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Part IV, continued: 

The specificity of the microflora of the root zone

  As indicated above, the chemical composition of rootexcretions, as well as that of the dying root hairs and cells of the root epidermis,varies in different plants. Consequently, root production will attract differentsoil microflora. Plants which excrete carbon compouonds attract to their roots amicroflora which differs from the microflora attracted by plants which mainly excretednitrogenous substances. With the presence of sugar in root excretions one kind ofbacterial and fungal species will develop and, in the presence of organic acids,others will develop. These microorganisms which utilize the available nutrient substancesmore quickly and fully will predominate in the plant rhizospheres.

  The qualitative composition of root excretions anddying root residues determines the characteristics of the quantitative and speciesmakeup of the microflora in the soil of the root region.

  The first to have observed the capacity of plants toaffect the microflora of soils was the writer, S.T. Aksakov.

  In 1896 he wrote: "The mushroom is the child ofthe forest. . . . As is well known, if one sows, plants-- in a word, if one plantsa forest in a bare field--the mushroom types characteristic of the varieties of theplanted forest will undoubtedly start to grow there. However, the shadow (as manyhave thought) cast by the branches of trees is not the only secret force by whichtrees cause mushrooms to grow around them. It is true that the shadow is one of theprimary reasons for this phenomenon. It protects the soil from the burning rays ofthe sun, and it create a moisture in the soil and even the dampness which is essentialfor the forest, as well as for the mushrooms. However, the main reason for the formationof the mushrooms, in my opinion, are the tree roots which, in their turn, havingmoistened the surrounding soil, give it the sap of the tree. This, in my opinion,is the secret of mushroom growth." (Notes and Observations on how to CollectMushrooms, vol. V, 1896).

  Later, data on this subject were published In the scientificliterature (Galakhov 1929; Danilov, 1943, 1949; Vasil'kov, 1953, and others). Detailedinformation on the interrelationship between edible mushrooms and higher plants inthe forest phytocoenoses of Latvia is given in the dissertation presented by Mazelaitis(1952), and information on the distribution of mushrooms in the forests of the Moscowarea in given in the book by Shiryamov "The Search and Collection of Mushroomsin the Forests of Moscow Area," (1948).

  An analysis of the obtained observation and studiesshows that the distribution of edible mushrooms is closely connected with the compositionof phytocoenoses. When the forest and plant species of an area are known, one candetermine beforehand which species of mushroom will grow in the area and the distributionof the mushroom types of interest to us. For instance, the white mushroom is encounteredin heather, mountain-cranberry, spruce-sorrel, and oak-bilberry forest. The ordinarybrown mushroom is found in pine forests with birch and heather; Boletus lutensis found in young pine forests having a certain grassy vegetation, etc (Vasil'kov,1953).

  We demonstrated experimentally the selective actionof plants. Different plants were grown under sterile conditions, in sand or cotton,and soaked with Knop's nutrient solution. Bacteria were introduced into the medium,either separately, or in the form of mixture. The results are given in Table 76.

Table 76
The effect of plants on the qualitative composition of bacteria
(number of cells in thousands per ml on the 20th day of growth)

Plant

Root-nodule bacteria-- clover

Root-nodule bacteria-- lucerne

Az. chroococcum

Pseudomonas flourescens strain No 1

Pseudomonas flourescens strain No 2

Wheat

10

100

0

200,000

1,000

Corn

0.1

200

0.01

15,000

4,000

Cotton

10,000

100,000

3.0

100,000

30,000

Sugar beets

--

1,000

1.5

100

100,000

Flax

0.1

10

0.01

300

100

Clover

100,000

10,000

6,000

1,000

1,000,000

Lucerne

1,000

100,000

5,000

1,000

100,000

Peas

10,000

1,000

3,000

1,000,000

1,000

  As can be seen from the given data, some bacteria growwell, others grow only slightly, while others show intermediate growth on the sameplant. Azotobacter, for instance, did not grow at all or grew very poorlyin vessels in which wheat, corn, and flax were planted, showed intermediate growthon cotton, and abundant growth on clover, lucerne, and peas.

  The reaction of two strains, Nos. 1 and 2, of the samespecies of nonsporeforming bacteria. Ps. fluorescens, to the root excretionsof plants differed. Strain No. 1 grew most abundantly in the rhizosphere of wheat,corn, and especially cotton, while strain No. 2 grew much better in the presenceof the root excretions of clover and lucerne. The root-nodule bacteria reacted positivelytoward the roots of cotton, clover, lucerne, and peas, and less so to the actionof the root system of sugar beets.

  We also grew a series of other sporeforming and nonsporeformingbacteria, mycobacteria and yeasts. The sporeforming bacteria Bac. mesentericus,Bac. subtilis, Bac. megatherium, as a rule, did not grow in a medium with theroot excretions of the experimental plants, but did not die in it. The mycobacterium,Mycob. album, grew fairly well in the rhizosphere of corn and wheat and somewhatbetter in that of clover and peas. Brewer's yeast did not grow at all with corn,wheat, and beans, but wild yeast of the genus Torula, on the other hand, grewbeautifully and multiplied.

  Similar data were obtained by Metz (1955) in experimentswith sterile cultures, nonsporeforming bacteria and, on other plants, their growthstopped. According to his data, the growth of Azotobacter is strongly suppressedin the rhizosphere of celandine, buttercups (Ranunculus acer L., Ran. repensL), peonies (P.officinalis L.), and fumitory, (Fumeria officinalisL.), and is less suppressed in the rhizosphere of Viola tricolor Wittr., Alliumschoenoprasum L., Rumex patientia L., and Epillobium montanum L.

  Plants such as Crepis virens K., Hieraciumpilosella L., Armoracia rusticana Gaertn, and others, which strongly suppressthe growth of sporeformong bacteria, do not have any deleterious effect on the growthof Azotobacter. On the contrary, many of them stimulate the growth of thismicrobe.

  A similar effect on the growth of Azotobacterby these or other plant species prevails under conditions of natural growth (Table77).

Table 77
Accumulation of azotobacter in soil with different plants
(number of cells per gram of soil)

Region

Plant

Cultivated soil

Fallow soil

Moscow Oblast', podsol

 

 

 

 

Wheat

0-10

0-10

 

Rye

0-10

0-10

 

Barley

50

60

 

Oats

70

50

 

Potatoes

100

50

 

Clover

1,000

80

 

Flax

0

10

Kola Peninsula, podsol

 

 

 

 

Clover

1,000

0

 

Cow parsnip

50

0

 

Barley

10

0

Kuibyshev Oblast' Serozem

 

 

 

 

Wheat

200

200

 

Barley

500

200

 

Lucerne

5,000

250

Moldavian SSR, Serozem

 

 

 

 

Wheat

500

800

 

Lucerne

600

800

 

Sudan grass

1,200

500

Central Asia Uzbek SSR, Serozem

 

 

 

 

Wheat

20

100

 

Corn

50

100

 

Lucerne

3,500

250

Vakhsh Valley, Serozem

 

 

 

 

Cotton

450

400

 

Lucerne

10,000

800

 

Rye grass

6,000

600

 

Orchard grass

300

400

 

Rice

12,000

1,200

Kirgiz SSR, Serozem

 

 

 

 

Wheat

380

250

 

Beets

500

300

 

Potatoes

180

150

 

Cotton

10

50

Trans-Volga region, Chestnut soil

 

 

 

 

Wheat

0-10

0

 

Millet

0

0

 

Sunflower

40

50

 

Corn

0-20

20

 

Lucerne

2,000

200

 

Sweet clover

1,500

200

Crimea, Southern coast

 

 

 

 

Tobacco

250

200

 

Vineyards

150

180

  The effect of grass plants on the growth and accumulationof Azotobacter in the soil is especially well demonstrated in monocultures.The longer a plant is cultivated, the more bacteria and fungi accumulate in the soil.Such a long-term accumulation of microbes was observed by us in the soils of CentralAsia, in the lucerne-cotton crop rotation, Usually, after lucerne is grown for threeyears whether in the pure form or in a grass mixture, cotton is cultivated for severalyears (six to nine). After this type of crop rotation, the microflora changes considerablyin relation to Azotobacter, and to other species as well. (Figure 78). Underlucerne, the number of Azotobacter cells increases and, under cotton, it decreases.However, Azotobacter does not completely vanish under cotton.

 

Figure 78. Growth of Azotobacter in soils with a crop rotation of lucerne-cotton in the fields of the Vakhsh valley, Tadzhik SSR:

1-- cotton, 2--lucerne.

 

  'This regularity of change in microbial forms is observedin monocultures of plants, for example, under lucerne and wheat in the chestnut soilsof the Trans-Volga region, or under clover-flax and oats in the podsol soils of theMoscow Oblast' (Figure 79).

 

Figure 79. Growth of root-nodule bacteria of clover on various plants:

1--clover; 2--wheat; 3--oats (monoculture) on the experimental fields of the Agricultural Academy im, Timiryazev.

 

  In the crop rotations of an ordinary farm with industrialcrops, one observes thesame results, but with less markedly expressed numerical indices(Krasil'nikov, 1940a).

  Sheloutnova (1938) and Wenzl (1934) observed the growthof Azotobacter under a culture of tobacco and vineyards; Mashkovtsev (1934)and Uppal and coworkers (1939) observed its growth under rice.

  We have studied the growth of Azotobacter inforest plantations in the fields of Kirgizia, Moldavia, Ukraine, and the Moscow Oblast'.The arboretum of a seven- to ten-year-old plantation, in sectors previously underlucerne or other vegetation possessing a considerable number of Azotobactercells was investigated. The growth of Azotobacter was quantitatively measuredon strips sown with different tree varieties: maple, ash, poplar, acacia, spruce,pine, oak, etc (Table 78).

Table 78
Growth of Azotobacter in soil under forest trees
(number of cells in 1 g of soil)

Soil

Plant

Azotobacter in forest strip

Azotobacter in plowed-up strip

Moldavian SSR, Chernozem

 

 

 

 

Oak forest

80

2,700

 

Acacia

150

2,800

 

Lime

0

3,000

Moscow Oblast', Podsol

 

 

 

 

Spruce

0

100

 

Birch

0

250

 

Oak

0

100

Kirgiz SSR, Serozem

 

 

 

 

Maple

60

6,000

 

Ash

500

5,000

 

Poplar

0

4,200

 

Acacia

1,500

4,000

 

Elm

0

6,000

 

Birch

0

3,500

 

Grass mixture

--

100,000

  As can be seen from the data given, afforestation,as a rule, removes Azotobacter from the soil. Only under certain species init preserved in small numbers,. Under certain plants, the acacia and the ash tree,Azotobacter grows moderately well, although it does not reach the numbersfound in plowed-up soils.

  Artificially planted birch, aspen, oak, and especiallyspruce and pine trees in podsol soils quickly remove Azotobacter. Azotobacteralso perishes in Southern chernozem soils, if the latter are afforested with oak,pine, hornbearn, etc.

  Fruit trees, such am apple, pear. plum, cherry, andothers, do not suppress the growth of Azotobacter and many of them are evenfavorable to its accumulation in the soil. The soils of the orchards investigatedby us in Moldavia (chernozem). in the central belt of RSFSR (podsol), in Crimea,and in other regions of the USSR contain larger quantities of Azotobacter than thesoils of fields which are intensively cultivated (Table 79).

Table 79
Growth of Azotobacter in soils under fruit trees
(number of cells in 1 g soil)

Region and soil

Azotobacter in orchard

Azotobacter in field

Moldavia, Chernozem

 

 

Kishinev region

2,500

450

Kalarash region

7,500

1,200

Slobodzei region

7,000

1,500

Rezinski region

5,000

650

 

 

 

Moscow Oblast', Serozem

 

 

Chashnikovo, experimental field

1,200

120

Snegiri

2,400

60

Kragnyi Mayak

1,400

0

 

 

 

Crimea

 

 

Alupke, schist

1,500

0

Yalta, humus

3,700

450

 

 

 

Sudak valley

 

 

Redish-brown, sepia brown

12,000

1,500

Steppe zone

4,200

540

Koktebel'. Heavy aluvium loam

2,100

250

Old Crimea. Gravel chernozem with low humus content

1,800

80

Perekop region. chestnut solonchak

3,600

240

  As can be mean from the above-mentioned, certain speciesof plants enhance the growth of Azotobacter in soil, others suppress it, andothers neither enhance nor suppress its growth.

  This type of plant classification is only relativeand only holds true for sterile cultures, where the microbes are subjected to a one-sidedaction by root excretions, with the exclusion of other external factors.

  Under conditions of natural growth in the field, theeffect of root excretions is to a large extent annulled by many factors and, firstof all, by microorganisms. Therefore, the summary effect in such cases will expressitself in a weaker form and sometimes will not be observed at all. A comparison ofthe effectiveness of plants should be performed under the same soil-climatic conditions.

  The same plant in different soils and in differingclimatic and geographical zones may act differently on Azotobacter. As wasnoted above, wheat acts negatively on the growth of Azotobacter under conditionsof sterility, in open ground in the Trans-Volga region on chestnut soils, and inthe central belt on podsol, but does not suppress the growth of Azotobacterunder the conditions prevailing in Kirgizia on chernozem soil, on the chernozem soilof the Kuibyshev Oblast', and in other places. Even in the same region and in thesame soil, the effect of wheat may differ, depending on the extent of the cultivationof the soil. Soils of the podsol zone, where cultivated, often contain a sufficientnumber of Azotobacter under wheat and other plants. Of great importance forthe growth of Azotobacter is the agrotechnical cultivation of soil.

  The root excretions of young plants differ from thoseduring the period of ripening and, therefore, the microflora during these periodsof growth will also differ.

  Only by consideration of a multitude of factors affectingthe growth of Azotobacter can the contradictory indices, obtained by differentinvestigators, on the distribution of this microorganism be explained.

  These considerations not only pertain to Azotobacterbut to all other microbial species in the rhizosphere of plants.

  The selective effect of plants is well demonstratedin the case of root-nodule bacteria. Korenyako (1942) tested three species of bacteria,Rh. trifoli. Rh. meliloti and Rh. leguminosarum, by introducing theminto containers in which clover, lucerne, peas, wheat, corn, and cotton were grownon sand. Root-nodule bacteria of lucerne grew equally well under lucerne and undercotton; Rh. trifolii grew abundantly under clover, lucerne, peas, and wheat,less well under flax, and hardly at all under corn. Root-nodule bacteria of peaswere found in large numbers under peas, clover, and wheat.

  These data were confirmed by experiments performedin open soil (podsol). Root-nodule bacteria of clover were introduced under clover,lucerne, peas, wheat, and corn. The growth of the bacteria in the rhizosphere wasfollowed through the entire vegetative period, The results are given in Table 80.


Table 80
Growth of Rh. trifolii in the rhizosphere of various plants
(in thousands in 1 g soil)

Date of analysis

Clover

Lucerne

Peas

Wheat

Corn

23 June

100

10

10

1

1

2 July

100

100

10

0.1

0.1

23 July

100

1,000

10

10

1.0

30 July

100

100

100

100

0.1

21 August

10,000

100

1,000

1,000

10

20 September

1,000

100

190

100

10

20 October

1,000

10

100

0.01

0.1

  The intense growth of root-nodule bacteria under leguminousplants was also noted by other investigators (Wilson and Wagner, 1936, and Lewis,1938).

  Rudin (1956) noted the activating action of corn sap,and especially that of pea sap, on root-nodule bacteria. According to his observations,the sap of inoculated peas having nodules on their roots is more active than thesap of noninoculated peas, The sap of corn roots enhances the virulence of the root-nodulebacteria of peas.

  According to our observations, the root-nodule bacteriaof lucerne grow well under timothy grass, cotton, and rye grass. Their number inthe serozem soil of Central Asia, reached hundreds of thousands in one gram of soil,i. e., almost the same as under lucerne. In the chestnut soils of the Trans-Volgaregion, these bacteria in the rhizosphere of timothy grass amount to tens of thousandsin one gram of soil, and under orchard grass, oats, and millet, their number wasconsiderably smaller.

  The number of root-nodule bacteria in the soil, afterleguminous plants, decreases, if the subsequent culture in the crop rotation is notfavorable to their growth this decrease in the number of bacteria in the soil takesplace at different rates, depending on the soil and on the plant species. Under flaxand wheat, the number of root-nodule bacteria of clover in the podsol of the MoscowOblast, decreases comparatively rapidly.

  The experimental data obtained by Chailakhyan and Megrabyan(1955) on the specific action of the roots of leguminous plants on root-nodule bacteriaare of interest. The authors found that the ground roots or the root sap of leguminousplants do not negatively affect the growth of root-nodule bacteria of their own species,but suppress the growth of bacteria of other foreign species (Table 81), The maximumexpression of this selective action by the roots of leguminous plants is observedduring budding and flowering stages, becoming less marked later.

Table 81
Suppressing effect of the roots of leguminous plants on the growth of root-nodule bacteria.
Zones of inhibition of growth around the roots, in mm
(according to Chailakhyan and Megrabyan, 1955)

Roots of plants

Root- nodule bacteria, Vetch

Root- nodule bacteria, Ono- brychis

Root- nodule bacteria, Lucerne

Root- nodule bacteria, Clover

Root- nodule bacteria, Peas

Root- nodule bacteria, Beans

Root- nodule bacteria, Soy

Root- nodule bacteria, Broad beans

Root- nodule bacteria, Trigo- nella

Root- nodule bacteria, Lupine

Vetch

0

4

3

4

6

3

3

7

4

4

Onobrychis

4

0

3

5

7

4

4

8

5

3

Lucerne

2

5

0

4

6

6

4

6

3

3

Clover

5

6

4

0

6

5

5

5

4

4

Peas

2

2

3

3

0

3

2

3

3

3

Beans

6

5

5

6

6

0

3

6

7

5

Soy

5

4

6

5

5

4

0

7

6

5

Broad beans

3

3

2

2

3

3

2

0

3

3

Trigonella

4

4

3

7

4

3

4

6

0

6

Lupine

4

5

4

4

7

4

3

7

4

0

  In our investigations we were unable to find such specificityin the selective antibacterial action of either the roots or the aerial parts ofleguminous plants.

  Torne and Brown (1937) tested the bacterial effectof the sap of a great number of plants, leguminous and others. According to theirdata, the extracts of leaves of many plants inhibit the growth of root-nodule bacteria.These plants include clover, cabbage, carrots, turnips, and others. The authors didnot observe any specificity in the inhibitory action of the sap. Lucerne sap inhibitedthe bacteria of its own species to the same extent as it inhibited that of clover,beans, and other plants. Root sap was less bactericidal than the sap of the aerialparts and, in many plants, the root extract had no toxicity for bacteria whatsoever.

  Among the other microbes which grow in the root zoneof vegetating plants we also studied mycolytic bacteria. These bacteria are characterizedby their ability to dissolve the mycelia, of fungi (Khudyakov, 1935; Novogrudskii,1936). They differ in classification and comprise a mixed group of bacteria. Theyalso possess a more or less defined specificity. Each species in this group of bacteriadissolves certain forms of fungi--saprophytes and phytopathogenic forms.

  According to Korenyako (1939). Raznitsina (1942), andKuzina (1951, 1955), mycolytic bacteria grow abundantly and accumulate under certainplants. Under the conditions prevailing in Central Asia, in serozem, soils underlucerne, their number varies from 100,000 to one million in one gram. Under cotton,on the other hand, their number decreases. In soil which has been plowed for sixto nine years, under this culture [cotton], the number of mycolytic bacteria is minimal,and under three-year-old lucerne it is maximal (Figure 80).

 

Figure 80. Accumulation of mycolytic bacteria in soil under lucerne and cotton (Central Asia)

1--under lucerne; 2--under cotton.

 

  Under the conditions prevailing in the central beltof the Soviet Union, many mycolytic bacteria grow well and accumulate in podsol soilsunder clover and under certain other leguminous plants. Some of these bacteria dissolvefungi of the genus Fusariam, and others dissolve fungi of the genus Helminthosporium.Mycolytic bacteria do not grow uinder wheat and especially under flax, and they areremoved from the soil relatively quickly.

  Daraseliya (1949) found a large accumulation of mycolyticbacteria in the rhizosphere of the tea plant. According to the author, the limiteddistribution of phytopathogenic fungi of the genus Fusarium in the soils oftea plantations is caused by the abundant growth of these bacterial antagonists.

  Nitrifying bacteria also grow in the rhizosphere ofplants, where, under certain species, for instance under leguminous and certain nonleguminousplants, their number is considerably higher than under cereals and certain vegetablecultures. In studying the species characteristics of denitrifying bacteria, whichwere isolated from the rhizosphere of lucerne, wheat, and millet at the Ershov Station(Saratov Oblast'), we successfully determined some of their characteristics. Thestrains which were isolated from the root zones of wheat were in the most cases moreactive than the strains growing under lucerne and millet. Nitrate reduction was completedby the former in three to five days, while the latter completed this reaction underthe same conditions in five to ten days. In the former case, the process was accompaniedby the violent evolution of gas (nitrogen), while in the latter, the formation ofgas was not observed or was negligible.

  This example of the growth of denitrifying bacteriais probably not general and was observed only under the soil-climatic conditionsof the given region. In the serozem soils of Central Asia and in the podsol soilsof the Moscow Oblast' we have not observed such specificity. Certain differencesin the properties of denitrifying bacteria, growing in the rhizosphere of lucerneand wheat, were observed in the chernozem soils of Moldavia.

  Among other species of nonsporulating bacteria growingin the rhizosphere of plants, specific forms also probably exist which have betteradapted themselves to the conditions of life under certain species of plants. Unfortunately,with the methods available we are unable to detect specialized forms under theseor other plants. Certain investigators have observed different concentrations ofphysiological groups of bacteria under different plant cultures.

  According to observations made by Starkey (1929, 1931),the process of the formation of nitrate from ammonium sulfate in the rhizosphereof certain plants, was more intensive than in others.

  According to our observations, the root system of peasactivates the process of the decomposition of cellulose. In our experiments we usedspecial growth containers, filled with turfy podsol soil. One wall of the container(made of glass) was covered with filter paper. The plant grew from the container,which had been placed in a sloping position, and the roots were established so thatthey grew on the surface of the paper. The consecutive stages of the process of thedestruction of the paper could be followed through the glass. Wheat, corn, peas,vetch, and beans were planted. The experiments showed that the roots of cereals donot exert a noticeable effect on the process of cellulose decomposition. Among theleguminous plants, beans proved to be ineffective, vetch only slightly stimulatory,while the roots of peas strikingly enhanced the process of the decomposition of paper(Figure 81).

  

Figure 81. Decomposition of the cellulose in soil under the influence of the root system of plants

a) destruction of paper around the roots of peas; b) destruction of paper in control soil, outside the root area.

 

  Lockhead and his associates (1950, 1955) demonstratedthat the composition of the microflora of the rhizosphere of various plants differswith respect to vitamin requirements. In the root area of some species of plants,bacteria prevail which require vitamin B1 or B2 and, in therhizosphere of other plants, bacteria requiring biotin, vitamin B12, cysteine,methionine, etc are prevalent.

  There are indications in the literature, that algaealso differ in their growth in the root area of plants. According to data obtainedby Shtina (1954a, 1955), rye, timothy grass, and potatoes mainly enhance the accumulationof diatom algae; in the rhizosphere clover and lupine enhance the growth of greenalgae and perennial grasses, and potatoes partially enhance the growth of blue-greenalgae (Table 82).

Table 82
Growth of algae in the root zone of plants
(in thousands per 1 g of soil)

Plant

Diatoms in rhizosphere

Green algae in rhizosphere

Blue-green algae in rhizosphere

Diatoms in control

Green algae in control

Blue-green algae in control

Ry e

42.0

79.8

8.2

18.0

65.0

8.0

Timothy grass, first year

43.6

63.6

8.6

28.6

78.1

6.6

Timothy grass, second year

73.2

147.6

8.6

28.6

78.1

6.6

Clover, first year

15.4

90.0

11.0

28.6

40.0

6.0

Clover, second year

37.4

105.0

6.8

39.5

58.4

1.1

Lupine

19.2

93.6

2.4

37.2

69.6

1.0

Potatoes

27.6

46.8

7.2

15.6

45.6

3.6

  There are many indications that if unfavorable conditionsprevail during the growth of the plant, more or less great numbers of phytopathogenicorganisms grow and accumulate in their rhizosphere.

  Timonin (1941) showed that under a culture of flaxconsiderable numbers of the following fungi often develop: Fusarium lini, Alternaria,Cephalosporium, and certain others causing plant diseases. Sanford and Broadfoot(1951) observed the growth of the phytopathogenic fungi Helminthosporium sativumand Fusarium culmorum in soil under wheat and oat monocultures. The fungusOphiobolus graminis, under the conditions which prevail in certain localitiesin Canada, is suppressed by oats and clover and, according to Winter (1940), it growsbetter in the rhizosphere of wheat, than in the soil outside the root zone. Martin(1950) found an accumulation of the fungi Fusarium solani, Pyrenochaeta sp.,and other species in the soil under citrus plantations. The author is of the opinionthat the weak growth of the seedlings and saplings of citrus plants in the soil ofcitrus groves is caused by the deleterious effect of the microflora. Cotton is favorableto the accumulation in the soil of the phytopathogenic fungi Verticillium dahliaeand Fusarium vasinfectum, causing it to wither, At the same time, lucernesuppresses the growth of these fungi. The accumulation of phytopathogenic fungi insoil under the influence of vegetative cover was also noted by other investigators(Lockhead and others, 1940-1950; Weindling, 1946; Eaton and Rigler, 1946; Timonin,1946; Grammer, 1955, and others).

  Under certain conditions, plants can accumulate microbialantagonists in the soil, which inhibit the growth of such useful species of microorganismsas Azotobacter, root-nodule bacteria, and mycorhizal fungi, which are theproducers of various biotic substances: vitamins, auxins, amino acids, and otherproducts of microbes.

  Plants may also favor the growth and accumulation inthe soil of the microbial antagonists of phytopathogenic bacteria, fungi, actinomycetes,and even viruses.

  Hildenbrand and West (1941) observed a lower rate inthe root-rot disease in strawberries when they were sown after soybean, and an increasedincidence of the disease when they were sown after clover. According to data obtainedby Cooper and Chilton (1950), in the root zone and on the roots of sugar cane, actinomycetes,antagonists of the fungus Pythium arrhenomonas, which is the causative agentof the root disease of this plant, grow abundantly. Their number in the rhizospherereaches 77,000 and more, while outside the rhizosphere, away from the root, it doesnot exceed 2,000 in one gram of soil.

  Similar data wam given by Sanford (1946, 1948), Weindling(1948), Fallings (1954), and others.

  Plants have a beneficient effect on soil. not onlyin relation to phytopathogenic microbes, but also in relation to pathogenic microbesaffecting men and animals.

  Bogopol'skii (1948, 1950) studied the effect of theroot system of various plants on the viability of bacteria of the colon group inthe soils of urban plantations, in the parks and squares of Kiev. According to hisobservations, certain grasses used for lawns considerably hastened the death of thesebacteria. For instance, in a plantation of sweet clover, after 60 days, 25 organismsout of one and a half million originally introduced were found, 45 were found underclover, and 110 were found under oats, of the total number originally introducedin the soil. Under orchard grass the colon bacillus almost totally disappeared (10organisms per gram), while In the control zone without plants, 180 bacteria per gramwere observed.

  According to our observations, the colon bacillus anda pyogenic staphlococci die in the soil under some plants quicker than under others(Table 83).

Table 83
Death of Staph. aureus and Bact. coli under the influence of plants
(number of cells per one gram of soil)

Bacteria

Time of stay in soil, days

Fallow soil

Two-year-old clover

Grass mixture

Staph aureus

 

 

 

 

 

0

2,500,000

2,500,000

2,500,000

 

5

100,000

10,000

50,000

 

10

10,000

100

10

 

20

1,000

0

0

 

30

100

0

0

 

50

0

0

0

Bact. coli

 

 

 

 

 

0

1,500,000

1,500,000

1,500,000

 

5

500,000

20,000

30,000

 

10

25,000

1,000

5,000

 

20

1,500

100

600

 

30

200

0

10

 

50

10

0

0

  On the tenth day after the introduction of the staphylococci,it could not be found under a grass mixture of lucerne and rye grass, nor under cloverwith timothy grass. Under clover it disappeared after 20 days and in fallow soil,after 50 days. The colon bacillus disappeared more quickly under clover than undera grass mixture or even under fallow soil. The same results were obtained by Mishustin(1954) in vegetation experiments with orchard grass, rye grass, brome grams, clover,and fescue grass. Two cultures were introduced into the soil, Bact. coli andBact. coli aerogenes. They died sooner under clover and fescue grass. Underother plants these bacteria died much later.

  Arkhipov (1951, 1954) studied the extent of the growthin soils of the bacterium which causes anthrax under different plants, under conditionsof vegetation experiments, and under field conditions. Wheat, rye, clover, vetch,lucerne, barley, Euagropyrum, potatoes, buckwheat, millet, lupine, flax, garlic,onions, etc were grown. Experiments showed that certain plants (garlic, winter wheat,rye, onions, rhubarb, and vetch) completely remove the anthrax bacillus from thesoil. Lucerne, spring wheat, hemp, and the castor-oil plant exert a weak inhibitoryeffect. Ornithopus sativus, carrots, radishes, rape, water cress, and othershave no effect whatsoever. Finally, such plants as potatoes, Eusgropyrum, horseradish,radishes, and turnips stimulated the growth and accumulation in the soil of the abovemicrobe. On the basis of the results of his own studies and data obtained from theliterature, V. V. Arkhipov recommended the sowing of winter wheat, rye, vetch, clover,rhubarb, garlic, and onions for the more rapid removal of anthrax bacilli from thesoil.

  The selective action of plants on the microflora isnot only caused by the specificity of the nutrient substances excreted by the rootsystem, but also by special antimicrobial compounds. In the chapter on toxicity ofsoils we shall give data showing that many plants form and excrete different toxicsubstances into the environment. Among them there are many compounds which stronglyinhibit the growth of certain species of microbes.

  Earlier we noted (1934b, 1939) that the root excretionsof wheat, corn, flax, and some other plants visibly supress the development of certainkinds of bacteria--Azotobacter, sporiferous bacteria, and other groups bothunder sterile experimental conditions and directly in the soil in their natural surroundings.It was shown that the root excretions of corn act differently to those of wheat.Under the influence of these excretions, the cells of the bacteria undergo a considerabledeformation, degeneration, involution, and subsequently die. Sometimes this degenerationof the culture is accompanied by the birth of new forms and species.

  The presence of antimicrobial substances in the rootexcretions of plants was observed by Sidoranko (1940b), Meshkov (1953) and others.The soil studies carried out for many years in the Wareham forests (in England) byRayner and Nelson-Jones (1949) showed that the obstacle standing in the way of afforestationof certain soil zones is not the lack or insufficiency of nutrient elements but thepresence of special organic substances. These substances, according to these authors,retard the growth not only of young saplings of various wood varieties but also thatof many microorganisms. Rayner has shown that by introducing organic fertilizer manure,or compost into these poisoned soils, the toxicity diminishes or even vanishes. Accordingto her observations, this decrease in toxicity takes place due to the activationof the microflora in the presence of fresh organic substance.

  Extensive studies of the antimicrobial properties oftoxins excreted by plants were made by Metz (1955). The root excretions and rootsap of 100 species of plants were studied. It was shown that 10 species out of 100(Armoracia rusticana Gaertn, Chelidonium majus L., Crepis virenK., Hieracium. pilosella L., Hypericum perforatum L.., Lampsanacommunis L., Ranunculus acer. L., Viola silvatica Schm., V.tricolor Wittr., Pulmonaria officinalis L.) greatly inhibited the growthof sporiferous bacilli or micrococci; 15 species--Brassica campestris L.,B. napus L., Campanula rapunculoides L., Capsella bursa pastorisMad., Fumaria officinalis L., Ranunculus repens L., Paeonia officinalisL., Sinapis alba L., Galium verum L., Allium schoenoprasum L.,and others inhibited their growth moderately; 31 species: Achillea millefoliumL., Aethusa cynapium L., Avena sativa L., Chrysanthemum lencanthemumL., Cichorium intybus L., Datura stramortium L., Hordeum sativumGessen and others inhibited the growth of these bacteria only slightly. The remainingspecies of plants did not have any inhibitory effect.

  Stiven, (1952) found that in the soil under the plantsTragopogon plumosus L., and Pentanisia variabilis Harw., the processesof nitrification and the growth of Bac. subtilis, Bac. coli, and others areinhibited considerably. Pure substances obtained from the root excretions of theseplants have the same effect.

  The action of root toxins is not specific, accordingto Metz. The roots inhibit the growth of bacteria, both when they were isolated fromthe rhizosphere of the given plant and when they were isolated from that of anotherspecies. For example, the roots of Chelidonium majus L. inhibit, to the samedegree. the growth of bacteria isolated from their own root zone and those from theroot zones of goutweed, violet, hawkweed, and others, and also bacteria from fallowsoil. However, the author notes, the root-zone microflora is less sensitive to theaction of the roots than organisms from outside this zone. The growth of bacteriaand mycobacteria, isolated from fallow soil or from soil outside the root zone, inthe majority of cases, is inhibited by the roots of many plants, while most of thebacteria of the rhizosphere are not inhibited at all.

  According to our observations, the roots of lucerneand peas, under conditions of growth in a sterile nutrient solution. excrete substanceswhich inhibit the growth of Azotobacter chroococcum and Pseudomonas fluorescens,certain species of rootnodule bacteria, and others.

  The growth rate of the bacteria from two-month-oldplants grown in a solution was determined (Table 84).

Table 84
Effect of root excretions of peas and lucerne on growth of bacteria

Bacteria

Peas

Lucerne

Az. chroococcum

 

 

strain No 54

++

-

strain No A

++

-

halophylic strain

-

-

garden strain

-

+

strain No 6

-

-

Az. Vinelandii

++++

+++

Ps. flourescens No 8a

+

-

Ps. aurantiaca

+++

+++

Root nodule bact. of:

 

 

kidney beans

+++++

++

peas

+

-

vetch

+

-

lucerne

+

-

Lathyrus

+

_

broad beans

+++++

+++

soy

-

+

lupine

-

-

sweet clover

+++++

+++

clover

++++

-

  The longer plants have been growing in a solution,the more strongly the solution affects the bacteria. After peas grew for three monthsin the solution, Azotobacter strain No 54 and strain A did not grow at all,nor did the root-nodule bacteria of clover and sweet clover.

  Antibacterial substances are also excreted by isolatedroots if the latter are grown in an artificial nutrient medium. We cultivated theroots of peas. lucerne, lupine, and certain other plants in Bonner's nutrient solutionfor one to three months and, after various periods of time, we determined the presenceof antibacterial substances in it. The bacteria were introduced into the solutionand, by means of plating, their viability and degree of multiplication were established,Cultures of Azotobacter strain No 54, a halophilic strain and a garden strain,and, in addition, root-nodule bacteria of peas, vetch, lucerne, and others, weresown into the solutions.

  The results of the experiments showed that the excretionsof roots grown in an isolated form act on the same species of bacteria as do theexcretions of non-isolated roots. Only the degree of their inhibition was weaker.The antibacterial spectrum of root excretions was the same in both cases. This indicatedthe fact that isolated and nonisolated roots excrete the same antimicrobial substances.

  Thus, by this experiment, it was observed that certainantimicrobial substances excreted into the medium, are directly synthesized in theroot system of the vegetating plant.


 


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