Transitions in nutation trajectory geometry in peppermint (Mentha x piperita L.) with respect to lunisolar acceleration.

Nutations of plant organs are significantly affected by the circatidal modulation in the gravitational force exerted by the Moon and Sun (lunisolar tidal acceleration, Etide). In a previous study on nutational rotations of stem apices, we observed abrupt alterations in their direction and irregularities of the recorded trajectories. Such transitions have not yet been analysed in detail. Peppermint plants were continuously recorded with time-lapse photography and aligned with contemporaneous time courses of the Etide estimates. Each nutational stem tip movement path was assigned to one of two groups, depending on its geometry, as: (i) regular elliptical movements and (ii) irregular movements (with a random type of trajectory). Analyses of the correlation between the plant nutation trajectory parameters and Etide, as well as of the trajectory geometry of the individual plants were performed. The trajectory geometry of young mint stem apices was related to the velocity of the apex rotation and significantly affected by the gravitational force estimated from the Etide. A low velocity of nutational movement, associated with the random character of the trajectory, usually occurred simultaneously with local minima or maxima of Etide. As the mint plant ages, the transitions in the stem tip trajectory were limited; no correspondence with Etide dynamics was observed. The results indicate that the plant tip geometry path transitions with respect to the changing gradient of lunisolar tidal acceleration could be interpreted as manifestation of a continuous accommodation of the shoot apical part to the state of minimum energy dissipation.