The breaking of grass leaves: a matter of survival for herbivores and for the plant

Context

Grasses show remarkable adaptation to grazing by herbivores. Their breaking properties are therefore the subject of particular interest among biomechanics researchers studying interactions between plants and herbivores (Vincent, J. F. V. 1991). When grass is mowed or grazed, cutting does not stop the growth of the leaf, which is amputated from its mature part (Gillet, M. 1980). The reconstruction of a photosynthetic surface large enough to meet the plant's needs is almost immediate. In the competition for light within grasslands, this constitutes an important ecological advantage for grasses (monocotyledons) over dicotyledons, whose bud (meristem) is at the tip of the plant and is therefore cut first. This is made possible by the location of the grass leaf's growth zone at the base of the plant and by a mechanism that protects this particularly tender area within the nested tubes formed by the sheaths of the older leaves in the leaf series.

Under the tension caused by grazing, the leaves fracture in the mature parts of the leaf blades, while the base remains intact, even though it is much more tender. In fact, the speed at which the leaf is pulled determines the area of breakage: when the speed is high, the break occurs in the mature area, but when the speed is slow and continuous force is exerted on the leaf, the growth area will be impacted and break (Lafarge and Durand 2010).

The ecological consequences of this biological mechanism, acquired during the evolution of plants subjected to grazing pressure from large herbivores, are considerable. Grasses, which appeared relatively late in the evolution of terrestrial flora, dominate many plant formations wherever light is abundant. Largely dominated by grasses, prairies, steppes, and savannas make up nearly 30% of continental land area and nearly 70% of cultivated land (source: FAO), not counting cereals, which also all belong to the same family.

Question

The dynamics of fracture and the location of the mature zone in relation to the growth zone remain poorly understood. Under what conditions of tensile force and force application does one type of fracture transition to another?

Objective

The objective of the internship will be to characterize the kinematics of extirpation and the growth zone, then to propose a physical model of these observations.

Methodology

To do this, we will conduct controlled force experiments on grass plants anchored to the ground. The force will be generated by pulling the leaf with a cable passing through a pulley and tensioned by weights (static analysis) or by a setup using falling weights at the end of cables of varying stiffness/length (dynamic analysis). At the same time, we will set up a biomimetic experiment using tubes to reproduce the kinematics of the phenomenon.

Impact and applications

Understanding the mechanism will enable us to resolve in detail the paradox of a leaf that is solid at its base yet capable of growing, since growth requires soft, fragile tissue. This mechanism shows promise for applications in engineering where it is necessary to mechanically protect an area and recreate an extension: telescopic antennas, walking robot legs, exposed sensors, fragile brushes.

Profile of the intern

Mechanics or Physics

Internship location and supervision

The internship will take place at LIPhy under the supervision of Philippe Marmottant (plant physics), in collaboration with Jean-Louis Durand from INRAe Lusignan (grass studies) and Bruno Moulia from INRAe Clermont-Ferrand (leaf mechanics).

Bibliography

Gillet, M. (1980). Les graminées fourragères : description, fonctionnement, applications à la culture de l’herbe.

Lafarge, M., & Durand, J. (2011). Comment l’herbe pousse : Développement végétatif, structures clonales et spatiales des graminées. Editions Quae.

Vincent, J. F. V. (1991). Strength and fracture of grasses. Journal of materials science, 26, 1947-1950.

Wright and Illius, 1995. A comparative study of the fracture properties of five grasses, Functional Ecology 9, 269-278

Wright, W. G., & Illius, A. W. (1995). A Comparative Study of the Fracture Properties of Five Grasses. Functional Ecology, 9(2), 269. https://doi.org/10.2307/2390573