Leaf movements and their relationship with the lunisolar gravitational force.

BACKGROUND: Observation of the diurnal ascent and descent of leaves of beans and other species, as well as experimental interventions into these movements, such as exposures to light at different times during the movement cycle, led to the concept of an endogenous 'clock' as a regulator of these oscillations. The physiological basis of leaf movement can be traced to processes that modulate cell volume in target tissues of the pulvinus and petiole. However, these elements of the leaf-movement process do not completely account for the rhythms that are generated following germination in constant light or dark conditions, or when plants are transferred to similar free-running conditions. SCOPE: To develop a new perspective on the regulation of leaf-movement rhythms, many of the published time courses of leaf movements that provided evidence for the concept of the endogenous clock were analysed in conjunction with the contemporaneous time courses of the lunisolar tidal acceleration at the relevant experimental locations. This was made possible by application of the Etide program, which estimates, with high temporal resolution, local gravitational changes as a consequence of the diurnal variations of the lunisolar gravitational force due to the orbits and relative positions of Earth, Moon and Sun. In all cases, it was evident that a synchronism exists between the times of the turning points of both the lunisolar tide and of the leaftide when the direction of leaf movement changes. This finding of synchrony leads to the hypothesis that the lunisolar tide is a regulator of the leaftide, and that the rhythm of leaf movement is not necessarily of endogenous origin but is an expression of an exogenous lunisolar 'clock' impressed upon the leaf-movement apparatus.
CONCLUSIONS: Correlation between leaftide and Etide time courses holds for leaf movement rhythms in natural conditions of the greenhouse, in conditions of constant light or dark, under microgravity conditions of the International Space Station, and also holds for rhythms that are atypical, such as pendulum and relaxation rhythms whose periods are longer or shorter than usual. Even the apparently spontaneous short-period, small-amplitude rhythms recorded from leaves under unusual growth conditions are consistent with the hypothesis of a lunisolar zeitgeber. Two hypotheses that could account for the synchronism between leaftide and Etide, and which are based on either quantum considerations or on classical Newtonian physics, are presented and discussed.