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PLANTS & ELECTRICITY.
Term Paper ID:24279
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Essay Subject:
Negative effects of electrical fields on plant development & growth: root elongation, electrotropism, flowering, more.... More...
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6 Pages / 1350 Words
11 sources, 14 Citations,
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Paper Abstract:
Negative effects of electrical fields on plant development & growth: root elongation, electrotropism, flowering, more.
Paper Introduction:
Recently there has been considerable public concern and scientific interest over the hazards associated with exposure of plants to extremely low frequency electrical fields (60-Hz), particularly those related to high voltage electric transmission lines. Using specific examples, this paper discusses the detrimental impact of such electric field on various aspects of plant development and growth including root elongation and electrotropism, phloem transport efficiency, species susceptibility and flowering rhythms.
Each plant cell consists of a highly conductive cytoplasmic core surrounded by a thin insulating plasma membrane which is in turn surrounded by a porous - but rigid - cell wall. The insulating plasma membrane plays a prominent role in the electrochemical balance between the cell cytoplasm and the
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"6 -Hz Electric Field Exposure Inhibits Net Apparent H+ -Ion Excretion from Excised Roots of Zea mays L." Radiation Research 123 (199 ): 22-31.---. Electric field effects have beensemiquantitatively and qualitatively related to the induction of 6 -Hztransmembrane potentials whose magnitude correlates positively with cellsize. Both cell wall loosening and the uptake of various solutes appearsto depend on the acidification of the environment immediately surroundingthe cells via the activity of enzyme driven pumps located on the plasmamembrane, specifically H+ - ATPase. The threshold Vim for an inhibitory effecton root growth rates in Pisum, Vicia, Cucumis and Cucurbita is estimated torange from 2.2 - 2.7 mV with associated field strengths ranging between 196- 328 V.m-1 (Brayman and Miller " Proportionality of" 2 7-2 8). McKee. H. Taken together, these studies suggest that 6 -Hz electric fieldexposure inhibits root growth by increasing the electrical potentential ofthe plasma membrane which subsequently inhibits H+ efflux at the plasmamembrane. Carstensen, Gary E. R. At field strengths of greater than 225 V.m-1, acidificationdue to H+ efflux decreases proportionally with increasing field strength.The field strength threshold for this inhibition is about 22 V.m-1 whichis nominally greater than that of the threshold for growth inhibition inZea (17 V.m-1). Works CitedBrayman, Andrew A., A. However, the inhibition of transportof substances originating in the roots which trigger photoperiodicflowering may also be involved. In susceptible species damage is limited to theaerial portions of the leaf tip and the rate of damage declines with timeof constant exposure to a given electric field intensity. "Leaf Export and Partitioning Changes Induced by Short-Term Inhibition of Phloem Transport." Journal of Experimental Botany 44 (1993): 1491-1496.Robertson, Dominique, and Morton W. Miller. Each plant cell consists of a highly conductive cytoplasmic coresurrounded by a thin insulating plasma membrane which is in turn surroundedby a porous - but rigid - cell wall. The induced membrane potential (Vim) arises when a cell is exposed toan applied electric field as a consequence of ionic movement in theconductive fluids inside and outside the plasma membrane. "Proportionality of 6 -Hz Electric Field Bioeffect Severity to Average Induced Transmembrane Potential Magnitude in a Root Model System." Radiation Research 117 (1989): 2 7-213.Johnson, Gregory J., Dennis T. Lastly, electric fields appear to inhibit photoperiodic flowerinduction in both short and long day plants (direct current (DC), 6(A.plant-1). E. Numerousexamples of such contradictory responses exist in the literature. 11 ). Plant also respond to electric fields by altering their direction ofgrowth in response to the anode or cathode. The majority of experimental studies on the impact of electricalfields on growth have used plant roots as model systems. Above thisthreshold, the duration of the inhibition increases with increasedmagnitude of the stimulating electric field. Roots representtissues which grow rapidly and well in simple, inorganic, chemicallydefined, aqueous media. The cells inside the roots are exposed to fields similar to thoseoutside the root and the roots appear to respond to field strength and notthe current density in the growth medium. Using specific examples, this paperdiscusses the detrimental impact of such electric field on various aspectsof plant development and growth including root elongation andelectrotropism, phloem transport efficiency, species susceptibility andflowering rhythms. Results ofdetailed studies into the timing of the DC effect, its stability andlimiting values, coupled with the afore mentioned electrode localization,suggest that DC interferes with the translocation of the "floral stimulus"from the leaves to the apex of the plant. Within the elongation zone of a root, cell dimensions increase withincreasing distance from the root tip, thus at a given field exposure,cells with different dimensions will exhibit different Vim such that growthrate inhibition rates will increase with increasing distance from the roottip (Brayman, Brulfert, and Miller 151-157). The mechanism bywhich this shift takes place is unknown (Pickard, Minchin, and Thorpe 1491-1496). "6 -Hz Electric Field Induced Inhibition and Recovery of Growth Processes in Roots of Pisum sativum." Bioelectromagnetics 2 (1981): 329-34 .Stenz, H. Kaufman, and Dominique Robertson. Minchin. The enzyme's activity is inhibited by the pharmacologicalagents vanadate and diethylstilbestrol, it is stimulated by metalliccations and forms a covalent phosphoenzyme intermediate during thetransport cycle (Brayman and Miller "6 -Hz Electric" 24). Further, the length ofinhibition is positively correlated with duration of exposure to the field(Pickard and Minchin 4 9-417). In a parallel study (Brayman and Miller "6 -Hz Electric" 22-31),acidification and H+ -ATPase activity in Zea mays root tips have also beeninvestigated. In general pollen tubes growparallel to the field, yet those of Vinca rosea grow towards the cathodeand the pollen tubes of Impatiens holstii grow towards the anode. The electrical nature of this membrane and the factthat externally applied electrical fields are localized in the plasmamembrane - and not the cell cytoplasm - suggests that they are thebiologically relevant site for studies into the impact of electric fieldson plant growth (Miller et al. H+ - ATPase influences acidificationand solute uptake via cation:H+ exchange and via anion:H+ cotransportmechanisms. The curvature of the roots towards thecathode is induced at field intensities of .5 - 3 V.cm-1 and occurs in thedistal elongation zone of the roots. For cellular expansion to takeplace, a turgor pressure sufficient to stretch the cell wall must bemaintained within the cell, a process that is achieved by water and soluteuptake. Oak Ridge, TN: Technical Information Center,: 1979.Machackova, Ivana, and Jan Krekule. Minchin, and M. These studies have employed electric shock (2 V.mm-1,current .5 mA, duration 5 seconds) to investigate this phenomena in Pisumsativum and reveal that phloem transport of photosynthate away from theleaves is inhibited by field strengths of .5 V.mm-1 or above. Thispolarization induces the opening of ion gated channels on the membranesurface and the admittance of messenger molecules which initiate thebiochemical signal that interrupts phloem translocation. One of the few studies documenting the impact of electric fields onwhole plants shows that a 6 -Hz high intensity electric fields causephysical damage to the leaf tips in Scotch Pine, Nutsedge and Onion atintensities of 2 KV.m-1 and above (Johnson, Poznaniak, and McKee 172-183).The severity of damage appears to be associated with a specific leaf tipgeometry and morphology, with sharp and pointed tips being more susceptiblethan blunt or rounded leafed species, the latter being unaffected by fieldsas high as 5 KV.m-1. II. When roots are placed in an extremely low frequency electric field,root growth is inhibited in a dose dependent manor (Robertson and Miller329-34 ). Brulfert, and Morton W. Thus, a moreintense field results in more polarization of the membrane and a greaterbiochemical signal. Such a phenomena would beexpected in light of the previous discussion of the inhibitory effects ofelectric fields on phloem transport. "Prediction of Damage Severity on Plants due to 6 -Hz High Intensity Electric Fields." Biological Effects of Extremely Low Frequency Electromagnetic Fields. The Effect of 6 -Hz Electric Fields on the Growth of Different Regions of the Cucurbit Root Elongation Zone." Radiation and Environmental Biophysics 25 (1983): 151-157.Brayman, Andrew A., and Morton W. In these experiments, the impact of the electric field ispolarity specific and initiated with the cathode connected to the leavesand the anode to the roots (Machackova and Krekule 381). Miller. "The Electroshock-Induced Inhibition of Phloem Translocation." Journal of Experimental Botany 43 (1992): 4 9-417.Pickard, William F., P. This hypothesis has gained support byrecent findings that direct electric current specifically interferes withthe uptake and translocation of charged substances - such as chloride ions- which may also be involved in the induction of photoperiodic flowering(Machackova, Tykva, and Krekule 1683-1686). In order to grow, each cell wall must loosen and undergo elasticextension in response to turgor pressure. These authors propose that the electricfields produces membrane polarization which varies over the surface of thecell, and increases in magnitude with increasing field strength. Technical Information Center: Oak Ridge, TN, 1979.Pickard, William F., and P. The small radial diameter of roots allowselectrical field effects to be isolated from their associated heatingeffects by regulating the temperature of the growth medium (Miller et al.1 9). These authors argue that the cathodalelectrotropism is a true physiological response and the reports of anodalattraction may be the result of unnoticed pH gradients in the experimentalset up and/or excessive electric field intensities. H. The insulating plasma membrane plays aprominent role in the electrochemical balance between the cell cytoplasmand the environment and this is reflected in the membranes intrinsicelectrical potential. "6 -Hz Electric Field Parameters Associated with the Perturbation of a Eukaryotic System." Biological Effects of Extremely Low Frequency Electromagnetic Fields. Weisenseel. As in the previous study, inhibition of acidificationcorrelates positively with increasing cell size and distance from the roottip, and further indicates that the cellular orientation to the electricalfield is an additional factor to be considerated (Brayman and Miller "6 -HzElectric" 29). "The Effect of Direct Electric Current on Transport of Chloride Ions, Sucrose and Assimilates in Chenopodium rubrum." Plant Cell Physiology 36 (1995): 1683-1686.Miller, Morton W., Edwin L. Poznaniak, and Guy W. A recentstudy using biologically and physiologically relevant electric fields hasrevealed that Lepidium sativum and Zea Mays roots exhibit electrotropism tothe cathode in weak DC-electric fields applied at right angles to the roots(Stenz and Weisenseel 335-344). Inhibition of this efflux - mediated by H+ -ATPase - preventsacidification, and stops solute and water uptake required for cell wallloosening and the persistent increase in turgor pressure associated withplant cell growth. "DC-Electric Fields Affect the Growth Direction and Statocyte Polarity of Root Tips (Lepidium sativum)." Journal of Plant Physiology 138 (1991): 335-344. "The Interaction Of Direct Electric Current with Endogenous Rhythms Of Flowering in Chenopodium rubrum." Journal of Plant Physiology 138 (1991): 365-369.Machackova, Ivana, Richard Tykva, and Jan Krekule. Electric fields have also been shown to have a deleterious effect onthe phloem pathway. "Induction of ELF Transmembrane Potentials in Relation to Power-Frequency Electric Field Bioeffects in a Plant Root Model System. In a later study, these authors show that the inhibition of phloemtransport in response to electric fields leads to a subsequent change inthe partitioning of mobilized photosynthate between sinks. Recently there has been considerable public concern and scientificinterest over the hazards associated with exposure of plants to extremelylow frequency electrical fields (6 -Hz), particularly those related to highvoltage electric transmission lines. The Vim issuperimposed on the static membrane potential (Vm) and results in bothhyperpolarization and depolarization of the transmembrane electric field.For healthy cells, the limiting value of Vim can be calculated using therelationship between cell dimension (d) and the applied field strength (Eo)as follows : Vim ( .5 (Eo x d). While these experiments have notyet been conducted on plant shoots, it is plausible that an identicalscenario exists in these tissues. E. Miller. Plant cellular growth is constrained by the rigidity of the cellwall. Thorpe. The key questionin understanding the impact of electric fields on plant growth is therelationship between plasma membrane potential (Vm) and the activity of H+- ATPase. G., and M. H. These effects are magnified with increasing cell sizeand increasing distance from the root tip.
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