Natural experiments indicate that geomagnetic variations cause spatial and temporal variations in coconut palm asymmetry.

In plants with alternately arranged foliage, such as the coconut palm (Cocos nucifera), leaves are attached to the stem in either an ascending clockwise (left handed [L]) or counterclockwise (right handed [R]) spiral (Fig. 1). Foliar spiral direction (FSD) is not genetically determined in coconut palms: All crosses (R 3 R, R 3L, L 3 R, L 3 L) yield R and L progeny in approximately equal numbers (Davis, 1962; Louis and Chidambaram, 1976; Toar et al., 1979). FSD is, thus, a classic case of morphological asymmetry in which dextral and sinistral forms are not inherited and are equally common within a species (Palmer, 2005). FSD would seem a simple stochastic process unworthy of further study if not for the observation by T.A. Davis, based on data collected from over 70,000 coconut palms in over 40 locations around the world, that the FSD of coconut palms varies with latitude: R trees predominate in the northern hemisphere and L trees predominate in the southern hemisphere (Davis and Davis, 1987). A reanalysis of Davis’s data indicated that these hemispheric asymmetries in FSD are significantly better correlated with magnetic (dip) latitude than with geographic or geomagnetic (centered dipole) latitude, suggesting that latitudinal asymmetries in FSD might be associated with the temporally varying component of the Earth’s magnetic field (Minorsky, 1998). Here, we report that asymmetries in FSD are also evident in populations of coconut palms on opposite sides of islands and that asymmetries between cohorts vary with an 11-year periodicity—two novel discoveries consistent with the hypothesis that geomagnetic variations underlie asymmetries in coconut palm FSD. Whereas the effects of the geomagnetic field on the orientation of magnetotactic bacteria and various animals, particularly insects and migratory birds, has been extensively studied, relatively little is known about the effects of geomagnetism on plants (Belyavskaya, 2004; Galland and Pazur, 2005). Important questions remain unanswered, such as whether or not plants perceive the geomagnetic field and, if they do, by what mechanisms and to what possible advantage, if any? At this early stage in our understanding of magnetoreception, several alternative mechanisms are being discussed by biophysicists concerning how cells might sense weak electromagnetic fields. Among the proposed modes of action are (1) torque on ferromagnetic particles; (2) modulation of biochemical reactions that involve spin-correlated radical pairs (radical-pair mechanism); (3) modulation of the transport rates and binding by ion-cyclotron resonance; and (4) quantum coherence mechanisms (for review, see Galland and Pazur, 2005). Regardless of the exact mechanisms involved in the physical reception of magnetic stimulation, considerable evidence suggests the involvement of biological membranes, in general (Balcavage et al., 1996; Volpe, 2003), and of Ca fluxes, in particular (Belova and Lednev, 2001; Bauréus Koch et al., 2003; Belyavskaya, 2004; Pazur et al., 2006), in magnetoreception. Evidence does exist that the electrical potentials of trees change in parallel, even in fine detail, with earth currents induced by variations in the Earth’s magnetic field. Pc1-type geomagnetic pulsations (0.2–5 Hz) of very small amplitude (0.05–0.1 nT), for example, have been recorded in oak (Quercus lobata) trees (FraserSmith, 1978). These extremely weak geomagnetic pulsations gave rise to electrical potential oscillations of approximately 100-mV amplitude (Fraser-Smith, 1978). These electrical signals were not artifactual: They were not found when the tree was replaced with a resistor or when a dead tree was used. Similar electrical periodicities had been measured previously in plants and correlated with 1to 10-Hz leaf movements (Semenenko, 1972), suggesting that plants are not simply passive antennae for geomagnetic variations, but that membrane functioning may be affected (Minorsky, 2001). Conceivably, a membrane transport process that might be affected by induced currents is the polar transport of auxin, a plant hormone that determines the phyllotaxy of plants (Reinhardt et al., 2003). Plant cells use electric currents to control their physiological polarity and direction of growth (Morris, 1980; Bandurski et al., 1992; Mina and Goldsworthy, 1992). Individual plant cells generate their own polar electric currents, but the direction of these currents can be changed by a brief application of a weak external current, after which the cell’s new current is in the same direction as the one that was applied (Mina and Goldsworthy, 1992). It is * Corresponding author; e-mail [email protected]; fax 914– 674–7520. The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Peter V. Minorsky ([email protected]). www.plantphysiol.org/cgi/doi/10.1104/pp.106.086835