The influence of fair weather electricity on the charging of wind-dispersed pollen

Wind-pollinated plants reproduce by capturing pollen suspended in the air. Pollen grains are electrostatically charged and electric fields are present around plants due to the ambient fair weather electric field. A charged airborne particle in an electric field experiences an electrostatic force equal to the product of its charge and the electric field at its location. Consequently, electrostatic forces might affect pollen capture. This study determines the electrostatic charge of pollen from a common weed, the English Plantain or Plantago lanceolata, in the presence of the Earth’s fair weather electric field. Most pollen grains carry a charge when released from the plant (Average 2 x 10 C, St. Dev 4.3 x 10 C), with some negative and others positive. In the presence of the 100 V/m electric field, the plant developed a strong negative surface charge which was, in part, transferred to the pollen. Pollen charge appears to be limited by the strength of the electric field at the pollen’s surface. When the electric field at the pollen’s surface exceeds the breakdown strength of air (3,000,000 V/m), the air ionizes and becomes conductive. The maximum charge depends on the pollen’s radius. No pollen grains were observed to exceed their predicted charge limit, with most pollen carrying less than one tenth of this charge. Generally, pollen grains are electrostatically charged and may experience significant forces in the amplified electric fields around pointy plants in the Earth’s fair weather electric field. Introduction Electrostatic forces have the potential to influence the capture of wind-dispersed pollen, and thus, plant reproduction. Pollen grains, which contain a plant’s male reproductive gametes, are transported by the wind to the stigma, the plant’s pollen-capturing female reproductive structure, of a downwind plant. The vast majority of pollen grains land on non-fertile surfaces. Typically, pollen release occurs during fair weather conditions. The 100 V/m fair weather electric field induces a strong negative surface charge on the top of the plant that increases in magnitude as the plant rises above its surroundings (Chalmers 1967). The plant’s charge will be concentrated on pointy plant features, such as a stigma which, for wind pollinated plants, is usually feathery or spiky in appearance. An electric field will be present around the plants, corresponding to the distribution of the plant’s charge. Consequently, if wind-dispersed pollen grains are charged, they will experience an electrostatic force as they encounter the electric fields around the plants. The force they experience will be equal to the product of their charge and the electric field at their location. This study investigates four aspects of pollen charge: (1) What is the charge on wind-dispersed pollen after it is released from a plant; (2) How much of the plant’s negative charge, generated by the Earth’s fair weather electric field, is passed on to the pollen; (3) What is the expected discharge rate of the pollen; and (4) What is the maximum charge carried by pollen. Materials and Methods An experiment was conducted in the laboratory to measure the charge on pollen and to determine if the Earth’s fair weather electric field influences pollen charging. The charge on the pollen was extrapolated from measurements of the pollen’s settling velocity (Uy) and the velocity (Ux) produced by a known uniform electric field (E). Plantago lanceolata spikes (flowers, with stems attached) were placed 5 cm above a set of vertically oriented parallel plates (19.9cm tall by 13.2 cm wide and separated by 1.89 cm using by ceramic standoffs). The plants were electrically grounded and were tapped, releasing pollen from the anthers, the pollen producing flower structures. When pollen entered the region between the plates, a horizontal “measuring” electric field of 63.4 kV/m was activated. The pollen’s velocity in the presence of the field, was videotaped under darkfield conditions at 60 fields/s using a 4900 COHU video camera equipped with a macro lens. Images were analyzed using National Instruments Image Acquisition (NI-IMAQ) Version 2 and IMAQ Vision software. For these pollen charge measurements, the pollen is assumed to be spherical (radius, a) with a density equal to that of water (1000 kg/m) and moving at low Reynolds number. Consequently, the pollen’s velocity (U) is proportional to the gravitational and electrostatic force (F). a πμ 6 U F = . (1) From the gravitational and settling velocities, the pollen’s charge (q) can be extrapolated E g U U y x p 2 μ 9 πμ 6 q ρ = ’ (2) where ρp is the pollen’s density, g is the acceleration of gravity, and μ is the viscosity of air (18 x 10 kg/(m s) in air (Vogel 1994). To determine if the plant’s negative charge is transferred to its pollen, a 100 V/m electric field was imposed on several electrically grounded Plantago lanceolata plants located at three heights (0 m, 0.14 m, and 0.9 m) above the ground. The 100 V/m electric field was created by placing a second pair of aluminum plates (13 cm tall by 9.8 cm wide and 5.08 cm apart) above the “measuring” plates (fig. 3.1). The Plantago spike was placed perpendicular to the 100 V/m plates. The stem of the Plantago spike was electrically grounded and was inserted through an insulated hole in the lower voltage plate. To recreate the field at 0 m above the ground, one of the plates was held at ground potential and the other was placed at a potential of 5.05 volts. To recreate the field at 0.14 meters, one plate was placed at a potential of 14 volts and the other plate was held at a potential of 18 volts. Thus, there was a potential difference of 14 volts between the plant and the plates, giving the plant a strong negative charge. To simulate a plant at 0.95 m, the plates were placed at 95.4 and 99.4 volts respectively. Assuming that the air is isotropic (the electrical properties are the same in all directions), the conductivity is independent of electric field strength, and the concentration of charge carriers is invariant, the discharge rate in air is dependent only on the conductivity and permittivity of air. The discharge rate for a charged airborne pollen grain is governed by the flow of current leaving it a Surface pol Surface pol Surface q dS E dS E dS j dt dq I ε σ σ σ = ⋅ = ⋅ = ⋅ = = ∫ ∫ ∫ , (3) where I is the discharge current, j is the current density (A/m), Epol is the electric field at the surface of the pollen, σ  is the conductivity of the air (assumed constant), and dS is an infinitesimal piece of surface area (S) on the pollen grain. The current equals the charge on the pollen (q) multiplied by the ratio of the air’s conductivity (σ, 1.8 x 10 C/(m V s) ) and permittivity (εa, 8.854 x 10 farads/m) (Nolan 1940). Solving for the charge remaining on the pollen grain (q) as a function of time (t),