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Asian Jr. of Microbiol. Biotech. Env. Sc. Vol. 20, No. (4) : 2018 : 1332-1343
© Global Science Publications
ISSN-0972-3005
*Corresponding author’s email: chandima.gomes@gmail.com
ELECTRICAL STIMULATION FOR THE GROWTH OF PLANTS: WITH
SPECIAL ATTENTION TO THE EFFECTS OF NEARBY LIGHTNING
ON MUSHROOMS
NOR AZREEN MOHD JAMIL1, 2, CHANDIMA GOMES3*, CHAN PICK KUEN1,
MOHD ZAINAL ABIDIN AB KADIR2 AND ASHEN GOMES4
1Agro-Biotechnology Institute, National Institute of Biotechnology Malaysia, C/O MARDI
Headquarters, 43400 Serdang, Selangor, Malaysia
2Center for Electromagnetic and Lightning Protection Research (CELP), Faculty of Engineering,
University Putra Malaysia, 43400 Serdang, Selangor
3School of Electrical & Information Engineering,University of the Witwatersrand, Johannesburg,
South Africa
4School of Electrical Engineering and Computer Science, KTH Royal Institute of Technology,
Stockholm, Sweden
(Received 10 August, 2018; accepted 25 September, 2018)
Key words: Mushroom, Electrical potential, Electrical stimulation, Growth enhancement, Lightning
Abstract – Electrical stimulation could trigger biological activities of plants, enhancing their production rate.
This paper addresses the application of this technology on mushroom which, possesses unique
developmental process that depends on hyphae morphogenesis at growth stages; thus their response to
electrical stimulation should be studied separately. The study analyses the information available on electrical
environment of mushrooms with the view of investigating the efficiency of external electric stimulation
approaches for the enhancement of their progress at various growth stages. Electric treatments at relatively
low strengths, applied for a short exposure time have resulted positive impacts on growth rates, yields,
length and size of various plants. In the case of mushrooms, the only information available on successful
external electrical stimulation technique is the application of high voltage pulses. The technique has shown
positive effects on the growth rate of varieties such as shiitake and nameko. This approach has been adopted
based on the unconfirmed claims that lightning in the vicinity develops cracks in mushroom hyphae and
stimulates their enzyme activities, which in turn boosts the growth rate. Based on the outcomes, we foresee
a good economic viability of mushroom production with these modern technologies and methodologies.
INTRODUCTION
Externally applied man-made electrical signals have
successfully improved the growth rate and yield of
plants in the last few decades, as the available
literature claims (Goldsworthy, 2006 and 1966; Pohl
and Todd, 1981). However, despite decade long
experiments in this regard, there are only few plant
varieties that has been studied so far, for their
response to electrical stimulation. This paper
focuses on the electrical stimulation of a particular
agro-product, mushroom, which is not categorised
as a plant.
Mushroom is a group of higher fungi that has
been classified as a unique biological form since
they pose unique cell structure, growth mechanism
and biological activities. Mushroom has a large
worldwide demand as an agro-commercial product.
There are nearly 10,000 identified species of
mushroom out of which only a limited number of
varieties are edible. Some species such as shiitake
(Lentinula edodes) are known for both their taste as a
food and medicinal properties, where as some, such
as tiger’s milk mushroom (Lignosus rhinoceros) are
known only for their medicinal value (Jamil et al.
2013). Figure 1 shows pictures of both shiitake and
tiger’s milk mushroom.
A few sources claim that some mushrooms are
able to fast multiply in the presence of lightning
(Takaki et al., 2009 and 2010; Tsukamoto et al., 2005).
The reason for this response of mushroom to the
presence of lightning is still in debate. Perhaps,
1333
Electrical Stimulation for the Growth of Plants: With Special Attention to the Effects of Nearby
lightning is considered as a sign of danger or threat
to the existence by mushroom. Therefore,
mushroom may attempt to boost their reproductive
efforts as a way of survival. Several investigations
have been conducted to determine the effects of
electricity in the form of artificial lightning (or
pulsed currents) in mushroom production (Takaki et
al., 2009, 2010 and 2014).
So far, the application of external electromagnetic
stimulation on mushroom has been limited to a few
approaches, which have mostly been done in
isolation in several parts of the world. However, the
lack of information on electrical stimulation
methods and a comprehensive database on the
research output stand as a barrier that hinders
further progress in the application of external
electric means for enhancing constructive processes
on mushrooms in an efficient manner. This paper
intends to fill this vacuum in the scientific
knowledgebase and recommend an appropriate
road map in carrying out pertinent research in the
future.
INFORMATION ANALYSIS
Application of electrical stimuli in agriculture
Exposure of seeds, plants, soil, water or nutrients to
strong electric fields, electric currents and magnetic
techniques is known as electro-culture. Electro-
culture techniques have been claimed beneficial to
crops as it can protect the agro system from diseases,
insects and frost; reduce the use of fertilizer or
pesticides; grow better quality of crops; enhance
growth rate; and also reduce the cost (Gandhare and
Patwardhan 2014, Sharaf-Eldin et al., 2015). Various
types of electro-culture medium have been used so
far, including antennas, static electricity, direct and
alternating current, magnets, radio frequencies,
sounds, electric wires, natural batteries,
magnetically or electrically charged rock dust and
paramagnetic rock dust (Artem, 2012). The
experimental studies in this regard have been
started extensively in the 80’s, with the injection of
low amplitude currents (Hart et al., 1981) and
changing of dielectric properties of seedlings (Hart
1983).
Existence of external electric charge could induce
action potential that cause short term transient
changes to the critical life processes of plant such as
water uptake, respiration, photosynthesis and
growth (Yan et al., 2009). The action potentials will
show no response if the electrical stimulus is below
a threshold whereas it exhibits maximum response
if the stimulus is in between certain threshold
minima and maxima (Davies, 2004). Electrical
charges also cause significant influx of calcium ion
(Ca2+), a crucial growth and development regulator
in living systems including plants and fungi, across
the plasma membrane (Lew, 2011). However, as per
the basic observations, the response of the plants to
the external stimuli depends on its amplitude,
frequency, and intensity (Balasa et al., 2011).
Biology of mushroom: The growth development
The kingdom of fungi is different from that of plants
even though they are both eukaryotic; A plant cell is
developed through splitting of a mother cell wall
into two daughter cells whereas the development of
fungi is through hyphae (the basic structure of
fungi) that branches only at their apex and
formation of cross walls which, only occurs at
specific angles to the long axis of the hyphae (Moore
et al., 2011). Hence, the fungal morphogenesis
depends on the position of hyphae branches (Chang
and Hayes, 2013). The growth of polarized tip of
mycelial fungi is related to their endogenous electric
field and ion currents (positive) that flow through
and around the fungal hyphae as a result of the
polarized distribution of ion channels that pump
Fig. 1. a. Shiitake mushroom. b. Tiger-milk mushroom
1334 JAMIL ET AL
within the plasma membrane (Rajnicek et al., 1994).
However, the position of emergence and growth
direction of the branches is controlled by their
reaction towards one or more tropisms (Moore et al.,
2011; Sanchez et al., 2006). This process, the ability of
a plant to direct the outgrowth of neuronal
processes through the use of an extracellular electric
field, is termed Neuronal Galvanotropism. This
phenomenon has been subjected to scientific
investigation for nearly a century and the outcomes
show that the process could direct the formation of
both axonic and dendritic processes in cell culture.
However, exposure to exogenous electric field could
affect the formation, growth polarity (direction),
frequency of branching of the hyphae either towards
anode or cathode (Moore, 2005). This reaction has
been shown by a variety of fungi when their hyphae
responded galvanotropically to exogenous electric
fields. One of the fungal systems that have been
reported to be influenced by exogenous electric
fields is split gill mushroom (Schizophyllum
commune) where its hyphae orient towards the
anode (McGillivray and Gow, 1986).
It should be noted that mushroom is a group of
spore-bearing or spore-less fungi that has the main
attraction to people as food and medicine.
Mushroom morphology and chemical composition
are very complex and varies in a large spectrum
(Sher et al., 2010). The growth of mushrooms
depends on the presence of several physical and
chemical properties such as water-holding capacity,
air, pH and osmotic potential (Pardo et al., 2010).
Mushroom also requires the right amount of
moisture content, temperature and high cationic
exchange capacity (Ibekwe et al., 2008; Pardo et al.,
2010).
Growth of mushrooms and lightning
There are several claims by farmers that the growth
of mushrooms is related with lightning as the
occurrence of lightning within several tens of meters
from mushroom plantation area has caused
extraordinary growth enhancement to the
mushrooms (Rumack and Spoerke, 1994; Tsukamoto
et al., 2005). As per the educated guesses made by
the researchers, the electrical stimulation by the
charge and current from lightning on mushroom
hyphae accelerate the mushroom development
through two possible ways (Tsukamoto et al., 2005):
1) generation of more cracks in mycelium hyphae as
the mushroom fruit bodies are generated from the
crack and; 2) activation of mushroom enzymes by
the electrical stimulation.
Lightning, a flow of transient current from cloud
level, brings few tens of Coulombs of charge into
ground during the total event. In the phases of
return strokes, the main charge transfer processes,
impulse currents which approximately follow
double exponential wave profiles, flow from cloud
to ground. Figure-2 shows a first negative stroke,
computed by the current model proposed in Heidler
(1985), which is known as Heidler function now.
Although lightning impulse currents have
somewhat large range of parameters, based on both
the randomness and the types of lightning currents
(negative first stroke, negative subsequent stroke
and positive stroke), the typical range of values
could be several tens of kilo amperes of amplitudes
and several tens of microseconds of time duration
(IEC 62305-1 2010, Rakov and Rachidi, 2009).
The impulse current is most often followed by a
relatively small slow varying current, which is
termed the continuing current. These continuing
currents have magnitudes in the order of few
hundred amperes and may be as short as few
milliseconds to as long as few hundred milliseconds
(Rakov and Rachidi 2009). During the injection of
lightning currents to objects at ground level, based
on the impedance of the current path large potential
gradients may develop along the object. Similar
potential gradients may occur along the ground
paths where the lightning currents flow before the
charge is neutralize. These potential gradients may
reach even few Mega Volts per meter (MV/m) along
the path of lightning current flow. Apart from the
potential rise along the current path, the
electromagnetic interaction between the lightning
channel and metal parts in the proximity may
generate induced voltages as high as many hundred
kilo Volts (kV) (Rakov and Rachidi, 2009). These
high electric fields and potentials may be the cause
Fig. 2. A typical lightning current waveform that
represents the negative first stroke, computed
according to the Heidler current function
1335
Electrical Stimulation for the Growth of Plants: With Special Attention to the Effects of Nearby
of mushroom rapidly producing more cells as a way
of protection against danger from being harmed or
destroyed by the inrush of electric energy (Ryall,
2010). It should also be noted that a lightning strike,
too close to the mushroom bed could completely
destroy the entire system due to the extremely high
temperature rise and the shockwave. Lightning
channel is known to raise to temperatures in the
order of 30,000 C for a very short period. The high
current, potential gradient, arcing, heating effect,
shockwave and the intense light emission affect both
human beings and animals (Gomes, 2012).
Growth stimulation of mushroom by pulsed
electric means
Several researchers have applied electricity at high
voltage levels to stimulate the growth of several
types of mushrooms as it has been reported in the
literature (Tsukamoto et al., 2003; Islam and Ohga,
2012). They have employed pulse power technology
as it provides impulse type electrical signals that
approximately resemble lightning transients, at high
voltage levels and short period of time (both voltage
or current impulses). Application of these controlled
high voltage impulses to the cultivation system of
several crops like tomatoes, lettuce, strawberries,
and various flowers has improved the yield (Takaki
et al., 2014). Repeated exposure of plant with short
pulses (microsecond to millisecond) at high voltage
has resulted membrane permealization (Balasa et al.
2011). Permealization of the cell membrane at a
certain stage, together with electrophoretic
movement of charged species between cellular
compartments may cause lethal damage or
induction of sub-lethal stress. Induction of sub-
lethal stress is very useful for metabolic stimulation.
However, suitable conditions have to be considered
including pulse shape and polarity, number, width
and inter-pulse time in order to obtain effective
response (Galindo, 2008). High voltage electrical
impulses are known to cause alteration in the
electrical properties of plants that affect plant
growth, yield and some traits quality (Takaki et al.
2014). Yi et al. (2012) reported that steady state
electric pulses at 4 V to 10 V volts, which has been
applied on cultured soil, has given rise to better
response on the growth of lettuce and hot pepper
plants compared to the observations at 2 V electric
field pulses. According to the results and inferences,
the electrical pulses have activated geochemical
cycle and phosphate solubilisation in the soils that
indirectly promotes plant growth. Eing et al. (2009)
has revealed that exposure of Arabidopsis thaliana (an
edible flowering plant) to 5 MV/m (or 50 kV/cm)
electric field pulses has caused the seedling being
destroyed completely when the plant was treated
for 100 nanoseconds (ns) which was claimed as due
to the electroporation of the plasma membrane of
the plant cells. This shows that when the electric
field strength in the vicinity is very high (even in the
absence of injected current), the possibility of
destruction of the plant is much more probable than
any positive effect. This destruction may be a result
of arcing into the plant bed, as the probability of air-
insulation breakdown is very high for electric fields
above 3 MV/m, even if the electrodes are plane-
plane. It should also be noted that, the
electroporation at higher energy and longer pulses
causes the stimulating effect to be dominated by
necrosis.
In contrast to the above observations related to
plants, fungi have demonstrated different responses
and behaviours to electric stimulation. At the right
amount of electric energy, the yield of mushrooms
has been increased by even 100%. This has been
demonstrated when Marx-IES pulsed power
generator at voltages of 50 kV to 130 kV with a pulse
width of 100 ns (square voltage pulse) were applied
to logs of mushroom spores of nameko variety
(Pholiota nameko) and shiitake variety (Lentinula
edodes). Application of single pulse of 50 kV and 100
kV has increased the yield of nameko mushroom by
1.7 times compared to the control. The total weight
of shiitake fruit body has been doubled compared to
the control at single pulsed stimulation of a 50 kV or
100 kV voltage impulses. However, the yield was
decreased by about 25% with the treatment of 100
kV to 130 kV. Increment of number of pulses from
one time to fifty times at a voltage of 50 kV has
further doubled the yield of shiitake mushroom
(Takaki et al., 2010, Takaki et al., 2014). The same
effect of high voltage pulses on shiitake mushroom
has also been reported in another study where
application of one time pulse of 90 kV on logs of
shiitake spore has increased the weight of the
mushroom fruit body.
Application of 90 kV pulsed high voltages on
natural logs of shiitake hyphae at different treatment
frequencies have found that the spreading ratio was
slightly increased (55.9%) compared to that of the
control where the spreading ratio of 53.9%, after one
time treatment. The ratio remained the same (about
55%) after two time treatment. The spreading ratio
of the hyphae was further increased (to 61.2%) after
1336 JAMIL ET AL
the treatment for three times. Further investigations
confirmed that the growth rate of shiitake hyphae
could further be increased with higher treatment
frequencies (Tsukamoto et al., 2003).
One time pulsed voltages at 50 kV by a generator
termed Small Population Lightning Generator
(SPLG) on matsutake mushroom (Tricholoma
matsutake) has resulted in an increment of 67% to
69% of fresh weight and 65% to 113% of growth
length (Islam and Ohga, 2012). The SPLG produces
0.5 Joules of energy that generates impulses similar
in wave profile to that of lightning (Islam and Ohga
2012; Robbins, 2013). The stimulation has been
applied along the ground (Islam and Ohga, 2012).
There were some studies that used pulsed high
voltage to increase mushroom yield by stimulation
at mycelium stage in a sawdust block. The treatment
was done on matured mycelium just before the
fruiting stage (Ohga, 2012; Takaki et al., 2009;
Tsukamoto et al., 2003).
Application of single-pulse at 100 kV on
mycelium of fried chicken mushroom (Lyophyllum
decastes) through a 3 mm diameter and 7 cm length
of needle electrode at the centre of the sawdust
block has resulted to the increment of 10% to 30% of
the mushroom yield (Tsukamoto et al., 2003; Takaki
et al., 2009). There were also notable increments on
the yield of fruit body for some other edible
mushrooms compared to the control such as
Pleurotus ostreatus (88%), Pleurotus abalonus (80%),
Pleurotus eryngii (64%), Lentinula edodes (64%),
Flammulina velutipes (57%), Hypsizygus marmoreus
(59%), Agrocybe cylindracea (45%), Pholiota nameko
(25%) and Grifola frondosa (25%) (Ohga, 2012).
Electrical stimulation has been observed to cause
acceleration on the synthesis of clump connection in
fungi and activation of some enzymes such as
laccase and protease in several other studies as well
(Islam and Ohga, 2012; Takaki et al., 2014). As per
the conclusions of the respective researchers, the
increment of chemical levels of fungi may contribute
to the specific activation of cell division and that
helped to increase the weight and height of fruit
bodies of treated mushroom (Islam and Ohga, 2012).
Growth stimulation by other electric means
The available information, so far, depicts that the
ability of mushroom to boost their growth is due to
the presence of cracks on mushroom hyphae or due
to the activation of enzyme activity during lightning
strike. The limited information in the literature
emphasizes that high voltage pulses are the only
technique that could produce the same effects as
lightning on mushrooms in stimulating the growth
successfully. However, the absence of information
on the application of electricity by other modes (eg.
continuous or pulsed current injection into the
growing media at low voltage values) should also be
of concern. It is interesting to observe the response
of mushrooms to various methods that have
successfully been applied in plant growth/yield
enhancement. Electric current injection is one of the
electrical stimulation techniques that have been
used by several researchers in their efforts to
improve plant growth. The range of voltage that has
been applied varies from several Volts to few kilo
Volts in the form of direct current (DC) or alternate
current (AC). Figure 3 shows some efforts in
increasing the growth rate of mycelium in a petri
dish by applying small magnitudes of DC and AC
(in milli-ampere to few ampere scales) into the agar
medium through two wire electrodes (unpublished
data by Jamil et al., 2016). The results are yet to be
released.
Fig. 3. Attempts to enhance the growth rate of mycelium
by the application of small DC and AC into the
growing medium (agar layer) through two
stainless steel wire electrodes (Jamil et al. 2016).
Poole (2010) has found a growth increment up to
4.5% when 5 V DC voltage is applied for 7 minutes
to bean sprout in water. Application of 1.5 V DC
voltage for 40 days to sweet pepper seedlings has
resulted in 35% increment of its stem length
(Gabdrakhmanova and Qussiny, 2011). The effect of
these steady voltage may be related to the activation
of plant hormones that stimulates the seedling
metabolism and also related to the increment of
nutritional cation uptakes (Artem, 2012).
Another technique that has been used to
stimulate plant growth was through uniform or
1337
Electrical Stimulation for the Growth of Plants: With Special Attention to the Effects of Nearby
sinusoidal alternating electric fields. Exposure of
alternating electric field at 50 Hz on tomato seeds at
12 kV/cm for 15s exposure time has double its
germination rate (Eing et al., 2009). This information
justifies the ability of external electrical stimulation
to manipulate plant activities even at non-pulsed
field variation.
Experiments on other plants with electric fields in
the form of pulses reveal that it could also alter
metabolism processes that lead to modification of
cell activities (Balasa et al. 2011; Yi et al., 2012). Low
amplitude pulses may generate small pores of
plasma membrane (1.5 nm in diameter) that reseal in
a very short time (nanosecond). Larger pores (about
50 nm) are generated by stronger pulse electric field
and will be resealed slowly or left permanently
opened. This causes the reestablishment of vital
functions of the cell (Balasa et al., 2011). Opening of
pores allow the efflux and influx of polar molecules.
Resealing of the plasma membrane after treatment
has results to several responses such as energy
release from the movement of ionic species,
hydrolysis of ATP to rebuild gradients of charges
across cell membranes and other physiological
events (Galindo, 2008). Therefore, electrical
parameters through various techniques such as
current injection, electric field and corona discharge
for both short and long time exposures especially at
low strength should also be considered to look for
effective growth enhancement on mushroom. As
low voltage is more viable than high voltage
applications, the feasibility of upscaling the
successful techniques to commercially producible
level is also of concern to the industrial sector. This
should also be an important point of concern in
developing new methodologies for triggering
mushrooms growth in the future.
DISCUSSION
The nature of applied electrical energy
It is evident that the observation of the rapid
enhancement of mushroom growth following a
lightning strike in the vicinity or a thunderstorm in
the area has prompted the researchers to select the
wave profile of the applied electric stimulation. The
pulsed power signals could be treated as transients
that approximately resembles the lightning impulse.
It should be noted that lightning is a current
generator (an electrical generator which produces a
current of which the parameters remain the same
irrespective of the impedance of the path through
which it flows), which produces voltage waveform
along the object that it flows based on the electrical
properties of the object (known as the load). For
good conductors, such as copper tapes, the
resistance (R) along the current path may be as low
as 10-4Ω/m whereas for bad conductors such as dry
wood, the value may be as high as 106Ω/m. Note that
the actual value of resistance of an object depends
on its resistivity and dimensions. The voltage
waveform also depends on the self-inductance (L) of
the path which is in the order of 10-6 H/m for a
copper tape of typical cross-section. The value of
inductance does not vary much based on the
materials.
As the lightning current flows down an object of
height h, the potential (V) along the object is given by
the following equation.
di(t)
V = R h i(t) + Lh
dt
where i(t) is the time dependent lightning current
and di(t)/dt is the time derivative of the current. If
the lightning current enters a good conductor the
first part in the right hand side becomes negligible
and the second part with current derivative
dominates. Thus for one meter of the object the
voltage (voltage gradient) becomes about 50 kV/m
(or 0.5 kV/cm) with typical value of lightning
current derivative for first negative stroke. For a
very poor conductor the first term in the right-hand
side dominates giving rise to voltage gradients in
the order of 50 GV/m (500 kV/cm). Therefore for a
wet log or moist soil/saw dust, which is poor but not
very bad conductor, an electric field in the order of
5 - 50 kV/cm is quite representative of the lightning
environment, considered that only a partial
lightning current flows into the mushroom bed. For
a log or sawdust bag of length 10 cm, this electric
field represents a voltage of 50 – 500 kV. However,
considered the breakdown voltage of air, 30 kV/cm
or 300 kV for a 10 cm distance the 100 kV threshold
or 10 kV/cm is a viable voltage or voltage gradient
respectively to be applied to any plant/mushroom
bedding system to ensure no arcing in the vicinity.
The impulse voltage width of nano-second range,
commonly adopted in most of the studies, is a total
deviation from the characteristics of lightning
impulse voltage. Figure 4a shows the voltage
gradient calculated for a wet log of length 1 m (with
resistance of 10Ω) applied with the current
waveform depicted in Figure 1. It could be seen that
the time duration of voltage in this case is in the
order of few hundred microseconds (half width
1338 JAMIL ET AL
approximately 100 μs). Even for an object of quite
low resistance, say 0.01Ω, the voltage waveform will
last for over 20 μs (neglecting the low voltage tail)
with half value width about 8-10 μs (Figure 4b). For
objects with larger values of resistance the voltage
waveform follows the profile of the current
waveform as the ‘R h i(t)’ term becomes significantly
dominant over ‘L h {di(t)/dt}’ term. Therefore, if the
researchers plan to follow the lightning impact on
the growth enhancement of mushroom, it is
suggested that they follow the above waveforms.
However, it is reasonable to assume that the energy
content of the applied waveform should be much
less than that of the total-current-generated voltage,
as only a fraction of the lightning current may enter
a log of mushroom nearby. The fractioning of the
energy content of the lightning impulse could be
achieved in two ways. The first is to reduce the
impulse amplitude and the second is to shrink the
time duration. By considering nano-second range
time scale for impulse duration, the scientists have
adopted the second option so far. One may suggest
that the first option is more realistic as plant
metabolism processes take place in the time scales
much larger than nano-second scale, thus micro-
second scale impulses may be more effective. The
following equation may be used to scale down a
nano-second scale square impulse of voltage V0 to
microsecond scale square impulse of voltage V1
with same energy dissipated in an object of same
resistance value.
Fig. 4. a. Time dependent voltage generated during the
passage of lightning current waveform given in
Figure-2, along a wooden log of resistivity 10 (a
wet wood). b. The same simulation for a log of
wood having resistivity 0.01 (a hypothetical
material)
V1 = V0 = T0
T1
√
Where T0 and T1 are the time durations of nano-
second scale and micro-second scale square
waveforms respectively. For an example, an impulse
of 50 kV magnitude and 10 ns duration becomes a
waveform of magnitude approximately 0.8 kV at 40
μs.
If it is required to calculate V1 in such a way that
the energy of the square waveform at micro-second
scale is a fraction of the energy of the lightning
impulse, then the following equation could be
applied.
Where τ is the time duration of the lightning
impulse and F is the fraction of the lightning current
assumed to be entering the mushroom bed.
Typically, F could be taken as 0.01. Figure-5 shows
the square waveform of width 40 μs having 1% of
the energy dissipated in the same resistor by the
voltage waveform given in Figure 4a.
Effects of injected voltage impulse
Several biological effects of the applied impulse that
may enhance plant growth has been discussed in the
previous sections. As the current due to the applied
voltage pulses grow along the growing media rather
than through the plants itself, it is reasonable to
expect that the changes observed in plant
Fig. 5. A square voltage waveform that generates the
same energy per resistance as that is generated by
the waveform given in Figure-4a.
1339
Electrical Stimulation for the Growth of Plants: With Special Attention to the Effects of Nearby
morphology may be caused by the strong transient
electric field generated in the vicinity of the current
path. Figure 6 shows a simulation done in ANSYS-
Maxwell software which shows the electric field
distribution along a long of wood as the current
waveform given in Figure-1 is applied axially into it.
The height, diameter, resistivity and relative
permittivity of the log has been considered as 1 m,
0.1 m, 100 & Ωm and 2 respectively. It can be seen
that the electric field strength on the surface reaches
to about 360 kV/m during the passage of impulse
current. The effects of such high electric fields on the
plant or mushroom morphology has not been
studied in details yet.
mushroom to grow faster. A detailed study on the
composition of ionised medium needs to be
conducted to fully understand the effects of
ionization on the mushroom growth.
NOx generation: The generation of large amount of
thermal energy during the passage of the lightning
current triggers many chemical reactions out of
which the fixation of atmospheric nitrogen plays an
important role.
During the return stroke and continuing current
phases of the lightning, thermal dissociation of
carbon dioxide occurs leading to the formation of
atomic oxygen and carbon monoxide.
CO2 → O + CO
In the presence of a third body M some of these
oxygen atoms recombines to form molecular oxygen
O + O + M → O2 + M
Some oxygen atoms, at high temperatures
(typically above 1500 C) associates with molecular
nitrogen to form NO. The production of NO
enhances in the presence of CO2.
O + N2 → NO + N
N + CO2 → NO + CO2
In the presence of atmospheric ozone, NO is
converted to NO2
NO + O3 → NO + O
A fraction of this NO2 dissociates into NO by
releasing energy.
NO2→NO + CO2
In the presence of a third body, a part of NO2
associates with OH to form nitric acid which
dissolves in rain water and reach ground.
NO2 + OH + M → HNO3 + M
(The above equations have been adopted from
Mvondo et al., 2001 and Drapcho et al., 1983). This
HNO3 provides the required nitrogen based
nutrients for plants. It is of interest to investigate
quantitatively, the correlation between the amount
of NOx received by a bed of mushrooms due to a
nearby lightning and the growth rate enhancement
of the relevant bed. If the distance and the current
parameters of the lightning strike could be obtained
(by interferometric techniques or by a sensitive and
accurate lightning detection system), an empirical
formula could be developed to relate the lightning
charge, distance and growth rate.
In addition to NOx, the presence of Ozone, a
product emitted during the lightning process as per
the reactions given above may affect the
biochemistry of plants. Ozone may penetrate the
leaves of trees in the proximity through stomata
during normal gas exchange. Being a strong oxidant,
Fig. 6. The electric field distribution as the lightning
current (given in Figure-2) enters a log of wood of
resistivity 100 & Ωm
Other effects of lightning that may influence the
growth enhancement
Although the popularly observed growth
enhancement of mushroom due to lightning (note
that still there are no published scientifically proven
evidence for such) is attributed to the electric fields/
potentials generated by the lightning current, one
should not overlook other possibilities as well. The
most prominent such lightning caused effects are
the ionization of the growing medium, NOx
generation, emission of ultraviolet radiation and
corona emission.
Ionization of the growing medium: Usually the
growing medium of mushroom is soil, clay or
wood. The passage of large transient current
through this medium changes the chemical and
electrical properties of this medium due to thermal
ionization (Ala et al., 2009; Burhanuddin et al., 2016,
Gonos and Stathopulos 2004). Sometimes, the
medium may even form fulgurites or fulgurite like
material which has totally different physical
structure to the original material (Burhanuddin et
al., 2016). The new material formed may provide
better growing media and nutrients for the
1340 JAMIL ET AL
ozone may give rise to several plant anomalies such
as chlorosis and necrosis (Felzer et al., 2007; Krupa et
al. 2001; Matyssek et al., 2008). Additionally, there
may be other plant health issues as well such as
flecks, stipples, bronzing, and reddening of the
leaves which deteriorate the metabolism of the cells
(Ainsworth et al., 2008; Booker et al., 2009). Similar to
the case of NOx, in this regard too, no research has
been done so far to find the effects of O3 on the
growth rate of mushroom, although such studies are
numerous with respect to plants and other living
organisms.
Emission of ultraviolet radiation: In the chemical
reactions taking place in the lightning current
channel, energy is released most often in the form of
electromagnetic radiation. For an example in the
reaction where NO2 dissociates into NO and O2,
energy is released with wavelength less than 420
nm. This is the violet and UV-A range. As per the
extremely high temperatures (about 30,000 C) that
has been observed in the lightning channel during
the return stroke phase, even UV-B and UV-C
radiation (with smaller wavelengths) is also emitted
into the space. It has been shown that high levels of
UV radiation (especially UV- (B) threatens the well-
being of plants as the radiation is capable of
damaging DNA, proteins, lipids and membranes
(Frohnmeyer and Staiger 2003, Hollósy 2002,
Stapleton, 1992; Zuk-Golaszewska et al., 2003). Thus,
the threat of extinction may prompt the plants to
enhance the growth rate and reproduction.
Interestingly, no research has been done to
investigate the effects of abnormally high exposure
of mushrooms to UV light (at least UV-A). Such
study could bring vital information on the growth
enhancement of mushrooms by non-electrical and
non-chemical means.
Corona Emission: The emission of charged fluid
particles (gases or liquids) from sharp points in the
presence of a high electric field is termed corona.
The presence of both the overhead thunder cloud
and the nearby lightning strikes may give rise to
partial discharges in various objects including the
tips and edges of plants, which produce corona. For
a long time this corona produced is known to the
scientific community as an effective chemical
catalyst (McMahon, 1968). The corona plays this role
by generating numerous free radicals that mediate
chemical reactions. Out of many chemical changes
that corona may make, only the effects of ozone
produced by corona on the plants have been studied
so far. Thus, it is another area that needs to be payed
attention with regard to both plants, in general and
mushroom.
Economic viability of growth enhancement due to
electric stimulation
A new technology or techno-methodology may give
rise to high yield in production output at laboratory
level. However, it could be treated as a successful
commercial outcome only if the set up could
adopted feasibly at mass production level. The
analysis of the methods adopted by many
researchers in increasing the yield of mushrooms
raises doubts on the practicality of equipment and
conditions needed for the observed output
enhancement, at mass production level. A custom
made impulse generator of nanosecond width pulse
trains needs high voltage supply and the operation
should be done in an EMI-free environment. Such
equipment may cost nearly USD 100,000 at current
rates in the market, which will be prohibitively high
capital investment for the farmer. The limitation of
the number of mushroom beds that can be treated in
parallel by such equipment is another constraint that
is added on the issue. The equipment also need
operators specifically trained for the purpose. All
workers in the farm and outsiders who may visit the
premises should be given strict advises on safety
requirements. The mechanical and electrical safety
of the equipment, which should continuously be
operated in the field is another concern.
Application of a small DC or AC signal to the
growing medium may be more practical, if such
method shows reasonable productivity
enhancement. Through a mesh network of electrical
connection, a medium size power supply could
provide current injection to a large number of
mushroom beds without raising any significant
safety issue. However, such technique is not yet
proven to be successful in enhancing the growth or
fruiting notably.
It is of interest to check the response of different
stages of mushroom to nearly uniform electric fields
such as that due to charged cloud overhead. If
corona generation has any positive impact on the
growth rate enhancement or increase in yield of
mushroom, a uniform electric field would provide
successful results. Such nearly uniform fields could
be produced by a Van der Graaf generator with
large dome. In contrast to other high voltage
generators Van der Graaf generators provide less
lethal static electricity. As the generator and plant
bed does not need resistive coupling, a single
1341
Electrical Stimulation for the Growth of Plants: With Special Attention to the Effects of Nearby
generator could provide required electric field
environment for many plant beds. In such
environment, passive corona generators could be
implemented by placing spikes or multi spike metal
components close to the plant beds while the Van
der Graaf generator is in operation. This concept is
pictorially represented in Fig. 7.
It will not be that profitable to apply
sophisticated electrical stimulation techniques to
enhance the growth rate or yield of common type
mushroom which has low cost per unit mass in the
market. Thus, it is advisable to concentrate the
research copes in this regard to more expensive
mushroom varieties with decent demand at the high
end of the potential customer base. In this respect, a
survey that has been done in the Malaysian market
recently revealed that instead of focusing on for-
food mushroom, investigating on varieties which
have medicinal value, such as tiger’s milk
mushroom, could bring much higher returns on
investment (Jamil et al., 2015; Lai et al., 2013). In the
case of edible mushrooms too, it would be more
advantageous to research on the growth
enhancement of varieties that have high demand as
exotic cuisine; matsutake, blue foot, golden
chanterelle, nameko, maitake etc.
Another important aspect of artificial electric
stimulation of mushroom that has been overlooked
so far is the impacts of environmental adaptation of
the species on its response to the proposed method.
Mushroom may positively respond to the electric
stimulation under well controlled environmental
conditions inside a laboratory, however, once the
method is applied to the same species grown under
exposed atmospheric conditions, the response may
be different. Recent research has revealed that many
plants adapt themselves to the varying climate
conditions either in short term or in long term, thus
their reactions to even soil nutrients may be changed
(Hall et al., 2016; Lange et al., 2016; Rossi 2015). This
is an important factor to be investigated before
investing on the proposed methods for production
enhancement of mushroom.
CONCLUSION
Short exposure time with low amplitude current
injection, electric field and pulsed electric field have
successfully enhanced the growth rate and yield of
many plants such as tomato, strawberry, lettuce and
flowers. For mushroom however, the improvements
so far were only due to high voltage electric pulses
that have been applied to the growing medium at
different stages of mushroom development. This
high voltage technique has been adopted following
the observations done by farmers on the growth
enhancement of mushrooms during thunderstorms.
However, further investigations are required to
determine the quantitative response of mushrooms
on several available electrical stimulation
techniques, especially at lower voltage levels, where
commercial scale application is feasible.
ACKNOWLEDGEMENT
We would like to thank National Institute of
Biotechnology Malaysia and Universiti Putra
Malaysia for the facilities provided to complete this
study. The IPS grant number GP-IPS/2016/9506500
and FRGS grant number 5540014 through which this
project has been funded are greatly acknowledged.
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