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VI. Modification of the Root Surface Microflora

VI. Modification of the Root Surface Microflora

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SOIL MICROORGANISMS A N D PLANT ROOTS



275



wheat, wherein, in order to establish experimental infection, the fungus

(Ophiobolus graminis Sacc.) was introduced into the soil along with the

organic substrate on which it had been grown. With such procedure,

there frequently was failure to establish infection. Stimulation of growth

of the antagonistic flora in the soil by the added organic material has

been considered responsible for the rapid disappearance from soil of t,he

fungal parasite introduced (Garrett, 1944).

2. B y Soil Treatment



Inasmuch as organic manures are known to affect the incidence of

certain root-infecting fungi, numerous studies have been made of the

effect of such treatments on the rhizosphere microflora. I n experiments

in which manured, untreated, and steam-sterilized soil samples were

established in containers cropped to wheat, Clark (1939) found that the

root flora was largely independent of the soil flora. Clark and Thom

(1939) believed that the effects of organic manuring were primarily

evident on the soil microflora, and that the root microflora of the plants

themselves was relatively little affected. In view of an observed independence of the root microflora from manurial treatments capable of

affecting differences in disease incidence, Stumbo et al. (1942) expressed

the opinion that factors of host nutrition were of greater importance than

microbial antagonisms in the rhizosphere in the control of take-all

disease of wheat. Timonin (1940b), Katznelson and Richardson (1943)

and Mitchell et al. (1941) also showed that organic manures did not

greatly affect the rhizosphere microflora. In some instances, however,

there do exist indirect influences of manurial treatments. Katznelson

(1946) has pointed out that manuring may affect the growth rate, vigor,

and maturity of the plant, and that stage of plant development is known

to influence t.he flora of the rhizosphere. Hildebrand and West (1941)

found that manurial treatment affecting the incidence of Ontario rot of

strawberries also affected the relative incidence of nutritional groups

within the rhizosphere.

Differences in host plant nutrition following organic fertilization may

induce changes in the rhizosphere microflora, which in turn may affect

disease incidence. Differences in plant nutrition may also affect incidence

of disease. Following primary invasion by root parasites, there typically

is secondary invasion by members of the soil population, and in such

circumstance there is abrupt disturbance of the rhizosphere flora. Frequently it is difficult properly to separate cause from effect. There appear instances in the root rot literature where this has been attempted

without sufficient information on the many factors involved in incidence

of root diseases.



276



FRANCIS E). CLARK



Various inorganic and physical treatments have been reported to

affect a t least certain fractions of the root microflora. I n experimental

soil initially of pH 5.0, Pohlman (1946) noted that nodules on roots of

alfalfa were largely concentrated in the soil layer receiving a high-lime

treatment, irrespective of whether this was the 8 to 16 or 16 to 24 inch

layer. Timonin (1947) found that various soil fumigants affected numbers of manganese-oxidizing bacteria on the roots of oats. Katznelson

and Richardson (1943) noted that although soil steaming markedly reduced microbial numbers in the soil in which tomato plants were grown

following the steaming, the numbers on healthy roots of the tomato

plants were as high as the bacterial numbers on roots in untreated, control soil. Such data indicate that steaming, as organic manuring, has

little effect on the root surface microflora.

3. By Plant Treatments



Inasmuch as stage and condition of plant growth have been found

to affect the rhizosphere microflora, it may be expected that plant treatments which affect the vigor or condition of plant growth may be used

to modify the rhizosphere flora. Such modifications have been accomplished experimentally. Eaton and Rigler (1946) found that with plant

mutilation (half-leming of fruiting plants us. de-flowering of fully leafed

plants), differences in the carbohydrate levels of cotton plants could be

established, and simultaneously, differences occurred in the surface

microfloras of the roots. For field cotton, plant mutilation has been

found to affect the incidence of saprophytic fungi in the rhizosphere

(Clark, 1942).



4. B y Application of Herbicides and Insecticides

Applications of plant growth regulators or herbicides, by affecting

plant development, can also be expected to alter root surface microfloras.

To date most of the investigations conducted on the microbiological

effects of herbicides have been concerned with their effects on the soil

population rat,her than on the rhizosphere population. Smith et al.

(1946) reported that 2,4-dichlorophenoxyacetic acid (2,4-D) had no

effect on numbers of fungi, actinomycetes, or of total bacteria when applied to soil a t concentrations up to 0.05 per cent, although there was

injury to nitrifying bacteria with applications of 0.01 per cent. Stevensen

and Mitchell (1945) found 0.02 per cent of 2,4-D to be bacteriostatic in

culture. Newman and Norman (1947) reported no effects of 2,4-D on

numbers of microorganisms present in soil, or on rates of nitrification,

until great excesses of the compound were added. Jones (1948) found

that although 2,4-D a t the rate of 25 pounds per acre had no effect on



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nitrification in soil to which no extra nitrogen had been added, applications a t the rate of 15 pounds per acre to soil with added nitrogen (urea.

or sodium nitrate) gave temporary inhibition of nitrate formation. Lewis

and Hamner (1946) found that Rhizobium cultures were not affected by

2,4-D, and concluded that the amounts of 2,4-D reaching the soil from

normal rates of application would have no important effect on microorganisms in the soil. Some subsequent workers, however, have reached

dissimilar conclusions. Employing rates of application of 2,4-D sublethal for plant growth, Payne and Fults (1947) found that 0.009 lb.

per acre mixed with the soil reduced the nodulation of beans grown in

such soil, and that nodulation was entirely prevented by 0.075 lb. rates.

Carlyle and Thorpe (1947) found that 2,4-D salts present in the soil

solution a t the rate of 0.5 p.p.m. (0.21 lbs. per acre) would seriously

restrict germination, limit growth, and practically inhibit nodulation of

legumes. In culture, however, rhizobia were not seriously inhibited in

growth until 2,4-D was applied a t a rate corresponding to 200 lbs. per

acre. Apparently, the inhibition of nodulation by 2,4-D must be ascribed

to an effect on the physiology of the plant or on the joint physiology of

the two symbionts, rather than to any direct bacteriocidal or bacteriostatic action on Rhieobium.

The writer is unaware of any report on the effect of lethal or sublethal

doses of 2,4-D on the rhizosphere flora of nonlegumes. A study in this

laboratory has indicated that for tomato plants treated with 2,4-D, there

is an increased rhizosphere flora in comparison with untreated plants,

and that this increased population persist-s in the rhizosphere for from

two to three weeks, after which there follows a general microbial invasion

and disintegration of the entire root system.

Knowledge concerning the effects of recently developed insecticides

upon the rhieosphere flora is also limited. Wilson and Choudhri (1946)

reported no effect of dichlorodiphemyltrichloroethane (DDT) on nodulation of legumes, nor were soil populations nor pure cultures of various

microorganisms adversely affected. Appleman and Sears (1946) likewise

failed to find intereference with nodulation of legumes with DDT applied

at the rate of 100 Ibs. per acre, but Payne and Fults (1947) reported that

applications of 103 Ibs. of DDT per acre greatly reduced the number of

nodules on bean roots. Wilson and Choudhri (1948) found that benzene

hexachloride, applied in the field a t rates recommended for the control

of wireworms, did not interfere with the nodulation of legumes.

More complete studies of the effects of herbicides and of insecticides

on the rhizosphere populations of both legumes and nonlegumes are

indicated.



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FRANCIS E. CLARK



VII. INFLUENCES

OF THE RHIZOSPHERE

FLORA

ON SUCCEEDING

OB ASSOCIATED

PLANTS

I n the preceding sections, the discussion for the most part has considered the microorganisms of the rhizosphere in relation to the plants

with which they are associated. It is also possible that organisms developing upon the roots of ine plant may influence another plant, growing

either successively or simultaneously in the same soil area. These microbiological influences may be exerted either through the establishment and

persistence in soil of the microorganisms themselves, or through changes

in the soil environment which persist even after the microflora initially

responsible for them no longer is alive.

The concluding section of this review considers (1) the persistence of

the rhizophilic flora in field soils, and (2) the persistence of changes in

the soil environment brought about by this flora.

1 . Persistence of the Rhizophilic Flora in Field Soils



There is little information available as to whether the microflorae

developing in association with plant roots persist in soil for any appreciable length of time after the death of the plants. The bulk of the available evidence indicates that the large numbers of microorganisms

associated with a growing crop do not long persist. Brown (1912) found

that the crop present on the soil was of more importance from the bact,eriological standpoint than the previous cropping of the soil. A rotation

of crops caused the development of greater numbers of organisms in soil

than did continuous cropping. For soils cropped to potatoes or oats,

Starkey (192913) noted a marked decline in microbial numbers subsequent

to maturity and death of the plants. It. is generally known that higher

microbial populations are observed for only brief periods following

organic matter additions to soil. Smith and Humfeld (1931), in a study

of the decomposition of green manures added to soil under glasshouse

conditions, noted t,hat the effects of added rye and vetch residues on

counts of microorganisms disappeared within a few days following addition. Where plants were grown on the soil, and both tops and roots

returned as a green manure, secondary increases in numbers of soil microorganisms were observed after 14 to 21 days. These effects disappeared

after an additional 7 to 14 days. Such data emphasize the rapidity with

which quantitative changes in the soil flora become obliterated. Under

field conditions, where as a rule conditions of moisture and temperature

are less favorable for microbiological activity than t.hose in the greenhouse, a longer period of time would be required. I n field plots in

Nebraska, Dawson et al. (1948) found that the stimulatory effect of



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crop residues on numbers of soil microorganisms was pronounced during

the first few months following addition, but had mostly disappeared

by nine months.

That qualitative changes accompany the quantitative has been

shown by Krassilnikov and Nikitina (1945), who found that an abrupt

change in types of microorganisms takes place during the decomposition

of plant roots. Doughty (1941) has called attention t o differences in

the rate of decomposition of plant roots, as measured by recovery of

coarse material and by production of carbon dioxide.

Although no lasting quantitative changes in microbial populations are

observed following crop growth, certain rhieophilic microorganisms have

been recovered a t varying intervals following crop removal. Smith

(1928) noted increased numbers of B. radiobacter in soil for several

weeks after the growth and harvesting of legumes. Nodule-forming bacteria inoculated on appropriate seed have been reported to become

established in soil and to persist therein for from one to inariy years following removal of the crop (Albrecht and Turk, 1930; Deherain, 1900;

Fred et al. 1926; Nobbe and Hiltner, 1898). Appleman and Sears (1947),

following study of numbers of nodule bacteria in field plots, concluded

that the largest numbers of rhieobia are found where the appropriate

host plants have recently been grown. Their data indicate that thc

presence of nodule bacteria in plots on which host plants have not been

grown recently should be attributed to applications of farm manure containing the organisms and not to any prolonged survival of nodule bacteria in soil.

2. Persistence of Changes in the Soil Environnient Brought



about by the Rhizosphere Flora

It has long been known that one species of plant may have a beneficial

or detrimental influence upon other species of plants that are growing

in close proximity to i t or that follow it in succession. General discussions of mixed cropping and of crop sequence effects have been given

by Nicol (1934), Miller (1938), and Ripley (1941).

There is evidence that both beneficial and deleterious effects upon

accompanying or following crops may result from changes in the soil

environment accomplished by the microflora accompanying certain crops.

The beneficial effect of a leguminous crop on another following frequently can be explained by the fixation of atmospheric nitrogen during

the growth of the legume. Although i t may be impossible to show change

in the total nitrogen content of a soil after legumes have been grown in

that soil for several years, the nitrate-supplying power of soils on which

legumes have previously been grown is generally greater than that of



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