Electrostatic forces in wind-pollination—Part 1: Measurement of the electrostatic charge on pollen☆
Introduction
For many plant species, successful reproduction depends on the release, the movement, and the deposition of pollen on the stigma of a receptive conspecific plant. Electrostatic fields are pervasive in the environment and electrostatic forces possibly influence the dispersal and deposition of wind-dispersed pollen grains. Electrostatic forces have been suggested to be involved in insect pollination (Corbet et al., 1982; Gan-Mor et al., 1995), and have been shown to influence deposition in agricultural wind-pollination settings when the pollen is artificially charged (Bechar et al., 1999; Vaknin et al., 2000, Vaknin et al., 2001; Law et al., 2000; Law, 2001; Gan-Mor et al., 2003).
This is the first in a series of two articles exploring the role of electrostatic forces in the capture of wind-dispersed pollen grains. The electrostatic force experienced by a charged wind-dispersed pollen grain is the product of two factors; its electrostatic charge and the electric field at its location. In this article, we explore the first of these factors, determining the electrostatic charge on wind-dispersed pollen grains. In the companion article (Bowker and Crenshaw, 2006), we develop a model of the electric fields around plants and then estimate the forces experienced by charged pollen grains and simulate their trajectories as they pass near plants.
While explored in detail by Bowker and Crenshaw (2006), during fair weather electric conditions (relatively clear skies, calm winds) when pollination typically occurs, a vertical electric field averaging roughly 100 V m−1 is present in nature. Negative charge is present on the ground (and on plants) while an equal positive charge is present in the air. The negative charge on plants is asymmetrically distributed, concentrated on points (e.g. the tips of branches, the edges of leaves, and the feathery stigmatic female reproductive structures) that extend above their surroundings. The electric field surrounding these charged features is distorted and magnified; sometimes reaching values >100 kV m−1 (Law, 1987, Law, 2001; Dai and Law, 1995; Bechar et al., 1999). Positively charged pollen grains encountering these regions would be strongly attracted to the plant and negatively charged pollen grains would be repelled.
The electrostatic charge of naturally released wind-dispersed pollen is not known. Investigators have suggested pollen charge distributions can be primarily positive (Erickson and Buchmann, 1983), primarily negative (McWilliam, 1959), or bipolar (Bowker and Crenshaw, 2003). However, at electrostatic equilibrium with the air, the charges on pollen grains are likely small (<0.002 fC), resulting from Boltzmann charging (Hinds, 1982) and from incurring a charge per volume equal to the ambient space charge (Chalmers, 1967). The electrostatic force acting on a airborne pollen grain with such a small charge, in a 200 kV m−1 electric field, is small (4×10−13 N) relative to gravity (4×10−11 N) (assuming 10 μm radius and a density of 1000 kg m−3).
The neutralization process, by which airborne pollen grains lose their charge and reach electrostatic equilibrium with the air, takes substantial amounts of time due to the low conductivity of air. The time for an object to lose 63% of its charge, is approximately 440 s (Bowker and Crenshaw, 2003). If pollen grains are charged at release, they may carry substantial charge through dispersal, if the dispersal time is short relative to the neutralization time.
Wind pollination is usually a localized process, with most pollen grains released from one plant traveling short distances (centimeters to meters depending on release height) and settling on neighboring plants. To a first approximation, the dispersal time is simply the time required for the pollen to fall through the air to the ground, depending on pollen settling velocity (typically 0.02–0.06 m s−1) and plant height (Crane, 1986; Young and Schmitt, 1995). Consequently, dispersal times are relatively short (a few seconds to a few minutes). Because the electrostatic neutralization time for an airborne pollen grain is long relative to its short average dispersal time, most pollen charged during release will retain a substantial charge throughout dispersal. They will not be at electrostatic equilibrium with the air and will not have sufficient time to become uncharged. Consequently, if pollen grains are charged at release, an electrostatic interaction with the plants in the environment is possible (Niklas, 1985; Crane, 1986).
The object of this paper was to determine the electrostatic charges on pollen grains immediately upon release (the non-equilibrium charges) for seven species of wind-pollinated plants: Plantago lanceolata (Bowker and Crenshaw, 2003), Cedrus deodara, Cedrus atlantica, Juniperus virginiana, Acer rubrum, Pinus taeda, and Ulmus alata. To corroborate the laboratory experiments, the charges on juniper pollen grains (J. virginiana) were also measured in nature immediately upon release from the plant. For the electrostatic force to be functionally significant in pollen capture, it must be comparable in magnitude to other active forces, such as the gravitational force. Thus, for pollen grains of various sizes with differing charges, we determined the electric field necessary for the electrostatic force to equal gravity. In the companion paper (Bowker and Crenshaw, 2006), we used the charges measured here to estimate the electrostatic force experienced by pollen grains and simulate their motion as they encounter the electric field around a plant.
Section snippets
Estimating pollen radius and charge from velocity measurements
Pollen charge can be determined by measuring its deflection and velocity in a uniform horizontal electric field (Swinbank et al., 1964; Leach, 1976). A pollen grain moving through the air experiences a drag force that depends on the Reynolds number (Re=LUρa/μ, where L is the diameter of a pollen grain, U is the relative velocity of the pollen grain, ρa is the density of air, and μ is the dynamic viscosity of air). The Re of a typical wind-dispersed pollen (e.g. radius 10 μm) settling at 0.03 m s−1
Results
A total of 13,465 average velocities were calculated for 9489 pollen grains from seven plant species. Most pollen grains carried a measurable electrostatic charge. Collectively, for all species in the laboratory, the average pollen grain carried a charge of positive 0.32 fC. The average of the absolute value of all charges was 0.84 fC. For all species and all trials, positively and negatively charged pollen grains were present (Fig. 2).
When the measurement electric field was off, the pollen
Discussion
The release, transport, and capture of wind-dispersed pollen are mechanical processes of great importance to many plant species which depend on successful pollen transfer for reproduction. Aerodynamic and gravitational forces govern the process, yet other forces such as electrostatic may be important, perhaps even dominating in some circumstances. Bechar et al. (1999), Law et al. (2000), Vaknin et al., 2000, Vaknin et al., 2001, and Gan-Mor et al. (2003) have shown that electrostatic forces can
Acknowledgements
The authors thank the Duke University Department of Zoology, the National Science Foundation, Sigma Xi, and Keithley Electronics for financial and equipment support, and S.E. Law, S. Vogel, G. Ybarra, and S. Perry for advice and assistance. This work was submitted in partial fulfillment of a Ph.D. in Zoology at Duke University (G.E.B.)
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The United States Environmental Protection Agency through its Office of Research and Development partially supported and collaborated in the research described in this paper. It has been subjected to Agency review and approved for publication.