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Mechanisms of foliaruptake of plant nutrients:Accomplishments andprospectsSeshadri Kannan aa Biology and Agriculture Division , BhabhaAtomic Research Centre , Bombay, 400 085,IndiaPublished online: 21 Nov 2008.

To cite this article: Seshadri Kannan (1980) Mechanisms of foliar uptake ofplant nutrients: Accomplishments and prospects, Journal of Plant Nutrition, 2:6,717-735

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JOURNAL OF PLANT NUTRITION, 2 ( 6 ) , 717-735 (1980)

MECHANISMS OF FOLIAR UPTAKE OF PLANT NUTRIENTS:

ACCOMPLISHMENTS AND PROSPECTS

KEY WORDS; Nutrient absorption by leaf, mechanisms,cuticular penetrations, absorption by leafcells, nutrient transport from leaves.

Seshadri KannanBiology and Agriculture DivisionBhabha Atomic Research Centre

Bombay 400 085, India

ABSTRACT

It has been established that the plant leaves and other

above-ground parts are capable of absorbing chemicals and

nutrients. However, there is great paucity of information

regarding the mechanisms of ion uptake by leaves and their

transport to other parts. Equally not well understood is

the phenomenon of retranslocation of elements from one leaf

to another during the different stages of plant growth. The

results of investigations carried out in this area of foliar

uptake and translocation in the last few years are described.

In this review, the importance of future studies to investi-

717

Copyright © 1980 by Marcel Dekker, Inc.

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gate the mechanisms involved in f o l i a r transport, the

influence of plant growth substances, and the effectiveness

of timely supply of specific nutrients during the grain

development stage, i s stressed.

INTRODUCTION'

The leaf i s the principal seat of several metabolic

a c t i v i t i e s in higher plants, and the mineral elements which

are both s t r u c t u r a l and functional in nature, reach t h i s

seat essentially to participate in the acquisition and

fixing up of the atmospheric carbon. This feature of plants

of terrestrial habitat differs from those which remain in

the aquatic medium, in that all the parts, namely, the leaf

and the roots of the latter carry out both the abstraction

and assimilation of nutrients. Leaves have been employed as

a portal of entry to the plant system, for growth substances

and herbicides to modify or control plant growth, and also

for systemic insecticides to prevent infestation. In all

these cases, the pathways and mechanisms are largely similar

to those for nutrient uptake.

The mechanisms of foliar uptake as well as the effici-

ency of foliar feeding of various nutrient elements have

been investigated more intensively during a period of 2

decades from the 1950s in a number of laboratories, espe-

cially by Wittwer and associates at Michigan State Univer-

sity, and the subject has been reviewed ' ' . However,

the concepts of plant nutrition relating to root absorption

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FOLIAR UPTAKE OF NUTRIENTS 719

have changed enormously in the l a s t few years, f i r s t l y by

the discovery of dual mechanisms of ion uptake , and secondly

when the plasma membranes from r o o t s could be i s o l a t e d and

ion binding and the r o l e of ATPases are b e t t e r understood .

Further, t h e r e have been breakthroughs in the mechanisms of

photosynthesis, significant of which i s the identification

of C, and C. pathways. The foliar uptake of nutrients i s

closely associated with these processes and the prospects of

foliar nutrition research in the light of these and other

advancements are great and would be equally rewarding. In

recent times, indiscriminate and heavy f e r t i l i z a t i o n of soil

have threatened our environment, prominent amongst these

being the increase in the nitrogen content of ground and

river waters, which affect the aquatic animal foods. Foliar

supply of nutrients when found effective, would minimise

these dangers.

PERMEABILITY OF CUTICLE - A PREREQUISITE TO FOLIAR ABSORPTION

The leaf is covered by a non-cellular cuticular struc-

ture which has been studied chemically and functionally ' •

Leece has observed that the differential foliar absorption

amongst plant species was due to the differences in cuticle

thickness, weight, surface wax and wettability, amongst

other factors. He has indicated that foliar absorption can

be enhanced by improved partitioning of the waxes via the

stomata. I t has been however emphasized earlier that th

stomates are also covered by a cuticle, and are not mere

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perforations, as widely misconceived. Yamada et al

isolated cuticles enzymically and reported that the pene-

tration rates of ions through both the astomatous (tomato

fruit) and the stomatous (onion leaf) cuticles were the same.

On the other hand, the penetration of Fe was found to be

very much higher through stomatous than through the astoma-

tous cuticular membranes isolated from the same plant spe-

21 15

cies, Euonymus japonicus , or Coffea arabica . Further,

molecular weight of the solute has significant effect on

the penetration, substances with higher molecualr weight

penetrating rather slowly . The ionic radius and degree of

hydration also influenced the penetration rates which gene-

rally followed the order of lyotropic series . Recently,

a method using a twin chamber has been developed for study-

ing the cuticular penetration, and i t is reported that the

cuticles from astomatous surfaces of citrus leaves were

more permeable to Fe than the stomatous ones . Permeability

studies with isolated cuticles are fraught with variations

amongst plant species. Although the stomates are covered

with cuticular invaginations, these would be thinner than

their continuations on the leaf, and therefore permeability

of these stomatal cuticles may be greater. It is to be

recognized that in intact leaves, permeability of air and

water molecules through the stomates is greater than the

other areas, and therefore stomatal penetration of solutes

in intact leaves will be greater.

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FOLIAR UPTAKE OF NUTRIENTS 721

Franke has indicated that there are specific channels

like ectodesmata in the cuticle, which would facilitate

cuticular entry to epidermal cells. Ectodesmata are demons-

trable as microscopical structures found mostly in the outer

walls of the epidermal cells of leaves and other above-

ground plant organs, and are distinguished by specific

physicochemical features. According to Franke , these are

not morphological structures of the wall but spaces in the

wall, and probably occur contiguous from cuticular spaces to

plasmalemma membranes. The role of ectodesmata (now known

as ectocythode) in foliar absorption and excretion has been

discussed in detail .

Studies on the penetration of substances through the

cuticles indicate that the process is only passive and not

energy—mediated, since these are devoid of living cells.

However, the enhancement of penetration of ions in the

presence of urea through cuticles has been attributed as

due to a "facilitated diffusion". The intact cuticles

behave differently from the enzymically isolated ones, and

may be holding water molecules in a different way when

attached to the living cells beneath. Furthermore, the

isolation process will alter the intracuticular arrangement

14

between the lipids and water molecules. Haas and Schonherr

have reported that water permeability of different plant

cuticular membranes are determined by the soluble lipids

contained within the membranes, and water permeability charac-

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t e r i s t i c s get altered with the age of the leaf. If cuticular

penetration of ions i s passive in an intact leaf, i t should

follow the pathway as water, simply by diffusion. Since

waterpermeability depends on the composition of soluble

lipids within the cuticle, the penetration of inorganic ions

and organic substances must be regulated by these l i p i d s .

There i s no report that describes such correlation. The

study on ion penetration through intact cuticular membranes

i s predisposed with d i f f i c u l t i e s and the results with leaf

are mere extrapolations.

ION UPTAKE BY LEAP CELLS

Foliar absorption of nutrients i s a multi-step process

consisting of passive and active components . However,

isolated and yet living systems offer better means of study-

ing the various uptake processes. Leaf cells have been enzy-

mically isolated and ion uptake has been studied under con—

20 22 32trolled conditions ' • . Leaf slices have been used by

Epstein group and have been found suitable for studying

the mechanisms of uptake at cellular level, comparable to

those by excised roots and stem slices (potato). The ion

•uptake processes by leaf slices resemble closely those by

root c e l l s , and the leaf slices 1 mm. thick have been

employed in several investigations on ion uptake ' '

I t has been found that the process of ion absorption by leaf

cells was metabolic and inhibited by respiratory inhibitors.

The leaf slices of a number of plant species have been used

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FOLIAR UPTAKE OF NUTRIENTS • 723

t o study the mechanisms of uptake of several n u t r i e n t i o n s .

The absorption of Cu, Zn and .'In by sugarcane leaf t i s s u e was

found t o follow a curve t y p i c a l of may ion uptake processes,

and Mn exhibited a second mechanism of uptake . The absorp-

t i o n of a l l c a t ions were found t o be coupled t o oxidative

phosphorylation. Light enhancement of ion uptake has been

noted in corn leaf t i s s u e and in enzymically i s o l a t e d

tobacco leaf cells ' . A light-stimulated chloride uptake

independent of stomatal opening was observed in chopped and

43

whole leaf (wheat) laminae . These effects of light indi-

cate that metabolic energy needed for ion absorption is supp-

lied through photosynthesis. However, certain differences

have been noted. In some cases light was found to enhance57 59

ion absorption under even anaerobic conditions ' . Jacoby17observed that there were differences in the distribution

0 0 AA /LO 00

of Na, Rb and K. In bean leaf slices, Na was concen-

trated in the vascular tissue while Rb was uniformly d i s t r i -

buted. Incubation under light however showed greater d i s t r i -

bution around stomata. Such a difference between light and42dark incubation was not obtained in the case of K. In this

connection, the work of Kholdebarin and Oertli is quite

interesting. They have shown that salt uptake and organic

acid synthesis were consistently higher in the light than in

the dark, and that the light-enhanced salt uptake in barley

leaf tissues was caused by additional source of ATP avail-

42able from photophosphorylation. Luttge e_t 1 measured

chloride uptake by leaf cells of a number of plants and

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found s i g n i f i c a n t difference in uptake between the C, and C

plants under the l i g h t and dark. Light always stimulated

chloride uptake in C p l a n t s , while both i n h i b i t i o n and

enhancement were obtained in C species, depending on the

physiological s t a t e of the leaves. They observed t h a t photo-

synthetic energy other than ATP or wP i s involved in chlo-

r i d e uptake by C. plant species.

TRANSLOCATIOH OF FOLIAR APPLIED K'UTSIEHTS

The transport of n u t r i e n t s from leaves to other parts

was investigated with isotopes by means of a leaf drop

method and l a t e r a leaf immersion technique was developed

so that there could be a continuous supply of ions to f a c i l i -

19t ä t e more precise k i n e t i c analysis of uptake mechanisms .

Foliar uptake and translocation of micronutrients were mea-

sured by supplying the isotopic solution into a p l a s t i c cup

having a 1 cm diam. opening at the bottom, while the cup was

fixed to the surface of the leaf . By t h i s method (Fig. 1)

larger q u a n t i t i e s of solute could be supplied and the 'free

space1 and 'exchangeable' components of absorption could be

eas i l y removed by washing with cold unlabelled solution

placed in the cup. Further, t h i s method does not a f f e c t the

r e s p i r a t i o n or photosynthesis carried on by the l e a f . Growth

substances can also be fed through the cup along with the

isotopes, when e f f e c t s of growth substances on ion transport

are studied. Our studies have shown t h a t the mobility of

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FOLIAR UPTAKE OF NUTRIENTS 725

FI6.1.

foliar applied nutrients follow the order K(Rb) >Mo>Mn>Fe

>Zn31'54.

The novenent of nicronutrients supplied to a corn leaf was

followed by means of a chromatogran scanner and i t was found that

28these elements generally moved towards the base of a leaf . It

was also found that ions supplied to one half of a corn leaf did

not migrate laterally, i.e., to the other half of the leaf across

the nidrib , but moved down to the base and perhaps to the main

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sten and returned to the other half. It has also been generally

observed that foliar applied elements like 7M are accumulated in

27the stem of plants

Xt i s generally known that the upward transport of

inorganic solutes takes place through the xylem under the

influence of the transpiration stream, and the carbohydrates

and inorganic substances move downwards from the leaves

through the phloem. Ringoet et al studied the movement of

45f o l i a r applied Ca using a semiconductor detector device,

and found greater accumulation in the vascular t i s s u e s . I t

45was shown that f o l i a r applied Ca moved in oat leaf upward

to the t i p through the xylem vessels and downward through

phloem . Penot and Gallou found that chloride was prefe-

r e n t i a l l y transported from leaves acropetally and i t took

place through phloem. In another study Penot and Beraud

reported that the movement of 32P, 35S, 85Rb, 45Ca and 36C1

in Pelargonium leaves was through the phloem tissue and not

cell-to-cell transfer.

GROWTH SUBSTANCES AND FOLIAR TRANSPORT

Plant growth substances are known to have diverse

effects on growth and metabolism. Studies on the effects of

growth regulators and retardants on ion absorption, espe-

cially by leaves, are very limited, although several years

ago substances like kinetin were found to enhance the mobili-

zation of organic and inorganic substances * ' . Halevy

and Wittwer16 showed that GAj (gibberellic acid) at 10~5H

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FOLIAR UPTAKE OF NUTRIENTS 727

given t o the r o o t increased f o l i a r absorption of Rb, and

NAA (1-naphthalene a c e t i c acid) a t 10~ K g r e a t l y enhanced i t s

tr a n s p o r t from leaf t o r o o t . The influence of a chemical,

DK30 (dimethylsulfoxide) which i s commonly used as a solvent

was examined on the penetration of Fe through leaf c u t i c l e s

and was found t o have no e f f e c t . However, in i n t a c t corn

pl a n t s DMSO (0.5 or 1%) was e f f e c t i v e in increasing the

59 7absorption of ?eSO. . The e f f e c t s of growth substances

l i k e GA,f CCC (Chloroethyl Choline chloride) and k i n e t i n

29were t e s t e d on f o l i a r applied Pe in bean p l a n t s . I t was

—5 -4

found that GA3 (10 H) and CCC (5 x 10 M) application to

the trifoliate leaf enhanced the absorption by primary leaf

while kinetin increased the transport to other parts. The

influence of growth substances, viz., ABA (abscisic acid),

ethrel, GA« and IAA on the transport of Rb placed in the25middle of corn leaves was examined and i t was found that

all these chemicals enhanced the rate of transport towards

the base of the leaf. Penot and Beraud have also reported

that growth substances like IAA, 2,4-D, NAA, BAP (benzyl

aminopurine) and GA all influenced" the direction of move-

ment of P, S, and Rb. In another study, Penot found

that the permeability of Mo in senescing leaf disks increased

and this effect was prevented by treatment with BAP. The

influence of ABA and a few growth substances on transport of

48foliar applied Fe and Rb was investigated and i t was found

that the cytokinins differed in their action on their

transport (Fig. 2).

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CONCLUDING REMARKS

The work on foliar uptake of nutrients presented here

brings forth several implications. Primarily there is ample

evidence to show that foliar absorption mechanisms largely

resemble those of root absorption. The operation of addi-

tional (second) mechanism in the case of leaf is also indi-

cated. Although cuticle is considered a barrier for foliar

uptake , light, moisture, temperature and also certain

chemicals would bring about alterations not only in the

frifoliates

Primary leaf

Fe trans location

ABAKINETIN

SO] .BA\

' trifoliate' root

•stem

PHLOEM

-- "XYLEM

FIG.2. Cytokinins and foliar translocation.

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FOLIAR UPTAKE OF NUTRIENTS 729

permeability of cuticles but also in the absorption by living

cells of the intact leaf. Following absorption by leaf cells,

translocation out of the leaf through living phloem cells

does take place in the case of several elements. The informa-

tion presented here lays great emphasis on the future role of

growth substances on foliar absorption.

Retranslocation of ions from the older leaves is an

important aspect and more research is needed especially with

regard to the elements like Fe, Kn and Zn which are not freely

mobile within plants. Some recent studies in this area have

been reported ' ' ',

Information is scant with regard to ionic interaction

in leaf. For instance, Na when translocated to leaf, becomes

toxic to plants, but this toxicity can perhaps be prevented

if some other element like K which can counteract Na, can be

supplied to the leaf.

Foliar nutrition has an important role in crop produc-

tion. Jones in his editorial has rightly pointed out in

his first sentence "Scientific Research Seems to run in

Cycles". This is very much true in the case of foliar

nutrition, which was vigorously pursued during 1960s, and

slowed down later. There appears to be a renewed interest

now because of several new considerations, like increasing

cost of fertilizers and shortage of irrigation water. Newer

surfactants are being discovered to increase the efficiency4

of foliar absorption and several laboratories are interested

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in foliar nutrition investigations ' * . There is evidence

that NaCl is absorbed by leaves of pine trees along the sea

coast, and this situation also should concern those engaged

in plant nutrition . More basic studies are needed to

understand the mechanisms of NaCl through leaf cuticle and

further absorption by leaf cells.

There are special situations where foliar supply of

nutrients may be effective. Foliar fertilization of soybeans

with N,P, K and 5 during the seed-filling period has been

found to be effective in increasing yield . Foliar appli-

cation could be used to prevent the depletion of nutrients

in the leaves and the resulting reduction in photosynthesis

during the crucial period of grain-filling due to poor root

absorption or retranslocation from the older leaves. Newer

compounds are being screened for efficacy and high molecular

weight phosphorus compounds have been found to increase the

yield of corn and soybeans * . Plants grown in saline condi-

tions respond better to foliar application of phosphorus to

44 34

peanut plants . Kendrick has rightly stated "We must

increase agricultural output if we are to provide food and

fibre for an expanding world population. But, as energy

sources become more restricted and costly, and greater

efforts are required to maintain the quality of the environ-

ment, i t is evident that we will need new knowledge and

different agricultural technologies". This is relevant to

the technology of feeding plants through the leaves.

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FOLIAR UPTAKE OF NUTRIENTS 731

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FOLIAR UPTAKE OF NUTRIENTS 733

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