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III. Effects of Environmental Factors

III. Effects of Environmental Factors

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104



M. G. HALE AND L. D. MOORE

TABLE VI

The Influence of Defoliation on the Quantities of Compounds Exuded by Roots

of Sugar Maples"**

1969'

Compound



Control



1970d

Defoliated



1. Carbohydrates



Fructose

Glucose

Sucrose

2. Amino aciddamides

Alanine

Cystine

Glutamine

Glycine

Homoserine

Lysine

Methionine

Phenylalanine

Threonine

Tyrosine

3. Organic acids

Acetic

Malonic

~



~



~



2.6 t 0.2

0

7.3 ? 0.1

0



*



0.2

0.1

2.3 f 0.3

0.5

0.1

1.1 2 0.2

0.8 2 0.1

0

0

Trace

0



*



49.7 t 10.1

0

~~



4.3



* 0.1'



0

2.9 t 0.Zp

0

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'

0

0

Trace

0



24.3 t 9.4

0



Control



Defoliated



4.3 2 1.2

Trace

7.9 f 0.9



6.1 t 0.9

Trace

6.0 ? 0.3



1.8 t 0.4



1.7 2 0.1

0

4.3 2 1 . 1

1 . 1 t 0.3

1.8 2 0.3



0

3.6 2 0.7

1.9 t 0.3

2.4 2 0.6

0

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

Trace

~~



0.5 2 0.2'

1.4 f 0.4



3.1 2 0.5

O P



1 . 1 k 0.2



58.1 2 13.3



0



~



OReprinted with permission for Smith (1971).

bData are micrograms x lo-' of each material released during 14 days per milligram oven-dry

root.

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.



105



ROOT EXUDATION



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

exudation.

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

(Table VII).

TABLE VII

Root Exudation Expressed as Total Nonstructural Carbohydrate (TNC) Equivalents"

Days

Clipping



0-10



Control

Moderate

Severe



0.595b

0.631

0.595



Control

Moderate

Severe



0.553

0.519

0.583



Control

Moderate

Severe



0.868

0.794

0.875



10-20



20-30



30-40



40-50



50-60



60-70



70-80



12 hours, day temperature 13"C, night temperature 7°C

0.500

0.432

0.412

0.426

0.407

0.400

0.551

0.515

0.473

0.487

0.487

0.476

0.515

0.513

0.539

0.538

0.524

0.534

12 hours, day temperature 24°C. night temperature 13°C

0.500

0.469

0.470

0.381

0.474

0.488

0.511

0.469

0.458

0.486

0.487

0.512

0.500

0.489

0.550

0.530

0.513

0.527

0.538

0.548

0.546

12 hours, day temperature 29.5"C, night temperature 18°C

0.700

0.637

0.560

0.567

0.576

0.585

0.558

0.645

0.634

0.603

0.625

0.597

0.608

0.589

0.727

0.729

0.678

0.656

0.628

0.635

0.615



0.465

0.560

0.519



'Reprinted with permission from Bokhari and Singh (1974).

'Data are milligrams per gram dry weight of roots.



Total



4.483

5.089

5.098

4.640

4.614

5.001



6.092

6.140

6.543



-



106



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

studies.



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.



ROOT EXUDATION



107



D. ANAEROBIOSIS



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

of mots.



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



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-



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