Microwave Radiation and Plants

7/26/2019 Microwave Radiation and Plants

1/54

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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2/54

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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3/54

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.



4/54

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



5/54

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



6/54

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



7/54

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.



8/54

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



9/54

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



10/54

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



11/54

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



12/54

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



13/54

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.



14/54

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



15/54

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



16/54

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



17/54

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.,



18/54

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



19/54

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



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

Documents

Microwave Radiation and Plants