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III. Effects of Environmental Factors
M. G. HALE AND L. D. MOORE
The Influence of Defoliation on the Quantities of Compounds Exuded by Roots
of Sugar Maples"**
2. Amino aciddamides
3. Organic acids
2.6 t 0.2
7.3 ? 0.1
2.3 f 0.3
1.1 2 0.2
0.8 2 0.1
49.7 t 10.1
2.9 t 0.Zp
0.5 ? 0.1
3.4 f 0.5
0.3 f 0.1
0.3 2 O . l e
1.8 t 0.2'
24.3 t 9.4
4.3 2 1.2
7.9 f 0.9
6.1 t 0.9
6.0 ? 0.3
1.8 t 0.4
1.7 2 0.1
4.3 2 1 . 1
1 . 1 t 0.3
1.8 2 0.3
3.6 2 0.7
1.9 t 0.3
2.4 2 0.6
1.5 f 0.1
2.7 t 0.6
3.4 2 0.3
0.9 2 0.7
63.2 2 11.1
0.5 2 0.2'
1.4 f 0.4
3.1 2 0.5
1 . 1 k 0.2
58.1 2 13.3
OReprinted with permission for Smith (1971).
bData are micrograms x lo-' of each material released during 14 days per milligram oven-dry
Mean and standard error of three replicate determinations using one Composite exudate sample
from 19 and 20 roots of control and defoliated tree, respectively.
Mean and standard error of three replicate determinations using one composite exudate sample
from 17 and 23 roots of control and defoliated tree, respectively.
eControl and defoliated figures significantly different at 95% level.
upon metabolically before it appears in exudates. Those factors that affect rates
of photosynthesis and translocation will have an indirect effect on exudation. For
a more thorough discussion of sources and mechanisms of exudation, see Hale
et al. (1978).
A few investigations have appeared since the previous review (Hale et al.
1971) relating to the environmental effects of temperature and light on shoots
with consequent changes in root exudates. For example, Shapovalov (1972)
explained the effect of temperature on exudation of scopoletin from soybean and
oat roots by setting apart three stages based on the Qlo of the exudation rate. In
the stage 20-24"C, the process appeared to be one of diffusion from free space;
from 23 to 30°C, he claimed the process activated diffusion, probably across the
plasmalemma; and in the range of 4O-6O0C, exudation probably increased
sharply as a result of denaturation of protein.
The complexity of temperature effects is compounded because of changes in
rates of photosynthesis and in rates of translocation of photosynthates to the
roots, in rates of enzymatic reactions that synthesize or degrade photosynthate,
and in changes in membrane permeability which may occur. For these reasons
and others it is difficult to establish a pattern for effects of environment on
Two interesting investigations need to be mentioned. Smith (1972) defoliated
sugar maple trees and measured the changes in exudation. Differences between
defoliated and nondefoliated trees were quantitative. Defoliated trees released
greater quantities of fructose, cystine, glutamine, lysine, phenylanine, and
tyrosine, whereas foliated trees exuded greater amounts of sucrose, glycine,
homoserine, methionine, threonine, and acetic acid (Table VI). Many of the
differences were not statistically significant, but considering the difficulty of
obtaining the amount of quantitative data presented as well as the conditions
under which it was obtained, the results are quite interesting.
Using more controlled conditions and a different plant, Bokhari and Singh
(1974) examined the effect of clipping and temperature on exudation from western wheat grass. Severe clipping and high temperature stimulated root exudation,
with more being exuded in the initial stages of growth than in the later stages of
growth. Over a period of 80 days, 1 g dry weight of roots exuded 4.5-6.5 mg of
reducing sugars. In terms of carbon balance in a sward system, grazing would
apparently cause a greater carbon loss through the roots than would nongrazing
Root Exudation Expressed as Total Nonstructural Carbohydrate (TNC) Equivalents"
12 hours, day temperature 13"C, night temperature 7°C
12 hours, day temperature 24°C. night temperature 13°C
12 hours, day temperature 29.5"C, night temperature 18°C
'Reprinted with permission from Bokhari and Singh (1974).
'Data are milligrams per gram dry weight of roots.
M . G . HALE AND L. D.MOORE
B. WATER STRESS
Experiments involving direct quantitative measures of the effects of water
stress on exudation have been few since Vancura (1964) showed that by allowing
a root system to develop water stress and then irrigating he could cause an
increase in root exudation. The roots of plants growing in the field are continually exposed to alternately water-stressed and release-of-stress conditions.
Methods must be devised to measure the effects of the cycles and their relationship to microbial colonization of roots.
In elegant studies of water stress on exudation from Ponderosa pine (Reid,
1974) and lodgepole pine (Reid and Mexal, 1977), polyethylene glycol (PEG
4000) was used to establish gradients of stress. Relationships between exudation,
water stress, and mycorrhizae were investigated. Decreasing water potentials in
the rooting medium caused a reduction in the amount of 14C02absorbed by the
leaves, probably as a result of stomata1 closure. Decreased absorption might also
account for the decrease in translocation of 14C to the roots at the lower water
potentials, and it might also account for decreased exudation. For Ponderosa pine
no label appeared in exudates at water potentials below -2.6 bars. The amounts
of 14C exuded peaked at 3 days after exposure to I4CO2for both Ponderosa and
lodgepole pine. Of the three treatments, 0, -2, and -4 bars (Reid and Mexal,
1977), the average cumulative exudation over the first 6 days was greatest at -4
bars and least at -2 bars, but when exudation was expressed as a proportion of
the total 14C translocated to the roots, exudation was greatest at 0 bar and
somewhat less at -2 and -4 bars. These results may be misleading because of
the low oxygen concentration in the rooting medium.
Effects of water stress on exudation are not clearly defined in the literature, but
it is apparent that there is an effect on both the amounts and kinds of exudates and
that this factor must be considered in interpretation of results from exudation
C. HYDROGEN ION CONCENTRATION
A change in pH affected the exudation of I4C applied as 14C02to the atmosphere surrounding the shoots of 8-day-old wheat plants. At pH 5.9, I4Ccontaining compounds exuded accounted for 20,306 dpm. At pH 6.4 the count
was reduced to 7057, and at pH 7.0 to 8595 (McDougall, 1970). Rovira and
Ridge (1973) found that addition of acetate buffer at pH 5 greatly increased
exudation. They attributed the increase to the acetate and not to a pH effect.
In examining exudation of cellulose by red clover roots, Bonish (1973) found
that salts decreased exudation of the enzyme to negligible amounts at pH below
5.5, but exudation increased as the pH rose. The general effects of pH on
exudation need further study.
Because of its effect on the basic metabolism of the root, oxygen deficiency
can cause changes in the kinds of compounds exuded. Kohl and Matthaei (1971)
found that, under partial anaerobiosis, lactate accumulated in roots of corn at the
expense of malate. Lactate was also found in the incubation medium of excised
root tips of corn. Under aerobic conditions lactate was not released into the
medium. Ethanol, a product of anaerobic metabolism in plants, was found by
Young er a / . (1977) to occur in the rhizosphere of seedlings of Lupinus angusrifoliu subjected to water logging for 36 hours.
E. MECHANICAL FORCES
To simulate soil pressure on roots and to avoid the difficulties of extracting
exuded compounds from soil, Barber and Gunn (1974) used glass ballotine.
Compared to unrestricted roots of barley and maize, those growing between the
glass ballotine exuded more amino acids and carbohydrates. The increase was
from 5% (in unrestricted roots) to 9% (in restricted roots) of dry matter increment
F. ENVIRONMENTAL POLLUTION
The relationship of air pollution to microbial ecology has been reviewed by
Babich and Stotzky (1974) and Smith (1976). They reported numerous effects of
air pollutants on microbial reproductive potential and morphology. There were,
however, no reports concerning the effects of air pollution on root exudation per
se. Manning er al. (1971) did report that pinto bean plants exposed to 0.1-0.15
pI of ozone for 8 hours a day for 28 days had poor root growth, and Rhizobium
nodules developed only on the nonfumigated bean plants. Similar results were
recorded when soybean plants were exposed to 75 pphm (parts per hundred
million) of ozone for 1 hour (Tingey and Blum, 1973). No one has yet attempted
to evaluate the effects of air pollution on root exudation, although air pollutants
such as ozone and sulfur dioxide readily alter the metabolism of higher plants.
IV. Foliar Application of Chemicals
A. EXUDATION OF FOLIARLY APPLIED PESTICIDES
Although foliar application of chemicals is common practice for protection of
shoots from pathogen and insect attack, little information is available concerning
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-