic genes under high CO2 can be excluded. It appears more likely that the high CO2-mediated decline in photosynthetic gene transcripts is due to a temporal shift in leaf ontogeny.

Ontogenetic changes of potato plants during acclimation to elevated carbon dioxide.

Transgenic potato plants (Solanum tuberosum cv. Desirée) with an antisense repression of the chloroplastic triosephosphate translocator were compared with wild-type plants. Plants were grown in chambers with either an atmosphere with ambient (400 mu bar) or elevated (1000 mu bar) CO2. After 7 weeks, the rate of CO2 assimilation between wild-type and transgenic plants in both CO2 concentrations was identical, but the tuber yield of both plant lines was increased by about 30%, when grown in elevated CO2. One explanation is that plants respond to the elevated CO2 only at a certain growth stage. Therefore, growth of wild-type plants was analysed between the second and the seventh week. Relative growth rate and CO2 assimilation were stimulated in elevated CO2 only in the second and the third weeks. During this period, the carbohydrate content of leaves grown with elevated CO2 was lower than that of leaves grown with ambient CO2. In plants grown in elevated CO2, the rate of CO2 assimilation started to decline after 5 weeks, and accumulation of carbohydrates began after 7 weeks. From this observation it was concluded that acclimation of potato plants to elevated CO2 is the result of accelerated development rather than of carbohydrate accumulation causing down-regulation of photosynthesis. For a detailed analysis for the cause of the stimulation of growth after 2 weeks, the contents of phosphorylated intermediates of wild-type plants and transgenics were measured. Stimulation of CO2 assimilation was accompanied by changes in the contents of phosphorylated intermediates, resulting in an increase in the amount of dihydroxyacetone phosphate, the metabolite which is exported from the chloroplast into the cytosol. An increase of dihydroxyacetone phosphate was found in wild-type plants in elevated CO2 when compared with ambient CO2 and in triosephosphate translocator antisense plants in ambient CO2, but not in the transgenic plants when grown in elevated CO2. These plants were not able to increase dihydroxyacetone phosphate further to cope with the increased CO2 supply. From these changes in phosphorylated intermediates in wild-type and transgenic plants it was concluded that starch and sucrose synthesis pathways can replace each other only at moderate carbon flux rates.

The role of transient starch in acclimation to elevated atmospheric CO2.

Although increased concentrations of CO2 stimulate photosynthesis, this stimulation is often lost during prolonged exposure to elevated carbon dioxide, leading to an attenuation of the potential gain in yield. Under these conditions, a wide variety of species accumulates non-structural carbohydrates in leaves. It has been proposed that starch accumulation directly inhibits photosynthesis, that the rate of sucrose and starch synthesis limits photosynthesis, or that accumulation of sugars triggers changes in gene expression resulting in lower activities of Rubisco and inhibition of photosynthesis. To distinguish these explanations, transgenic plants unable to accumulate transient starch due to leaf mesophyll-specific antisense expression of AGP B were grown at ambient and elevated carbon dioxide. There was a positive correlation between the capacity for starch synthesis and the rate of photosynthesis at elevated CO2 concentrations, showing that the capability to synthesize leaf starch is essential for photosynthesis in elevated carbon dioxide. The results show that in elevated carbon dioxide, photosynthesis is restricted by the rate of end product synthesis. Accumulation of starch is not responsible for inhibition of photosynthesis. Although transgenic plants contained increased levels of hexoses, transcripts of photosynthetic genes were not downregulated and Rubisco activity was not decreased arguing against a role of sugar sensing in acclimation to high CO2.