UPTAKE BY CITRUS LEAVES
A thasi s pr e;scnte:~ in l)r.;rtial fulfilment of the r oquircments for tho Deer oc of
Master of Horticultural Science at Massey University
Palmerston North New Zoalan0.
by
Shnli0re .. m Ku1:1ar Thapa
Pineap~le sweet oran~e seedlings and rooted leaf bud cuttings of Meyer lo~ons were used to investigate the effects of some cf the factors affecting mac:;nesium u1.:;take by leaves. Magnesiun was cteterrnined by thiazol e yellow nethot of Drosdoff and Nearpass ( 1948) anc1 u:,take was usually m0asured 24 hours after spray treat• ent.
It was shown that the addi t ion of wetting a5ents to
nacnesiuD nitr2.te sprays si t;nificant1.y increased the u1)tako of mac;nesiun '.)y l eaves. The nonionic wetter (Terrie GN9) at the very l ow (0.01% a.i.) anr~ high (0.08 - 0.1% a.i.) concentratio.ns di e~ not affect nc1.c;nesiu,:1 uptake, whereas at interi11ediate con-centrations, ma5nesium uptake was increased.
Use of the humectant clycerine at 1 or 2 percent sie nif-icantly increase,, the uptake of nagnesiu1a by leaves, compared with sprays to which no: slycerine was added, but had no bene-ficial effect o,ver sprays which contained a nonionic wetter (Te·rric GN9).
uptake of magnesium by :!_oavos, compared with afternoon sprays.
A significant increase in l eaf magnesium concentration occurred after 2 hours of a magnosiur:1 nitrate spray appli ca-tion. Loaf magnesium concentration rose steeply for 24 hours after spraying, thereafter remaining constant. (Becaus;) it was not possible to measure the degr ee of magnesium transport out of the l eaf, it i s not cl ear whether m&gnesium uptake, in fact, stopped after 24 hours).
Of the three magnesium salts used, magnesium nitrate and magnof;ium c hloridG sprays rGsll.ll t ed in groa tor magnesium upta.i..:e by l 0aves, compar ed with magnesium sulphate sprays.
Uptake varied wi th tho cone entra.tion of magnesium in. the l eaves. The lower the concentration of ~agnesium in the leaves, the l oss the uptake of magnesiur:;i by loaves, and tho hie;hcr the cone entration o.f magnesium, the higher tho uptake of magnesium.
in the high nitros2n ; lants waE dcubl o that of tho low nitrogen pl ants. This may to a direct effect of the low l oaf nitroccn or an inc1.irec t one .h1c to tho in•:uc cc'. low lce.f ;-1ac;nosim1 in those plants.
--I gratefully ecknc~lotl~G tho continuous assistanc0 ant guidance of Hr D.A. Slaie of the Dopartucnt of Horticulture, Massey University in conplotinc this project. My sincere thanks also go to Professor J.A. Veale of the Department of Horticurlture for his nsGful sugiestions and ac1.vico.
This stutly was made possibl e by financial assistance from t~e Ne~ Zoal anJ Department of
1 •
2.
INTRODUCTION
CHAFER 1
CHAPTER 2 FOLIAR APPLICATION OF NUTRIENTS
· CHAPTER 3 LITERATURE REVIEW
3.1. Pathways of ponetrc..tion of nutrients, her:Jie.i.dos and other substances into the leaf.
3.
1. 1.3.
1. 2.3
.1.3.
3. 1.5.
Entry through stomata Structure of tho cuticle 3.1.2.1. 3.1.2.2. 3.1.2.3. St:r-ucturo wa.11 Structure Mechanisms
3.1.5.1.
3.1.5.2.
3.1.5.3.
Physical nature of tho pl:mt cuticle
Chemical nature of tho plant cuticle
Tho role of the cuticle
anc1.. na.ture of the. col l
of the plasma-mem :Jr ane of foliar penetra.tion Mechanism.s of po
netra-tio.n in tho cuticle Mech2..nism.s· o,f pe
netra-tion in the cell wall Mechanisms of
penetra-tion in the p lasma-membrane
3.2. 1. Pl.s.nt factors 3.2.1.1.
30
of tho ~lant cuticle 30 3.2. 1.2. AGO C• f the l oaf
3
.
2
.1.
3
.
Leaf surfaces and rJorpholocy3.2.1.4. Mineral status of tho
lc:i.f
3.2.2 .. External factors
3
.
2
.
2
.1.
3
.
2
.
2
.
2
.
3
.
2
.
2
.
3
.
3.2.2.4.3
.
2
.
2
.
5
.
3.2.2.6.3
.
2
.
2
.7.
The citrus leaf U12tak.e of ma.c;nesium anr_~, other crops3.4. 1. Glasshouse 3.4.1.1.
3
.
4
.1.
2
.
Licht
Temperature Humidity
pH of the s~r~y so lu-tions
Surfactants Humectants
Solute characteristics
b;:t: 1-oaves of citrus
and luborator;:t: studios Tho effect of different magnesium s.:1.l ts on magnesium absorption The effect of differ-ant spreaders and hygroscopic aeonts on
ma.gnes ium abs ori)tion
Tho effect of sprc.l.ying at different hours of the d~y
Tho effect of nitrogen lovol
The effect of macncsiun 53
55
57
level 60
Plant responses to fi 81.d concU t ions 62
3
.
4
.
2
.
1
.
3.4.2
.
2.
3
.
4
.
2
.
3
.
Responses to citrus Miscellaneous responses The effect of nitrogen level
Tho effect of magnesium lovel
M~rne.sium nobility studies Literature review Summary
62
66
69
MATERIALS AND KCTH0DS
4. 1 • 'rhe experi1:1ents
4,2. Con11Josition an~l ;,reraration of concentratecl nutrient stock solution for experiments
4
.
3
.
.
Techniques usorl_ in crowinc pinea;;ple sweetoranee seedlings
4
.
6
.
4
.
7
.
Sowin5 of seeds Planting out
Fcojin3 the ~lents
0rowi n,:; ?_!__!'1_eyer l oaon l eaf bud. cuttine;s Exeerimontal ncthods
4.5,1. 4.5.2.
Spraying techniques Ex1Jerimental desi gn Leaf sampl es and analysis 4.6.1.
4
.
6
.
2
.
4
.
6
.
3
.
4.6.4,Sampling techniques Cloa
ning-Ashing
Techniques of leaf magnesium analysis
Statistics
5. 1. Ex~eriment I
Tho effect of different wetting a~ents on the uptako of m~~nosiuw by l eaves.
5.2. Ex~erirnont II
Tho effect of :iiffercmt concentrations of a noni onic wetter (Terrie GN9) on tho uptake of magnesium by leaves.
5
.
3
.
Ex]'cffirirnnt III93
93
96
Tho eff0ct of different concentrations of
cl ycorine on tho uptake of rna~nesius by l eaves. 101
5
.
4
.
Ex?erinont IVThe effect of hm-:1.idi ty on tho uptake o-f magncsiu• by l eaves.
5
.
~.
~xp~riment VThe effect of srraying at different times of the d~y on the uptake of magnosiun by l eaves.
5
.
6
.
Exuerinent VI103
105
The rat e of uptake of ma[';nesium by l eaves. 107
5
.
7
.
~ _periment_~IIThe effect of different 3aGnesium salts on
tho uptake o-f mac;nosiun by leaves.
5
.
8
.
Experiment VIIIThe effect of l eaf magnesium l evel on the
uptake of magnesium by leaves
110
6.
7.
Tho ofL:c t of l eaf ni troGon l.evel on the
CHAFER 6
DISCUSSION OF THE DATA
CONCLUSION
APPENDICES BIBLIOGRAPHY
CHAPTER
7.
115
117
TABLE
Sto~atal pore si zes of citrus l oaves
(Turrell,
1
94
7).
2 Th8 effect of tlifforent raacnosium salts on na5nesium absorption
(Fi.sher anc Walker,
1
9
55).
3 Th~ effect of three spreaders an magnesium
atsorption from a 5 per cent Mgso
4.7H2o spray applic~tion (Fisher and Walker,
1955)
4 The effect of glycerine, carbowax and m2thyl cel losolve on cagnesium atsorption fro• a
5 per cent Msso
4.?H2
o
spro.y applicati on (Fisher end Walker,(1955)
5 The rat o of e1acnosi.um al:sorpti.on over a 12 day period (Fi.sher and Walkeri
1955)
6 Tho effect of Mgso
4.?H2o solution applied at different hours 6urinB the day
(OlGnd and Opiand,
19
56
).
7 Moan concentration of elements(% dry weight) in l oaves (Ford,
196?).
8 Effects o-f mat;nesium treahi.ents on the
concentration of magnesiun in the l eaves of
PAGE
48
50
52
53
55
57
62
Valoncict orange (Embl eton anc1 Jones,
1959)
65
9 Magnesium concentration in grape leaves asaffected :.)y spray and soi l application of
Mgso
(ForE:hey,
1
95.9
)
.
7
0
1 1 ~-~0 an con contra t ion of clo:-:ieot:__: (% dYIY •-,;,,1· .',ht)Jo• L ' . . J . D
in l eovcs, stc1.TIE.i :--:nrJ root::. (Fora,
1966)
.
7.
3
t M ( ..,-,, ) ~r O 1 · t · cen . g ~u
3 2.b~2
•
pray•
pp 1ca 10n.9~
-12a Tho effect of different ~ctting agents usca
~t 80~ wettability of th~ loaf area.
13 The effect of rJifferent concontro.t iono of D nonionic wettorr (Torrie GN9) on the upta}:o of i:nar;nosi um by l eaves from a 2.5 per cont
13a The visual nsscfJ;::iii1ent of the ,,ottine of tho upper surface of the citrus leaves by
To rric GN9.
14
The effect of different concentrations of97
99
gl ycerine on ths uptako of ma~nasium by l eaves from a
2
.
5
per centhl
g(No
3
)
2.
6E
20
spray application.15
The effect of humidity on the uptcko ofmari:nosium by leaves from n
2
.
5
per cent102
1 6 The effect of s::raying at ,1iffcrent tiri1os of
the day on the u11take 0f ::1a1:.,nesiw,1 by leaves
frorn a 2.5 percent
Mg(No
3)2.6H2
o
spraya~plication. 106
17
The rate of uptake of ~asnesiu• by l eavesfrom a 2.5 percent Mc,(~m
3)2.6H20 srray
a~plication. 108
18 The effect of different TiaGnesiug salts on the
uptake of • aGnesium by l oavos.
19 Th(: effect of leaf ;:1aGnesiu::1 l ovel on the uptake of ua~nesiuc by leavos frora a 2.5 po r-cent
Mc
(
No
3)2.6H2
o
sprny application. 20 The effect of leaf nitroeen level on thouptake of nacnesium by leaves from a 2.5
per-1 per-1 per-1
113
FIGURE
rrjpothotical structure of tho functional
asject s nf the rlant cuticle (Foy et al; 1967) , 10
1 a A c1.i asrai7! showin:.c, the su~stances th2.t r1ny compose tho cell wall (Miller,
1
938)
.
2 Tho level of P32 activity found in the ;otiol e fol lowinc, a 4-hour l}ericc' of trn.nslocation fron tho blac'l,) as a function of pH of tho an-'lie,~ solutisn (Sv.r-.:i_nson am1. \'/hitney,
1953
)
3
The rate of a~sorJt ion of nitrocen, ;hosfborus4
and macnosiu• from s}rays applieC to tho l ower sur fac '..: ,:: f McIntosh a~:_;_-·le leaves
(Fisher an~ Valker,
1955
)
,
Standard wettins chart for citrus l oav0s
5
The standarf curve of wettability of tho uppersurface of the citrus l eaves fer different
vret tin~: at~onts .
6 The effect 8f different concontruticns of a nonionic wetter (Torrie GN9) on ~ho uptake of uagnesiura by l eaves.
6a The effect of different concentrations of a
nonionic wetter (Terrie
G
N9
)
on the wettinG 0£ the uprer surface of the citrus l e&ves.7
The rate of uptake of macnesium by l eaves.8
Tho effect of l oaf magnesium 1cvol on theuptake of m,:1,.enesiu1.1 by l oaves.
PLATE
Pinoappi o sweot orange seodlinG supflied with hich and low lGvel of nitrocen.
APPENDIX
Comi)ositio:n cf concentrated. nutric,nt stock solution for exJerirnonts I, II, III, V, VI, VII -?tlllt VIII.
2 Couposi tio,n of cone ontrated nutrient stock solution for experiment IX.
3 Quan ti tati ve r.iicro c.~eterminntion o,f ;nacnesium in plant tissue and soil extracts.
4 5
A rapid colorimetric method. Nitr'Jge:n analysis.
Analysis of vnriance of the eff0ct of diffo r-ont '!lotting agents on the uptake of magnosiur.1 by leaves.
6
Analysis of variance of the effect of diffe r-ent wettinc agents used on the uptake ofmagnesium by l eaves, at
80%
wettability of tho l eaf area.7 Analysis of variance of tho effect of diffe r-ent concentrations of a n,.,nionic (Terrie GN9) on the uptake of magnesium by leaves.
7a Analysis of variance o·f the visual assessment of the wettins of the upper surface of the citrus loaves hy Terrie GN9.
8 Analysis of variance of the effect of differ-ent concentrations of vlycerino on the uptake
of magnesium by leaves.
PAGE
134
.
135
136
13
9
142
142
143
143
9 Analysis of variance of tho effect of humid
-i ty on tho uptake ,::,f m?.cnesium ty l oaves.
10 Analysis cf varianc0 of the effect 0f spray
-ins at differunt times of the day.
11 Analysi s of varianc o of the r:1 te of uptake of
JQ[uosium by leaves.
12 Analysis 0f variance of th& effect of differ
-ent mnEnesium. salts on the uptake of me.cnesium
ty leaves.
1
3
Analysi s of variance of the effect of l eafrnagnosiu• lovol on the uptake of magnesium
l,y l0aves.
14
Analysis of variance of the effect of l oafnitrogen Level on the uptclrn of ma6nosium
cy leaves.
14
6
1
4
6
1
47
148
CHAPTER 1
1. INTRODUCTION
Nutrient sprays, these days, are becoming increasingly important to supplement the mineral requirements o:f the
crops to increase crop production.··
Nutrient sprays may be important in two directions.
(i) Where soil application of fertilizers is not responsive
or very slow.
(ii) To prevent the development of a deficiency symptom vary
sonn before the trouble is expected or immediately it has app€ared.
But the responses of nutrient sprays are influenced by
environmental factors (both physical and chemical) and plant
!actors. Magnesium absorption is not an exception to these
factors. Leaves of same plant species do not sho,w responses
to magnesium sal.t sprays, while o.thers do • . Soil application of magnesium salts on the other hand, has been slow. in action or has not been effective or partially effective. Foliage
application of magnesium sal.ts appears to be superior to soil
application in increasing the concentration of magnesium in
the leaves and in reducing deficiency syptoms. But the
respoilS'es are not consistent.
The present study, therefore, was undertaken to
affect the magnesium absorption by citrus leaves. The
literature review, description of the methods and the discuss
-ion of the results have been presented with the aim of provid-ing as much background information as possible in order to facilitate further detailed studies of magnesium absorption.
CHAPTER 2
2. FCi:'" IAR APP:::,ICATI8~! OP :nuTRIEl·TTS
Among same of the early work on the use of mineral
nutrients to plants as foliage application is that of
Johnson (1924)i on pineapple plants in Hawaii, where
pine-apple plants were grown on soil. rich in manganese. Within
three to six months after planting, pl.ants developed a
serioms injury known as pineapple-yellows o.r "manganese
yellows:11• Johnson was able to control that chlo-rosis by
simpJ.y applying sprays of ferrous sulphate- (FeSO
4
?H20) to theleaves. Th~. response was quick greening of the sprayed
leaves, which indicated that at l.east some. af the iron had
been absorbe-d by the I.eaves.
Much af the work on iron sprays has been do.ne with a
view to, sup.plying iron to plants suffering from what is
called li.m.e induced chl.or6·sis. But due to, immobili.zatio·n of
iron once it has pe.netrat.e.d the l.eaf eel 1.s, none of this work
has given satisfactory results. For example, Guest and
Chapman
(1949)
tried more than thirty iron compounds indipping tests, using orange, grapefruit and lemon leafy shoots,
which had be-en affected by lime induced chl.o·rosis. None of
them caused complete recovery. Hilgeman
(1969)
also haddis2,}'lpou..nting results with ferrous sulphate and two i.ron
che]l.ate compounds, when these we-re sprayed on chJLoro,tic
Zinc sprays have given satisfactory responses to control zinc deficiency in a wide range of fruit crops. For example, tho studies of Parker (1937) on mottl e leaf of grapefruit, of Reed and Parker (1936) on mottle leaf of orange, of Dhingra and Others (1967) on citrus chlorosis, of Labanauskas and Puffer (1964) in correcting manganese and zinc deficiency in Valencia ornago, of Paulochova - Kralikova (1966) on little leaf of appl es and of Hoffmann and Sc1J11ish (1966) on control of zinc deficiency in appl0s, are all examples of investiga-tions with responsive species.
Coppor sprays have been satisfactory in correcting
copper deficiency in many plants. The studies of Dunne
(1938)
on Wither tip. or summer dioback of apple trees, of Bada~in (1966),. Tarasov and Kovalenk:o(1
967
)
on tho control of shoot dessication of apples, of Lee (1
96~
-
)
on cop.per do.ficiency in Venturia Country citrus and of Kiely(19
6
6)
on exanthoma of citrus, have all shown that copper sprays have been satis -factory in correcting deficiency symptoms.Manganese sulphate sprays havo controlled manganes
deficiency of many plants. Camp and Poach
(1938)
in less than30
days, abtainod compl0tcly gr een l eav&s when manganesedeficiency of grapefruit by manganese sulphate sprays.
Working with Newtcwnapplc, Uriu and Koch
(1964)
correctedthe symptom.s for tho entire year by zinc plus manganoso
sprays applied at l east twice in early .spring.
Boron deficiency of several fruit and vegetable crops
has been cofrtrolled by tho foliar sprays of borax or boric
acid. Askew and Chittcdon
(1936),
with a single spray of1 per cent hydrated boirax, obtained fo,ur to five fold increase
in boron level in the fruit and prevented the internal cork
sy,mpt.oo of born deficiency :in apple. Early seaso:n sprays of
boron on appl.o have resulted in subst2ntial. increase of boron
in fruits and considerab~e amounts in l eaves (Bramlage and
Thomson,
1962) •
Urea sprays have been studied extensively to furnish a
considerable. part of ni trogen needs of several crop plants.
The response to urea sprays :is variab1o. Urea sprays to
apple trees have increased leaf chlorophyll and leaf total
nitrogen as compared with unsprayed trees (Hamil to.n et al...,
1943;
Ludders and Bunomann,1967),
improved vegetativegrowth ~ lilkcnbaumer and Hohmann,
1964;
Fisher et al.,1948;
Fisher and Cook,1950;
and Fisher>1952).
However,urea sprays to grapes proved disappoint~ (Weinberger et a1.,
1949).
Burrell and Boynton
(1943)
doubled the potassiumSulphate. Ganja ot al . ,
(1
96
6)
controlled soil induced potassium do£ici0ncy of citrus in green houso, with potas s-iWll nitrate sprays.Magnesium has boon supplied both by soil and foliage app li-cation. Findings indicate that foliae;o application of magnesium i s bettor than soi l application in controlling magnesium deficiency in plants.
CHAPTER 3
3.
LITERATURE REVIEW~1. Pathways oJ'...E..~netration of nutrients, herbicides and other substances into the leaf
Bo.th surfaces of tho l oaf are penetrable, but all areas of tho l eaf arc not equally permeable. Prefer Gntia:t areas of foliar absorption named in the literature for various substances (herbicides, other pesticides, nutrients, and fluorochroines) arc:
( a) diroc tl.y over tho veins,
(b) over ant:icli.nal epidermal walls,
(c) glandular and non-glandular trichromes, (d) open stomata,
(e) hydathodos, lonticols, and natural fissures, (f) insect punctures and o,thor imperfections i.n the
cuticle.
Penetration. may be classified as stomatal o,r cuticular, then cellular. Opinion is divided as to whether stomatal or cuticul.ar entry is more important as a genoralizat1on. Both are known to occur under appropriate circumstances.
3.
1 • 1, En try through stoma taNAA (1-naphthalsnoacetic acid) (Harley et al., 1957). This
32 42 86
path was not indicat0d for P , K , Rb (Toubner et al . ,
1957) o,r urea, R0<.lney (1952)~ Volk and McAuliffe (1954).
Many workers claim that stomatal penetration does not
occur unless the surface tension of the solution i s r educef.
considerably by the additian of surfactants (Weaver and
De-Rose, 1946; Turrell, 1947; Norman et al., 1950; van
Overbeek anct Blondeau, 1954). Only open stoaata havo been
reported to be penetrable by oils (Rohrbau0h, (1934)
Minshall and Helson, (1949); not, or only slightly by water
an:.l to varyins ,le1:.:rees by aqueous solutions containing su
r-factants. Cor.1plotoly closec~ stoma ta can exclude al l fluids
(van Ovo.rbeek, 1956). SarGent and Blackman (1962), on the
other hand, pr esentec'. a new theory as r egards to penetration
of 2, 4-D. Accordins to them, 2,4-D does not need to have
a passage between two guard cel ls of stomata, but can
penetr ate throuch cuard and accessory cells.
3. 1.2. Structure of the cuticl.e
3.1.2. 1. Physical nature af the plant cuticle
Accor L1ing to Schieferstein and Loom.is ( 1959),
Brongniart (1934) has described the cuticle as the outer
cellul.ose-free covering o,f l eaves, distinct from the
cuticularized layer s that might be formed be-ne.ath i t . In
s·imple thin !Il:embrane of low elasticity and of poor adherence
to the cell wall (Roelofsen
1950),
according to van Overbeek(1956),
and in higher pl.ants the cuticle is usually complexas sketched in Fig. 1
The true cuticle, according to Lee and Priestley
(1924),
is formed by the oxidation of oily materials which generally
permeate the walls of living cells. The oils are assumed to
be products of ce.11 metabolism, and to have diffused through
the cellulose pectin structure of the cell wa.ll. Lee and
Priestley
(1924)
discussed the cuticle formation and concludedthat various lipoidal substances, formed or mobilized in the
epidermis, migrate to the surface of the plant. There they
tend to oxidize, saturating the double bonds by polymarization
ar oxidation, and thereby cause a "varnish like" covering to
form.
In addition to the cuticle, 1eaves may show marked
accu-mulations of readily soluble surface waxes. The way in which
wax is deposited on the surface af the leaf is not clear. In
many species characteristically shaped projections of wax
arise from the surface,, but the specific sites from. which these
projections are extruded have not been found by Shieferstein
an.d Loomis
(1959).
Their observations suggest that wax isextruded at random through the thin areas of the cuticle.
But
Hall and Donaldson (1962) c]..aimed that epidermal cells af
Fig. 1 .
Hypothetical
structure
of the
functional
.
f the plant
cuticle.
asuec~s o
.
The waxy
rodlets
...
having
.
'""loom"
·u ••may ureven
;.t
of
leaves
contact. of
a
1
f surface
spray
droplet with the
ea
[image:28.536.105.492.87.599.2]wax is extruded. They suggested that wax from a number o·f pores, may form a single wax particle. The configuration of those particles appears to be responsible for wat er
repelling properties of a h0avily bloomed surface. Cutin and waxy material have also been found on the free surfaces of the leaf mcsaphyll cells and on the inner walls of the epi-dermis vhore these are exposed to internal air spaces. The internal cuticle is continuous with the external cuticle through tho stomatal. apertures, whose bounding cells, the guard cells, arc covered with a cuticle on their free surface.
Sc.ct t et al. ( 1948) found tho ou tor cpidernal walls of leavos of citrus sincnsis Osbeck to be cutinized and coated with wax, which se0mod to be excreted through minute canals similar to those in the fruit rind. By treating sections of loaves with IKI-H
2so4, they showed the canals to be lined with protoplasmic threads; and protoplasmic activities •;r.ithin these canals were attributed to the excretion and the maintenance of the waxy surface.
Environmental factors can influence the formation of
cuticle. Stevens (1932) measured the cutic~e thickness of several varioti.es of cranberry for thrc_e consecutive seasons and found up to 20% variation with the season, similar in all
varnish-l ike conuition ch8.racter.1istic of c:1 mot urc cutj_clc.
Further, light anc1 humidity were shoY.:n to affect the
t1icknesG and consistency of the cuticle by their
inf luence upon the o~idation and condensation of fatty
acids, a process involv0d in cuticle format ion. Working
with NicQ_t_iana gl~ll.Q../:2 and Hedei1a. heJ,_ix, Skosr,
(19
55)
obRervcd that the deposition of cuticle wos continuous
until tho leaf reac~1ccl morphological maturits,; beyond
maturity no furtlwr deposition occu1°red. LeaveG grown in
the sun produced heavie r cuticles of greater wa.x content
than l eaves gro~n in tho ohade. Th0 tomperGture conditions
under v,hich pl P,nts vrn re grmrn 1nc re sho'.rn to inflLwnce the
deposition of cuticle 2ind rrnx. T~10 1.110::,t cut icl e 1::as
produced at a median tcmpcrnturG, c1nd tllo r.roatost
percent&ge of wa:~ at a i1i 3h tumrerature. Pl crnts undergoing
water stress produced cuticles containing a greater
proportion of waxeo than plants ~ith mor~ favourable
moisture conditions.
the
3.1
.2.2. Che~ical nature of/plant cuticleAccording to Foy et a~.!
(1
96
7),
Frey-Wyssling(1948)
has summarisecl the chemical nature of cutinizcd plant cell
wal lsy which are composed of four distinct substances~ all
of which may vary in distribution within the wall. These
substances are: (a) cutin, (b) outin waxes, (c) pectin and
(a) cellulose. All constituents contribute t heir own
physio.,chemi cal properties to the surface layer of the
(a) Cutin:- Cutin has boen described as the polymerisation and condensation of
c
18 hydroxy fatty acids which ar esynth0sized in the protoplasm and penetrate the wall as procutin. Extruding on the surface, these precurssors ~re 0,xidizcd and polymerised, forming a sponge-like frane of
submicroscopic dimensions. Cutins contain reactive end
groups v1hich enable them to form esters and ethers. Also, cutin may contain an appreciable amount of dicarboxylic acids and hydroxy carboxylic acids. Having many such polar groups such cutin may absorb water and swell and this increases permeability (van Overbeek, 1956 ) .
Lee (1925) made a chemical study of cutin and showed i t
to be a complex mixture of fatty substances, consisting primarily of free fatty acids, fatty acids combined with
ma-nohydric alcohols, and soaps. Legg and Wheeler (1925)
iso]at.ed two, acids as major compo·nents of cutin and suggested the names "cuti.c acid" (c26H
50o6) and cutinic acid (c13H22o3). Other acids were isolated in much smaller quantities.
(b) Cutinwaxes;- Crafts and Foy (1962) state that cutin
waxes. are short chain eqters and aJlcohols o,f relatively low
moJ.ecul.ar weight, lacking reactive· end groups and unabl.e to pol,Ymerise. Cutinwaxes are aptically negative, stainable in
li~id dyes, melt above 220°c, and do not absorb ultra violet
Foy et
ca
l. (1967)
state that waxes are hydrophobicin nature and hence 1.1esistant to wet ting v1ith pure water.
The waxy rodlets of the leaves f orm s 11bloornli v1hich may
prevent contact of a spray droplet with the leaf surface (Fig.1 ), but the addition of a suitable surfactant to the
oil or aqueous sprays may facilitat0 the wetting of waxy
leaf surfaces. Further enhanced wetting does not mean
enhanced penetration, because surface wax deposits may
interfere_ with wetting but l i t t le with penetration,
provided good surface contact is ensured.
(c) ]?ectins:- Crafts and Fof
(1
962
)
st ate that pectinsconsist of long chain polygal~cturonic acid molecules
having side carboxyl groups which can forn1 salts and impart
to pectins base exchange properties. Polygalacturonic
acid and its methylated derivative are soluble in water,
but its calcium salt is insoluble. Pectin substances have little tendency to crystal l ize; they occur in an amorphous
state in pl ant cell walls, and they are responsible for the
strong water holding properties of the walls .
(d) ~lulo~e~:- Foy ~t al.
(1967)
describe the celluloseas composed of long chain molecules which arc r~l_at~ ~e.l:Y stable. These molecules are organised into micelles. The
micelles are associated into microfibrils, and because of
its microf ibrillar organisation, cellulose imparts tensile
i.t i.s this prapcrty af cell walls that r esists c-xpansion and results in turgor. Turgor, in turn, enables the plants ta grow er ect against the force o-f gravity, to extend roots
into the. soil, to absorb water and nutrients, and to main-tain its foliar organs in posi tio,ns favourab].e for maxi.mum absorption of carba-ndi..oxide and light.
3.1.2.3, The role a,f the. cuticle
Epidermal cells of l eaves and sto@s, unlike roots, are
mc.:,-r e or l_ess covered with thick cu tic le, whic.h ma~' be s. uper-im.posed by wax excrusions. The lipid character of these waxes and cutin layer can produce an obstacle for tho pene-tration of hydrolphi.lic substances. It is belie.ved that the obstacle created by tho cuticle is so groat that the penetration of the hydrophilic substance occurs onl...y through stomata. However, the stomatal passage allows solutions only to e.nter in st0111ata1. chambers and interccl1.ular spaces, but n0t in the cells, because the out.er wal1.s of the cells lining these cavities are also covered by an internal cuticle
(Scott,
1950).
Also, the hydrophoibi.c nature of the cut:icl.elining the stomatal p0;res normally does not permit. tho passage o.f tho aqueous solutions. Use of detergents may induce this passage (Dybi.ng and Currier,
1961).
Thus, under natural conditions, absorption of solutes must take· itsThe question at O;nce arises whether there is any po.re in
the cutic1.u, providing pathways for penetration.
Electron-microscopy studies reveal that there are no pores in the
cuticle other than local thin spa.ts, punctures, breaks, and
fissures made by insects. However, Hal]. and Donalds.on (1962)
have found true perforations i.n the epidermal cells of
Trifolium.
repens
and Brassica o1e.racea. Wax is said to beext.rud e.d through these pores.
Orgell (1955) has described cutic].a as being im.bricate
arrangements of lipoid platelates cemented together by
hydroph:U.:ic pcctane-ous substances. Thus, an intercuticu.l.ar
penetration should be possible for a polar so1ution. But
there is no direct proof of such a pathway. Thero.fore, as a
rule, in foliar absorption, substances to be absorbed have, in
contrast to the situation in roots, to penetrate a lipid like
l.ayers·. This means that an "intracuticul.ar pencrtration11
without pathways through distinct pares has to be performed
before solutions can c.nter tho cellulose wall.a (Wittwer and
Teubner, 1959).
3.1.3.
Structure and nature of the cell wallAccording to Jensen (1967), celL walls vary greatly in
composition and maTphology. However, the cell wall consists
of three layers, middle lamella, primary wall and secondary
and e1astic. It increases in area as the cell grows. A
secondary wall is formed between the cytoplasm and the primary wall, when the growth of the coll ceases. The secondary wall is often thick (5-10 /u) and rigid, providing great tensile
strength to the cell. The primary walls of two cells are joined by a common layer call ed the middle lamella.
The coll wall when first formed is very thin, but increas-es in thicknincreas-ess through the deposition by the protoplasm of
new particles upon those already present. Thickness of the
eel~ wall sometimes becomes so great, that it almost fills the cell cavity (Anderson, 1927).
The chemical nature of the cell wall is loss understood and thought to be a complex one. Miller (193_8) provides a
diagram· which shows the substances that may compose. the cell
Material ( ( that may ( ( enter into ( ( the com- ( ( position ( (
of the (
( cell wal l ( ( ( ( ( ( ( ( ( (
Cellulose ( lformalr typical or trve celluloses
~cl.
( EYdroc elluloso2. 8 types
( o~~c elluloses Y
( Crnrc1po,und celluloses - Lignoc oll.ul.oscs
( Homicelluloses (Skel etal )P3ntosans )In tho cell
( or (ho ni ) xy lan ) walls o f
( -pseudocellulosos(ct:Jlluloses)A:i:'.aban )wood and
( )~l~ctosans)seed coats. (Reserve
(hemi - )Pentosans )In tho cel l (celluloser,)~ylau )waLls of
)Araban ) endospn:rms
Suberin and cutin
Pecti.c Substances (Pee.tic acids
(Pectatos
(Pectosc (Pectin
)Mallilosans ) & you:u.g fibres
.Galactosans) of wood & bark
Other c ons.t.i t-ue.nts:
(Resins, gums, tRnnis·, minertls,
(Colo·uring mat ters, pr')toins,,, fG.ts, phospho li-(p:ides, oth0r o::1l oils a:1d callose.
Cellulose, composed of thousnn~s of r 0~e3tins clucoso
units , is tho major structural eloBont of tho cel l wall,
ospeciP.lly i n the c,_,se nf pri,1:~.ry Vl.'.'.11 (Jonsen, 1967).
Cellulose has '.·ecn r 0p.:,1rt0,:1. t:., '.,o L:>os8ly O!:J.'.)edc':.or1 in
thu :anr tix i n tho f.:;rr,1 of :,!icrofi~ .. r ils (Fr oy-1Vyssling ancl
Muhlothc.l er; 1965) Tho ;.;~i crofi~)rils , in the seconC.>.ry wal l which cont.?,i!1s 60 t~ 94 por cont cellulose 2.nd only a l i t t le mrtrtix, ar e cl,)sol y pc.eked n.w: interwoven into a net work.
Ther e exists intor sp:ccs r.,etweon t:.c i:J.-icrofibrils 2-nd
botweon the el omc21t.:-.ry fi:;rils (Frcy-Wyssling and Muhlethal er,
1965). Tho sizo of the intorcicol l~r spaces between the elementary fi~rils ic ahout 10 A0 3nct ther efor e they shoul d be penetrabl 0 by smal l mol ecules such as wator and halogon~~ns.
Tho size of the intorfi½r i l l~r spacos between tho microfibrils 0
however, has ,_ oen r oacl].ed up to 100 A • Hence, l arger
molecules should p.:1.Gs freely if the spac cs ar v not filled with non-cellulose mP.torial (Frey-Wyssling and Muhletllal or,
1965). Other c o;upononts of tho primary vml l ancl tho r:1iddle
lamel la ar e tho pectic substances . Thoso ar c lar0o mol ecules
made up of r cp<w.tin5 units called hexuronic 2.cids, which ar e derivatives of hoxose sugars (Jensen, 1967). Still other
cocrpounds ar e foun~ in the walls of many calls. Chief among them ar c tho i':axes, which constitute the surface
Ectodesnata:- The discovery of extoiesraata in the
ou~er
wall of the epidermal cells has provided a now outlook for foliae;o penetration. Franko ( 1961) has studied tho occurrence and ci stributi0n of octodosnata i n tho opido~m~G of Plant€'..0omajo~ and HeJ_xino sol oirolii. Leaf struc turos, such as guard cel ls, conical hairs, anticlinal walls and tho opidermal cells adjacent to tho l eaf vei ns have Leen observed to contain large nurabors of ectodosmata. Since ectodosmata extend from the cuticle, which they do notperforate, through the wall t o the lumina of epidermal cel ls, they seem to provide an almost direct connection of tho
3.1.4.
Structure of the plasma-membraneThe cytoplasm is surrounded by a layer callc(1
plasna-membrane ur plasmalelllila which i s thin, flexible and not
directly visible to the light micro·scope. The plasmamembrane,
~ccording to (Jensen, 1967) is composed of protein and lipid.
Their molecules are arranged in the membrane in such a way
that the protein molecul.es are aligned on the exterior of the
membrane and-are bound to the lipid moiecules which occupy
the centre.
Frey-Wyssling and Muhlethaler
(1965)
have mentioned avariety o,f functions which are performed by the plasmamembrane~
It controls semipermeability, resorption, excretion and se cre-tion, leading to the formation of slime and a whole. series of cell-wall substances, and it is al.so capable of breaking down substances enzymically.
Jensen (1967) has mentioned the advantages a,f the
semiper-moability of the membrane in that it prevents the organic
materials of the cell such as sugars and soluble proteins
from leaking o:ut of the cell whiJu~ allo,wing water and sal.ts
to enter, and this maintains the integrity of the cell i.n
rel..at:i.o,n to the surrounding enviromaent.
The piasmamembrane provides no passage as such for tho
adsorbed in the- plasmamembrane and then taken into the cyto-plaso by a process requiring metabolic energy (Jensen,
1967).
3.
1.5.
Mechanisms of fa:J:.iar penetration3.
1.5.
1. Mechanisms of p,:-,ne!_rqtion in the cuticleMany workers, in order to study the penetrability and mechanism of cuticular penetration, have ex~uined the isolated cuticles (Skoss,
1955;
Schieferstein and Loor.li.s.1959;
Goodman and Addy,
1962;
Darlington and Ciruli.s,1963;
Yamada, Wittwer and Bukovac>1964,
and Silva Fernandes,1965).
These studi.es have confirmed the penetrability o,f the cut.icular membranes.permea-bility of the isolated cuticlGs, brought about by tho changes
in physical and chmlical characteristics o-f isolated cuticles,
during tho process of thGir i solation by chemical moans.
Somo. pormeabili ty, surfac o binding, and ion exchange
characteristics have beon reported for onzy~ically i solated
cuticalar membranes (Yamada, Wittwer and Bukovac, 1964;
Yan.all a, Bukovac and Wi t tvrer , 1964). They include the
sto~atous groon onion leaf cuticle anc the astomatous tomato
fruit cuticle. Cations and anions wore bound to- the inner
surfaces of cuticlos to much greater extent than on tho outer
sur faces. With di alyzing membranes, no such differences between the two surfaces occurred. Approximately, throe
tiaos more cati0:ns were bound on the inner surface
(7
.
8
9
2 m
1
u mole per c~) of tho tomato cuticle than the outer surface(2. 73 m
1
u r1ole/cm2). The possi ble explanation of suchdifferential ion binding by surfaces of isolated culicular
membranes has been given as the mo-rphola,gical differences in
the cuticular surfaces. The outer surfaces were smooth, while the inno.r surfaces were irregular, having protrusio.ns and
cellu].ar fragments. A further possibl e explanation for the
differential ion binding by the two surfaces as given by
y·aoada,, Bukovac and Wittwer ( 1964), has been rel at ed to, the physio-chemica1. propertie$ o·f the two surfaces. The outer
It has boon rcpo:rted that cuticular pcnetratia.n from outsic1e to inside is gro.ater than from inside to o,utsicle. This type of behaviour was first observed in tho stomata free cuticl.es of Hedera helix leaves in which more water penetrated inward, i.o. towards tho protoplast than autwarc (Schieferstein and Loomis,
1959).
A similar typo o-f result was obtaine~l for the penetration of -r~~ioactivo cations (ca45 Rb86 ) and anions
s
35o
4,
cl36 )
an'.l also for unclissociatocl organic compounds through onzym-ically isolated cuticles of astomatous tomato fruit and stomatous green onion l oaves (Yanada, Wittwer and Bukovac,1964;
1965).
Again the penetration of cations from outside to insiclc was more pronounc od than tho anions and tho rate of penetration through different cuticul.ar surfaces was directly rolateJ to the extent of ion binding on the surface which was opposite the· site of the initial entry (Yamada, Wittwer and Bukovac,1964).
Crafts and Foy
(1962)
have shown tho evidence of a gradient from low polarity on tho Gx.terior t.o a rolativol.y high polarity in the layers born.oring tho epidermal coll-wal l. In any case t.he negative c_harges characteristics o,f cuticular membranes offer reasonable exple,nations for permoabilityHowever, Goo~nan and A~dy
(1
9
62)
obtained an opposi teresult for the penetration of ,1 number of organic compounds
through appl a cuticles i. o. the pcnctr,;tion of o.rc;anic co;:;1
-pounds fror;1 inside to o:utsi(1.c was significantly gr ontor than
from outsic'.o to inside. In this caso tho cuticl.es were
isolc:.tcc.l wi th nmrnoniurtl oxal ate anc. oxalic acid. Such
chem-ically se:paratec:. cuticular mGmbranos might be diffor ent from
thoso isol~ted cnzyraically and their structure might have been
al tered, thus causing the reversed bchaviror. Further, the
permeability to organi c compounds might be different from that
of inorganic ions.
Urea, cts r eported 1.:y YarJada, Wi ttwcr and Bukovac ( 1965)
penetrates the cuticular menbrane:, with a higher velocity than
couJ.~1 bo expoctod fro• the siiilplo c.1.iffusion.. The penetration
of urea, thus, through tho isolated tomato frui t cuticle was
10-20 times gr eater than tho penetration of inorganic i ons
(Rb+;
c.:t-+ ,
er,
so
4 --·) They concluded that urea mi ght have penetre,ted through the facilatcd diffusi on. Thero is supp0J
rt-:L.ng evidence for fncilatccl diffusion in that urc.a promotes the
uptake of other nutrients similta.neously applied (Wittwer and
Teubner,
1959;
Labanauskas and Puffer,1964).
The effect ofurea on the permeability as reported by Yamada, Wittwer and
Bukovac ( 1965), is based on tho l.oose.ning a-f the membrane structure by changing ester, ether, and di ethe.r bonds· between
the !llacromole-culcs of cutin.
rrnd un1issociato,~ IJOl oculos cc.n penetrate tho isol.::i.t oc1
cuticular m0m.branes by c1oans of sim.plo diffusion, while uroa
seems to pono.trc3.tc by a process of facilatoc~ diffusion. In intKct tissues sinilar physical processes might be involved
during penetration.
3.1.5.2. Mechanisms of __p_o_n_<:tr;at1:~n in the coll W-:1.l l
As soon as the substances penetrate the cuticle and
cuticular layers, they roach tho c ollul.oso layers of tho wall. Tho interfibrillar spaces between tho cellulose fi!,rils are thought to be large enough for the penetration by diffusion.
Still i t is believed that w~ll. penetration takes pl ac o th.rough
sopa.rA.tecl pathways. These pathways ar0 ectodesrnatn. FrarJrn
(1961) strongly believes that nutrients follow these pathways.
However, Sutcliffe (1962) states that tho contonts of tho cell wall. s-uch as cellulose, homice.llulosc, pol.yuronido-s and
phospholipids arc capable of binding salts in tho farms which are readi1.y e.xchange2,ble, and hence the wall. acts as a Donan system containing an appreciable concentration a,f negative charges. The movement of ions tokes place froiu th0 oxt0rnal mediucr, by di.ffusion and exchange, to tho surface of the cytoplasm.
3
.
1.5
.
3
.
Machanisms of _penotz:.at1:on in the 1?l asma-membranebarrier, tho plasmalor:lli1a on r c.::tchinG tl10 protoplasts.
AccorC:i115 to Jonson ( 1967) the pl2.smar:1onbrano provides passctge for physical penetration of water nolcculos only ani a largo number of subst-'3.ncos ar o incorporr.~tocl ';,y a process r equiring i:1ot;lbolic energy. This process rnay involve a carrier systen which involves the pickins up., 2..t tho oxtornc1.l surface o,f the plasme.me1..1":::rano, of a substance to be transportc(l by a carrier into the int crnal surface o.f tho oembraue.
Jenson ( 1967) provides the cvhlo-nc o of another nochanism, which may be r esponsi bl e for tho activ5 uptc.ke of tho sub -stances. This is cal ler'. Pinocytosis, which involvc::s the invaginations of cel l mcmLr ano and formation of vesicl es
within the cytopl asm of tho cel l . The; vesicle may br eak down and tho material cont:,.inec1.. in 1.t i s finally utilisoc", by the cell~
Tho ovL\enco cf active absorption o,f rul:idiuo ~n~~ phosphate by the prinary leaves of tllo bc.s:.n plant has been
dCL,onstrato::1 by Jyung an.d Wittwer ( 1964) and for tobacco l eaves by Jyung et al. , ( 1965).
Tho energy require~~ for active absorption ma;y 1:,e obtained fron respiration and photosynthesis. If the energy is pra,vided by r espiration, lack of oxygen sh.oulcl r o:1uce tho uptake, anc1. if the energy is provided by photosynthesis tho presence of CO
2 and light shoulrl accelerate the uptake. The fact that the light iflproves tho uptak0 of several substances has been
1965; Jyung ot al . , 1965; Middl eton and Sanderson, 1965).
Ener gy is thought t o be mostly provides in tho form of
ATPP Honcc, the acLli t ion of thi.s compound must increase tho up:tako. This has boon :-1o~i1o,nstratecl for the uptake of Rb+ by
the ce-lls onzy:mical ly isolator'.. fro:n tobacco lcavos, whor e +
ad.J.i tion of ATP, incrcasuc1 Rb uptake by 13% (Jyung ot al . , 1965). Also, application of high enor gy pr oc'..ucts o,f photo.
-synthesis along with tho substances to bo absorbod, has
increased tho upt ake of the latter. For oxa:,1ple, feiac'.ing with sucrose enhanced tho uptake of l eucine by apple l eaves in the dark (Kamimura and Goodman, 1964). Similarly, ins t oad of o•xygen, the acldi tion of intormo·:1.i ates of tho r espirator y motabolis1;i,
enhances tho upt alrn of the sullstanco to bo absorbed.. Thus , feecling with succinate increasoc: the Rb-+ uptake by cells of enzymically i.sol.ated from tobacco l oaves in tho dark and light (Jyung ot al., 1965).
From tho datails c1.oscriboc.1 above, i t appears that the foll.owing stages ar o involver! in the over al l process of foliar uptake.
I. In the first sta6o, substances applied to the surface of tho l oaves penetrate the cuticle, and the cel l wall through a physical process of diffusion.
III. In t.he third sta6G, substanc cs aro t.J.lrnn into the
cyto-plasm, by the processes requiring metabolic energy.
During stage I ancl II, sut.stancos only fill u~ the free
space ant~ merely adsorb. Thoy G.r c not ta.ken up into t.hc
cyto-plasm. This has been shown from tho fact that c1.uring these
stagesi substances ar o again washc<t out (Vickery anc1 Mercer,
1964).
The fe..ct th9.t tho ovorall foliage penetration involves
tho above three stages can be seen by illustrating tho work of Vickery and Hore Cff ( 1 964
i
ancl Allen ( 1964) • Vickery andMercer
(1964)
r ccoeise thrc0 stages of uptake of sucrose bybean loaf. (:i) an initial r 2,picl uptake of sucrose.. (ii.) a
linear phRse (iii) a phase in which uptake 0.0crcases. Phase
{ i) and (ii) may correspond to tho stage I anc1- II of the coll
wall penetration and adsorption, and the third phase I!lay
correspond to the activo uptake, which requires metabolic
All.en (
1964)
has provided much information on these vario,usphas.es of uptake of r:tetallic io,ns by apple leaves. He r c cog-copper
nise:s four phases of uptake of .~ by appl.e. l.cavos. His results
conf:i.rm that :Ln phase (i.) and (ii), the proco.ss involven. is only adsorption. In phase (iii) there is ~ocrcase in uptake,
which corrGspornls to active uptake requiring metabolic energy.
material. from within the leaf is simultanoously released into ctue
tho solution /4o tho broakngo of some barrier to diffusion.
Finally, after the incorporation of the. substances into tho proto,plasts of the epidermal cells, a trans1ocation of the
absorbed mat0rial.s to th8 o.thor areas of the l eaves: or to other
plant parts takos place regularly.
3.2.
Factors affecting penetration and movement3.2.1. Plant factors
3.2.1.1. Physio-chemical nature of tho
~~n.t
cuticlePlant spocios cFffor in the physio-chemical nature of the
cuticle. Environmental factors pl.ay an irnp())rtant role in tho structure and camposi tion o.f the cuticle. For exa1:1ple, the cuticle, is generally thicker on leaves which have developed :iLn bright sunlight. than which develo,ped in shac1.e, and the wax
deposition in the former has been found to be eroater than in tho latter (Skoss,
1955).
The point is that in tho case of sun grown plants, ther e is high rato o,f transpirati.on andhence p1ants go through a per iod of water stre.ss, which causes heavier cutinization of leavcsw
Crafts and Foy ( 1962) stat·e that deve.I.opmentally the
abrasion !'lay r1ctorioratc tho. cuticle 2,ncl make i t imperfect as
a layer.
S~lva Fernandes (1965) observed that tho extent of
cuticle thickne8s awl wax deposi t ion has an iuportant effect
in tho ponetrati.on. No penetration o,f copper soluti.ons from
copper acetate or sulphate or i:rnrcury fro,J solution of
phenyl-ncrcuric acetate occurro-::l through astomataus mcnbranos con-taining
o.
l mg/cm:2 or more of cutin. Wax offered an IDport-ant barrier for the; penetration o,f mercury through tho mem-brane, but the effect of the wax depended on its composition.
Ponotration of mercury incrcaso1 by an increase in the
percentage of esters in the wax.
Crafts ,.md Foy ( 1962) have mentioned that both inward
and outward movement of aqueous solutions is restricted by
the heavy and 1J1ore o-r l0ss continuous deposits of cuticular
wax. Tho irregular surface wax deposits inf1uencc the
wetting and contact angle botwecm droplet of solution and l.caf surface.
The contact angle between water and tho waxy corn:panent
will be highc-r than between water and the cutin component
i.e. water wets cutin bettor than wax. This is only because
o,f binding capacity of polar groups for water through hydro
-gen bonds, while hydrocarbon chains only attract water through
tho much weaker van dor Waal's forces. (van Ovorbe8k, 1956).
o-sition of cutin components, they also diffe-r in wottability an~ this ruay affect ponutration.
3.2.1.2. _Af!,_G o·f tho l eaf
Th0ro iE an al•ost universal agr0emcnt that the younger loaves show cr eator aLsorption than tho olc'..er ones. Tho nutrients include urea, Cain
(1
956
);
phosphorus, Fisher and Walker (1955); Koontz anr':. Biddulph (1957, and Thorne (1958); MagncsiUIJ, Oland and Opland(1956)
ancl Zinc, Wallihan and Heymann-HorschLcrg (1956). The reason for the greaterabsorption ty the youncor leaves nay bo thinner cuticle-s and
less wax ~oposition. In advancing ace l oaf cuticles become thicker, and thereby nay inhib~t penetrRtion. But tho ~cp o-siti.on of cuticle as roportec'. by Skoss
(1955)
talrns place only to a certai n stac;e of rn.orphclo·gical maturity, beyond that no further deposition occurs.3
.
2. 1.
3
.
L.e.~f . s~rfac e_s -~nd mprpho_logy_Horizontal, hairy, pubosc ont leaves retain r.ia:xe spray than the vertical. anct smooth l oav0s (Ennis et al., 1952); and this results in gr eat er absorption in tho former typo of leaves than in the latter. Upper anc lower surfaces of ieaves also exhibit rhfferent rate of absorption. Usually the 1.a.wcr
Chapraan, 194-9; Cook and Bo.ynta:n, 195•2; · Volk and McAuliffe, 1954; Bennett and Thomas, 1954; Gustafson, 1956; Cain, 1956;
Gu.s:tafson and Schlossinger,
1956).
Tllis is porhaps duo to tho presence o:f thinner cuticl.o., numerous stomata, pronincnt vo.ins and roughness of the lowc-r surface of the l oaves. HOJwovor, species differ as to which surface func ti.ans most in absorptio,n ( Gustafson, 1957).
3.2.1
.4.
Mineral . status of tho 1.er1fApple leaves havinc; highor status 0;f nitrogen have shown greater absorption of urea, Cook and Boynton (1952), magnesium, Forshey (
1959;
1963) than the leaves o,f loVIor status cfnitrogen. '!'his increase in a·:Jsorption has boon partly attributec. to the difference in gro,wth af the plants because in above studi o.s, plants grown in high nitrogen conditions produced rapid and quicker growth of the plants and loaves wore comparatively larger and greener than tho low ni trogon plants. Under these concl:itions the cuticle might be expected to be thinner than on tho slower growing smaller I.eaves of l.o,w nitrogen tr0es and thus, more easily penetrable.
phos-phorru:: · than thoso grown in phosphorus c1..oficiont J:J.cdi a.
However, the results of Koontz and Biddulph (
1
957)
inc1.icatc that, plants grown in rich phosphorus ncdia translocate filOJre phospho,rus than those grawn in phosphorus deficient media.3.2.2.
External factors3.2.2.1.
Li1;htLisht may affect tho absorption both directly or
indirectly. It cray favour tho absorption directly by stimu-1.atinG thC; openin0 of stomata or indirectly by supplying
enorGY throuch photosynthesis. Licht may also affect tho structure of loaves and thG condition of stomata and thereby nay affect absorption. Tho fact that lisbt intensity and quality inprove the rate of al:sorptio,n of various substances has been dm:,mnstratec:. by several workers. (Sari:;e.nt and
Blackman,
1
9
62;
196
5
,
Kamirluraanc
Goodcran,1964;
Middleton and Sanderson,1
9
65).
3.2.2.2. Tomp~ratur o
C~rrior and Dyb{ng (1 959) state that warm t emperatures,
if not oxcossivo cnhanco penetration by affecting physio -chemical processes (increased rato of diffusi on, lororod
viscosity etc. ), physiological factors (accel eration of
photosynthoci.s, phloem translocation, accumul ation processes,
protoplasmic s tr c.aming and grovrth).
Teubnor ot al . (1957) found increased mineral absorptian and transport by boan and tomato l oaves wh~n the tomporatur o
\Fas increased from 50°F to 70°F. Wi th inc:toasing tor,1peratur o
tho penctra_ticm of 2, 4-D (Barrier an:i Loomis, 1957; Rico, 1948) and NAA (Harley et al., 1957) also incroasod.
It has boon pointed out by van Overbeck (1956) that tho penetration through tho layers of oriented fatty molecules is highly t emperature dopondont. If fatty molecules ori ented in l ayers as micelles ar c in tho solid. state thoy ho,Vtl .a lower permeability. In tho liquid state they have a higher pormoab -ility;when totally disorganised,as at lethal temperature~ such l ayers can no longer servo as permeation barriers. Since this temperature effect is on tho bGrri er itself, the principles stated by van Overbook (1956) must hoJ:..d for any material,
3.2.2.3,
~umidityA rcl c:l ti vely high hur.iidi ty has incronsed the ponatration of a nm:iber of foliage appliod subst2,nccs. Currier and
Dybing
(19
59)
have statod tho effects of relatively highhumidity on ponetration. It delays drying of the spray
doposi ts,, prevents water stress in the plant and IDP..Y increase
cuticul.ar permeability.
Increased hydration of cutin raartix, according to van
Overbeck
(195
6
),
may spread tho wax components further apart;because the wax components aro not elastic ns i.s the spongy cutin framework. This, in turn, will increase the
por~eabil-i ty of tho cuticle. High turgidity of epidor~Al cells and other underlying tissues would havo a similar result.
Con-versely, lowor water content of cutin and underlying tissue
will cause shrin..l{age, and th.e wax uni ts will bo pulled close together and this will decrease the cuticle permeability. Thie proposed ''Valve system" has great significance for the
water baJLanco o·f the 1.caf. When the cuticle is dry, tho valve
effect restricts water loss; when the cuticle is wet, tho valve system allows water uptake.
Similarly, various substances are known to penetrate the
cuticle most efficiently in saturated atmosphere. For example,
(Rico,
1948;
Holly,1956).
High humidity also has significnntly increased the uptake of strontium: - 89 and caesium 137 than low humidity (Middleton Rild Sanderson,
1965).
However, no effect of hum-idity has been found on the uptake of suge.r by toi.:lato l eaves(Went and Carter,
194
8
).
3,2.2.4.
p~ of tho ~pray &olutionsThe effects of solution pH on permeability and pene tra-tion c:.r e cor:1pl:icated by the facts that whether the results are caused by effects on degree pf dissociation of solutes, or whether tho changes in porr:ieability are brought about by the nombrane itself. In cases, where the latter influence may be
more pronincnt, there i s still the question of whether the
effect is due to hydration of the plasma-membrane or to a hydrolysis effects on membrane components.
Tho rosul ts o,f Cook and Boynton (
1952)
suggest that the per cent urea absorbed by apple loaf at pH5.6
is greater than at pH8.
Similarly a pH of2
to3
as compared to higher pH of appli.ed phosphate solution facilitated more rapid uptake byleaves (Silberstein and Wittwer,
1951;
Swanson and Whitney,1953;
Fisher and Walker,1955).
As shown in Fig. 2 the amountQ) +:> :::s s:! ·rl E ... Cl) +:> s:! :::s 0 C) ~ 8 H :> H 8 0 <i:
Fig.2. The
level of
P32
activity found
in
the
petiole
following a 4-hr. period
of translocation
from
t
he
blade as a function
of pH of the applied solution.
Ea
c
h p
oint
an
average of at least 3
p
lants.
:Dat
a
represented
by the symbols 0,
e,
and
Ataken
from
separate
experiments (Swanson and
W
hitney, 1953).
3500
(I
3000
~
0\
2500
0
2000
01500
1000
500
0
0
\A
1
2
3
4
5
6 7pH
Tho role of pH in promoting absorptioh has been attributed to two f?.c tors, tho ef feet o•f high acidity in suppressing the dissociation of phosphoric aci~ and a possible dir ect effect of pH on the permcf.:'..bility of e:pidermal. and subjacent tissues
(Swanson and Whitney,
1
9
53).
In case of divalent ions (Ca++,Mg++) the effects of pH on relative absorption docs not appear to h~ve been elaborately studied. Mor 8 investigations in this line arc needed.
3.2.2.5.
SurfactantsSurfactants arc used to increase tho effectiveness of spray solutions applied to tho foliage. According to Currier and Dybing (1959)~ tho r esponse to surfactants mQy be due to one or more of the following:
(a) i~proving coverage;
(b) removing air filn1s between spray end leaf surface;
(c) reducing interfacial tension between relatively polar and apoln.r submicroscopic regi ons cf tho cuticle;
(d) enducing sto~atal entry;
(o) increasing the perr.ioability of the plasr.1a-me•brano through stimulati.on or incipient toxicity;
(f) facilitating cell-wall novcment in the region of the wall cytoplasm interface;
(g) acting as a cosolvont;
