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IV. Foliar Application of Chemicals

IV. Foliar Application of Chemicals

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108



M. G . HALE AND L. D.MOORE



the effects of such applications on protecting the health of plant roots, even

though earlier studies have demonstrated that rhizosphere populations and root

exudation patterns have been changed by the application of pesticides. The

earlier work has been reviewed by Hale et al. (1978). Various foliarly applied

pesticides and nutrients affect exudates and rhizosphere populations. Root exudation of growth regulators amounts to 10-15% of the amount applied to the foliage

by whatever means (Foy et al., 1971). Unfortunately, no work has been done on

exudation of applied pesticides since the 1971 review (Hale er al., 1971).



B . EFFECTS OF FOLIARLY APPLIED CHEMICALS ON

EXUDATION OF ENDOGENOUS COMPOUNDS



In subsequent investigations (Balasubramanian and Rangaswami, 1973) foliar

applications of 0.1% NaNO , 0.1% Na2P04, 25 mg of 2,4-dichlorophenoxyacetic acid (2,4-D) per liter, and 200 mg of Dithane 278 per liter (Table VIII)

were studied to determine their effects on the exudation of amino acids and

sugars from roots of sorghum and sunnhemp. NaNO, decreased the amounts of

amino acids exuded by sorghum but increased the amounts of amino acids

exuded by sunnhemp. Na2PO4decreased but 2,4-D increased amino acid exudation. For sorghum, fungal populations in the rhizosphere increased with applications to the foliage of 2,4-D and NaNO,; bacteria increased with applications

of 2,4-D, and actinomycetes increased with all applications. For sunnhemp, all

three groups of organisms increased with applications of 2,4-D and NaNO,.

The 2,4-D effects were correlated with increased populations of microorganisms

in the rhizosphere, whereas the application of the other compounds did not lead

to such a correlation.

Hale et af. (1977) found that applications of 100 rng of 2,4-D per liter increased cholesterol exudation from peanut roots, and both 2,4-D and 200 mg of

gibberellic acid per liter decreased fatty acid exudation.

Reported effects of herbicides on exudation and root rot interaction in Sanilac

navy bean (Wyse et al., 1976) showed EPTC and dinoseb to increase exudation

of electrolytes, amino acids, and sugars from root and hypocotyls and to increase

root rot 42-84%. However, Jalali (1976) applied six growth regulators and

herbicides to wheat and found that chloramphenicol, and to a lesser extent 2,4-D,

reduced rhizosphere populations by suppressing exudation of ribose, maltose,

and raffinose, which were exuded abundantly from root-rot-infected roots (Table

IX). Lee and Lockwood (1977) applied chloramben, which increased exudation

and reduced plant height and stand of soybeans in media infested with

Thielaviopsis basicola. Compared with the controls, chloramben at 2 pg/ml

caused roots to exude 540% amino acids, 205% electrolytes, 80% carbohydrate,

123% fatty acids, and 132% nucleic acids. The exudates caused more en-



ROOT EXUDATION



109



TABLE VIII

Names of Chemicals Mentioned in Text by Common Name or Abbreviation

Common name

abbreviation



oi



2,4-D

GA

EPTC

Dinoseb

Chloramphenicol

Alachlor

Kinetin

ABA

IAA

Dithane 278



Chemical name



2,4-Dichlorophenoxyaceticacid

Gibberellic acid

S-Ethyl dipropylthiocarbamate

2;sec-Butyl-4,6-dinitrophenol



D(-)-Threo-2,2-dichloro-N-[phydrox-a-(hydroxymethyl)~

p-dinitrophenmethyll acetemide

2-Chloro-2’,6’ diethyl-N-(methoxymethy1)acetanilide

6-Furfurylamino purine

Abscissic acid

Indole-3-acetic acid

Zinc ethylenebisdithiocarbamate



doconidia to germinate. Alachlor and dinoseb also enhanced root rot, but not as

strikingly as chloramben.

Fungitoxicants reportedly restrict the development of mycorrhizal development on wheat roots (Jalali and Domsch, 1975). It was observed that a number of

foliarly applied fungicides caused suppression of the total amino acid content of

wheat root exudates, although release of some individual acids was enhanced. It

is conceivable that many chemicals used in plant protection could have measurable effects on plant metabolism and root exudation.



C. GROWTH REGULATOR EFFECTS



Cytokinins have been found in root exudates (Van Staden, 1976; Itai and

Vaadia, 1965), and various forms of nitrogen applied to the leaves of rice plants

increased cytokinin exudation (Yoshida and Takashi, 1974). Since microorganisms in the rhizosphere may release cytokinins (Phillips and Torrey, 1970,

1972; Lalove and Hall, 1973; Azcon and Barea, 1975; Vancura etal., 1977) and

since cytokinins are synthesized in roots, the effects of cytokinin on mobilization

of metabolites and on exudation were investigated by Thompson (1978). Kinetin

at

M and low6M was applied to the roots of 57-day-old peanut plants. The

exudation of fatty acids decreased with application of

M kinetin, but the

fatty acid concentration in the roots increased. Kinetin at

M did not have

these effects.

Abscissic acid (ABA) and indole-3-acetic acid (IAA) are also exuded (Tietz,

1975); the amount exuded depends on the cultural methods. Exudation of IAA

from 5-day-old pea rots was 1 I .O mg/kg fresh weight of roots in sand, but only



TABLE M

Effects of Foliar Sprays on Carbohydrate Exuded from Roots of Wheat Grown under Axenic Conditions"

Foliar treatments

"HI)

Carbohydrates

Pentose monosaccharides

Ribose

Xylose

Arabinose

Hexose monosaccharides

Glucose

Fructose

Galactose

Rhamnose

Disaccharides

Maltose

Sucrose

Trisaccharides

Raffinose



2 SO



(350 mM)



+



4.3b

15.2

- 100

-



+



4.5

3.5

4.1

4.0



-



17.3



-



+

-



- 100

-



8.4



Na2HPO

(325 mM)



KCI

(255 mM)



- 30.6



-



8.1

9.2



-



+1.8

9.1

3.1



-



+



+



C O ( W2 )2

(455 mM)



-



2.4-D

(2.26mM)



-



-



13.8

11.7

2.3



11.5



-



-



3.0



-



23.5

9.0

+ 8.0

- 100



-



- 1.3

- 9.0

- 100



-



- 5.8



+



1.9



-



- 100



-



- 100



- 100

- 100



i



+



1.7

3.27



- 6.7



1.2

4.8

8.1



Chloramphenicol

(3.41 nM)



- 23.5



Control

( d 5 0 plants)



9.8

10.1



96.99

117.24

100. 10



2.1

2.2

+ 12.7

-100



147.77

121.93

99.93

100.03



27.1



-



-



-



5.6

3.4

2.7

10.2



- 100



-



13.7



-



- 100



- 100



- 100



92.73

83.71



- 100



- 100



- 100



87.03



-



Reprinted with permission from Jalali (1976).

*Values are percentage reduction or increase compared with that in controls.



-



ROOT EXUDATION



111



4.1 pglkg fresh weight in water culture. ABA applied to the foliage translocated in only small amounts to the roots, and the ABA that was exuded was

apparently synthesized in the roots. ABA has been shown to increase the

hydraulic conductivity of roots (Glinka, 1973), and it may have an effect on

outward loss of water and solutes, particularly under water stress conditions

during which the abscissic acidlcytokinin ratio increases (Itai and Benzioni,

1 976).

The production of biologically active substances by bacteria that predominate

in the rhizosphere may play an important role not only in plant growth but also in

root exudation. As an example, IAA, gibberellin-like substances, biotin, and

pantothenic acid were produced by strains of bacteria isolated from the root

surfaces and rhizosphere of maize (Hussain and Vancura, 1970). That plant

growth regulators affect cell membrane permeability has been reported by Gregory and Cocking (1966), Etherton ( 1970), Kennedy and Harvey (1972), and

Wood and Paleg ( 1 972).

V. Biotic Factors Affecting Root Exudation



A. RHIZOSPHERE ORGANISMS



Rhizosphere organisms effect higher plants by ( a ) altering the morphology of

the root system, ( b ) changing the phase equilibria of soil and hence the nutrients

so that they are more readily available to plants and are more readily transported,

(c) changing the chemical composition of the soil participating in symbiotic

processes, and ( d ) physically blocking the roots surfaces (Nye and Tinker,

1977). To this list should be added the effect on exudation of soil-borne microorganisms.

Soil microorganisms may affect the permeability of root cells by ( a ) damaging

root tissues, ( b ) altering root metabolism, (c) preferentially utilizing certain

exudates, or ( d ) excreting toxins (Rovira, 1969; Rovira and Davey, 1974).

Changes in root exudation caused by microorganisms may indirectly effect general plant health, resistance to disease, or development of other rhizosphere

microflora.

Darbyshire and Greaves (1973) reported that the magnitude of the rhizosphere

response in terms of microbial number is markedly influenced by biological as

well as chemical factors. The rhizosphere can be altered by plant species or

variety and by the physiological age and the metabolic state of the plants. Such

factors as soil moisture, soil temperature, aeration, and soil fertility have direct

effects on the rhizosphere population as well as effects on the plant and root

exudation. Similarly, light, relative humidity, and air temperature can also indirectly affect the rhizosphere population.



112



M . G.HALE AND L. D. MOORE



That plant root exudates serve as nutrient sources for rhizosphere microorganisms is well known (Bowen and Rovira, 1976; Darbyshire and Greaves,

1973; Rovira, 1969). But root exudates can also either stimulate or inhibit the

growth of microorganisms. For example, root exudates of Crotaluriue

medicaginea reportedly stimulate the growth of Penicillium herquei, Aspergillus

niger, and Alternaria humicola, but significantly reduce the growth of

Trichoderma lignorum (Sullia, 1973).

Reid ( 1974) reported that mycorrhizal Ponderosa pine roots had significantly

lower 14C specific radioactivity than did nonmycorrhizal roots when the shoots

were exposed to I4CO2. Harley (1969), however, proposed that mycorrhizal

roots, relative to noninfected roots, acted as metabolic sinks for photosynthetically fixed carbon. Results with lodgepole pine seemed to confirm that mycorrhizal roots are sinks (Reid and Mexal, 1977).

The quantity of total carbon in root exudates of maize and wheat has been

shown to increase approximately two to two and one-half times in the presence of

microorganisms when compared with axenically cultured plants. The use of

carbon compounds in the exudate by Pseudomas putida apparently increased the

concentration gradient between the root and the nutrient solution, and there was

an increase in exudation (Vancura et al., 1977).

In recent years there has been significant interest concerning the roles of

rhizosphere microorganisms in plant nutrition (Barber, 1978; Tinker and Sanders, 1975). Although soil-borne bacteria have an uncertain or small effect,

mycorrhizal fungi readily improve plant nutrition, usually by increasing the

phosphate supply. Mosse (1973) and Tinker (1975) have demonstrated that a

position growth response of plants to vesicular-arbuscular mycorrhizae was associated with phosphate nutrition.

Asanuma et al. (1978) examined the effects of dilute paddy soil suspensions

on the uptake of nitrogen and phosphorus by rice seedlings. More nitrogen was

absorbed by the sterile plants at 25 or 50 ppm nitrogen, whereas the inoculated

plants absorbed more nitrogen when it was supplied at 100 or 200 ppm. The

amount of phosphorus absorbed by the rice seedlings was affected by the concentration supplied in the nutrient solution and by the presence of microorganisms.

Microorganisms do not always have a positive effect on ion uptake. While the

uptake of manganese, iron, and zinc by barley grown in solution culture was

stimulated by the presence of microorganisms (Barber and Lee, 1974), the uptake of both phosphate and sulfate by pea plants was limited by Trichoderma

viride (Brannstrom, 1977). Iron transport in the pea plants was apparently retarded,

Enzyme activity in the roots of higher plants may be altered by rhizosphere

microorganisms. Vagnerova and Macura (1974) found that protease activity was

nil in the roots of axenic wheat. Roots colonized by microorganisms, however,

had appreciable protease activity. The activity was detected exclusively in the



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