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Chapter 6. Soil Microorganisms and Plant Roots

Chapter 6. Soil Microorganisms and Plant Roots

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242



FRANCIS E. CLARK



VI. Modification of the Root Surface Microflora . . . . . . . . . . .

1. By Seed or Soil Inoculations . . . . . . . . . . . . . . . .

2. By Soil Treatment . . . . . . . . . . . . . . . . . . . .

3. By Plant Treatments . . . . . . . . . . . . . . . . . . .

4. By Application of Herbicides and Insecticides . . . . . . . . .

VII. Influences of the Rhieosphere Flora on Succeeding or Aswciated Plants .

1 . Persistence of the Rhieophilic Flora in Field Soils . . . . . . . .

2. Persistence of Changes in the Soil Environment Brought About by

the Rhizosphere Flora . . . . . . . . . . . . . . . . . .

References. . . . . . . . . . . . . . . . . . . . . . . .



274

274

275



276

276

278

278

279

282



I. INTRODUCTION

For many years the plant root has been regarded as an absorbing and

anchoring organ, between which and the surrounding soil there exist

numerous relationships. Concepts concerning these relationships must be

subjected periodically to scrutiny. Certain clarifications have already

been made. I n the uptake of nutrients by plant roots, it was formerly

considered that mineral salts entered the cell sap of epidermal cells and

root hairs by processes of simple diffusion from the soil solution. Such

processes have been found inadequate to account for the transfer of

nutrient materials into the root, particularly since solutes may be encountered in the cell solution in higher concentration than in the soil

solution; accordingly, other physical as well as metabolic explanations

are now offered. Further, with the discovery of the phenomenon of base

exchange, it has become recognized that cations come largely from absorption sites in the clay lattice and not simply from the soil solution.

A concept relatively little emphasized is that plant roots are in contact

not merely with a physicochemical environment, but with a microbiological environment as well.

Root hairs, as well as root surfaces, are covered with a mantle of

microorganisms, which affect the growth and welfare of the plant in many

ways, and the activities of which in turn are subject to influences of

higher plants. The concentration of soil microorganisms upon plant root

surfaces is intense, and a t times may reach such proportions that the

roots themselves possess no direct contact with the soil solution. Such is

the case with tree roots that become encompassed by mycorrhizal-forming

fungi. For other roots, the ensheathment is less complete, but, nevertheless, the region of contact between plant roots and the surrounding

soil remains a region of microbiological interest. It is here that the soil

microorganisms may exert their most direct influences upon plants.

Here any beneficial products of microbiological decompositions or syntheses are in immediate contact with the root absorbing surfaces; here

also, any toxic or injurious substance of microbial origin, or any com-



SOIL MICROORGANLSMS AND PLANT ROOTS



243



petition by microorganisms for nutrients, directly challenges t,he growing

plant.

It is beyond the purpose of this review to consider either the manifold

activities involved in microbiological transformations of organic and

inorganic subst.ances in soil, or the physiological responses of plant root$.

Its purpose primarily is to consider the extent t o which the root surface

microflora, by reason either of its composition or its proximity, influence

plant growth and welfare. There is also offered a brief characterization

of the microflora encouraged by growing roots, and a discussion of the

persistence of this microflora under field conditions.

1. Historical Summary



Interest in the microbiology of plant roots dates rather sharply from

Hellreigel and Wilfarth’s report in 1888 on the symbiosis between root

nodule bacteria and leguminous plants. Successful explanation of the

fixation of atmospheric nitrogen by symbiotic bacteria served not only t o

stimulate research in soil microbiology as a branch of science, but it also

led microbiologists to expect that similar relationships, equally clear-cut

and dramatic, might be established between soil bacteria and other plants.

I n the decade following Hellreigel and Wilfarth’s contribution, claims

were made that nonlegumes could be inoculated profitably with diverse

bacteria. Although such claims failed to win general acceptance, they

have not failed to attract enthusiastic supporters even to this day. A

further debate, more academic than that concerning the bacterial inoculation of nonlegumes, developed at the opening of the twentieth century as

to whether there was a specialized microflora associated with plant roots

or whether there was merely a stimulation of the general soil microflora.

This controversy was nonproductive excepting for its emphasis upon the

association of microorganisms and plant roots, and the resulting designation of this ecologic region by Hiltner in 1904 as the rhizosphere. The

rhizosphere is defined as that soil region inside which the soil is subject

to the specific influence of plant roots.

Knowledge concerning the microflora of the rhizosphere developed

slowly during the first quarter of the current century. There were scattered observations that the bacteria associated with roots might affect

plant development. The possibility was suggested that the root microflora, by its contribution to the carbonic acid production in soil, might

affect the solution and availability of mineral salts. It was also admitted

that the rhizosphere population might compete directly with plants in the

assimilation of nutrient elements from soil, and there was observed a

depression of mineralization of nitrogen in soil under the influence of

plant roots.



244



FRANCIS E. CLARK



A series of papers by Starkey (1929a-l929c, 1931a, b) in the period

1929-1931 called attention to the many microbiological problems afforded

by the rhizosphere. Stmarkeyboth reviewed the earlier literature, and

presented his own extensive data concerning microbial numbers and

activity in the root zone and the unequal stimulation by roots of different

microbial types. He observed that type of plant, age and condition of

any given plant, and proximity to roots influenced microbial activity

in soil; he also emphasized the possible importance of the rhizosphere

flora to the growing plant.

More recent investigations on the rhizosphere have been fostered by

a diversity of interests. Russian microbiologists have developed an extensive program of seed treatment wit,h nonsymbiotic bacteria, claiming

that such inoculation increases yields; they have also been interested in

the relation of the rhizosphere microflora to the formation of a stable soil

structure. Some workers in t,he U S . Department of Agriculture have

studied the saprophytic microflora of the rhizosphere and its relationship

to differences in resistance of plant roots to soil-borne plant pathogens.

Canadian investigators have been similarly intereeted, and t.hey have

also considered the rhizosphere in relation to the nutritive requirements

of soil bacteria. Workers in several countries have considered the role

of the rhizosphere flora in the uptake or availability of nutrient. materials

to plants.

Krassilnikov (1940) has emphasized the extent to which bacteria

overgrow root surfaces and has reviewed the literature concerning influences of microorganisms on the growth of plants. Kabznelson et aZ.

(1948) have reviewed the more recent literature concerning the microflora of plant roots, particularly that dealing with t,he preferential stimulation by higher plants of certain types of soil organisms.

2. Characterization of the Rhizosphere



The diversity of interests in rhizosphere microbiology has led to a

profusion of experimental techniques and to differences in expression and

interpretation of results. It becomes desirable, therefore, to attempt

some orientation of the biology of the rhizosphere before undertaking to

discuss, firstly, t,hose activities of microorganisms that may affect plant

welfare, and secondly, those alterations within the soil microflora that

are occasioned by plant growth and development.

Hiltner’s definition of the rhizosphere, stated above, refers primarily

to soil adjacent to plant) roots. Nevertheless, nearly all microbiological

studies of the rhizosphere by cultural procedures have included both roots

and soil as material for study. Referring indirectly to the rhizosphere,

Thorn and Smith (1939) speak of “that ball of earth, filled by the roots



245



SOIL MICROORGANISMS AND PLANT ROOTS



of a particular plant, with the microorganisms that accompany them."

Photomicrographs of Cholodny slides recovered after burial within the

root zone show large numbers of microorganisms to be associated intimately with the root surfaces, very much as an enveloping sheath, with

the surrounding soil relatively sparsely colonized. Both Starkey (1938)

and Linford (1942) have published excellent photomicrographs of associations of microorganisms and roots as shown upon glass slides exposed

within the root zones of growing plants.

Cultural data also show the importance of proximity to roots upon

the magnitude of microbial populations encountered. Representative

data for the cotton plant rhizosphere are shown in Table I. Starkey

(f931a) has published microbiological data for other plants which serve

equally well to emphasize the immediate localization of microorganisms

on root surfaces.

Perot,ti (1926) , in attempting to establish boundaries for the rhizosphere, has considered it to be bounded on one side by the general soil

region, or edaphosphere, and on the other, by the root tissues, or histosphere. Although a sharp line cannot be drawn between the edaphosphere

and the rhizosphere, practically, the boundary may be considered to be

reached wherever plant roots can no longer be shown to have an observable influence on the soil flora. I n Table I, with sample (a) considered

as representative of t,he soil flora apart from and uninfluenced by roots,



TABLE I

Occurrence of Certain Types of Bacteria in Relation to Proximity to Cotton Roots"



Total

bacteria

Description of sample

(a) Soil, 10 to 15 cm. distant

from roots . . . . . . . . . . . . . . . . . . . .

(b) Soil, 5 to 10 cm. distant

from roots . . . . . . . . . . . . . . . . . . .

(c) Soil, 2.5 to 5 cm. distant

from roots . . . . . . . . . . . . . . . . . .

(d) Soil, 0.5 to 2.5 cm. distant

from roots . . . . . . . . . . . . . . . . . .

(e) Soil, 0 t o 0.5 cm. distant

from roots . . . . . . . . . . . . . . . . . . .

( f ) Root surface scrapings . . . . .

a



b



I



Dyetolerant

bacteria



I1



Ratio



II/I



Spores

of

bacillus b Ratio



111



III/I



52.6



7.4



0.141



17.0



0.323



47.9



9.0



0.188



18.0



0.376



45.6



7.7



0.165



17.0



0.373



54.7



15.0



0.274



11.0



0203



129.9

510.0



91.3

440.0



0.702

0.863



13.7



0.105




0.1



After Clark (1940).

Hundred thousands per g. of air-dry soil or per g. of root surface scrapings.



246



FRANCIS E. CLARK



samples (b) and (c) would not be included in the rhizosphere on the

basis of microbiological data presented for them. If appreciable cultural

or direct microscopic differences are not required as criteria in delimit,ing

the rhizosphere, then it must be considered to be the entire field soil at

least to the depth penetrated by plant roots, and perhaps even further,

inasmuch as such soil is, in the main, under the influence of roots. Past

experience and usage is justifiably against such an extensive definition.

The histosphere, which in Perotti’s scheme serves as t.he inner boundary of the rhizosphere, has in the past been considered normally to be

free of microbial cells. Exceptions, to be discussed shortly, occur in

parasitism, in mycorrhizae, in bacteriorrhizae and the nodulation of

legumes, and, of course, during the terminal disintegration and decay of

the root system. Some recent work makes it appear probable that the

interior tissues of plant roots are more commonly colonized by microorganisms than has heretofore been recognized.

Perotti (1926), and almost wit,hout exception other writers publishing

on the rhizosphere, include under that term the det.ritus on the root

surface, sloughed or dying cells, and the microorganisms closely attached

to the root surfaces. Inasmuch as nutrient material coming from the

roots is primarily responsible for this immediate microbial concentration,

this mantle of microorganisms attached to the root surface is undoubtedly

the most important part of what has been called the rhizosphere microflora. Accepting the root surfaces as responsible for the bulk of the

microorganisms found in those combined soil and root samples taken

collectively as the rhizosphere, it would be more logical to speak of this

microflora in terms of the root surfaces than in terms of the soil adjacent

to the root# of plants. This goal can be obtained by the use of the term,

rhizoplune. The rhizoplane is defined as the external surfaces of plant

roots together with any closely adhering particles of soil or debris.

The introduction of the term, rhizoplane, by shifting the microbiological emphasis from the soil around the root to the root surfaces themselves, provides those working either with roots or root surface scrapings

with a means of considering their data on other than a soil basis. The

frequent employment in the literature of the phrase “the microorganisms

associated with root surfaces” indicates an unwillingness to ascribe t o

the surrounding soil the microbes present on root surfaces. Certain workers (Graf, 1930; Poschenreider, 1940) have used the terms “outer rhizosphere” and “closer rhizosphere” in describing sites of microbial

concentration. Berezova (1941) considered there were two distinct zones

in the rhizosphere supporting appreciably different microbial populations

- o n e zone was the adjacent soil, the second, the root surface itself. The

writer is of the opinion that by using the term rhizosphere to describe



SOIL MICROORGANISMS AND PLANT ROOTS



247



the Soil region adjacent to plant roots, and the term rhizoplane to denote

the plant root surfaces, much of the uncertainty and confusion in the

literature can be avoided.



11. TYPESOF RELATIONSHIPS

BETWEEN MICROORGANISMS

AND PLANT

ROOTS

Microorganisms associated with plant growth cannot be classified

rigidly as to whether they occur within, on the surface of, or only at a

distance from plant roots. The site a t which any given microorganism

occurs may vary with changes in environmental or physiological conditions. Nor can soil microorganisms be classified rigidly as to the type

of relationship exist,ing between them and plant roots. These relationships may range all the way from chance association to symbiotism or to

parasitism.

1 . Symbiotism

Symbiotism between microorganisms and plant roots, in the sense of

mutualism or benefit to both partners of the associat,ion, is best known

in mycorrhizal formations and in the nodulation of legumes by

Rhizobium. A mycorrhiza is defined as the symbiotic association of a

fungus with the roots of a seed plant, with the symbiosis probably of

mutual benefit, though not unquestionably proved to be so. Where the

hyphae of the fungus form an interwoven mass investing the root tips,

they constitute an ectotrophic mycorrhiza, where they penetrate the

parenchyma of the roots, t.hey constitute an endotrophic mycorrhiza. A

mycorrhiza is the morphologic combination of a fungus and a root. The

term mycotrophy denotes the processes of plant nutrition by means of

mycorrhizae, or the tendency of fungi and roots to form associations of

functional benefit. The literature on mycorrhizae and mycotrophy is

voluminous. A number of review articles have been published; it will

suffice here to mention those of Rayner (1927), Hat.ch (1937), and

Schmidt (1947).

Bacterial associations with roots, analogous to those formed by fungi

in mycotrophy, are termed bacteriorrhizae. Very little is known concerning the actual existence or extent of bacteriorrhizae, apart from the

root hair invasions and the resulting localized nodule formations by

rhizobia. Nodules on legumes caused by rhizobia may be considered to

represent a localized bacteriorrhizal condition. Because of their practical

importance, the rhizobia have been studied more extensively than any

other group of soil microorganisms. They are commonly referred to as

the legume bacteria, and their symbiotic relationships with their host

plants are generally acknowledged. Monographs on the nitrogen-fixing



248



FRANCIS E. CLARK



bacteria have been prepared by Fred et al. (1932), and by Wilson (1940).

Whether actinomycetes form symbiotic associations with roots similar

to those formed by fungi and bacteria is unknown. There remains the

possibility of the eventual discovery of an “actinorrhiza.” Lutman and

Wheeler (1948) observed invasions of cells of potato tubers by microbial

filaments, which they first thought to be actinomycelial in nature, but

which yielded bacilli upon culture. Sanford (1948) has recently reported

that the interior tissues of the stems and tap roots of potato, lucerne,

and sweet clover contain a mixed bacterial flora. T e y e t and Hollis

(1948) also have encountered bacteria in healthy storage organs of

potato and other plants.



2 Parasitism

The occurrence of fungi, actinomyces, bacteria, and viruses injurious

to plant roots has long been recognized. References and discussions concerning the phytopathogenic microflora of plant roots are available in

the numerous textbooks and monographs on plant diseases.

3. Commensalism



Because of the excretion of inorganic and organic substances, and the

sloughing of root caps, root hairs, cortical and epidermal cells, all of

which supply available energy material for microbial utilization, a great

number of microorganisms are encouraged to develop immediately upon

the root surfaces, and the majority of these microorganisms are undoubtedly commensalistic in their relationship to the higher plant, living a t

the expense of secreted or excreted material, but not invading the root

tissues with either injurious or beneficial results. Certain workers have

emphasized the scavenger role of microorganisms on root surfaces ; others,

the existence of a successive symbiotic effect, rather than a true or

simultaneous symbiotism.

Undoubtedly the relationships between microorganisms and plant

roots are variable, depending upon environmental and physiological conditions. Microorganisms normally commensalistic may become parasitic

with decreased vigor of plant growth. Similarly, mycorrhizal invasions

vary from absolute parasitism to complete symbiosis, depending upon

t.he vitality of the fungus and the health of the higher plant (McArdle,

1932). Thornton (1935) has suggested that eventually it will be possible

to trace an evolutionary gradient from the purely ecological relationships

that exist between free-living soil bacteria and higher plants through

that too little studied population of organisms existing outside but in

close contact with the roots of higher plants, to those microorganisms

such as mycorrhizal fungi and rhizobia that live symbiotically within



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