Twining Plants Capitalize on Circumnutations to Assess Mechanical Properties of a Candidate Support, and Initiate a Thigmotropic Twining Response.

Climbing plants prioritize vertical to radial growth, resulting in loss of mechanical stability, compensated for via attachment to supporting structures, for example by twining. While much is known about the twining process itself, little is understood about what signals underpin the onset of twining. In our study we measure the force exerted by stem twining beans (Phaseolous vulgairs) while encountering a rod-shaped obstacle during circumnutations – inherent large periodic movements of shoots, commonly identified with facilitating search of new supports. Forces are measured until either twining is initiated, or the shoot slips off the support, where circumnutations resume. Force trajectories are generally characterized by a sine-like form, with different amplitude and period. We find twining generally occurs after a minimal contact time of half a circumnutation period, with accumulated strains increasing gradually, suggesting twining initiation as a possible thigmotropic response to the mechanical interaction between circumnutating shoot and resisting support.

Experimental observations are outfitted with a minimal model for circumnutating shoots pushing against a rod, informed by mechanical properties of bean shoots. Simulations, based on a previously developed numerical framework for morpho-elastic rods, recover force trajectories with the observed sine form, where the amplitude is set by mechanical properties, and the period is set by the circumnutation period, thus directly relating observed forces to circumnutations. We find consistent differences in the period of simulated and experimental force trajectories, in line with an internal response to accumulated internal stresses. A further analysis of shoot position right after slip events reveals a smaller jump angle compared to the expected, based on the measured circumnutations period prior to contact. This discrepancy increases with contact time, pointing at decreasing rotation rates during contact.

Put together this study suggests that climbing plants capitalize on circumnutations to push against a newly encountered support, enabling them to assess its mechanical stability, and initiate twining according to accumulated stresses or strains.