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 conditions, and are, therefore, surrounded by electric 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 forces assisted pollen deposition on various flower parts, including precipitation (Ish-Am and Eisikowitch, per- the stigma (Fig. 1B). These processes depend sonal information). upon physical variables such as the magnitude Schroeder (1995), stated that electrostatic and spacing of the charge source, the dielectric forces may be associated with the movement of properties of the media, and the geometry of the avocado pollen within a given flower and the flower (Dai and Law 1995). It also depends in the subsequent transfer of pollen grains to upon environmental variables such as atmo- other flowers by the honey bee. He conducted spheric ion concentrations and mobility, and simple studies on the effects of electrostatic local components of the earth's ambient elec- forces on pollen collection from avocado tric field (Law et al. 1996). Other factors such flowers, and proposed that it is most probable as flower size, size and hairiness of the bee, that the pollen adheres to insect bodies not level of bee activity and relative humidity are only by the stickiness of the pollenkit but also yet to be tested. by the action of electrostatic forces. The electrostatic force can also function as Endress (1997) who worked on pollination a short-term sticking factor, especially if the of Dillenia (Dilleniaceae), suggested that the pollen grain is deposited on a dry stigma, pointed ends of the stylar branches with the enabling it to remain on the receptive surface stigmas may also be seen as an adaptation to long enough for proper germination (Woittiez enhance the efficacy of the electrostatic forces and Willemse 1979). involved in the transfer of the dry pollen from Chaloner (1986) suggested the possibility anthers to bees and from bees to stigmas. that features of exine ornament in pollen of 136 Y. Vaknin et al.: The role of electrostatic forces in pollination

A pollen transfer from the bee to the stigma because the stigma will provide the strongest attraction for the charged pollen grains on the body of the insect. However, on the basis of calculations of relaxation time (the time it takes the charges to flow from the ground through the plant and into the flowers) conducted by Dai and Law (1995) the authors of the present paper do not F = K q~q2 agree with this statement. Experimental measurements of style and petal conductivity of yellow crooked-necked summer squash (13 mr2 2 J ÷ (Ohm) and 100 m~, respectively) revealed that the relaxation times were 3 ns for the style and b 25 ns for the petals (Dai and Law 1995). This B difference in relaxation time is so small that it is negligible since the charged body covers the last + ,

Fig. 1. A scheme of electrostatic induction. (A) A Measurements of the forces involved positively charged sphere approaches an earthed in electrostatic pollination sphere and induces an opposite charge; (B) a positively charged bee, carrying pollen grains on its Several attempts have been made to measure body, approaches an earthed flower and induces an the forces of attraction induced by the foraging opposite charge, especially on the edges of the flower. honey bee as it approaches the flower (Corbet In both cases the induction causes temporary forces et al. 1982, Schwartz 1991, Gan-Mor et al. of attraction between the charged bodies (F). These 1995). In a series of experiments, Corbet et al. forces are dependent on the magnitude of the charges (1982) subjected pollen grains of oilseed rape (q~ and q2) and on the distance between the charged (Brassica napus L.) on anthers and honey bees bodies (r). K the dielectric coefficient to electrostatic potentials of hundreds of volts as suggested by the measurement of Yes'kov entomophilous flowers may have some func- and Sapozhnikov (1976). They showed that a tional significance in the context of electrostat- dead honey bee connected to a potential of ically assisted pollination. According to this, 750 V (Volt) could cause detachment of oilseed the exine ornament could function to separate rape pollen from an earthed anther to the bee's the charged pollen from either the surface of body over a distance of 630 lam. Under the the bee or the flower, to which it is attracted. same potential, pollen on the bee's body Accordingly, this would delay charge sharing, jumped to an earthed stigma over a distance and so prolong adherence of pollen to an of 376 Ixm. oppositely charged surface. However, this is In the case of the honey bee, Gan-Mor pure speculation and requires experimental et al. (1995) were able to show that the validation. average charge on a honey bee after active Hardin (1976) and Corbet et al. (1982) flight through the air was 23.1 pC (pico suggested that when stigmas protrude from the Coulomb), with a maximum of 93 pC. The flower and provide the lowest-impedance path forces required for detaching pollen were to earth, the electric field induced by an 4x 10 -l°N (Newton), 3x 10 -l°N, and approaching charged bee will be the strongest 39 x 10-1°N for avocado, Eucalyptus camal- around the stigma. Such a field may enhance dulensis, and lisianthus (Eustoma grandiflo- Y. Vaknin et al.: The role of electrostatic forces in pollination 137 rum), respectively. Mathematical modeling In another model, the electric field in a showed that there were cases when the accu- system comprising a charged cloud approach- mulated charge on the honey bee was sufficient ing an earthed date (Phoenix dactylifera L.) for non-contact pollen detachment (Gan-Mor flower was calculated (Bechar 1996, Bechar et al. 1995). et al. 1999). A 3-D finite element model was constructed, and a pollen-grain trajectory initiated from the cloud and ending on the Electrostatic pollination and flower morphology flower was simulated. The model flower was The geometries involved in electrodeposition simplified by defining its geometry as a 4 mm processes of pollen grains are very complex diameter sphere with a cylindrical pistil, and difficult to model mathematically. Never- 0.4 mm in diameter attached to its top. It theless, two mathematical models have been was found that the maximal electric field was developed in order to describe the complex concentrated on the pistil top, and it increased system of a charged body (insect or pollen as the pistil length increased. The simulation cloud) approaching a flower which has a also showed that when electrostatic charging protruding stigma. was applied, the pollen density on the pistil A 3-D finite-element model for the analysis top was much higher (up to 225%) than of the transient electric field produced by a without charging. Two experimental systems cloud of charged pollen particles as it ap- were set up, in order to test this model. One proaches and enters a model squash flower experiment with an artificial flower used a was constructed by Dai and Law (1995). The 19-ram-diameter steel sphere with a 1.9-mm- model was coupled with the dynamic space diameter, and 19-mm-long rod to represent charge and the resulting transient boundary the style (on a scale of 4.75:1). The artificial potential on the flower surface. Exactly mod- flower was exposed to three treatments: It was eling the charged cloud and its neighboring dusted with uncharged date pollen; with pol- flower becomes difficult because of the ex- len charged by electrostatic corona at 40 kV tremely complex geometry of the flower. and 11 mA (for more details see Bright et al. Therefore, several assumptions were made to 1978); and with pollen charged by electrostatic simplify the physical model: (1) the cloud's corona at 80 kV and 27 mA. It was found that space charge was represented by a uniform as the voltage and the electrostatic charge space charge density; (2) the charged cloud density in the pollen cloud increased, the had a cylindrical shape; (3) the flower con- density of pollen on the pistil increased, and sisted of a style surrounded by five segmented the ratio of the pollen density on the pistil to cone petals. For instantaneous introduction of that on the corolla also increased. In another a charged pollen cloud, results showed that experiment, almond flowers were exposed to the electric field immediately above the flower the same pollination regimes as those above stigma was approximately three times that and similar differences in pollen densities on above the petal edges. The model also showed the stigma were obtained (Fig. 2; Vaknin that the electric field near a flower was 1999). affected by the geometry of the flower: as the opening angle between the petals and the Electrostatic pollination in agricultural systems style increased, the electric field near the stigma rose while the one near the petals Pollination of crop plants is a major factor in decreased. According to this model, the great- achieving sufficient crop set. Crop plants are er electric fields which prevail when the flower pollinated by various means: most of them, is more open and exposed enable pollen such as almond (Amygdalus communis L.) and electrodeposition onto the stigma to be max- apple (Malus sylvestris Mill.) by insects (main- imized. ly honey bees, Apis mellifera L.) (McGregor 138 Y. Vaknin et al.: The role of electrostatic forces in pollination

Fig. 2. Pollen grains of almond deposited on the stigmas of almond flowers. (A) The flower was dusted with uncharged pollen; (B) the flower was dusted with pollen charged at 80 kV and 27 mA

1976, Free 1993), others, such as date, olive Law et al. (1996), stressed the need for an (Olea europea L.) and pistachio (Pistacia efficient mechanized pollination method as a vera L) by wind. solution for pollination deficiencies in modern In recent years, insufficient pollination has agriculture. They suggested using the tech- been found to be one of the important causes niques used in electrostatic deposition of pow- of low yields in many field and orchard ders and liquids, which were available in both species (Shivanna and Sawhney 1997). Some industry and agriculture. These techniques species require management of pollinating generally comprise three distinct operations: agents, while others benefit from artificial (1) the generation and electrification of a means of pollination, one of which is pollen charged particulate cloud or stream; (2) the supplementation (Hopping and Jerram conveyance of these particles to the vicinity of 1980a, b), involving three major steps: (1) the desired target object; and (3) deposition of pollen collection; (2) pollen storage; (3) pollen the charged particulates on the target (Law deposition on receptive stigmas. 1989). Over the past 35 years, electrostatic forces Law and Cooper (1989) had shown that for have been employed in numerous technologies, plants growing directly in the soil, electrical including the transportation, collection, or conduction must be adequate to establish the separation of materials in the form of powder appropriate distribution of charges to main- or small droplets (Bright et al. 1978). tain the plant at earth potential during its Y. Vaknin et al.: The role of electrostatic forces in pollination 139

interaction with an incoming electrostatic discharge nozzles that induced an electrostatic charge. Also, several experiments showed that charge on the pollen. Although no scientific electrical grounding of plants is adequate for experiment was carried out, it seems that the satisfactory development of electrostatic farmers in California are now beginning to crop-spraying. use this technique regularly, and some even Three widely used particle charging meth- report increases in yield (Vaknin, personal ods are suggested for electrostatic pollen sup- communication). plementation: (1) electrostatic induction of Electrostatic dusting of larch (Larix eurol- charge onto a conductive liquid dispersion epis Henry.) was described by Philippe and medium containing suspended pollen grains Baldet (1997). In this experiment, electrostat- (Banerjee and Law 1996); (2) triboelectrifica- ically charged pollen was blown to the flower- tion (i.e. frictional charging) of pollen grains ing trees by an electrostatic gun and its (Banerjee and Law 1988); and (3) ionized-field effectiveness was compared with that of con- charging of the pollen grains by feeding them ventional pollen blowing. Results showed that in an air stream past a high-voltage electrode electrostatic dusting deposited three times which incorporates spikes at which corona more pollen on the flowers (15 vs five pollen discharges are formed by the locally intense grains per bract) and enhanced full seed electric field (Bright et al. 1978). Emerging percentage (32 vs 23%) without reduction in pollen grains are charged by ion bombardment pollen viability, although the amount of pollen in the corona region, and are then transported used was much smaller in the electrostatic by both aerodynamic and electrostatic forces procedure. towards the flowering plant. As the mass of Bechar et al. (1999) used electrostatic dust- negatively charged pollen grains approaches ing techniques in pollination experiments on the targeted plant, they induce positive charg- dates, and found that by applying electrostat- ing by creating an electron flow inside the plant ically charged pollen they could raise fruit set down into the soil, which keep the earthed by an average of 85 to 175%. SEM analysis of plant at zero potential, leaving all the exposed date flowers which were dusted electrostatical- plant surfaces with a temporary positive ly or non-electrostatically showed that the charge. The resulting electric field forces the stigmas which were dusted electrostatically negatively charged pollen grains in the cloud were totally covered with pollen grains where- front towards the positively charged plant as the ones which were dusted non-electrostat- parts. ically received fewer pollen grains. Similar In the last 20 years, several groups of results were observed with almond flowers researchers have tried various techniques for (Fig. 2; see also Bechar 1996). supplementary pollination. Detailed experiments on several aspects of Although experiments on mechanical dust- pollen supplementation in almond and pista- ing of flowering apple trees with apple pollen chio were conducted by the authors of this succeeded in increasing fruit set, when the paper. They included pollen collection and pollen grains were electrostatically charged, storage (Vaknin and Eisikowitch 2000) and pollen deposition in the trees did not increase pollen dilution prior to its application (Vaknin (Legge 1975, cited by Williams and Legge et al. 1999). They had also shown that the use 1979). of electrostatic techniques in pollination of Oltman (1997) described, in a commercial almond and pistachio could raise yields by an paper, mechanical dusting techniques for var- average of 10 and 20% respectively (Vaknin ious commercially grown plants, including et al. a, Vaknin et al. b, in preparation). almond, plum (Prunus spp.), apple, olive, Usually, an increase of 10% or more in fruit walnut (Juglans regia L.), and pistachio. The set is sufficient to cover all expenses of pollen machinery consisted of pollen blowers with supplementation and it leaves the growers with 140 Y. Vaknin et al.: The role of electrostatic forces in pollination a substantial profit. However, an excess of This study was partially funded by grant no. pollen supplementation could be detrimental US-1996-91 from the United States - Israel Bina- as it has been found in walnuts (Juglans regia tional Agricultural Research and Development L.; McGranahan et al. 1994). The balance Fund (BARD). between sufficient and excess pollen supplementation requires further investigation for References each crop individually. These studies have just Banerjee S., Law S. E. (1988) Characterization of begun to reveal the potential of this technol- chargeability of biological particulates by tribo- ogy. More research on this subject will not electrification. IEEE Transactions on Industry only lead to improved crops in agriculture, but Applications 34: 1201-1205. will also add valuable information on the Banerjee S., Law S. E. (1996) Electrostatic induc- intricacies of electrostatics in pollination biol- tion charging of pollen suspensions. IEEE/IAS ogy. Conference (in press). Bechar A. 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