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IV. Foliar Application of Chemicals
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-
Names of Chemicals Mentioned in Text by Common Name or Abbreviation
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
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
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
Effects of Foliar Sprays on Carbohydrate Exuded from Roots of Wheat Grown under Axenic Conditions"
C O ( W2 )2
( d 5 0 plants)
Reprinted with permission from Jalali (1976).
*Values are percentage reduction or increase compared with that in controls.
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,
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
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.
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