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The Role of Electrostatic Forces in Pollination

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The Role of Electrostatic Forces in Pollination Plant Syst. Evol. 222:133-142 (2000) Plant Systematics and Evolution © Springer-Verlag 2000 Printed in Austria The role of electrostatic forces in pollination Y. Vaknin 1, S. Gan-Mor 2, A. Bechar 3, B. Ronen 2, and D. Eisikowitch 1 1Department of Plant Sciences, George S. Wise Faculty of Life Sciences, Tel-Aviv University, Tel-Aviv, Israel 2Institute of Agricultural Engineering, Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel 3Department of Industrial Engineering and Management, Ben Gurion University of the Negev, Beer-Sheva, Israel Received June 9, 1999 Accepted October 18, 1999 Abstract. This paper reviews research on the role related electric fields and potentials. In elec- of electrostatic forces in pollination, both in natural trostatic charging, electrons remain relatively and in agricultural systems. Researchers from stationary and are spread on the surface of the various fields of biological studies have reported object and concentrated on sharp edges. An phenomena which they related to electrostatic object becomes electrostatically charged either forces. The theory of electrostatically mediated by having electrons added to it, thus becoming pollen transfer between insect pollinators and the flowers they visit is described, including recent "negatively" charged or by having them studies which confirmed that the accumulated removed from it, thus becoming "positively" charges on airborne honey bees are sufficient for charged. Charges of the same sign repel each non-contact pollen detachment by electro- other and of opposing signs attract each other. static forces (i.e., electrostatic pollination). The The electrostatic force between two charged most important morphological features in flower bodies (F) is described in Coulomb's equation: adaptiveness to electrostatic pollination were deter- F = K(qlq2/r2). This force is dependent on the mined by means of two theoretical models of a magnitude of the charges (ql and qe), the flower exposed to an approaching charged cloud of distance between the charged bodies (r), and pollen; they are style length and flower opening. on the dielectric coefficient (K). Electrostatic Supplementary pollination by using electrostatic interactions play a major role in a variety of techniques is reported, and its possible importance biological processes (Honig and Nicholls in modern agriculture is discussed. 1995), including the pollination of plants, both Key words: Electric charges, electrodeposition, in nature and in agriculture. electrostatic pollination, electrostatic powder Normally plants possess small negative coating, flower morphology. surface charges under clear fair-day condi- tions, and are, therefore, surrounded by elec- tric fields of low intensity (Maw 1962). Under Introduction unstable weather conditions, as on a cloudy or Electrostatics deals with the electric forces rainy day, the electric fields can change their which involve electrons and ions, and with the polarity and the surface charges become pos- 134 Y. Vaknin et al.: The role of electrostatic forces in pollination itive (Warnke 1977). The magnitude of the position on the branch (Niklas and Buchmann electric fields depends in part on the chemical 1985). Electrostatic forces have also been composition of the plant, its height and the shown to influence pollen separation or clump- environment (Erickson and Buchmann 1983). ing, which could affect its aerodynamic prop- The distribution of the electric field around the erties (Erickson and Buchmann 1983). It was plant varies with its shape, and the plant's later speculated that as pollen grains were electrical fields should be greatest near sharp carried through the air they acquired a strong points such as plant terminals including flow- positive charge and were thereby electrostati- ers (Dai and Law 1995). cally attracted to the negatively charged female Foraging bees usually possess electrically flowers that were concentrated on the termi- positive surface charges (Erickson 1975, Yes'- nals of the plants they reached (Erickson and kov and Sapozhnikov 1976, Warnke 1977, Buchmann 1983). Therefore, together with Schwartz 1991, Gan-Mor et al. 1995). When a airflow, electrostatic attraction could form an bee flies through the air it is confronted with efficient mechanism for pollen capture by electrical currents and its body will be electro- anemophiles (Erickson and Buchmann 1983). statically charged with "frictional electricity" However, most reports on the possible in- (Warnke 1977). Warnke (1977) and Thorp volvement of static electricity in wind pollina- (1979) suggested that in the case of pollen- tion are speculative, since it is very hard to seeking insects, accumulation of pollen on the distinguish between aerodynamic and electro- surfaces of the insects and pollen distribution static forces in the direction of pollen grains by the insects are enhanced by the forces of towards the receptive stigma. attraction between the insect's positively Buchmann and Hurley (1978) described the charged body surface and the generally nega- probable role of electrostatics in buzz pollina- tively charged plant with its pollen. tion (vibrational pollination); they considered the attachment of the small, light and dry pollen grains of buzz-pollinated plants (e.g. Electrostatic pollination in natural systems senna Cassia sp., shooting star Dodecatheon Stanley and Linskens (1974), suggested that sp., deadly nightshade Solanum sp. and many while there was no evidence to support the others) to the body of the pollen-collecting view that electropotential gradients between female bee to be largely electrostatic in nature. pollen carried by insects and flower stigmas Pollen deposition on the stigma of these plants were involved in pollination, it was possible is also assumed to be mediated electrostatically that long-distance transport of pollen to the (Buchmann 1983, Erickson and Buchmann flowers was influenced by electrical relation- 1983). However, the actual process of pollen ships. Several researchers in various fields of transfer and adhesion to the dry stigmatic pollination study reported the possible surface is as yet unknown. involvement of electrostatics in natural polli- Eisikowitch (1981) observed that some- nation mechanisms. times, under sunny conditions, bumble bees Wind pollination is associated with the leaving the flowers of oilseed rape (Brassica aerodynamics of particle transport and capture napus L.) created clouds of pollen grains which (Whitehead 1969, Niklas 1985). It involves burst out of the flowers. Some of this pollen airflow patterns which bring conspecific pollen was assumed to have reached and adhered to to stigmas of wind pollinated flowers, and is the bumble bee's body, drawn by electrostatic influenced by various factors, including the attractive forces (Eisikowitch, personal infor- overall shape of the plant (Niklas and Buch- mation). Although wind or spontaneous selling mann 1985, Niklas et al. 1986), the size and was considered as the main agent of pollination shape of the inflorescence (Niklas and Buch- of oilseed rape, seed yield increased when mann 1988), and flower morphology and flowers were subjected to insect pollination. Y. Vaknin et al.: The role of electrostatic forces in pollination 135 Corbet et al. (1982) experimented with a Theory of electrostatic pollination living bumble bee (Bombus sp.) held down to a The theoretical basis for electrostatic induction cork on a wax block with silken threads. suggests that when a charged body is brought Oilseed rape pollen was sprinkled over the bee into the vicinity of an earthed electrode, a and some of the pollen grains that were falling charge of the opposing sign is induced to flow near the bee rapidly drifted towards it and from earth up onto the electrode, to ensure adhered to its body. Electrostatic forces of that the electrode remains at ground potential attraction were thought to be responsible for in the presence of the charged body (Law 1975) this phenomenon also. (Fig. 1A). In avocado (Persea americana Mill.), in the Based on this theory, a detailed mechanism pistillate stage, the style is erect and receptive, of pollination involving electrostatics has been while the anthers bend up towards the style suggested (Hardin 1976, Corbet et al. 1982, and dehiscence occurs. Ish-Am and Eisiko- Erickson and Buchmann 1983). As an insect witch (1993) found that bees collecting nectar, or nectar and pollen, visited both pistillate and carrying an electrical charge approaches a staminate flowers, and, because of the flower flower, charge of opposite polarity flows into structure, they were forced to touch both pistil the plant stem and flowers and induces an and anthers. Their visits to flowers in the electric field between the insect and the flower. staminate stage were very short, and they This electric field grows in strength as the gap touched the anthers for less than 1 s while between them narrows, and the forces of hovering, or while landing instantaneously on attraction temporarily created between the the flower on the top most position. During airborne insect and the flower initiate the these visits, their buzzing sounded lower than detachment of pollen grains from the anther usual, and a small cloud of pollen sometimes and their deposition on the body of the insect; surrounded them. The possible role of this the same forces initiate the detachment of the phenomenon was not discussed, although it is pollen grains from the insect's body and their likely that electrostatic
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