Mechanical Stress Induces Remodeling of Vascular Networks in Growing Leaves

International audienceDifferentiation into well-defined patterns and tissue growth are recognized as key processes in organismal development. However, it is unclear whether patterns are passively, homogeneously dilated by growth or whether they remodel during tissue expansion. Leaf vascu-lar networks are well-fitted to investigate this issue, since leaves are approximately two-dimensional and grow manyfold in size. Here we study experimentally and computationally how vein patterns affect growth. We first model the growing vasculature as a network of viscoelastic rods and consider its response to external mechanical stress. We use the so-called texture tensor to quantify the local network geometry and reveal that growth is heterogeneous , resembling non-affine deformations in composite materials. We then apply mechanical forces to growing leaves after veins have differentiated, which respond by anisotropic growth and reorientation of the network in the direction of external stress. External mechanical stress appears to make growth more homogeneous, in contrast with the model with viscoelastic rods. However, we reconcile the model with experimental data by incorporating randomness in rod thickness and a threshold in the rod growth law, making the rods viscoelastoplastic. Altogether, we show that the higher stiffness of veins leads to their reorientation along external forces, along with a reduction in growth heterogeneity. This process may lead to the reinforcement of leaves against mechanical stress. More generally , our work contributes to a framework whereby growth and patterns are coordinated through the differences in mechanical properties between cell types

The multiscale nature of leaf growth fields

Plant leaves are out of equilibrium active solid sheets that grow in a decentralized fashion by deforming its unit cells while maintaining a typical shape. Here, the authors measure the surface growth of Tobacco leaves at high spatial and temporal resolution, and find that growth dynamics is dominated by sharp fluctuations at the cellular scale, suggesting that it is regulated and correlated in space and time

The multiscale nature of leaf growth fields

AbstractA growing leaf is a prototypical active solid, as its active units, the cells, locally deform during the out-of-equilibrium process of growth. During this local growth, leaves increase their area by orders of magnitude, yet maintain a proper shape, usually flat. How this is achieved in the lack of a central control, is unknown. Here we measure the in-plane growth tensor of Tobacco leaves and study the statistics of growth-rate, isotropy and directionality. We show that growth strongly fluctuates in time and position, and include multiple shrinkage events. We identify the characteristic scales of the fluctuations. We show that the area-growth distribution is broad and non-Gaussian, and use multiscale statistical methods to show how growth homogenizes at larger/longer scales. In contrast, we show that growth isotropy does not homogenize in time. Mechanical analysis shows that with such growth statistics, a leaf can stay flat only if the fluctuations are regulated/correlated.</jats:p

Geometry and Mechanics in the Opening of Chiral Seed Pods

Two joined latex strips show complex twisting behavior similar to that of seed pods.</jats:p

Formation of Indium Carbide Cluster Ions: Experimental and Computational Study

We report the observation and structural analysis of novel indium carbide gas phase cluster ions generated by bombardment of a clean indium surface by keV C₆₀^– ions. Positive In_<i>m</i>C_<i>n</i>⁺ (<i>m</i> = 1–21, 1 ≤ <i>n</i> ≤ 9) ions were ejected off the surface and analyzed mass spectrometrically. C₆₀^– ion beam irradiation is shown to be an efficient way of producing new kinds of gas phase carbide ions with relatively balanced stoichiometries. The rise kinetics of the ion signal (immediate jump within the beam opening time to a plateau value) indicates that the formation/ejection of the carbide ions constitute a single impact event. In₃C₂⁺ was found to be the most abundant carbide cluster ion. Optimal geometries of the different clusters were derived via density functional theory calculations. The acetylenic/cumulenic nature of the impact emitted cluster ions is manifested by the high abundance of In₂C₂⁺, In₃C₂⁺, and the calculated structures for In_<i>m</i>C_<i>n</i>⁺ (<i>m</i> = 3–4, <i>n</i> = 2–8). Odd/even intensity alternations in the In₃C_<i>n</i>⁺ (<i>n</i> = 1–8) and In₄C_<i>n</i>⁺ (<i>n</i> = 1–9) abundances are observed and rationalized by the calculations