isnard sandrine
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Twining plants use their helical stems to clasp supports and to generate a squeezing force, providing stability against gravity. To elucidate the mechanism that allows force generation, we measured the squeezing forces exerted by the twiner Dioscorea bulbifera while following its growth using time-lapse photography. We show that the development of the squeezing force is accompanied by stiffening of the stem and the expansion of stipules at the leaf base. We use a simple thin rod model to show that despite their small size and sparse distribution, stipules impose a stem deformation sufficient to account for the measured squeezing force. We further demonstrate that tensioning of the stem helix, although counterintuitive, is the most effective mechanism for generating large squeezing forces in twining plants. Our observations and model point to a general mechanism for the generation of the twining force: a modest radial stem expansion during primary growth, or the growth of lateral structures such as leaf bases, causes a delayed stem tensioning that creates the squeezing forces necessary for twining plants to ascend their supports. Our study thus provides the long-sought answer to the question of how twining plants ascend smooth supports without the use of adhesive or hook-like structures. |
| Mechanical design - Squeezing force measurements We developed a mechanical pole, inspired by the TWIFOR of Matista & Silk (1997), to measure the in vivo squeezing forces exerted by twining vines. A mechanical pole consisted of a PVC pipe from which a section had been cut above the bottom end . This section was split longitudinally and one-half cylinder was rigidly reassembled with the pole. The remaining half-cylinder was anchored to the rest of the pole via two thin-beam load cells (LCL-113G, Omega Engineering, Stamford, CT, USA), forming a hemicylindrical force plate. Vines completed roughly two gyres before touching the force plate.
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| Dioscorea bulbifera twining around a mechanical pole concomitantly with development of squeezing force for 6 days - Squeezing force starts to develop on day 2 after the first gyre has been formed around the support, and increases over time during the formation of new gyres above the force plate. (20 fps with 10 min frame interval). Original Publication (URL link) |
Isnard, S., Cobb, A. R., Holbrook, N. M., Zwieniecki, M and Dumais, J. 2009. Tensioning the helix: a mechanism for force generation in twining plants. Proceedings of the Royal Society B: Biological Sciences 276: 2643-2650.
Isnard, S., Cobb, A., Dumais, J., Zwieniecki, M. & Holbrook, N.M. 2007. “Pulvinus-induced tightening plays an important role in the generation of the squeezing force in the monocotyledonous twiner Dioscorea bulbifera” - Society for Experimental Biology (SEB), Glasgow, Ecosse. (31 Mars-4 Avril). Abstract, pp. 111.
Matista AA, Silk WK. 1997. An electronic device for continuous, in vivo measurement of forces exerted by twining vines. American Journal of Botany 84(8): 1164-1168.
