Foliar Nutrition

T h e concept t h a t leaves a n d other aerial p l a n t parts m a y function as n u t r i e n t absorbing organs of sufficient i m p o r t a n c e to alter m a r k e d l y the n u t r i e n t status of a p l a n t is still held in reservation b y some p l a n t physiologists. Yet t h e absorption of n u t r i e n t s from sprays b y above-g r o u n d p l a n t parts has been k n o w n a n d accepted, a n d the principle has been utilized in a limited w a y in practical crop production for m a n y decades, one of t h e earliest records being t h a t of F o r s y t h in 1803

( 2 8 0 ) . Ballard a n d Volk, in 1914, used w i n t e r sprays of sodium n i t r a t e on d o r m a n t apple trees, a n d Lewis ( 1 4 5 ) , in 1936, published a note on t h e absorption of phosphate b y lettuce leaves.

I n recent years it has become increasingly popular to a p p l y n u t r i e n t s as foliar sprays (37, 2 8 1 ) . T h e usefulness of foliar application of n u t r i -ents depends on t h e following circumstances: (a) the existence of special problems t h a t m a y not be coped w i t h as well b y application of t h e fertilizer to t h e soil or b y soil m a n a g e m e n t ; (b) satisfactory response to the n u t r i e n t sprays; a n d (3) economical materials a n d m e t h -ods of application.

Foliar sprays of iron, zinc, m a n g a n e s e , a n d copper compounds a r e used in preference to soil applications because of fixation problems. A n -other special problem is slow response to soil application a n d need for a t e m p o r a r y control m e t h o d in the period before the soil t r e a t m e n t takes effect. T h i s is t h e m a i n reason for using foliar sprays of E p s o m salts on apple trees. M a g n e s i u m deficiency in fruit crops is often induced b y high potash m a n u r i n g , w h i c h results in a high soil K : M g ratio. U n d e r these circumstances it takes several years, a n d h e a v y soil dressings, to correct t h e deficiency b y soil t r e a t m e n t , w h e r e a s three to four foliar sprays of 2 % Epsom salts will raise the leaf m a g n e s i u m level from deficiency to sufficiency in one season ( 3 5 ) . This is clearly illustrated in T a b l e X X I I . W i t h r e g a r d to response, Bukovac a n d W i t t w e r (48) h a v e studied t h e absorption, transport, a n d mobility of foliar-applied isotopes of r u b i d i u m , sodium, potassium, phosphorus, chlorine, sulfur,

zinc, copper, m a n g a n e s e , iron, m o l y b d e n u m , calcium, strontium, a n d b a r i u m w i t h b e a n as the test crop. Using as a criterion t h e percentage of t h e foliar-applied radioactive isotope w h i c h is recovered in non-treated p l a n t parts, a n d autoradiography to p o r t r a y gross distribution in t h e plant, it was found that r u b i d i u m , sodium, a n d potassium w e r e the most readily absorbed a n d most highly mobile. Calcium, strontium, a n d b a r i u m although absorbed b y t h e leaf w e r e not exported from t h e leaf a n d w e r e considered immobile. Phosphorus, chlorine, sulfur, zinc, copper, m a n g a n e s e , iron, a n d m o l y b d e n u m w e r e i n t e r m e d i a t e w i t h

T A B L E X X I I

EFFECT OF FOLIAR SPRAYS OF 2 % EPSOM SALTS ON THE MAGNESIUM STATUS OF APPLE LEAVES*

MgO as % dry matter Before After

treatment, treatment, Response to Increase over Treatments May 19, 1948 Aug. 30, 1948 treatment control Control, no spray 0.252 0.202 - 0 . 0 5 0 — 1 Foliage spray6 0.287 0.302 + 0 . 0 1 5 0.100 2 Foliage sprays 0.273 0.343 + 0 . 0 7 0 0.141 3 Foliage sprays 0.260 0.493 + 0 . 2 3 3 0.291 4 Foliage sprays 0.262 0.800 + 0 . 5 3 8 0.598

a From Bould and Tolhurst (35).

6 Applied at fortnightly intervals beginning at petal fall.

decreasing mobility in the order given. T h e economics of foliar sprays versus soil dressings can be illustrated b y t h e use of iron chelates for t h e control of lime-induced chlorosis in fruit trees. W h e r e a s it takes from Y² to ί p o u n d of ferric ethylenediaminetetraacetic acid (Fe-E D T A ) , applied as a soil dressing to a single m a t u r e apple (Malus sylvestris) or p e a r (Pyrus communis) tree, to control lime-induced chlorosis, 1 p o u n d of F e - E D T A will m a k e 100 gal of foliar spray. T h i s will allow m a n y m o r e trees to be treated, assuming 3 to 4 a n n u a l sprays as being necessary for a seasonal control of chlorosis.

A. FACTORS T H A T A F F E C T T H E ABSORPTION OF FOLIAR-APPLIED N U T R I E N T S

Reference m a y h e r e be m a d e to P a r t I I I of the Chapter b y Steward a n d Sutcliffe in Vol. I I of this treatise ( 2 3 1 ) .

Position of leaf

Surface of leaf Position of leaf Upper Lower

Tip 0.104 0.062

Margin 0.130 0.102

Midrib 0.148 0.104

Base 0.206 0.090

a After Wittwer (280) from data of Tukey et al.

6 Values = micrograms in roots 2 hours after treatment.

surface, showed t h a t both sides of t h e leaf blade functioned equally well in the absorption of u r e a ( 2 6 0 ) . D a m a g i n g the epidermal hairs b y gentle b r u s h i n g increased absorption of u r e a tenfold. F u r t h e r m o r e , absorption w a s t h r e e to ten times greater d u r i n g t h e n i g h t t h a n d u r i n g t h e day, a n d three times greater in t h e m o r n i n g t h a n in t h e afternoon.

This suggests t h a t i n t e r n a l factors in the leaf, w h i c h undergo d i u r n a l fluctuations, m a y p l a y a n i m p o r t a n t role i n foliar absorption of urea.

Oland a n d Opland ( 1 7 9 ) , studying t h e u p t a k e of m a g n e s i u m b y apple leaves, found t h a t y o u n g leaves absorb m a g n e s i u m readily, whereas old leaves absorb v e r y little or n o n e if t h e y are sprayed d u r i n g the day.

W h e n sprayed in the evening, just before darkness, old leaves absorb large a m o u n t s of m a g n e s i u m . T h i s effect of the t i m e of spraying could be related to h u m i d i t y a n d the n a t u r e of t h e salt used in addition to d i u r n a l changes in i n t e r n a l composition. Allen (5) has shown t h a t It h a s been shown b y Cook a n d Boynton (60) t h a t lower leaf surfaces of apple absorb m o r e u r e a t h a n u p p e r surfaces; t h a t lower surfaces of leaves w h i c h w e r e g r o w n u n d e r high nitrogen conditions w e r e m o r e efficient in absorption t h a n w e r e low nitrogen leaves, a n d t h a t t h e lower surfaces of basal leaves w e r e less efficient t h a n w e r e lower surfaces of t e r m i n a l leaves. Different areas of the same leaf m a y h a v e v a r y i n g absorption rates (see T a b l e X X I I I ) . As a possible ex-planation for differences in the rates of foliar absorption a m o n g species, Gustaf son ( 9 1 ) has suggested t h a t stomatal pores function as the p r i m a r y sites of e n t r y into the leaf. T h e r e is, however, considerable controversy on this point. Experiments, using species w h e r e t h e stomatal n u m b e r on the lower leaf surface greatly exceeded t h a t on the u p p e r

T A B L E X X I I I

ABSORPTION AND TRANSPORT OF P3 2-LABELED O-PHOSPHORIC ACID BY DIFFERENT REGIONS OF BEAN LEAVES^ B

w h e n attached leaves of apple rootstocks w e r e m o m e n t a r i l y dipped in a solution of the appropriate salt, m a g n e s i u m was taken u p b y leaves m o r e r a p i d l y from t h e chloride or n i t r a t e t h a n from t h e sulfate. T h e a m o u n t of m a g n e s i u m initially retained on the surfaces of leaves w h i c h w e r e dipped in a 0.1 M solution w a s independent of the salt used. T h e differential u p t a k e w a s due to the relative h u m i d i t y a n d the behavior of t h e salts in relation to it. A t 20°C, the relative h u m i d i t y of air in equilibrium w i t h saturated solutions of m a g n e s i u m sulfate, acetate, nitrate, a n d chloride is 82, 65, 55, a n d 3 3 % , respectively. Therefore at 20°C, a relative h u m i d i t y of 3 2 % would cause solutions of all four salts to crystallize out, a n d a relative h u m i d i t y of 6 0 % would cause the sulfate a n d acetate to crystallize out, b u t would allow the nitrate a n d chloride to r e m a i n in solution. T h u s , assuming t h a t m a g n e s i u m can enter the leaf only from solutions, differences in behavior of the t h r e e salts can be accounted for on t h e basis of t h e different conditions u n d e r w h i c h their solutions d r y out on the leaf surface, i.e., relative h u m i d i t y .

It was thought at one t i m e t h a t the outer epidermal cell walls of apple leaves w e r e covered with a continuous l a y e r of cutin. Roberts et al. (203) h a v e since shown the presence of pectinaceous substances w h i c h form a continuous p a t h reaching from the outside of the leaf a n d extending to the walls of the vein extensions. T h e epidermal cell walls of t h e apple leaf, therefore, can no longer be considered as covered w i t h a continuous cuticle w h i c h prevents the absorption of w a t e r a n d salts. T h e a m o u n t a n d location of the pectinaceous substances present in the leaves m a y account for the entrance of water-soluble materials such as nutrients, hormones, and fungicides.

T h e widespread occurrence of epidermal plasmodesma (strands of protoplasm) m a y provide t h e most probable exchange sites a n d the p a t h w a y for e n t r y of nutrients t h r o u g h leaf surfaces. T h e y increase in frequency as t h e y o u n g leaf develops, u n t i l a m a x i m u m is reached in vigorous bright green leaves, a n d t h e y decline as the leaf yellows.

Skoss, according to W i t t w e r ( 2 8 0 ) , has reported t h a t intracellular plasmodesma m a y even penetrate the cuticle.

It is k n o w n t h a t different p l a n t species, a n d different varieties of the same species, v a r y as regards their ease of wetting. This is due to t h e chemical a n d physical n a t u r e of t h e leaf cuticle a n d the degree of pubescence. T h e addition of surfactants (wetting agents) to foliar sprays m a y increase or decrease t h e absorption of foliarapplied n u -trients. Cook a n d Boynton ( 6 0 ) , working w i t h M c i n t o s h apple leaves found t h a t the absorption of urea solutions b y lower leaf surfaces in

Approximate

absorption Absorption

Nutrient Plant treated _(%) time Authority

Phosphorus Apple 32-48 7 Days Fisher and Walker (1955)

Apple 75-100 30 Days Eggert et al. (1952) Bean and squash 40-50 6 Days Mayberry (1951)

Swede 50 10-12 Days Thorne (1955)

Nitrogen Tobacco 25 6 Hours Volk and McAuliffe

(as urea) (1954)

Cucumber, bean, 100 3-12 Hours Hinsvark et al.

tomato, corn (1953)

Potassium Bean and squash 25-35 72 Hours Mayberry (1951)

"From Wittwer (280).

p l a n t parts, a n d w i t h one exception all additives u p to a concentration of 1.0% reduced absorption. Similar results w e r e obtained b y Koontz a n d Biddulph (135) with red k i d n e y bean (Phaseolus vulgaris) using nonionic a n d cationic surface active agents, but Fisher a n d W a l k e r (73) m o r e t h a n doubled t h e absorption of P3 2 labeled monopotassium phos-p h a t e ( 0 . 2 % ) b y adding glycerine (1 or 2 % ) or T r i t o n X-100 (an alkyl a r y l poly ether alcohol) to t h e s p r a y solution applied to apple leaves. Generally speaking, the addition of wetting agents to n u t r i e n t foliage sprays enhances t h e u p t a k e of the n u t r i e n t , particularly cations.

T h e percentage absorption of some foliar-applied n u t r i e n t s is given in T a b l e X X I V , a n d the rate of absorption of foliar-applied nitrogen as urea, phosphorus as K H²P 0⁴, a n d m a g n e s i u m as M g S 0⁴- 7 H²0 is shown in Fig. 17.

4-hour periods was increased, on t h e average, m o r e t h a n 1 0 0 % b y the addition of T w e e n 80 (a sorbitan mono-oleate polyoxyalkylene de-rivative) at 0 . 1 % or b y T w e e n 20 (a sorbitan m o n o l a u r a t e polyoxy-alkylene derivative) at 0 . 0 1 % . T h e effect of t h e wetting agent in in-creasing absorption w a s a p p a r e n t only w h e n the leaves h a d not been sprayed previously w i t h solutions containing wetting agents or oils.

On t h e other h a n d , T e u b n e r et al. (244) tested the effect of a n u m b e r of surfactants on the absorption of phosphate b y bean leaves, as m e a s u r e d b y the percentage of applied P3 2 recovered in nontreated

T A B L E X X I V

TOTAL ABSORPTION FROM FOLIAR SPRAYS AS PERCENTAGES OF AMOUNTS OF ISOTOPICALLY LABELED NUTRIENT APPLIED"

90-DAYS

FIG. 17. The rate of absorption of nitrogen, phosphorus, and magnesium from sprays applied to the lower surface of Mcintosh apple leaves. From Fisher and Walker (73).

Not only are n u t r i e n t s taken u p from foliage sprays, b u t t h e y m a y also be lost b y leaching d u r i n g rainfall (62, 156) (see T a b l e X X V ) . Long et al. (149) g r e w plants in solutions labeled w i t h P3 2 a n d K4 2. T h e plants w e r e t h e n leached w i t h a mist of distilled w a t e r for 4^-48 hours a n d t h e runoff was collected a n d analyzed. Intact plants lost no

T A B L E X X V

AMOUNT OF LEAF COMPONENTS LEACHED FROM APPLE TREES BY RAIN DURING 1954, CALCULATED PER HECTARE OF TREE-COVERED AREA

Κ Na Ca Organic

Variety (kg) (kg) (kg) constituents Bramley seedling 30.0 9.0 10.5 7.4

Cox's orange 25.0 9.0 — —

Rain water 1.7 4.5 3 . 8 4.0

aF r o m Dalbro (62).

phosphate, b u t cuttings lost from 1.5 to 1 2 . 8 % of the absorbed P3 2. After 12 hours of root absorption i n t h e dark, subsequent leaching for 4 hours removed u p to 7 1 % of the previously absorbed K^{4 2}. Analysis of the leachates also indicated the presence of a m i n o acids a n d reducing substances. It would a p p e a r t h a t potassium is the n u t r i e n t most readily lost b y leaching.

PART 2. M I N E R A L N U T R I T I O N OF P L A N T S I N C U L T U R E M E D I A by E . J . H E W I T T

VIII. Early Experiments and the Development of Nutrient Culture Methods