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

High power room temperature operation of a Tm:YAG laser longitudinally pumped by a 20W diode bar

We report efficient room temperature operation of a Tm:YAG laser end-pumped by a 20W diode-bar. An output power of 3.5W TEM00 has been obtained for 14.5W of incident pump powe

Efficient single-frequency operation and frequency stabilisation of a Q-switched diode pumped Nd:YAG laser

Efficient second-harmonic conversion of the 1064-nm output of a diode-pumped cw single-frequency Nd:YAG laser to 532 nm was obtained by frequency locking the laser to a monolithic ring resonator constructed of magnesium-oxide-doped lithium niobate. The conversion efficiency from the fundamental to the second harmonic was 65%. Two hundred milliwatts of cw single-frequency 532-nm light were produced from 310 mW of power of 1064-nm light. This represents a conversion efficiency of 20% from the 1-W diode laser used to pump the Nd:YAG laser to single-frequency 532-nm output. No signs of degradation of the 532-nm power or photo-refractive damage in the crystal were observed for over 500 h of operation of the system at green output powers greater than 100 mW