Recirculation cells in a wide channel

International audienceSecondary flow cells are commonly observed in straight laboratory channels, where they are often associated with duct corners. Here, we present velocity measurements acquired with an acoustic Doppler current profiler in a straight reach of the Seine river (France). We show that a remarkably regular series of stationary flow cells spans across the entire channel. They are arranged in pairs of counter-rotating vortices aligned with the primary flow. Their existence away from the river banks contradicts the usual interpretation of these secondary flow structures, which invokes the influence of boundaries. Based on these measurements, we use a depth-averaged model to evaluate the momentum transfer by these structures, and find that it is comparable with the classical turbulent transfer

Inclination not force is sensed by plants during shoot gravitropism

International audienceGravity perception plays a key role in how plants develop and adapt to environmental changes. However, more than a century after the pioneering work of Darwin, little is known on the sensing mechanism. Using a centrifugal device combined with growth kinematics imaging, we show that shoot gravitropic responses to steady levels of gravity in four representative angiosperm species is independent of gravity intensity. All gravitropic responses tested are dependent only on the angle of inclination from the direction of gravity. We thus demonstrate that shoot gravitropism is stimulated by sensing inclination not gravitational force or acceleration as previously believed. This contrasts with the otolith system in the internal ear of vertebrates and explains the robustness of the control of growth direction by plants despite perturbations like wind shaking. Our results will help retarget the search for the molecular mechanism linking shifting statoliths to signal transduction

Écoulements secondaires dans les rivières: influence sur le transport de quantité de mouvement et de soluté.

River flow can induce secondary currents orthogonal to the main flow direction. The acoustic Doppler effect allows measurements of these weak currents without modifying the flow. In a straight reach of the Seine river we use an acoustic Doppler current profiler (ADCP) mounted on a small raft to evaluate the time-averaged velocity in the river cross-section. These measurements reveal secondary flows organised in periodic counter-rotative pair of cells with a size comparable to the water depth and a velocity of about 1% of the streamwise velocity. The observation of these cells in rivers is reminiscent of previous laboratory measurements made by Blanckaert (2010). We complement these observations with new measurements in a smaller river using an unidirectional acoustic profiler fixed at the water surface. These measurements reveal secondary flow cells similar to the ones observed in the Seine river. Their influence on momentum transfer is then investigated in the framework of the shallow-water approximation. This approach is used to reproduce the streamwise velocity profile over the cross-section. We show that secondary flow cells transport as much momentum as turbulence in rivers. Then, using numerical simulations, we extend this result to the dispersion of solutes by a series of counter-rotative cells. We discuss finally the origins of these recirculation cells. L'écoulement d'une rivière peut générer des circulations secondaires perpendiculaires à sa direction principale. L'effet acoustique Doppler permet de mesurer ces circulations lentes sans perturber l'écoulement. Sur une portion rectiligne de la Seine, nous utilisons un profileur acoustique (ADCP) placé sur un radeau. Ce dispositif nous permet de mesurer la vitesse moyennée en temps au travers de la section à partir des deux faisceaux alignés avec l'écoulement principal. Ces mesures révèlent des courants secondaires organisés en cellules de recirculation périodiques, dont le sens de rotation est alterné. Leur taille est comparable à la hauteur d'eau et leur vitesse est de l'ordre de 1% de celle du courant principal. L'observation de ces cellules, inédite en rivière, rappelle les mesures de Blanckaert (2010) en laboratoire. Ces observations sont complétées par de nouvelles mesures dans une rivière plus petite en utilisant un profileur acoustique unidirectionnel fixé à la surface de l'eau. À nouveau, ces mesures révèlent la présence de cellules comparables à celles observées dans la Seine. Leur influence sur le transport de quantité de mouvement est ensuite étudiée dans le cadre des équations de Saint-Venant. Cette approche permet de reproduire le profil de vitesse au travers de la section. Nous montrons ainsi que ces cellules constituent un mécanisme de transfert dont l'intensité est comparable à celle du transfert turbulent. À l'aide de simulations numériques, nous étendons ce résultat à la dispersion de matière dissoute par une série de cellules contra-rotatives. Enfin, nous discutons les différents mécanismes susceptibles de former ces cellules de recirculation

Posture control in land plants: growth, position sensing, proprioception, balance, and elasticity

The colonization of the atmosphere by land plants was a major evolutionary step. The mechanisms that allow for vertical growth through air and the establishment and control of a stable erect habit are just starting to be understood. A key mechanism was found to be continuous posture control to counterbalance the mechanical and developmental challenges of maintaining a growing upright structure. An interdisciplinary systems biology approach was invaluable in understanding the underlying principles and in designing pertinent experiments. Since this discovery previously held views of gravitropic perception had to be reexamined and this has led to the description of proprioception in plants. In this review, we take a purposefully pedagogical approach to present the dynamics involved from the cellular to whole-plant level. We show how the textbook model of how plants sense gravitational force has been replaced by a model of position sensing, a clinometer mechanism that involves both passive avalanches and active motion of statoliths, granular starch-filled plastids, in statocytes. Moreover, there is a transmission of information between statocytes and other specialized cells that sense the degree of organ curvature and reset asymmetric growth to straighten and realign the structure. We give an overview of how plants have used the interplay of active posture control and elastic sagging to generate a whole range of spatial displays during their life cycles. Finally, a position-integrating mechanism has been discovered that prevents directional plant growth from being disrupted by wind-induced oscillations

On the role of gravity in shoot gravisensing

A plant accidentally put in a horizontal position rapidly bends and deforms to recover a vertical position. The ability of plants to feel gravity thus plays a key role in their development and adaptation to environmental changes (gravitropism and posture control). A crucial step in this gravisensing occurs in specific cells, the statocytes, which contain dense organites filled with starch granules (amyloplasts). The amyloplasts being denser than the surrounding intracellular fluid, they sediment at the bottom of the cell and are supposed to indicate the direction of gravity with respect to the cells (Morita 2010). However the mechanisms at work in statocytes and the link with the active bending of the plant at the macroscopic scale still need a better understanding (Moulia and Fournier 2009, Blancaflor 2015). In this study, we use an experimental approach to study gravitropic motions at the plant scale, and more specifically to investigate quantitatively the plant sensitivity to gravitropic stimuli and identify the sensed variable (e.g mechanical pressure by amyloplats, velocity or position of the amyloplasts in the cell ….). An original experimental setup called ``gravitron'' has been developed to investigate the response of plant shoots to changes in both gravity intensity and direction. The system is based on an instrumented rotating table allowing full kinematical tracking of the tropic mouvement. These records were then interpreted in term of gravisensitivity using the relevant dimensionless quantity introduced by recent quantitative studies on gravitropic control (Bastien et al. 2013, 2014

Revealing the hierarchy of processes and time-scales that control the tropic response of shoots to gravi-stimulations

Experiments on shoot gravitropism at the plant and cell scale reveal the existence of a memory process and provide a unifying framework for predicting the bending response of shoots to arbitrary gravi-stimulations. Abstract Gravity is a major abiotic cue for plant growth. However, little is known about the responses of plants to various patterns of gravi-stimulation, with apparent contradictions being observed between the dose-like responses recorded under transient stimuli in microgravity environments and the responses under steady-state inclinations recorded on earth. Of particular importance is how the gravitropic response of an organ is affected by the temporal dynamics of downstream processes in the signalling pathway, such as statolith motion in statocytes or the redistribution of auxin transporters. Here, we used a combination of experiments on the whole-plant scale and live-cell imaging techniques on wheat coleoptiles in centrifuge devices to investigate both the kinematics of shoot-bending induced by transient inclination, and the motion of the statoliths in response to cell inclination. Unlike previous observations in microgravity, the response of shoots to transient inclinations appears to be independent of the level of gravity, with a response time much longer than the duration of statolith sedimentation. This reveals the existence of a memory process in the gravitropic signalling pathway, independent of statolith dynamics. By combining this memory process with statolith motion, a mathematical model is built that unifies the different laws found in the literature and that predicts the early bending response of shoots to arbitrary gravi-stimulations

Revealing the time scale hierarchy that controls the tropic response of shoots to gravi-stimulations

Gravity is a major abiotic cue for plant growth. Yet, little is known on the response of plants to various pattern of gravistimulation, with apparent contradiction between dose-like responses observed under transient stimuli in microgravity environments and response under steady inclination observed on earth. Of particular importance is how the gravitropic response of the organ is affected by the temporal dynamics of downstream processes in the signalling pathway, like statoliths motion in statocytes or auxin transporter redistribution. Here, we use a combination of plant scale experiments and live cell imaging techniques on centrifuge devices, to investigate on the same organ both the kinematics of shoots bending induced by transient inclinations, and the motion of the statoliths in response to cell inclination. Unlike previous observations in microgravity, the response of shoots to transient inclinations appears independent of the gravity level, with a time scale much longer than the statolith sedimentation time. This reveals the existence of a new memory process in the gravitropic signalling pathway, independent of statoliths dynamics. By combining this memory process with statoliths motion, a mathematical model is built that unifies the different laws found in literature and predicts the early bending response of shoots to arbitrary gravistimulations

Le contrôle postural des organes aériens : processus complexe impliquant la perception de l’inclinaison et de la courbure

Le contrôle postural est un des mécanismes essentiels des plantes terrestres pour maintenir le port érigé de leur système aérien face aux variations de leur environnement. En effet, ce processus actif participe au redressement des axes végétatifs face aux perturbations liées à l’augmentation de leur masse ou à des variations d’inclinaison (verse, pente). D’un point de vue agronomique, le contrôle postural est donc impliqué dans la capacité des cultures à être résilientes à la verse et celle des arbres à produire des fûts rectilignes. Le redressement des axes végétatifs est un processus complexe et le moteur de ce redressement est la croissance : la croissance primaire longitudinale localisée près du méristème apical, et la croissance secondaire, croissance en diamètre, qu’on retrouve chez la plupart des dicotylédones herbacées et chez tous les ligneux, intervient sur les zones en maturation proches de la base de la tige. La question est de déterminer quels sont les signaux permettant à la plante de réguler cette croissance pour maintenir ce port érigé. Grâce à une technologie de digitalisation numérique 3D et du suivi cinématique du redressement des tiges chez plusieurs espèces, nous avons pu montrer que le redressement impliquait un ensemble coordonné de courbures et de décourbures actives. En fonction des espèces, des formes transitoires ont été observées au cours du redressement, certaines tiges ne dépassant jamais localement la verticale, lorsque d'autres présentent des formes en C, voire en S. L’élaboration d’un modèle mathématique, a pu reproduire le contrôle complet des mouvements de redressement sur 11 espèces de plantes à fleurs terrestres, et sur des organes allant de la germination du blé à des troncs de peupliers. Ce modèle montre que la dynamique du mouvement et la forme finale de la plante est contrôlée par le ratio entre la sensibilité à la gravité et la sensibilité proprioceptive, ajusté à la taille de la plante (Bastien et al, 2013, 2014). Le premier terme, graviceptif, amène la tige à se courber vers le haut, tant qu'elle n'est pas verticale, et le deuxième, proprioceptif, tend au contraire à réduire la courbure pour maintenir la tige rectiligne. C’est cet équilibre propriogravitropique, entre la verticalité et la courbure de la plante, qui permet d’atteindre un état stationnaire et la forme finale de la tig

Assessment of Head Impacts and Muscle Activity in Soccer Using a T3 Inertial Sensor and a Portable Electromyography (EMG) System: A Preliminary Study

Heading the ball is an important skill in soccer. Head impacts are of concern because of the potential adverse health effects. Many elite players now wear GPS (that include inertial monitoring units) on the upper spine for location tracking and workload measurement. By measuring the maximum acceleration of the head and the upper spine, we calculated the acceleration ratio as an attenuation index for participants (n = 8) of different skill levels during a front heading activity. This would allow for in-field estimates of head impacts to be made and concussive events detected. For novice participants, the ratio was as high as 8.3 (mean value 5.0 ± 1.8), whereas, for experienced players, the mean ratio was 3.2 ± 1.5. Elite players stiffen the neck muscles to increase the ball velocity and so the torso acts as a support structure. Electromyography (EMG) signals that were recorded from the neck and shoulder before and after a training intervention showed a major increase in mean average muscle activity (146%, p = 3.39 × 10−6). This was accompanied by a major decrease in acceleration ratio (34.41%, p = 0.008). The average head-ball impact velocity was 1.95 ± 0.53 m/s determined while using optical motion capture. For this low velocity, the impact force was 102 ± 19 N, 13% of the published concussive force. The voluntary action of neck muscles decreases isolated head movements during heading. Coaches and trainers may use this evidence in their development of junior players

Importance of root cap on root response to mechanical impedance

Roots grow in a complex soil environment, which exhibits spatial and temporal variability in structure. The soil conditions such as soil strength, water and nutrient availability present significant challenges to root growth. Roots and in particular root apex must sense the physical and chemical characteristics of these changes, to optimize the development of root system. Even if the influence of soil strength on root growth and root reorientation are well described, the influence of root apex comprising root cap and its production is not well understood. In this study we developed a model experimental medium and applied an interdisciplinary approach to investigate the implication of root cap in the response to medium strength. We focus our studies on the in vivo analysis of the Arabidopsis thaliana root growth dynamics and root trajectory following the contact of the root apex with a permanent obstacle, provided by a lower harder layer of Phytagel medium and which is able to block the initially straight root trajectory. In response to contact with the harder layer, the root apex reorientation, which might be interpreted as buckling effect, is preceded by a decrease of root growth rate, illustrating an active root response. Interestingly, the root growth of Arabidopsis fez-2 mutants, which exhibit a reduction of root cap cell layers, is less affected and root tip reorientation occurred progressively. This response could be interpreted both as a better resistance to buckling and a higher sensitiveness to strength of fez-2 mutant. Conversely, smb-3 (SOMBRERO) roots, which exhibit an increased number of root cap cells, exhibit a S-shape in response to contact with the harder layer. This could be interpreted as higher sensitivity to buckling. Thus, the present study highlights the influence of root cap shape and integrity on root mechanical response