Dorothea Samtleben- The Cosmic Microwave Background Radiation
Microwave Radiation and Plants
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The Effect of Externally Applied Electrostatic Fields, Microwave Radiation and ElectricCurrents on Plants and Other Organisms, with Special Reference to Weed ControlAuthor(s): M. F. Diprose, F. A. Benson and A. J. WillisReviewed work(s):Source: Botanical Review, Vol. 50, No. 2 (Apr. - Jun., 1984), pp. 171-223Published by: Springeron behalf of New York Botanical Garden PressStable URL: http://www.jstor.org/stable/4354034.
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T H
BOT NIC L R V I W
VOL.
50
APRIL-JUNE,
1984 No.
2
The Effect of
Externally
Applied Electrostatic
Fields,
Microwave Radiation
and Electric Currents on
Plants and other
Organisms, with
Special
Reference to Weed Control
M. F.
DIPROSE,
F. A.
BENSON
Departmentof
Electronicand
ElectricalEngineering,University f
Sheffield
Mappin
Street, Sheffield,SI 3JD United
Kingdom
AND
A. J. WILLIS
Department
of Botany, University f Sheffield,
WesternBank
Sheffield,
S1O
2TN
UnitedKingdom
Abstract
.----------------------------------------172
Sommaire.------------------------------------------------173
I. Introduction
..14...
..............
....
174
II. Definition
of Forms
of
Electrical Energy
..
. 175
A.
Electrostatic
Fields
.---------------
.----.--..--..-----175
B.
Microwave Radiation
.
176
C.
Electrical
Discharges and Direct Electric
Shocks
.
177
III.
Electrostatic
Fields
and
their Lethal Effects
on
Plants
..-------------------------------------------
77
A.
Introduction
..----------------------------------------------------------------177
B.
Plant
Growth
in
the
Presence of
Electric Fields .....
178
C. The
Lethal Effects of
Electric Fields on
Plants
.
.
179
D
.
C
onclusions
..18------------------------------------------------------------------------------------------------------86
(i) Summary
186
(ii)
Practical Weed
Control
by
High
Electric
Fields
........................................ ......
187
IV. Microwaves and
Weed
Control
...---------------------------------187
A.
Uses of
Microwaves in Agriculture
---------------------------------........-...............
187
B.
Laboratory Experiments with
Microwave Radiation
on Plants and Seeds 190
Copies
of
this issue
[50(2)] may
be
obtained from:
Scientific
Publications
Office,
The
New York Botanical
Garden, Bronx,
NY
10458 USA.
PRICE
(Includes
postage
and
handling
fee. Valid until 31 December
1984):
U.S.
Orders:
$10.75.
Non-U.S.
Orders:
$11.50.
(Payment,
U.S.
currency
only
and either
drawn
on
a U.S. bank or
made
out
by
intemational
money order,
should
accompany
order. Thank
you.)
The
Botanical
Review
50: 171-223,
April-June, 1984
171
?
1984
The
New
York Botanical
Garden
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172
THE BOTANICAL
REVIEW
C. Field Experiments
with
Microwaves for W eed Control
................................................96
D. The Effect of
Microwave
Radiation
upon
Soil
Microorganisms
and
N em ato des ..----
--- --
---
---
--
---
--- -- ---
--- -----
--- ---
--
--- ---------- ---
202
E. Conclusions ..204------- . ................. 204
(i)
Summary.
-------------------------------------------
-----
204
(ii)
Thermal or
Non-thermal Mechanisms
of Death 206
(iii)
Practical
Weed Control
by
Microwaves .................
207
V. The Effect of
Electric
Currents
Applied Directly
to
Plants
and Soils
.
209
A.
Soils,
Plants and
Applied
Currents
.
.....
209
B. Weed Control
by
High Voltages
...........
212
C. Conclusions
.............................................................
215
(i) Summary
----------------------------------------------------215
(ii) Practical Weed Control by Electric Discharges
and Currents
......
216
VI. Acknowledgments ................ 217
VII. Literature Cited.
-----------------..----------------------------------------------------------------------------
17
Abstract
A
wide-ranging
review is
presented
of
the effects
of
various forms of
externally applied
electrical
energy upon plants
and
other
organisms.
Al-
though investigations involving
both
small and
large
amounts
of
energy
directed at the targets are considered, a particular emphasis of this review
is the feasibility of each
type
of
electrical stimulation
for weed
control.
Electrostatic fields
ranging
from 100
V
m-I
to 800
kV
m-I
have been
applied
to
plants
under
laboratory
conditions
and
in
field trials since the
1880's.
Some
beneficial effects have
been
reported (e.g. increase
in
yield
from both cereal
and vegetable crops), but the results have been erratic
and
the
electrical
conditions
leading
to definite benefits on a
large scale
could not be
confidently predicted
from
early studies. High electric fields
are
reported
to
damage plants
if
currents
greater
than 10-6
A
are induced
to flow through leaves causing corona discharges from the tips. The nature
of the
damage
and
the
effects on metabolic
processes are discussed. The
results from
experiments on the growth of plants
in
which the density
and
charge
of air ions have
been varied are
also reviewed.
The
effects of microwave
radiation
(mostly 2450 MHz) upon seeds,
plants
and
other
organisms
in
soil are discussed. These effects depend
upon
the
power
density
of the
radiation and the electrical properties of
the
targets.
Factors such as
size of seeds
and plants, shape and moisture
content are important, as are the properties of the soil irradiated (notably
water content).
Although microwaves can be effective in killing plants
and also
seeds that are buried
several centimeters deep in soil, high power
equipment
is
required and
treatment times are long e.g. a 60 kW machine
could take
up
to
92.6
hours
per hectare. Other experiments reported show
that microwave
radiation can kill
nematodes in the soil and that it is also
very effective
in
killing fungi and bacteria. The potential of the various
possible
uses of
microwave radiation in agriculture is also described.
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ELECTROSTATIC
FIELDS,
MICROWAVE
RADIATION
173
Electric currents
have been
caused to
flow
throughplants
by
the
ap-
plication of
electrodesto
the
leaves. The effects
range
from
nil,
when
50-
100 V and 1 or 2 ,uAare used, to very strikingwhen voltages from 5 to
15 kV
areapplied
causing
currents
of several
amperes
o flow and
resulting
in therapid
destructionof
the
target.Smallelectric
currents
passed
through
soil
containingplants
are
reported
o increasetheir
growth.
The
effects
of
small current on the
growth
of
individual leaves are reviewed.
The
use
of
high
voltage
tractor-borne
quipment
for
weed control is
also consid-
ered.
Sommaire
Une revuea
larges
themes
presente
les effets
des diverses
formes
d'ap-
plication externe
d'energieelectriquesur les
plantes
et
autres
organismes.
Bien
que
desrecherches
omportant
a
la fois de
petites
et
grandesquantites
d'energie
dirigees
sur les cibles en
question
soient
prises
en
consideration,
un
des
aspects
particuliers
de
cette
revue est
la
possibilite
d'application
de
chaquetype de
stimulation
electriqueau
controle
des
mauvaisesherbes.
Depuis
environ
1880,
les
plantes
ont ete
soumises,
soit en
laboratoire,
soit lors d'essais sur le terrain, a des champs electrostatiquesallant de
100
V
m'
a 800
kV m'.
Quelques effets
benefiques
ont ete
enregistres,
par
exemple, l'accroissement
de
la
production
des
recoltes
de
cereales
et
de
legumes; mais les
resultatsetaient
irreguliers
et les
conditions
elec-
triques
conduisanta des
profits
bien
determines
a grandeechellene
pou-
vaient
pas
etre
predites avec
confiance
des
premieres etudes.
On s'est
apercu
que
de
grands
champs
electriquespouvaient deteriorer
es
plantes
si des courants
superieurs
a 10-6 A
etaient
amenes
a
circuler
dans
les
feuilles, entrainant
des
dechargesde la
couronne
a
partirdes
pointes. La
nature des degats ainsi que les effets sur les procedes metaboliquessont
ici
etudies, de
meme
que les
resultats
des
experiences sur
la
croissance
des
plantes
pour lesquelles la
densite et
la
chargedes ions
dans
I'airont
ete
changes.
Les
effets du
rayonnement
par
micro-ondes
(pour
la plupart
de
2450
MHz)
sur
les
graines,
les
plantes
et
autres
organismesdu sol
sont
ici
exposes. Ces
effets
dependent
de la
densite electriquedu
rayonnementet
des
proprietes
electriquesdes
objectifs.
Des
facteurstels
que la
taille des
graines et des plantes,leur forme et leurteneuren humidite sont impor-
tants, comme le
sont les
proprietesd'un sol
irradie
(notamment
sa
teneur
en
eau).
Bien
que les
micro-ondes
puissent
etre efficaces
pour
tuer des
plantes et aussi
des
graines
enterreesa plusieurs
centimetres de
profon-
deur,un
important
appareillage
electrique est
necessaire
et
les
temps de
traitement
sont
longs; par
exemple une
machine de 60 kW
peut
prendre
jusqu'a
92,6
h
ha-'.
D'autres
experiences
demontrerent
que le
rayonne-
ment
par micro-ondes
pouvait
tuer les
nematodes
dans la terre
et qu'il
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174
THE
BOTANICAL
REVIEW
etait tres
efficace
pour
detruire
es
champignons
et les
bacteries
du
sous-
sol.
Le
potentiel
des
diverses
utilisations
possibles
de la
radiation
par
micro-ondes en agriculture st egalementdecrit.
Des courants
electriques
ont
ete
amenes a
circuler
a
traversdes
plantes
par
l'application
d'electrodes
sur
les
feuilles. Les
effets,
nuls
quand
50
a
100
V
et
1
a
2
,uA
sont
utilises,
sont
par
contre
frappants
quand
des
voltages de 5 a
15
kV
sont
appliques,entrainant
a
circulationde
courants
de
plusieurs
amp'eres
et la
destruction
rapide
des cibles. On
remarque
cependant
que,
de
petits
courants
electriques
envoyes
dans un
sol conte-
nant des
plantes, accelerent
eur
croissance;
es
effets d'un
faible
courant
sur acroissancedesfeuilles ndividuellessontici reexamines.L'utilisation
d'un
appareillagea
haut
voltage
mobile
pour
le
controle
des
mauvaises
herbes
est aussi
pris
en
consideration.
I.
Introduction
Mechanical
methods
of
removing
weeds,
e.g.
hoeing
and
tilling, have
been
practiced
for
centuries
(Crofts,
1975).
The
use of
chemical
methods
for weed
control
has
been
expanding
at
a fast
rate
since the
mid
1940's
(House,
1967).
In
recent
years electrical
methods have
been
investigated
in
this
connection,
some of
which
promise
to be of
practical
value. This
review
discusses
the
control of
weeds,
and
also of
plant
growth
more
generally,
by
means of
such
methods.
Weeds can
be
subjected o
electrical
energyby use of
electrostatic ields,
microwaves,
electric
discharges
or
direct
electric
shocks
using either
al-
ternating
current
(a.c.)
or
direct
current
(d.c.). These
techniques
have
several
advantages
over
present
methods of
weed
control
and,
although
they cannot be regardedas a panaceafor all problems,electricalmeans
appear to offer
viable
and useful
additions to
the
stock
of
weed
control
methods.
During
and
following
the
applicationof an
electric
shock
or
microwave
or
laser
irradiation, he
energy,
rapidly
absorbedby
the
"load,"
is
largely
dissipated
as
heat
in
the
plant.
No
residue is
left to
contaminate
the soil,
an
important
feature
n
view of
rising
concern
about
the
contributionby
herbicides to
environmental
pollution.
The
electrical
energy
can also
be
directedto where it is required,even underfairlyadverseweathercon-
ditions.
Electrodes
carrying he
currents,
or
the
radiation
being
directed
towards
the
ground
are,
unlike
sprayed
herbicides,
not
blown
by
winds,
so
areas
can be
treated
under a
wider
range
of
weather
conditions than
previously
possible.
Microwave
radiation
penetratesthe
soil
to a
depth
of
several
centimeters
depending
upon
the
applied
power,
its
wavelength,
and the
composition
and
moisture
content
of the
soil.
This
microwave
treatment
may
prove
better
than
the
use of
flame
guns
for
destroying
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ELECTROSTATICIELDS,MICROWAVE
RADIATION
175
weed seeds of the
type
that
can
withstand
high temperatures, .g.
as
great
as
1
270C
f
slightly
below the surfaceor if in cracks n the soil
(Sampson
and Parker,1930). Microwavepower would reach these seeds and still
be as effective
in
killing
them as if
they
were on the
surface.
While electrical
methods
of
weed
control are a
relatively
recent inno-
vation, some
forms
of electrical and
electromagneticenergy
have
long
been
known to be
essential
in
agriculture-notably electromagneticra-
diation
in the visible
wavelength region
(Borthwick, 1965)
necessary
for
photosynthesis.Other
plant processes
in which
light
is involved
include
photoperiodismandgermination.
Furthermore,higherfrequency
electro-
magneticwaves such as y-rays may be importantin evolution, leading
to the formation
of
new
species, by
causing
mutationswhen absorbed
e.g.
by seeds
(Nelson, 1965).
The naturalelectricalstate
of
the
atmospheremay
be
important
o
plant
growth (Wheaton, 1970). The air contains both
positive and
negative
ions,continuouslyprovidedby
radioactivedecay of elements
n
the earth's
surfaceand
some
cosmic radiation
(Chalmers, 1967).
Ion densities
vary
throughout
he
day
but
over
open
land are
in
the
order of 1.5-4.0
x
103
ions cm-3.
Kotaka and Krueger 1968) have examined the effectsof
vary-
ing ion densities
of air
on
the growth of
barley, oats and lettuce. They
report that increases
n
ion density lead to
vigorous, accelerated
growth.
Alternativelyplants exposed to
ion-depleted atmospheres ack vigor
and
have soft
leaves (Kruegeret al., 1965).
The effects of electrical
energy are therefore
mportant
to
plant life
in
many ways and
electromagnetic adiation s vital to it. For weed
control,
however, larger
than normal applicationsof electrostaticfields,
electric
discharges,and
microwave radiationto plants
are necessary.Thesetreat-
ments arereviewed below, following a short explanationof the technical
definitions of the
various
forms
of electrical
energy.
II.
Definition
of
Forms of
Electrical Energy
Small electric
potentials
exist
across the
membranesof plant
cells and
within whole
plants. These can give
rise to potentials of 200-300 mV
betweendifferent
parts
of the
plant (Lund,
1947). Such naturallyoccurring
phenomena
are
not
considered
n
this
review which deals with the various
waysin whichplants can be subjected o electricalenergy rom an
external
source-typically many
orders of
magnitudegreater han internally
gen-
erated
electrical
effects.
II.A
ELECTROSTATIC FIELDS
An
electrostatic field is an
electric
field that is static
in
time, i.e. its
strength
does not
vary through
time
(Nussbaum, 1966). The field can be
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176
THE
BOTANICALREVIEW
formedby placing
two
conductors
apart
from each other
(they
need
not
necessarilybe parallelor
both
planar)and connecting
them to a
voltage
source.
The
electric
field
strength
for
parallel planar
conductors is de-
fined as
V
E
=
d
where E is the electric field strength
n
the air gap between the
electrodes
(conductors)
measured
in
volts
per meter,
V
the
voltage applied
to the
conductors
n
volts,
and
d the distance between
them
in
meters. For the
parallel-plateconfiguration he field is uniformbetween the conductors,
but
differentexpressions
exist
to
describe other more
complex
non-uni-
form
field patternsthatarise,
for
instance,
with one
planar
electrode
(e.g.
earth)
and an overhead
wire,
or when a
plant
is
introducedbetween
two
parallelplates.
II.B
MICROWAVE
RADIATION
Microwave radiation s the name given to
electromagnetic adiationof
which the wavelengthsare
much
greater
han
those
of
light,
i.e.
they
are
measured
n
mm
or cm
(Glazier
and
Lamont, 1958).
A
wavelength
com-
monly
used
in
weed control s 12.25
cm which
corresponds
o a
frequency
of 2450
MHz.
Most of the
papers
referred
o
in
this review that describe
microwave
experiments
use this
frequency.
This is one of
the
frequencies
allocated, by
international
agreement,
to
microwave
power
devices
for
domestic and
industrial
applications.
A
wide
range
of
equipment
s
com-
mercially
available which
produces
the
necessaryradiation powers.
The
power absorbed,
Pabs,
by an irradiated dielectric load which is converted
to
heat
is
given by
Pabs
=
0.556fE2Er"
x
10-10
Wm-3
where
f
is the
frequency
n
Hz;
E
the
field strength
of
radiation at point
of
absorption
n
Vm-
1; r"
is the relative
dielectric oss factor(Nelson and
Wolff, 1964; White, 1970).
Thus
the power absorbeddepends upon several
factors ncludingprop-
erties of the load itself such as the relative dielectricloss factor,and the
homogeneity of the medium. The electric field
strengthat any point in
the
load
depends upon
the
load
shape and the
dielectric
properties
of
the
medium,
as well as the
strength and frequency of the irradiating ields.
The radiation
penetratesall parts
of
the load
simultaneouslywhich is
in
contrastto the normal
heating process where heat
is conducted from the
outside
of
the load towards the
center. The
speed
of action is
one of the
main
benefits of microwave radiation.
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ELECTROSTATICIELDS,MICROWAVE
RADIATION
177
II.C ELECTRICAL
DISCHARGES
AND DIRECT ELECTRIC
SHOCKS
Electricdischarges n the context of this review are takento mean the
discharge
of
a
quantity
of
electricity
into
the
plant
from
a
high voltage
electrical
energy device,
usually,
but not
necessarily,
a
capacitor
(Duffin,
1965). When the electric
charge
s
pulsed,
the
generator
has to
chargeup
a capacitor,and when
that
operation
s
completed
the
energy
s
transferred
to the load. Repetition
rates
depend upon
the
generatorsize,
the
storage
capacitance
and the
properties
of the load under
discharge.
Russian re-
searchers
have
used
this method
with
up
to 60
kV
pulses
of 1
As
duration.
The discharge
electrodes need
not
necessarily
touch the load
if
very high
voltages are used, since
the
discharge
can
jump
across
an
air
gap.
Alter-
natively, a pulsed high voltage
discharge
can
be
applied
from
the
sec-
ondarywindingsof a high
voltage
transformer,
without
using
a
capacitor,
by
a
suitable
control of the
input voltage
to
the
primarywindings.
Direct electric
treatment is a continuous
process whereby
a
generator
is
physically
connected to the
plant by
two
electrodes,
or
by
one
electrode,
the
ground
being
used as the other. The
voltage
can
range
from 500
V
to
several
kilovolts as
required,
and can
be
a.c.
or d.c. The
current,
I
(A),
flowing throughthe plantis determinedby its electricalresistance,R (Q),
and
the
applied
voltage,
V
(volts),
so
that
V
R
The
units of alternatingvoltage and
current
are
volts
or
amps r.m.s. The
latterterm standsfor"rootmean
square"and describes
heeffectivevalue
of
a
voltage
or current
waveform that varies
continually
with time
(e.g.
an alternatingcurrentwhich has the same effecton a load as a 1 A direct
current
has
a value of
1
A
rms).
III.
Electrostatic Fields and their
Lethal Effects on
Plants
III.A
INTRODUCTION
A
number
of
studies
of the
effects of electric fields on the
growth of a
range
of
plants
have been
made. The Great Plains area of
the United
States sometimes suffersfrom severe dust storms, in which the normal
electrical
balance of the
atmosphere
is
considerablydisturbed and very
high electric field strengths
exist near the
ground level. Miller (1938)
reports that, after these
storms, damage
observed in some of the crops
was attributed
to electrical
action. Shlantaand Moore
(1972) have also
observed
damage (browned
eaf tips) in the
"natural" rassof a mountain
meadow
in
New
Mexico
after
storms. Suchobservations
have led to the
investigationof the
possibilities of a "lethal
electrotropism"n plants and
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178
THE
BOTANICALREVIEW
to
the
demonstrations
that
growth
can
be
stopped
and leaves and
whole
plants killed
by
higher
electric
fields.
III.B
PLANT GROWTH IN THE
PRESENCE OF
ELECTRIC FIELDS
Earlyresearch
suggested
hat the
effects
of
subjectingplants
to
electric
fields were beneficial.
This
topic
has been the
subject
of research
and
debate
for
many years(Ellis
and
Turner,1978;Pohl,
1977;
Sidaway, 1975;
Wheaton,
1970),
but the
beneficial effect
is
not now
generallyaccepted.
Fromthe mid
eighteenth
o
the
early
twentieth
century,
indings
ndicated
that increasedyields from both cereal and vegetablecropscould be ob-
tained by
applying electrostatic
fields to
plants
while
they
were
growing
(Hendrick,
1918; J0rgensen
and
Priestley,
1914;
J0rgensen
and
Stiles,
1917;
Newman, 191 1; Shibusawaand
Shibata,
1930).
Between 1885
and 1903
Lemstrom(1904) undertook
various
experi-
ments on the
effects of
electrical fields on
plants
in
different
places
in
Europe-from Finland
and Sweden
n
the
north,
to
England
and
Burgundy
in
the
south.His
results,
while
encouraging,
were not
very
consistent
(one
factor
being
the
unreliability
of
earlyhigh
voltage
equipment).
His
general
conclusion was that electricfields, appliedto cropsby wiresstrungabove
the
growing
areas,
were
beneficial,
producinghealthy
plants
with
increased
yields.
This applied
to root
crops,
vegetables,cereals and
strawberries.
He
proposed
that the best
times for
applying
the
electrostatic
field
were
for
four
hours
in
the
early
morning
and
another four
during
the
late
afternoon.
The
electricity
could be
applied
all
day
duringcloudy weather
and
during
nights
of
moist weather.
Application
duringdry conditions
and
in
strongsunshine,
however,
could have
adverse effects.
Lemstrom'sstudies were extendedby severalworkers.In general,the
same
encouragingresults were
obtained.
Blackman's
(1924) field
trials
with
cereals and
clover-hay
between
1917
and 1920
produced 18
sets of
results of which
14
showedincreased
yields;
9 of
these increases
were of
at
least
30%.
In
pot-culture
experiments
carriedout on
maize,
wheat and
barley,
the
average
yield
increase,
when
these
cereals
were
subjected to
weak
electric
currents,
was over
11%
(Blackman
and Legg,
1924). The
Board
of
Agriculture
and
Fisheries of
the
U.K. even
established a
Com-
mittee forElectroculturen 1918;however, its finalreport n 1937 (Board
of
Agriculture
and
Fisheries,
1918-1937)
stated
that the results
of 18
years
of
research
were not
conclusive
and it
was not
possible
to predict
confidentlythe
benefits of
electrical
treatmentsto
crops.
The
encouraging
results of the
European
workers
were not,
however,
shared
by
their
American
colleagues,who, in
two
fairly
comprehensive
sets
of
controlled
trials, found no
significant
beneficial
effects of electric
fields
on
growth
(Briggs
et al.,
1926;Collins et
al.,
1929). In view
of these
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ELECTROSTATIC
IELDS,MICROWAVE
RADIATION
179
findings
and the
inconsistency
of other
results,
the
use
of
electricaltreat-
ments of plants lost
favor.
Apart
from
a few
supporters
Sidaway,
1969;
SidawayandAsprey,1968),the methodsdid not attractattentionalthough
more
recently
interest
has
been
shown
in
the
effects of air
ions,
rather
than electric
fields,
on
plant growth
(Bachman
et
al.,
1971;
Kotaka
and
Krueger,1968;
Kotaka et
al., 1965; Krueger
et
al., 1965,
1978).
III.C THE LETHAL
EFFECTS
OF
ELECTRIC
FIELDS ON PLANTS
The term "lethal
electrotropism"was
suggestedby
Murr
(1964a)
fol-
lowing a
series
of
experiments
on
the effects
of
electric fields on
plants.
Intheseexperiments Murr,1963a, 1963b)anelectric ieldwasestablished
between two aluminium
wire
grids
(0.24 m2).
A
lower
electrode
was
situated below the
soil
in
a plot
in which
seedlings
of orchard
grass
(Dac-
tylis
glomerata)
were
planted.
An
upper
electrode was
suspended
above
the soil and
adjusted
n
height
to
vary
the
electric field
strength,
but
was
never
more than 10 cm above the
tops
of
the
plants.
Temperature
and
light
intensity
werecontrolled
(unspecified)
and a 16-hour
day length
was
used.
The
control
plots
had
the same
electrode
arrangement
s the
"active"
ones, but withoutthe voltagesapplied.The top electrodewas madepos-
itive and
the
bottom
one
connected to the
negative
of
the
power
supply
simulating
the
earth's natural
electric field
(Chalmers,1967).
Murr
(1963a,
1963b)
observed
that
during
continuous
exposure
to the
electrostatic ields the leaf
tips
of
the
seedlingsbegan
to
brown,
as if
burnt,
and
he noted the
similarity
to
mineral
deficiency
symptoms.Between
the
region of
leaf tip
"burning"and
the normal tissue
was a
small strip of a
much
deeper green
color than
usual. He
also found that
damage
spread
downwards romthe tipat a fasterratethan thegrowthof theplant(Murr,
1963b).
The
plants were clipped
to a
height of
2.5 cm after two
weeks,
twice
more at
weekly
intervals, and
the
dry weights of the
clippings ob-
tained. Murr
defined a
damagefactor as the
proportion
of
the
dry
weight
of
electrifiedto
control
samples
(averageof three
results)
expressed as a
percentage.
For
orchard
grass,
damage was found to rise
to
25%at a field
strength
of 50
kV
m-1
and
then
rapidly increasedto
50%
at
75
kV
m-l.
Similarresults
were
obtained with
seedlings
of
reed
canary
grass(Phalaris
arundinacea) Murr,
1963b).
Transversesections
of some
leaves showed
that the epidermalcells had been destroyed n the
brownedtip and
dam-
aged
in
the dark
green
zones. There
was
completeabsenceof cell
structure
in
the
tip area
and chloroplast
derangement n the
darkgreen
band.
Murr believed
a
possible cause of this
damage
was the
migration of
ionized salts to
the leaf
tip under the
action of
the electric field.
The
resulting
concentrationunbalance
might
upsetnormal
osmotic
phenom-
ena and
cause
rupture
of the
cells.
To
investigatethis
further
he
applied
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180 THEBOTANICAL
REVIEW
Table I
The
concentrationof elements
n
the
leaftipsof
orchard
grass Dactylis
glomerata)
seedlings afterexposure to electrostatic ields(after Murr,1964b)
Electrostatic
strfielgth
Phosphorus Nitrogen
Iron
Zinc
Aluminum
(kV
m-')
(percentage of control
samples)
30
104
95
119
177
104
50
94
97
169
258 233
75
97
98 202
317
324
field
strengths
of
30,
50 and 75
kV
m-1
to
seedlings
of
orchard
grass
(in
the same
experimental
regime
as
before)
and
took
clippings
at
2, 3,
4
and
5
weeks
(Murr, 1964b).
Dried
samples
were
used for mass
spectrometric
and
micro-Kjeldahl
analysis.
The
results
were
contrary
to
expectation,
and there were
no
significant
differences
in
the
quantities
of
phosphorus,
nitrogen,
calcium, magnesium or
potassium between the
electrified and
control
samples.
The
minor elements
iron,
zinc
and aluminum did show
increases,
however,
which
ranged from
104%
to 324%
(Table
I).
Murr
(1964a,
1964b)
concluded that
the
lethal
damage
was not
caused
by
the
drift of the
ionized
salts, leading
to the
bursting
of
cells
and
de-
hydration,
but
suggested
that
metabolism
was accelerated and a
"general
tissue
deterioration" ensued. He
regarded
(1964b)
the increase of
iron,
zinc
and
aluminum in the
leaf tips of
exposed plants to
be associated
with
accelerated
"metallo-enzyme
activity"
affecting
respiration,
ultimately
leading to tissue
destruction. He also
interpreted
changes
in
the
density
of chloroplasts as evidence for "metabolic acceleration" (Murr, 1964a).
However,
while the
highest
densities of
chloroplasts,
in
the
tips
of leaves
of
orchard
grass
damaged
by exposure
to an
electrostatic
field
of
40 kV
m-
I
(Murr,
1964a),
were
found
in
the
deep green regions
(below
the
brown
tip),
even
here
the
chloroplast
density was lower than
that
in
untreated
leaves.
The deep green
of
treated leaves
was
suggested (Murr,
1964b)
as
attributable to
the effect of
traces of ozone in
oxidizing porphyrin
groups,
electrostatic
fields
leading to
an
increase of
porphyrin.
The deep green color of leaves of plants exposed to static electric fields
had been
reported
by several earlier
investigators.
Priestley
(1907) noted
that
young blades of
wheat from
electrified plots were, in
the
opinion of
many observers, darker
green than the
control
plants. In
continuation
of
his
experiments,
Priestley (1910)
again
remarked on
the color
difference,
reporting that
other
workers had
observed a
darker
green, this
being
especially
noticeable in
wheat.
Blackman and
J0rgensen
(1917), experi-
menting
with
oats,
reported that,
after one
month,
the electrified
crop
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ELECTROSTATIC
FIELDS,
MICROWAVE
RADIATION
181
was taller
and greener
than oats
not
subjected
to
electric fields.
Again
in
an
experiment at
Rothamsted
Experimental
Station
in
1917,
Blackman
(1924) foundthatbarleywas tallerandgreenerafter20 daysof application
of the electric
field
than that
in the
neighboring
control
plot,
although
the
visual
differencebetween
the
crops
subsequently
became
less
marked.
Priestley
19 10)
suggested without
proof)
that the
deepergreen
n
wheat
might result
from
a
slight
but continuous amount of nitrates
being
added
to the
soil
by
the overhead
discharge,perhaps
ormed
in
a similar
manner
as the combination of
oxygen
and
nitrogen produced
by
thunderstorms
is
washed
into
the
soil by rain. One soil test
(Priestley, 1907)
indicated
some three times the amount of nitrogen n the soil beneathan overhead
discharge
than
in
the
soil
in
the control
plots,
but
unfortunately
mea-
surementswere taken
only
after the
crops
had
been harvested. However
Blackman
1924)
reported
no
appreciable
differences
n
soil
nitrogen
con-
tent before and after the
application
of electric fields to
oats.
A
further
unconfirmed
uggestion
by
Priestley
(1907)
is
that
plants
exposed
to elec-
trostatic
ields
may
utilize
atmospheric
nitrogen
directly,
perhapsby
com-
bination of
gaseousnitrogen
with
carbohydrates
within
the
plant.
Hart and
Schottenfeld
(1979)
also observed
that leaves
of
pole
beans
(Phaseolus
multiflorus)
ecame darker
greenwhen
exposed to
electrostatic
fields
resulting
in
corona
current
from a
few
points on the leaf
edges.
Prolonged
exposure
caused loss of
turgorand
collapse
of
the plants.
The
pole
bean
plantswere
grown
in
individual
containers
(20?C;
30%
relative
humidity).Soil
moisture
content was
monitored
by
measuring he
resis-
tance
between two brass
posts
set 4.5
cm
apart.The
plants were 5-10 cm
high
at the
time of
treatment
(estimatedfrom
Fig. 1.
of Hartand
Schot-
tenfield, 1979), and the mesh
electrode
was about 5 cm
above the
top of
the plant. With the electrodechargednegatively, plants sustained2 ,uA
of
corona
currentfor 8 hours
with no
visible
effects,and with
20
AA
of
corona current
here was
little
change
in
soil
resistanceafter 5
hours
but
the
plant
began to
droop and one
leaf
became dark
green and
lost "tex-
ture."
At 50
,A,
however,
leaf
discoloration
developed
rapidly and
stem
collapse
occurred
within 45
minutes. With
theelectrode
charged
positively
and 20
,A
of
corona
current,wilting
occurred
morerapidly
and there
was
a
large
ncrease
n
soil
resistance about
35%
n
3
hours).If
slightly
droop-
ing plantswerewaterednear their stems they becameturgidagain.Hart
and
Schottenfeld
attributed he
effects
o
severe water
oss from
the leaves
caused
by the
corona
current.
Bachman
and
Reichmanis (1
973a) applied
various
strengthsof
electric
field to
singleleaves of
barley that
were
five days old
and 5 cm
longwith
the cut end of
the leaf
in
contact withthe
negative
electrode,and
a
positive
electrode
suspendedabove the
leaf. Their
results
ndicated that
with fields
less
than 40
kV
m-
I
leaf tip
burning
would not
occur,butat a
field
strength
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182
THE BOTANICAL
REVIEW
of 400
kV
m-
damage
was
almost
instantaneous
afteronly
a few
seconds
of
exposure).
Above 800 kV
m-
'
the
density
of
positive
air ions and
level
of ozone wereshownto buildup very rapidly.The time of the appearance
of
damage
was
found
by
Bachmanand
Reichmanis
to be
proportional
o
1
V2. They
also investigated
he
speed
of
propagation
of the
damage
down
the leaf and found this
to
depend
on the field
strength.Although
the
damage
at the
tip appeared
very quickly,
the
subsequentspread
was at
a
reducedrate,e.g. for 250
kV m-I
damage
appeared
after
10
seconds and
after
2
minutes it
was
about
0.8
mm
down
the
leaf;
after
60
minutes it
was only about
2 mm in
extent.
Similarly for 166
kV
m-l
the
damage
appearedafter 20 seconds and after 2 minutes it was about 0.2 mm in
extent;
after
60
minutes
it
was
only
about
1.1
mm
down the
leaf.
Damage
n
air
was
compared
o
that
in
nitrogen
and
hydrogen
by passing
these
gases
over the leaf
during
electrification,
and the
steady progression
of destructionwas observed.
The
spread
of
damage
was much
higher
in
the
presence
of
hydrogen
than
in
nitrogen
or
air, e.g.
for
2
minutes'
exposure
with 10 kV
voltage,
1 mm
of
damagedtip
was observed
in
air,
and more than
4
mm in
hydrogen.
From these results,
Bachman and
Reichmanis (1973a) concluded that
the
damage
was
probablycausedby glowand brush
dischargesat the leaf-
air
interface,
so
they
made
subsequent
observations
using
a
microscope
in
a dark
room. They observed an
ivory-colored glow
around the tip of
the
leaf and
usually
two
purple-coloredbrush
discharges,one on either
side of
the leaf at the
junction
of
the damaged and
undamaged tissue.
These
observations,
in
their
view, confirmedthat damage
was caused by
glow discharges.This
damage
mechanism is substantially
different rom
that earlier
proposed by Murr
(1964b) of
acceleratedrespirationcausing
membrane ruptureand cell dehydration.The results also differin other
respects. Murr
(1963a,
1963b) reporteddamage at 40 kV
m-l and below
and an
intermediatezone of
dark
green,
while
Bachmanand
Reichmanis
stated that no
damage
occurredbelow
40
kV
m'-I
and
made no
mention
of color
changes.
After observingthe physical
characteristicsof
the damage, Bachman
and Reichmanis
(1973b)
concentratedon the
growth
rates
of plantssub-
jected
to
electrostatic
fields.
By using
both
sloping
and horizontal
upper
electrodes,they found that barley seedlingsgrew until they were about 2
cm
from the
electrode and then
stopped so that the plant
tops took the
profile
of the
electrode. At this
point the field strength
corresponded o
about 800
kV m-
1.
Similarly,
growthratesabove 200 kV m-
I
were found
to be
inhibited when
comparedwith
controls, with 300 and
400 kV m-
I
having strong
effects. By extrapolating hese
growth rate
results they pre-
dicted that a
zero value
should occur at
approximately800
kV m-
I
which
agreed
with
their earlier
findings.
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ELECTROSTATIC FIELDS, MICROWAVE RADIATION
183
Bankoske
et al.
(1976)
investigated
the effectsof 60
Hz
electromagnetic
fieldsupon
plants
under
power
transmission ines.
Theycarefullydesigned
theirlaboratory xposurechamber o have an even field distributionwhen
empty
and then calculatedand measured he effectof
placingplants
nside.
The field
pattern was
disturbed
considerably
and enhanced
in
the
im-
mediate
vicinity
of the
plants. Only
with
a
dense
growth
of
plants
with
a
uniform
top
surface
could the electric
field be defined as the
applied
voltage divided by the
distance between
the
top
of the
plants
and the
electrode
above.
Single plants
or small
groups
caused considerable dis-
tortion
but
if
the
plant height
was
less
than 25% of the
electrode
spacing
no more field enhancement occurredthan for a single plant beneath a
transmission
line.
Plants
with
sharp-pointed
eaves have a
greater
con-
centrationof electric
field
aroundtheir
tips
and
consequently
breakdown
owing
to corona
currentat lower
voltages
than broad-leaved
plants.
To define experimentalconditions Bankoskeet al.
give "undisturbed"
field
strengths,
.e. the
field
strength
with an
empty
chamber,
which bears
no direct relation to the actual
field strengths
at the surfacesof
leaves
in
the
chamber.
This is also
recognized by
Hart and
Schottenfeld
(1979)
who
preferred o state electricalexposure conditions
in
terms of
AA
of
corona
currentrather
than
unrepresentative
ield
strength
values.
Murr
(1965a)
uses
a
very simple
model of
a column
of
dielectric slabs
to
represent
he
air, leaves,
roots and soil between the
electrodes.This is
unrealistic
except
in
the one
previously
mentioned case of a
very
dense
growthof
plants whose top surface forms a new
ground plane. Most of
Murr's
experimentsare
with
a
single plant
or
groups of separatedplants
(e.g. Fig. 5, Murr, 1965a)
and so his values for
field
strengths are not
valid,
especially his dynamicfield strengthvalues.
Enhancementof field strengthowing to the radiusofcurvatureof objects
placed
in
electric fields means
that
it is
impossible to
relate the responses
of
different
species
of
plants,
or
even different
specimens
of one
species,
to
particular
values
of electrostaticfield
strength.Each
plant and part of
a
plant
will
experience
different
electrical forces at the
leaf surface
when
placed
in
a
uniformelectric
field.Comparisonbetween
experimentsbased
on field
strength
values is
extremely difficult.
Bankoske
et al.
(1976) determinedthat corona
currentwas responsible
for leaf tip burningof corn(Zea mays) and alfalfa(Medicagosativa). The
alternating
field
(60 Hz)
had an
undisturbed value of
50
kV
m-1 and
plants
introduced
into this had
their uppermost leaf
tips damaged after
exposure
for seven
days.Corona
dischargeswere
photographed t the tips
of
leaves when
plants
were
placed
in
"undisturbed"ield strengthsof 25-
50
kV
m-',
with
22.5
kV m-1
as a
damage inception
level. Kentucky
bluegrass
(Poa pratensis)
showed leaf
tip burning after a few days in
"undisturbed" ield
strengths
of 50
kV
m-l, 60
Hz.
The
damage did not
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184
THE
BOTANICAL REVIEW
progress further down the
leaves after one week and
was 5-7 mm
in
extent.
At
25 kV
m-l (60
Hz) damage
was limited
to 1-2 mm.
When
placed in "undisturbed"electric field strengthsgreaterthan 30 kV m-'
leaves
were
seen
to flutter
owing
to corona-inducedmotion.
Murr
1965b, 1966a,
1966b, 1966c) nvestigated
he
effectof
"reversed"
(i.e.
upperelectrodenegative)electrostatic ields,
alternating 60 Hz)
fields
and
magnetic
fields
upon
the
growth
of
young plants.
He
concluded that
it was possible to stimulate their
growth
if
the electricalconditions were
carefully
chosen.
If
the
electric fields became too
large,
corona
current
flowed
and caused damage
to leaf
surfaces.
The
thresholdvalues
per plant
fordamagewere 5 x
10-7
A for orchardgrass(Dactylisglomerata) Murr,
1965a),
1.5
x
10-8 A for
sweet corn (Zea mays)and 3
x
10-8
A
for wax
beans (Phaseolus
vulgaris)
Murr, 1966c).
Murr
proposed (1965b) a modification to his earlier theories
(1963a,
1963b, 1964a, 1964b) to
include
the effect
of corona
current.
It
was
suggested that this caused
damage
to
the
epidermal
layer
of a leaf
by
stimulating respiration
and metabolism. Moderate electric
fields led to
epidermal
damage
and were believed to
result
in
respiratory
action suffi-
cient
to
stimulate
growth
but
not to cause substantial eaf tissue
damage.
Raising
electric
field strengths was
thought
to
cause
over-stimulation,
enzyme
toxicity
and sufficient
epidermaldamage
to
result
n
death of leaf
tissue. Murr
(1966a)
concluded that currentbelow 10-16
A
per plant
had
no
effect but growth stimulation occurred
in
the
range
10-I5
to
10-9
A
per plant.At 10-8 to 10-6 A
per plant,
leaf
damage occurs, and plant or
leaf
destruction occurs at
10-5
A
and above.
Blackman and Legg (1924), while examining
the stimulation of
the
growth
of
barley by electric fields, used currents
from 0.3
x
10-9 A
to
175 x 10-9 A per plant to maximize the response. The highercurrents,
however, were found to be injurious:above a level
of 10-8
A
per
plant
there was
damage
to
the plant tissue. Collins et
al. (1929) claimed
that
passing 75
x
10-9
A
per plant
had
no effect on
maize, but with
larger
currentssome
plants
showed
injury.
Scott
(1967) reported
hat
attempts
to
modify growth
by using
electro-
static fields
wereinconclusive, most having no
effect or retarding
growth.
He
also noted that
lethaldamagemay be caused by
coronadischarge
rom
leaf tips.
Krueger
and
others (Kotakaand
Krueger,
1968; Krueger
et
al.,
1978)
have
experimented
with
changes
in
the
density
of
air ions.
It
is believed
that the effects observed
in
the
past may
have been due more
to
the
atmospheric
ion
conditions that the
electric fields
per
se.
They
showed
that
increasing
the
air
ion
density (from
the
normal values of
approxi-
mately
4.0
x
103 ions
cm-3
to
approximately
3.5
x
107
ions
cm-3)
re-
sulted
n
speededup respiration
and
growth
ratesof
young barley
seedlings
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ELECTROSTATICIELDS,
MICROWAVE
RADIATION 185
(Kotaka
et
al.,
1965)
as
compared
to controls. Increases n air ion
density
have also been
reported
to increase
the
chlorophyll
content
slightly
and
the cytochrome content considerablyin treated plants (Kruegeret al.,
1963), to speed
up
chlorosis
of
plants grown
in
iron-depleted
environ-
ments (Kotakaand
Krueger,1968; Krueger
t
al., 1964),
and to stimulate
ATP
metabolism
(Kotaka
et
al., 1968)
in
isolated
spinach chloroplasts.
Complementing
these
results, Krueger
et al.
(1965)
found that
barley
seedlingsgrown
n ion-free
atmospheres
how retarded
rowth,
ack
rigidi-
ty
and have soft leaves. Blackmanand
Legg
(1924)
and
Hicks
(1957)
have
experimented
with
plants
and
trees
in
areas surrounded
by
wire
mesh.
All reportedretardedgrowth,lack of turgorand soft leaves. A wire mesh
surrounding lants
would act like a
Faraday
cage
so the
atmosphere
nside
it would be
relatively
free of electric fields and air ions.
Krueger
et al.
(1978) exposed
barley seedlings (Hordeumvulgare
var.
CaliforniaMari-
out)
to
electric
fields
in
an air ion-free chamber and to electric fields
in
an air ion-enriched environment
(1.7
x
105
small
negative
ions
cm-3;
current
averaged
10-11
A
per plant). They
found
an increase
in
growth
rate
of the
ion-treated plants compared
with
those
in
an
electric field
alone,
but no differencebetween the latter and control
groupsgrown
with
an ion-free
and
no
electric
field
regime.
They
concluded that the air ions
werebiologically active and responsible or the
increases
n
plant growth,
although they
were unable to
distinguish
whether the effects were
due
to
the air ions
alone
or to
their
presenceallowing
electric currentsto flow
through
the
plant. They pointed
out that
many previous
workers
had
examined the
responses
of
plants
when
subjected
o electric
fields but had
not taken into
account the presence
and
role of air
ions.
Bachman et al.
(1971) found that under
their experimentalconditions
ozone appearedwith currentsof 5 MA hrougha singleleaf or 0.2 PA per
leaf
if
therewere 20 plants. They recognized
that the electric field at the
tip
of
a
barley
leaf
would be much
higher
than that calculated from the
separation
of leaf and
electrode
owing
to the small
radius of curvatureof
the leaf
tip.
Also the
edge
of a
barley
leaf is covered with
tiny "spikes"
whose diameter s
about 1000 times smaller
than the leaf tip radius.Thus
very high electric fields would exist around
the "spike" tips and corona
discharges
will
appear
at much lower
potentials
than would be needed for
discharges rom the leaf tip (equivalentto a few hundredV m'-Ibetween
planarelectrodes),so ozone
will
be produced
and lethal dischargecurrents
can
flow
at
quite
moderate
exposure levels.
Ozone can
damage plants
and is
produced at ground level by point
dischargesduringthunderstorms.However,
Shlanta and Moore (1972)
commenting
on
the
leaf
tip burning
of
grassconsiderthatit is the discharge
that
burns he
grassrather han the
ozone.AlthoughBachmanet al. believe
that
it
may
inhibit
growth
and
Menser et al.
(1963) report ozone damage
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186
THE
BOTANICAL
REVIEW
to
tobacco
leaves,
complete
death of a
plant
from
the
gas
has not
been
suggested.
Bankoskeet
al.
(1976)
measured he amounts
of ozone
present
when leaf tip burningoccurredduringcoronadischarge rom leaves. The
level was
16
parts per
billion
(ppb) compared
to an ambient level
of
8
ppb.
It was concluded hat ozone was
unlikely
to
have caused
any damage.
Blackman
et al.
(1923) passed
small currents
0.5
x
10-10
A
per plant)
through
barley seedlings
(Hordeumvulgare
var.
Goldthorpe).
The
current
was
generatedby means
of a
needle connected to a
high
voltage
source
(ca.
10
kV)
suspended
20
cm
above the
plants, applied
with the
needle
electrode
positive
for
one
hour or
three hours and for three hours
with
the needle negative.The positive polarityproduceddefiniteincreasesin
growth rate
over controls (no
electric field
or current)which
increased
withtime and
persisted or
up
to four hours
after
the
currentwas
stopped.
The negative
polarityresulted
n
an initial
increasefollowed
by
a
steadily
decreasing
rate
of
growth
compared
with controls.
When
the
currentwas
switched
off, however,
growth
rates
began
to
rise
again
still
above
the
control values but
less than those
of
the
positively
charged
set. Further
work led
them
to concludethatneither he
"electricwind" nor
the
gaseous
byproducts i.e. ozone
and
nitrogenoxides)
of the
corona
dischargewere
responsible
or the
effecton the
growth
of the
plants
when a
current
passed.
III.D
CONCLUSIONS
III.D
(i) Summary
Considerable
effort was
expended
in
the first third of
this
century
in-
vestigating
the effects of
electricfields on
the growth
of plants(Jorgensen
and
Priestley, 1914;
Lemstrom,
1904; Newman,
1911).
Although some
of theresultswereveryencouraging e.g.fieldtrialswith cerealsand clover
hay
(Blackman, 1924)
showed
increases
in
yield of
over 30%
in half of
the
experimentsand
yield increases
n
another
28%of them], other
work-
ers
could measureno
effectsat all
(Briggs
t
al., 1926;Collins et
al., 1929).
For
18 years
Lemstrom
(1904) studied
the
effects of
exposureto electric
fields
and
reported
healthy plants and
increasedyields
(includingcereals,
vegetables
and root
crops). The time of day
when
electricitywas applied
was
importantsince
exposureat
midday or
in hot sunshine
could decrease
crop yields. However, a reportof a subcommittee of the Board of Agri-
culture
and
Fisheries
(Committee of
Electroculture,Final
report of work
carriedout
1918-1937)
indicated
that, after
prolonged
research,benefits
of
electrical
treatmentscould
not be
confidently
predicted.
Recently
Kruegeret al. (1978)
have
shown that the density
and type of
air
ions
are
important
for
plant growth,
and
suggested that ions rather
than electric
fields
produced
the
various
effects.
Growth and respiration
rates (Kotaka et
al.,
1965) as well as
ATP
metabolism (Kotaka et al.,
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ELECTROSTATIC
FIELDS,
MICROWAVE
RADIATION 187
1968)
can be
increased
by
air ions.
Barley
seedlings grown
in
ion-depleted
atmospheres
(Krueger et
al., 1965)
show retarded
growth, and
plants
grown in Faraday cages (Hicks, 1957) are also weaker than ones grown
outside.
Murr
(1 963a, 1964a)
studied
the
effects
of
large
electric fields
on
leaves of
orchard grass
(Dactylis glomerata)
and
reported burning
of leaf
tips.
He
proposed
that
an
over-respiration process
was
causing
the burn-
ing, but Hart and Schottenfeld
(1979)
believed
that
the
effects
were due
to loss
of
water
from
the leaf
tips
when corona current flowed
caused
by
the
high
electric
fields. Currents below
10-16 A
had no effects on
plants
(Murr,
1966a)
while 10-15 to
10-9
A
stimulated
growth. Higher
current
values destroyed the plants.
III.D
(ii) Practical weedcontrol by high electric
ields
Although plants have been killed by the
application
of
very high electric
fields it is not
possible
to
use this method
for weed
control.
This
method
requires arrays
of
wires suspended some
2 m
or more
above
crops, all
charged to tens
of kV.
Outside of a
laboratory
this
system
is
cumbersome
and
dangerous.
In
addition
broad-leaved
weeds
in
cereal crops
would be
affected less than the long, thin crop plants with pointed tips which would
be
destroyed
first.
If
it can be shown
that air ions stimulate growth then
it may be
possible
to use
them, especially
in
greenhouses.
IV.
Microwaves and Weed Control
IV.A
USES OF MICROWAVES
IN
AGRICULTURE
Microwave radiation has been suggested as a possible solution to many
and varied
problems
in
agriculture, including prevention
of frost
damage,
rapid crop drying,
reducing
hard seed
(impermeable seed coats), pest
control and weed control. Its
advantages
include the
rapid penetration
to
all
parts
of
the
"load,"
it leaves no residue after
application
and it
can
be directed at its
target.
In
weed
control,
microwave radiation is
not
affected
by winds,
thus
extending
the
periods
of
application compared
to
conventional
spraying
methods.
Furthermore,
it
can
kill
the roots
of
weeds
and also seeds buried to a depth of several centimeters in the soil as well
as
nematodes and
fungi. Although
the
primary
concern of this review
is
weed
control some
examples of other uses of microwave
radiation are
indicated
below,
illustrative of the versatility of the technique.
In
studies
of
the possible microwave
protection of plants from
cold,
Bosisio
and Barthakur
(1969) placed two wax bean plants
(Phaseolus
vulgaris)
in
a cold chamber
maintained at
-
5?C for 4 hours. One
plant
was
treated with 2450 MHz
radiation of
power density 15 mW
cm-2
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188
THE
BOTANICAL
REVIEW
whichwas
sufficient
o maintainthe leaf
temperature
t
25?C.
The
authors
calculatedthat a
power
level
of 2
mW cm-2
was
being
absorbed
by
the
leaf, and a minimum of 1 mW cm-2 represented he thresholdfor leaf
protectionfrom frost at
-
5?C.
After transfer o normal
temperatures,
he
irradiatedplantremained
healthy
whereas the
unprotected
one had been
destroyed
by
the cold.
The
investigators
estimatedthat
for frost
protection
125
kW
of
power would
be
neededperhectarewith
high capital
cost but
low
running
cost.
In a
later field
test Bosisio et al.
(1970)
irradiated
our-month-old
corn
(Zea
mays) in a
plot 8
x
8
m
and
protected
50%of it for
60
hours
against
winds of between 8 and 33 km h- 1, temperatures of
-
1C to
-
6?C and
1.5 cm
of snow.
A
2.4 kW
generatorwas used and the
irradiating
ntenna
was erected 2
m above
one corner of
the plot.
Power levels of
10
mW
cm-2 at 6
m
from the
antenna were the
threshold for
protection
of the
crop.
Plants
directlyexposed
to
the
prevailing
winds
were,
however,killed,
even
with
150 mW
cm-2
radiation
intensity.
Microwave reatmentwould
be
very
expensive
and
pose
health
hazards.
Although
Bosisio et al.
(1970)
measured
only
1
mW
cm-2
at 3
m
from
the
edge
of
the
plot, they
had
suggested
earlier
(Bosisio and
Barthakur,
1969)
that farming
areas using
microwave
radiation
would have to be
kept clear
of
people
duringirradiations.
Distributing
the
energy
equally
over
large
areas
would also be
a
problem.
Boulangeret
al.
(1969) produced a
design
study of a
microwave grain
drying
system.
Reducing grainmoisture content
from
20%
to
15% ook
less than
15 minutes by microwave
methods (12
kW,
2450 MHz), 30
minutes by
a
high
frequency
(HF) dielectricsystem
(13
MHz
at
10
kW)
and 150
minutes
by conventional hot-air
techniques.
Some
wheat
insects
and their larvae were also controlled (Triboliumconfusum,Sitophilus
granariusand
Cryptolesteserrugineus).
Costs
for the electronic
system
were
estimated
to
be half
those
of
conventional
methods. Baking and
milling
tests
showed
little differences
between
the methods.
Fanslow and
Saul
(1971),
however,
using
a
cavity
with 1.8 kW
at 2450 MHz
and
0.6
kW
at 915
MHz,
found
that
there was a
practical
imit to the
increase in
drying
speed
since too
rapid
heating
led
to crackingof
the
kernel and
swelling
of the
grain,
with
consequentreduction
n
market
grade.At
very
highratesof dryingthey found thatvaryingthe air flowthrough he grain
had
little effect on
drying
rateswhereas
Boulanger
et al.
(1969)report hat
the
drying
of
wheat
depends
upon
the
change
n
vaporpressure
determined
by
the
air flow.
Microwave
radiation has been
used
to
kill
insect
pests.
Hightower et
al.
(1974)
employed
2450
MHz
radiation
to control
powder
post
beetles
in
imported
hardwoods,
particularly
ellow pine and fir.
Heating
rates of
30?C
min'-I
were achieved
and the
beetles and larvae
could be controlled
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7/26/2019 Microwave Radiation and Plants
20/54
ELECTROSTATICIELDS,
MICROWAVE
RADIATION
189
when wood
temperatures
of
50?Cwere
obtained.
They
predicted
that
a
30
kW
unit could treat
a 7.5 cm
wide board at the rate of 1.5
m
min-I
with costs comparable o kiln dryingmethods butconsiderably asterand
in
less space.
Nelson
(1976a)
and
others
(Iritani
and
Woodbury, 1954;
Nelson and
Stetson,
1974; Nelson et
al.,
1966;
Rai et
al.,
1971, 1972)
have
studied
the
effects of
radiation
in
treating nsect
pests
in
grain
and
found
that, often,
much lower
frequencies
of radiation are more
suitable
(e.g. 39
MHz)
because the dielectric
properties
of
insects
and
grains
differ
more from one
another
at lower
frequencies
han
at microwave
frequen-
cies and so
selective
killing
is easier. In relation
to
these
investigations,
measurementshave been made on the dielectricand electricalproperties
of
grains,
seeds,
insects,
larvae
and soils
(Nelson
1973a, 1973b;
Nelson
and
Charity,
1972).
The
proportion of hard seed
in
alfalfa
(Medicago
sativa)
has been re-
duced
(Nelson,
1976b)
to
levels
of
5-15%
from
40-60%
following
ex-
posure
to 39 MHz
and 2450
MHz
radiation. The
success was
related to
seed moisture
content,
the
greatestreduction
n
hard
seed
being
n
samples
with the
lowest
moisture
content. The
optimum
temperature
was
found
to
be 75?C.
The
benefits of
treatment asted for
up
to four
yearsafter the
application.
The
effect was believed to be
associated
with the
increase of
water
sorption
by the seeds.
In
studies on the seeds
of trees
(Kashyap
and
Lewis,
1974), germinationhas been
found to
increase