Magnetic Field And Its Impact On Plant Growth And Development

Abstract

Plant magnetobiology has become an important topic to study and manage biological effects on plants.Studies over the past several years have paved its way asa new advancementfor achieving enhanced growth and development in crop plants including horticultural plants.An emphasis has been made upon the use of this technique with different magnetic field intensity and exposure. The precisemechanism of its action is not known, but biological and chemical cues are being studied as possible mechanisms. This article presents an overview of ‘magneto priming’ for the enhancement of growth and yield of various plants.

Introduction

All the living organisms in this ecosystem are under the influence of Earth’s geomagnetic field (GMF) and it is believed that all the life forms have evolved in the presence of GMF- whether it be geotaxis in magnetotactic bacteria, cellular responses in humans or stimulation of growth in plants (LefèvreandBazylinski2013).Amagnetic field (MF) is a vector field in the neighborhood of constant magnets or electric currents that is specified by both direction and strength and is characterized by magnetic flux density (measurement in T) and MF strength (measurement in amperes [A]/meter [m]).The magnitude of the Earth's magnetic field at its surface ranges from 25 to 65 μT (0.25 to 0.65 gauss). A German botanist in 1930 Ssawostin first reported faster growth stimulation of wheat coleoptiles under magnetic fields followed by Murphy in 1942 concluding that germination rate of different types of seeds would accelerate underMF treatment.Developments in the area ofmagnetobiology began in the 1960’s with the development of space biology and the effects on different life forms separately including studies on migratory birds (Wiltschkoand Wiltschko 1972)and honeybees(Gould 1980). In the subsequent period, magnetoorientation of magnetotactic bacteria (Blackmore 1982)was studied under bacterial magnetotaxis.

Initial studies of MFeffectsonplants were conducted by KrylovandTarakonova(1960)who experimented seed germination under MF treatment and suggested that the effects were mostly anauxin-likeand termed the phenomenon, magnetotropism. There have been several studies on the effects of plants and these have been reviewed (Maffei 2014).To investigate the magneto sensitivity of plants, four different types of magnetic fields have mainly been employed:(1) weak static homogeneous magnetic fields, (2) strong homogeneous magnetic fields, (3) strong in-homogeneous magnetic fields and (4) extremely low frequency (ELF) magnetic fields of low to moderate magnetic flux densities.

Studieshave shown that magnetic fields have significant effect on seed germination, plant growth, development, and yield, depending upon a specific species and on the characteristics of field exposure such as intensity and duration with difference in their growth pattern (Teixeira da Silva and Dobránszki 2016). For example, a significant decrease in the cell numberof onion (Allium cepa) meristemshas been observed. Barley (Hordeum vulgare) seedlings grown in Helmholtz coils with a 10 nT MF intensity showed a decrease in fresh weight of shoots and roots, as well as dry weight of shoots and roots in comparison with GMF controls. In another study, it was concluded that very low MF was capable of delaying both organ formation and development (Lebedev et al., 1977).Belyavskaya (2004) found that weak electromagnetic fields suppressed the growth of plants, reduced cell division, intensified protein synthesis and disintegration in plant roots. Sunflower (Helianthus annuus) seedlings exposed to 20 μT vertical MF showed small, but significant increases in total fresh weights, shoot fresh weights, and root fresh weights, whereas dry weights and germination rates remained unaffected (Fischer et al., 2004). However, the best results have been observed for the plants exposed to Electro magnets.Electromagnetic Fields (EMFs) have magnetic and electrical properties that surround objects with an electrical charge which will interact with other objects within that field. At present, there is a growing tendency to use either strong homogeneous or in-homogeneous field for the treatment of seeds for improving their performance (Shine et al. 2012).

Effects on germination rate and root and shoot initiation

Several studies proved the positive effect of Magnetoprimed seeds with significant andrapid germination and early root and early morphogenesis giving rise to the possibility that magnetism can increase the speed of plant development. Many studies have suggested positive effects in MF treated seeds of agricultural importance or with ornamental value. Groundnut seeds had shown an increase in germination rate and vigorous seedling growth, Similar studies on Onion and rice seeds (Vokharia et al., 1991; Alexander and Doijode 1995) have been observed. There are reports sharing that the magnetic field exposure increases germination of extremely low viability seeds and improvement in their quality and sprouting rates (Carbonell et al., 2008; Alexander and Doijode 1995).In case of annual grains like chickpea which is considered financially risky by farmers because of its tendency to face diseases in its early growth stages has also shown response to MF in germination and early growth characteristics(Cakmak et al., 2011) with high survival Percentage. Enhanced photosynthetic efficiency and high growth of crops like wheat and maize has also been reported(De Souza et al., 2008; Shine and Guruprasad 2012). Studies have also demonstrated that the energy involved during germination is increased due to magnetic fields.Germination energy and germination rate of tobacco seeds increased linearly with an induction of 0.15 T at expositions of 10, 20 and 30 min with and without preliminary soaking (Aladjadjiyan and Ylieva 2003).

Based on the treatment dose and type of magnet used, stationary magnets with high magnetic fields demonstrated the best results as compared to WMF and super Weak MF.50 mT and 200 mT for 2 h exposure increased the activities of hydrolyzing enzymes in Helianthus annuus, which were responsible for the quick seed germination, improved seedling vigour and better root characteristics of treated seeds in this plant(Vashisth and Nagarajan 2010). By the application of power frequency magnetic fields (100 μT), growth of germinated Viciafabaseedlings was enhanced, supported by an increased mitotic index (Rajendra et al., 2005).Exposure of maize seeds to stationary magnetic field strength of 125 or 250 mT enhanced the germination parameters (Zepeda-Bautista et al.,2010). The effects of electromagnetic irradiation on seed vigour of maize hybrids, as well as the response of each genotype were assessed in this study. It was also reported that pre-sowingtreatment increased corn seed vigour through emergence rate, establishment percentage, and dry mass of seedling aerial part, according to the combination of MF intensity, time and the genotype.

There have been several experimental studies on the effect of different MF capacitance on increased growth rates in different species (Table 1). In maize, plants showed higher growth and biomass than control plants, with asignificant increase inmaize plants which were exposed continuously to 125 or 250 mT(Florez et al., 2007). In barley, a stimulating effect on the first stages of seedling growth was observed. Germinating barley seeds were treated with 125 mTMFfor different time periods (1, 10, 20, and 60 min, 24 h, and chronic exposure). Interesting results were obtained with increase in seedling length and seedling weight (Martinez et al., 2002). Pea plants when exposed to 125 or 250 mT stationary MF (1, 10, and 20 min, 1 h and 24 h) and continuous exposure showed better growth in case of continuously exposed plants to the MF compared to control plants(Carbonell et al., 2008).

Several in vitro studies have shown that MFs can affect the development of cells and tissues cultured in vitro. In soybean, plant regeneration and growth of shoot-tip cultures exposed to MFs (2.9–4.6 mT) for 2.2 and 6.6 s showed an increase in regeneration frequency (87% and 74%, respectively, as compared to 62% in the control (Atak et al. 2003).The root induction also showed similar positive effect of MF with 26% and 36% increase over control. Shoot and root formation rates of Paulownia tissue culture increased when nodes were exposed to external MFs (2.9–4.8 mT for 2.2, 6.6, and 19.8 s during the culture period) compared to the control (Yaycili and Alikamanoglu 2005; Çelik et al., 2008).In vitro shoot cultures of P. tomentosa exposed to a magnetic flux density of 2.9–4.8 mT for 2.2 s recorded a higher growth from 61.9 to 82.5%; with an increase in total chl, chl a, and chl b content and doubled total RNA content of the treated tissues over the control (Çelik et al., 2008). These studies suggest that in vitro plant tissues respond to MF exposure in terms of enhanced growth parameters.

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In our studies on an important ornamental orchid, we investigated positive influence of high intensity magnetic field (200mT) on in vitrogamma irradiated Dendrobium sonia cultures. Pre-optmizedmagnetic field strength of 200 mT was applied for different duration (15 min, 30 min, 45min, and 60 min), on freshly isolated protocorm like bodies (Shikha et al.,Unpublished). Our preliminary results suggested that MF treatment significantly altered growth, O- and OH- radical level, photosynthetic activities and water use efficiency in irradiated cultures as compared to MF untreatedcontrol cultures(Shikha et al.,Unpublished).

Mechanisms of MF stimulation of plant growth and development

Free radical production and photosynthetic efficiency

Every aspect of plant growth requires energy and in cases of stress the total energy produced could be distributed to the defence pathway(Bailey-Serres and Mittler 2006).In general, the electron transport chain (ETC) in chloroplasts operates in an O2 sufficient environment such that if ETC is overloaded it will result in leakage of electrons and generation of free radicals.This free radical produced upon stress is shown to be decreased after Magnetic treatments and alleviating the chlorophyll content and photosynthetic efficiency(Shine and Guruprasad., 2012). The reduction in the free radical content was reported in maize and soybean seedlings after the treatment with 200 mTfor 1 h and 150 mTfor 1 h(Shine and Guruprasad 2011) Photosynthetic parameters were tracked by calculating energy by performance index through three independent functional steps of photosynthesis, the density of reaction centers in the chlorophyll bed (RC/ABS), excitation trapped per photon absorbed (ϕpo) and efficiency with which a trapped exciton can move an electron into the electron transport chain further than QA- (Ψo)(Srivastava et al., 1999). This proved that magnetopriming of soybean seeds enhanced the PI up to 48 and 63% in 150 mT (1 h) and 200 mT (1 h) respectively over control plant, specifically showing enhancement of PI attributable to higher efficiency of ϕpo.Increased Rubisco content and higher light harvesting efficiency in the treated plants leading in an increase in biomass of plants from MFtreatment(Baby et al., 2011).

Water and nutrient uptake

With the most prime requirement of plant of nutrients, supply is only possible through water, and studies have shown that magnetic exposure can enhance intake of these essential nutrients (Ijaz et al., 2012). Even in salt stress these plants could show robust and vigorous growth.The water in the region is heavily salted, which interrupts plant uptake. By exposing the water to magnets, the salt ions change and dissolve, creating purer water that is more easily taken up by the plant.The magnetic field changes water properties due to displacement and polarization of water atoms. Cai et al.(2009)reported that MF caused changes in physicochemical properties of water; these changes included decreasing water surface tension and increase viscosity suggesting an increase of activation energy and water molecule size due to extra hydrogen bond formation. Fundamental to all biological systems is the biochemical activities and the biomolecule (chlorophyll) can be affected by MF as the photochemical activity, respiration ratio and enzyme activity are all influenced under MF (Phirke et al., 1996; Dhawi 2003). In our study on In Dendrobium sonia, IRGAbased studies with MH treated PLBs showed that that the water use efficiency was higher in MF treated cultures(Shikha et al., Unpublished).

Modifications at cellular and molecularlevel

Several studies reported that MF affectsatcellular and molecular level leading to plant growth enhancement (Dhawi et al., 2009). The negative charges around the DNA molecule, as any charged entity, increase the potential of MF impact and the theoretical concept of how MF affects the DNA is that MF prolongs free radical ions’ lifetime, by inducing the singlet-triplet transition of unpaired electrons leading to oxidative stress (Sahebjameipour et al.,2007).

Several studies reported a decrease in DNA level following low level exposure of magnetic field in arbour seedlings (Racuciu et al., 2008)and date palm (Dhawi and Al-Khayri et al., 2009).The mutagenic effect of MF is indirect because of the limited physical ability of non-ionizing radiation to induce double brake in DNA. Study by Pingpinget al.(2007) suggested that MF increases cell membrane permeability, which may increase uptake of water and nutrition. The effect of magnetic fields can be seen in protein synthesis activation leading to further development of root system (Phirke and Kudbe et a.l, 1996). The MF induced changes in cellular level leading to increase in cell viability, organization and differentiation (Vizcaino et al., 2003). In addition, MF affects cell proliferation and cellular metabolism (Atak et al., 2003) gene expression(Paul et al., 2006) and enzyme activity (Ataket al.,2007).

Magnetoreception

The phenomenon ‘‘ion cyclotron resonance’’is suggested to be the mechanism involved in magnetoreception. The movement will be a circulation of ions in a plane perpendicular to an external magnetic field with their lamor frequencies (Galland and Pazur 2005). When this frequency matches with that of the electromagnetic field, there is an absorption of energy from the external field. It is also suggested that exposure to magnetic field can modulate the internal potential energy of a biological tissue which could contribute to improve overall growth and development (Kavi 1983).

Another evidence is that of the Plant cryptochromes being involved in magnetoreception(Ahmad et al. 2007). In Arabidopsis, cryptochromes are encoded by two similar genes, cry1 and cry2. CRY2 protein levels in seedlings decrease rapidly upon illumination by blue light, presumably as a result of protein degradation of the light-activated form of the receptor (Ahmad et al., 2007).The expression changes of three Arabidopsis cryptochrome-signaling-related genes (PHYB, CO and FT) suggest that the effects of a near-null MF are cryptochrome-related, which may be revealed by a modification of the active state of cryptochrome and the subsequent signaling cascade plant cryptochrome has been suggested to act as a magnetoreceptor (Xu et al., 2012).

Based upon the premise that cryptochromes form radical pairs subsequent to photoexcitation, the MF mediated sensitive responses are suggested to be the result of this radical pair formation (Galland and Pazur2005; Maffei 2014). It is also the most suggested mechanism for magnetic interactions to have an effect on chemical reactions. It is opined that “the kinetics and quantum yields of photo-induced flavin-tryptophan radical pairs in cryptochrome are indeed magnetically sensitive and cryptochrome is a good candidate as a chemical magnetoreceptor” (Maeda et al., 2012). As of now, cryptochromes are considered as the likely mediators of the MF induced biological effects (Maeda et al., 2012).

Magnetoreception is a research frontier with immense application in plants. There is a great amount of information on the physiological effects and biomass accumulation, and cyptochrome and radical-pair based mechanistic view are being established as plausible mechanisms. Molecular studies in this field are needed to elucidate the biological effects. Extensive research is also necessary to extend MF-induced biological effects field-leveldemonstration forpositive outcomeon plant growth and productivity.

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