Ohad, A.; Meroz, Y.

* Measuring the mechanical interactions of growing plants with the environment is a critical step towards deciphering their complex growth strategies negotiating their environment. Yet, available force-measurement systems have limited capacity to capture weak (sub-mN) forces in freely moving plant organs - such as the forces applied by a growing shoot pushing at an obstacle. * We developed a measurement system based on the deflection of a pendulum (rod) by a freely moving plant shoot. Crucially, unlike many other force-measurement systems, the organ is not tethered to any part of the device. Moreover, force is measured along 2 axes, as opposed to 1 axes in commonly used methods such as cantilevers. Two orthogonal cameras track the 3D position of the rod and shoot. From the tracked coordinates we calculate the rod deflection angle and, using a mechanical torque equilibrium equation, extract the force applied by the plant over time. * We demonstrate the system with bean <phaseolus vulgaris> shoots, measuring the forces they apply on a candidate support (the rod) during inherent circumnutation motions prior to twining. The force detection range can be tuned be altering rod mass and length. This system is relevant for measuring weak forces in macro-sized systems (such as growth or turgor pressures), but can be adjusted to suit a variety of sizes and forces.