Session G06: Plant Biomechanics

Directional abscission of dandelion seed

Dandelion (Taraxacum officinale) is one of the many species of plants that has successfully leveraged natural wind as their primary driver for seed dispersal. One of the important aspects of the wind-driven dispersal mechanism is how the seed is released, or abscissed, from the plant. Past studies have explored the effects of wind speed, temperature, and humidity on dandelion seed abscission. In our studies, we explored the effect of the wind direction. Our results show that the dandelion seeds' detachment from the receptacle is sensitive to the wind direction. Selective dispersion effected by wind direction suggests that dandelions could benefit from a wider distribution of its seed. This could in turn have important implications for their population dynamics.   

Reconfiguration of plumed-seed inspired poroelastic cluster in fluid flow

Plumed seeds, such as dandelion or milkweed seeds, can fly a distance of kilometers while other types of wind-dispersal seeds disperse a few hundred meters at maximum. The plumed seed consists of many microscopically thin and elastic plumes appended to an achene, and it is well known that this porous structure enables superior flight performance by augmenting counter-gravity drag forces, which is essential for long-distance dispersals. Although the effects of porosity have been of interest recently, it remains unexplored how the elastic deformation of individual plumes and the resulting reconfiguration of the whole seed change aerodynamic loading. In this study, we numerically examine the effect of elasticity on reconfiguration and drag force for a porous cluster of elastic cylinders, by adopting simplified two-dimensional models. In contrast to common plants or elastic fins that streamline and reduce drag forces when subjected to a fluid flow, the poroelastic cluster reconfigures to enlarge the frontal area and therefore increases the total drag force.   

Analysis of osmotically driven pipe flows with application to conifer needles

The long-distance sugar transport in vascular plants takes place in the phloem vascular tissue, roughly a collection of pipes (sieve elements) with semipermeable walls, where the flow is driven by osmotic uptake of water from the neighboring xylem vascular tissue. In analytical models of sugar transport through the sieve elements, interaction with the xylem and axial sugar diffusion are typically not taken into account. However, in the transfusion tissue surrounding the vasculature of conifer needles, the typical length scales are so short and the geometry so complex that interaction with the neighboring xylem cells and axial diffusion become important and can even be dominant. We discuss the limitations in the sugar transport induced by the opposing pressure gradient in the xylem, and we introduce appropriate dimensionless numbers in the form of Peclet and Nusselt numbers (from diffusion and sugar flux respectively) characterizing the flow. For values of the parameters appropriate to the transfusion tissue, the method of [Segel, J. theor. Biol. (1970) 29, 233-250] gives approximate solutions by regular perturbation theory. For parameters corresponding to the sieve elements in the phloem, the solutions have boundary layers, which can be determined by singular perturbation theory.   

Sucrose transport inside the phloem: Bridging hydrodynamic and geometric characteristics

The delivery of the products of photosynthesis is achieved through a hydraulic system called the phloem. This semi-permeable plant tissue consists of living cells that contract and expand in response to fluid pressure fluctuations. The M$\ddot{u}$nch pressure flow theory, which is based on osmosis providing the necessary pressure gradient to drive the mass flow of carbohydrates, is currently the most accepted model of sucrose transport. When this hypothesis is combined with the conservation of fluid mass and momentum as well as sucrose mass, many simplifications must be invoked to mathematically close the problem. The work to be presented will focus on describing this mass flow using the Navier-Stokes and the sucrose continuity equations in cylindrical coordinates inside an elastic membrane with a concentration-dependant viscosity. It is demonstrated that the interaction between the hydrodynamic and geometrical characteristics of the phloem has a significant effect on the speed of flow. These results offer a novel perspective about the evolutionary adaptation of plant hydraulic traits to optimize phloem carbohydrate transport efficiency.   

Curvature-induced stiffening in leaf

Water stress directly affects crop growth and is used as a key indicator in evaluating the crop yield. There were several non-destructive approaches suggested to utilize the relationship between water stress and leaf vibration frequency, but the tendency of the frequency due to the dryness was controversial. Here, we proposed a new perspective of leaf surface curvature effect to vibration frequency in order to understand this discrepancy. The change of leaf curvature due to dryness was measured through a 3D scan of individual leaves each day it underwent drought conditions. The leaves were observed to undergo two types of drying patterns. In some leaves, drought conditions led to an increase in upward transversal curvature, while in others, they wilted without significant change in surface curvature. We show that topological changes due to dryness influence the overall stiffness of the leaf and contribute to increasing the frequency when vibrating. These results indicate that the morphological feature of leaves is key to the vibration frequency.