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Part IV, continued: 

Microbial inhibitors and their action on plants

  These data show how large and versatile is the microflorawhich produces biotic substances. With the aid of these substances, the microorganismsgrowing in the soil activate the growth, nourishment, and many other vital processesof both higher and lower organisms.

  In soils, as well as in other natural substrate, thereare microorganism-inhibitors. which, in course of their metabolic activity, suppressthe growth and development of higher and lower plants. They form special substanceswhich are toxic to plant tissues and organs. Toxins, or phytotoxins formed by phytopathogenicfungi and bacteria, were studied long ago by numerous investigators (see Kuprevich,1947; Sukhorukov, 1952; Bilai, 1953, Gorlenko, 1953; Goiman, 1954). However, thequestion of whether these organisms produce toxic substances directly in soil remainedunsolved in the literature. It is known that many species of fungi and bacteria formtoxins which act on animal organisms. Growing on food products, fodder, and on variousplant residues, they excrete toxic substances. Upon feeding these products to animals,one often observes a strong case of poisoning (Pidoplichko, 1953).

  Investigations show that microbial inhibitors may poisonplants with their toxins under conditions of their natural growth in soil, if favorableconditions for such growth are formed. They suppress germination of seeds, the growthof sprouts and plant growth in general and decrease the total crop. Consequently,when there is a massive growth of these organisms, they may become an important factorin determining the fertility of soil and the crop yield of plants.

  The suppressing action of microbial inhibitors is alsomanifested in the growth of lower plants--fungi, bacteria, algae and others. In suchcases, the microbes are called antagonists.

  Microbial inhibitors are found among various groupsof lower forms: bacteria, fungi, and actinomycetes. Greig-Smith (1911) establishedthe fact that toxic substances formed by certain bacteria, suppress the growth ofplants. Ressel (1933) ascribed great importance to Protozoa, which consume microbialcells. Hutchinson and Thaysen (1918), Lewis (1920), and Laudenberger (1952) noticedin certain nonsporiferous bacteria of the genus Pseudomonas the ability tosynthesize potent toxic substances. Johnson and Murwin (1931), and later Braun (1950),discovered this ability in Pseudomonas tabaci, the causative agent of tobaccodisease. The ability to form toxic substances was also found in other bacterial groups.

  Among the group of nonsporiferous bacterial inhibitors,representatives of the genera Bacterium and Pseudomonas, are comparativelyoften encountered and lose often those of the genus Rhizobium. They are oftenfound in the rhizosphere of vegetative plants. We studied more than 300 culturesof these organisms isolated at different times from different soils, from the chestnutsoils of the Trans-Volga region, the serozem soils of Central Asia, and the podsolsoils of the Moscow and other regions.

  Of this number of cultures which were studied, about100 suppress to a greater or smaller degree the growth of plants and the germinationof seeds, Strongly expressed herbicidal properties were possessed by certain strainsof Ps. flourescens, Ps. pyocyanea and Bacterium sp. They completelyor almost completely inhibited the germination of needs of clover, vetch, and wheat(Figure 86). The seeds merely sprouted and died, or did not show any signs of germination.

 

Figure 86. Suppression of the germination process of clover seeds by nonsporiferous bacterial inhibitors, Posudomonas sp.

a--control; b--in the presence of bacterial inhibitors.

 

  The toxic properties of many sporiferous bacteria aresharply expressed, We studied more than 350 cultures, isolated from various soilsof the Soviet Union. The bacteria were grown in liquid nutrient media. The seedswere treated by soaking them for several hours in the culture fluid. Seeds of plantstreated with culture fluid were germinated on cotton or on paper which had been wettedwith water.

  The toxic or herbicidal effect of the bacterial fluidrevealed itself in the suppression of growth and the lowering of the percentage ofgerminating seeds, Analyses have shown that approximately 20-30 per cent of the culturesinvestigated possessed inhibitory proportion. Among the bacteria isolated from turfypodsol soils, the number of inhibitors was large (about 34-45 per cent).

  The nature and strength of their effect vary in differentcultures. Some organisms completely or almost completely inhibit the germinationof seeds (Figure 87), others are less inhibitory, and still others do not show anyinhibitory effect whatsoever.

 

Figure 87. Effect of sporiferous bacterial inhibitors on the germination of plant seeds. The seeds were soaked in the bacteria-culture fluid and germinated on cotton or on paper wetted with water:

A--effect of Bac. mesentericus (strain 50) on germination of wheat seeds: 1--control, seeds soaked in water; 2--seeds soaked in culture fluid; B--effect of Bac. mesentericus (strain 67) on the germination of seeds of peas: 3--seeds soaked in culture fluid; 4--control, seeds soaked in water.

 

  The species of the inhibitors studied by us mainlybelonged to Bac. mesentericus and Bac. subtilis.

  The capacity to suppress the germination of seeds andthe growth of seedlings is revealed in various degrees among strains belonging tothe same species. Among cultures of the bacterium Bac. messentericus, we foundmore than 180 strains isolated from various soils, including 100 strains from thepodsol soil of the Chashnikovo Experimental Station. Among these were some very stronginhibitors, while others did not inhibit plants growth at all. Some of them inhibitedthe germination of wheat seeds, others those of peas, vetch or clover, while someof thorn inhibited the germination of wheat, peas, vetch and clover seeds. In Table95 data are presented from an experiment with vetch and clover.

Table 95
Effect of metabolic products of bacteria on the growth of plants
(calculated for the 30th day of growth, in cm)

Bacterial cultures

Clover: height of shoots

Clover: length of roots

Vetch: height of shoots

Vetch: length of roots

Control

5.0

4.0

24.5

12.0

Bac. subtilis strain 7

4.5

0

26.0

6.0

Bac. subtilis strain 15

5.5

0.5

22.0

3.5

Bac. brevis. strain 3

4.8

0-0.2

25.0

2.5

Bac. mesentericus

5.2

0

25.5

1.0

  Among sporiferous and nonsporiferous bacteria, thereare sometimes strains possessing an organotropic or selective herbicidal action.They either suppress the growth of only this root system or of only the aerial parts.We found cultures which completely inhibited the growth of the roots of vetch andwheat. The seeds sprouted without the formation of roots, while the latter were verymuch reduced (Figure 88). The aerial part developed more or less normally as longas the seeds contained nutrient substances.

 

Figure 88. Suppression of growth of wheat roots by bacterial inhibitors:

a--experiment; b--control.

 

  Some bacterial strains in our collection (three nonsporiferousand four sporiferous strains) inhibited the growth of the aerial parts, but did notaffect the root system. The needs germinated a root, while the aerial part was stronglyreduced (Figure 89).

 

Figure 89. Inhibition of growth parts of vetch by cultures of bacterial inhibitors:

a--experimental, b--control plant.

 

  Certain strains of bacteria suppress the sporulationprocess of lower organisms, the formation of zygotes in phycomycetes and the formationof spores in yeasts (Krasil'nikov, 1947 a). It is possible that there are microbeswhich inhibit the fruiting process of higher plants as well.

  In our collection of actinomycetes there are strainswhich cause chlorosis of higher plants by the action of their metabolic products.Chlorosis appeared in corn and wheat after treating the seeds before sowing witha culture fluid of certain species of actinomycetes and, even more markedly, withpurified preparations of antibiotics. If the seeds of these plants are kept in asolution of an antibiotic for two, to four hours before sowing, the seedlings arecompletely colorless, without the slightest sign of the formation of chlorophyll.The growth of much plants is suppressed and, soon ceases altogether. In some casesthe plants, recover, become green, and continue to grow more or less normally,

  If the seeds are treated with weaker solutions of theantibiotic, one obtains seedlings which are slightly green and somewhat etiolated.Strongly etiolated plants are obtained upon the treatment of seeds with streptomycin.Soaking seeds for two hours, in a solution of one microgram per ml causes the completeetiolation of the seedlings. The latter do not become green for a period of 15-30days and finally die. A suppression of chlorophyll synthesis is caused by aureomycin,terramycin and other antibiotics.

  We obtained the etiolation of duckweed by growing itin a nutrient solution to which an antibiotic had been added. Depending on the concentrationof the antibiotic, the growth of the plants was inhibited to a greater or smallerdegree. The extent of the appearance of the green color also varies, from slightchlorosis to full colorlessness.

  Certain toxins of microbial origin cause the phenomenonof chlorosis in grapevines, According to our observations, this phenomenon may bedue to fungi of the genus Fusarium. We found certain strains, the toxins ofwhich caused the etiolation of shoots, of cuttings, and grape stock, when treatedbefore planting in the soil. The plants that grew from them had light green leaveswith a yellowish hue, their development was slow, and other deviations were observedwhich are characteristic of chlorosis of grape vines (Krasil'nikov and Kublitskaya,1956).

  This picture of the etiolation of cuttings was observedby us after the treatment of the vine with antibiotics of actinomycete origin. Certainstrains of gray and pigmented actinomycetes synthesized substances which inhibitthe formation of chlorophyll in the leaves of grapevines. Cuttings, when immersedwith their basal ends in the crude fluid culture and subsequently planted in thesoil developed and showed obvious signs of etiolation.

  The inhibition by antibiotics of the formation of chlorophyllin plants has been mentioned by certain other investigators. Provasoli, Huntner,and Schatz (1948) obtained colorless cultures of Euglena sp. under the influenceof streptomycin. The antibiotic was added to the nutrient solution in small quantities;under its influence the chloroplast of the cells was destroyed, as a result of whichcompletely etiolated forms of organisms were obtained.

  The phenomenon of chlorosis as an effect of streptomycinwas observed in cereals (wheat, corn, etc) by Von Euler (1947) and Hagborn (1956).They wetted the seeds of plants in the antibiotic solution and planted them in thesoil. The seedlings were devoid of green color.

  Berezova and Sudakova found that the death of the growingtip of flax is not connected with boron starvation, but is the result of poisoningby toxins formed by bacteria.

  Kugushova has shown that on the roots of lucerne, bacteriamay grow which, by their excretions, cause the failing off of the buttons (accordingto Berezova, 1953 a).

  Inhibitors, suppressing the growth of plants and thegermination of seeds, are encountered in great numbers among actinomycetes. In thisgroup of microorganisms, cultures with strong herbicidal properties are most oftenfound among the orange A. aurantiacus, among the gray A. griseus, andamong other species and groups (Table 96).

Table 96
Effect of the culture fluid of actinomycetes on the germination of plant seeds

Number of germinated seeds by % of control------>

beans

corn

clover

lucerne

wheat

A. aurantiacus, strain 1149

86

60

66

77

12

A. aurantiacus, strain 1306

44

60

50

88

12

A. griseus, strain 2283

142

60

100

100

100

A. griseus, strain 293

86

120

83

111

100

A. globisporus, strain 070

114

80

50

100

87

Control

100

100

100

100

100

  Experiments with seedlings have shown a more or lesssimilar picture. Some cultures of actinomycetes strongly suppress growth, while othersonly slightly or not at all (Table 97).

Table 97
Effect of filtrates of actinomycetes on plant seedlings
(in length of plant parts in cm)

Actinomycetes

Wheat, rootlets

Wheat, sprouts

Corn, rootlets

Corn, sprouts

Beans, rootlets

Beans, sprouts

A. aurantiacus, strain 1149

1.5

2.0

12.0

7.0

15.0

13.7

A. aurantiacus, strain 1306

1.5

7.5

0.7

3.5

1.7

3.8

A. griseus, strain 2241

12.4

10.5

13.7

15.0

11.0

8.8

Control

14.0

15.0

15.0

12.0

18.5

14.0

  Toxic substances of actinomycetes and other microorganismsexert a suppressing effect on single isolated organs or parts of plants; on leaves,cuttings, etc. If one immerses the cuttings, or cuts off the leaves, then, aftera certain period of time, they wither and die. By the speed of the withering anddeath of these parts one may judge the strength of the action of the toxic substances.

  In our experiments we used cuttings of various plants,of beans, peas and corn, and branches of lemon, apple, pear, and apricot trees, etc.

  If one puts on the surface of an uncut leaf a pieceof cotton which has been wetted with toxin, after a few hours spots appear of a necroticnature. The stronger the poison, the more sharply the necrotic spots on the leafare expressed. This method was used by us in testing the toxic substances formedby microorganisms.

  Among the inhibiting factors of great importance arethe phages: bacteriophages and actinophages. The studies of Rautenshtein (1955),Khavina (1954), and certain others show that these agents are widely distributedin soils, where they are detected in considerable numbers. There is reason to believethat they suppress and lyse cells of bacteria or actinomycetes as readily as underconditions of pure cultures.

  For example. root-nodule bacteria become inactive whenphages multiply abundantly in the soil. Under conditions of the experiment, theymultiply to a considerable extent. We have counted tens and even hundreds of thousandsin one gram of soil. According to Demolon and Dunez (1934), phages of root-nodulebacteria of clover and lucerne, under certain conditions, saturate the soil to suchan extent that the soil becomes much less fertile for these plants, root-nodule bacteriado not develop in it, and there is only a slight or no formation of nodules on theirroots, and when they do develop they have an abnormal appearance. The authors areof the opinion that the observed clover-lucerne soil exhaustion is caused by theaccumulation of phages. According to certain data, phages penetrate the plant, and,by interfering with plant metabolism, lower crop production (Vandecaveye and others,1940).

  There is data in the literature on the formation oftoxic substances by fungi. Leng (1949) has shown the poisoning effect of the Penicilliumfungi on the seedlings and of cereals. The most active inhibitors in these experimentswere P. notatum, and P. oxalicum. Monnaci and Torini (1932) and Diachum(1934) note the formation of toxins by fungi, which act on cereals under conditionsof their growth in soil.

  Producers of toxic substances are known among variousgroups of soil microflora. An important place is occupied by representatives of thegenus Fusarium. The substances formed by them were obtained in a chemicallypure form having a known structure; for example, lateritin, C6H46O7N2; avenacein, C25H44O7N2; fructigenin,C26H44O7N2; sambucynin, C24H42O7N2,and enniatins, lycomarasmin, yavanicin, etc.

  These substances act differently on plants and animals.Some of them are specific (Goiman, 1954).

  Fusaria are very widespread in nature. The probablyplay an important role in the toxicoses of soils. Their inhibitory effect on thegrowth of plants was observed by many authors (Rehm, 1953; Laundoldt, 1952; Sukhorukov,1952). The significance of these fungi for the fertility of soils is not only determinedby their ability to synthesize toxins and excrete them into the soil but also bytheir phytopathogenic properties.

  Bilai (1955) described in his monograph many strainsof the genus Fusarium which have a deleterious effect on the germination ofseeds and on the growth of seedlings of rye, oats, and barley. The products of theirmetabolism, obtained in the form of filtrates, were tested under various conditions.The results of the author's experiments are given in Table 98.

Table 98
Effect of filtrates of Fusarium cultures on the germination of plant seeds
(in length of plant parts in cm)

Fungal culture

Rye, rootlets

Rye, sprouts

Barley, rootlets

Barley, sprouts

Control

21.5

4.25

29.8

3.6

Fus. poal., strain 2

3.8

1.9

--

--

Fus. poal., strain 5

8.3

2.6

16.0

3.6

Fus. poal., strain 9

11.7

2.5

11.8

2.3

Fus. poal., strain 41

2.4

1.6

--

--

Fus. poal., strain 45

15.0

5.4

8.4

1.2

Fus. sporitrichioides, strain 28

6.1

1.4

18.4

2.1

Fus. sporitrichioides, strain 30

11.3

3.2

6.0

1.5

Fus. sporitrichioides, strain 51

15.3

6.3

11.2

1.5

  As can be seen from the table, the filtrates of somestrains affect the seedlings of rye, while others act predominantly on the growthof barley. Certain strains suppress the growth of rye and wheat to the same extentas that of barley or oats.

  Klechetov (192 6) in studying the phenomenon of theflax exhaustion of soils found the growth of the fungi Fusarium, Thielaviopsisbasicola, Cladosporium herbarum, Alternaria, and Macrosporium in thesesoils; these fungi, according to the author, form toxic substances and are the reasonfor the death of the sown flax.

  A considerable role in the exhaustion of soils andin the lowering of plant yields is attributed in the literature to the fungi of thegenus Fusarium. Kvashina (1938), Kurtesova (1940), and Ioffe (1950).

  Kublitskaya (1955) studied the degree of the distributionof fungi of the genus Fusarium in the soils of Central Asia (Uzbek SSR) undergrapes. She isolated 52 cultures and many of them proved to be toxic for grapevines,causing poisoning and death to the cuttings and stock under the conditions of growthin soil. Certain strains caused chlorosis under experimental conditions.

  Strongly expressed herbicidal properties are exhibitedby fungi of the genus Pythium. According to Likais (1952), Pythium debaryanumforms toxins in the soil which inhibit the root systems of plants.

  Mirchink (1950) studied a large collection of fungiisolated from turfy podsol soils of the Moscow district and found among them manytoxigenic forms. The most toxic and the most widespread fungi in these soils arerepresentatives of the genus Penicillium and, secondly, Fusarium andTrichoderma. Fungi of the genus Trichoderma (T. lignorum) and certainrepresentatives of the genus Fusarium strongly suppress the germination ofwheat seeds, as a result of which the number of germinating seeds decreases by 68per cent and more. The length of sprouts in the presence of the metabolic productsof Trichoderma is 3.5 cm; in the presence of the fungus Fusarium, 4.0cm; and in the control, 4.6 cm. The Penicillia inhibitors are often found in theturfy podsol soil in a great number of species. Some of them are very toxic for wheat,which can be seen in Table 99 and in the photograph (Figure 90).

Table 99
Toxic effect of fungi of the genus Penicillium on wheat seeds

Fungi

Per cent of germinated seeds

Mean length of sprouts, cm

Control (nutrient medium)

100

4.6

Control (water)

100

4.6

P. cyclopium

0

--

P. paxilli

54

2.6

P. ochro-chloron

74

1.5

P. martensii

74

3.0

P. nigricans, strain II/14

100

1.0

P. nigricans, strain II/35

87

0.6

P. nigricans, strain VIII/8

90

1.0


 

Figure 90. Effect of the culture fluid of the fungus Penicillium nigricans on the germination of wheat seeds:

a--control; b--treated seeds.

 

  Active toxin producers in soil are fungi of the generaTrichoderma, Trichothecium, Botrytis, and others. From cultures of Helminthosporium(H. victoriae), the toxin victorin was isolated, which inhibits the growth ofroots and seedlings of oats at a dilution of 1:1,000,000. This substance is formedby the fungus directly in the soil (Weeler Luke, 1954; Tyler, 1948). Toxic substancesharmful to plants were found among the representatives of Verticillium. Themost well studied among them is V. alboatrum. Its toxic substance was foundby Bewley (1922), It causes the withering of tomatoes, cotton, tobacco, and otherplants. Green (1954) discovered two substances in this fungus-- a protein and a polysaccharide.The former is excreted into the medium and the latter enters the tissues of the plants,The poisoning effect of this fungus was also noted by Sukhorukov (1952) and others,

  Among members of the genus Trichothecium werefound the toxic substances trichothecin and others, which inhibit plants and certainmicrobes, Similar substances were found in Deuterophoma tracheiphilus, causing"malsecco" in citrus plants, They were also encountered in many other fungi(Hossayon, 1953; Freeman and Morrison, 1949, Gelman 1954).

  It is obvious that the importance of microbial inhibitorsin soil toxicosis will be mainly determined by the degree of their growth and activity.

  The distribution of microbial inhibitors and theiraccumulation in the soil has been but occasionally studied; as were microbial activators.Monnaci and Torni (1932) found about 60 per cent of the soil fungi isolated and investigatedby them, to be inhibitors.

  According to our data, there are a great number ofinhibitors among the fungi, bacteria, and actinomycetes in soils. Out of 1,500 culturesof actinomycetes, more than 200 inhibited, to a larger or smaller degree, the germinationof beet or wheat seeds and 16 strains completely suppressed their germination; 21cultures strongly suppressed and 58 weakly suppressed the growth of clover and lucerne,The total number of inhibitors among actinomycetes is comparatively small, on theaverage 5-15 per cent,

  One finds inhibitors among sporiferous bacteria considerablymore often, Out of 560 strains studied, belonging mainly to three or four species,Bac. mesentericus, Bac. subtilis, Bac. cereus, and Bac. brevis. 178strongly suppressed the germination of clover seeds, more than 200 cultures suppressedto some degree the germination of peas. According to our data, there are about 40per cent inhibitors among the sporiferous bacteria of Bac. mesentericus andBac. subtilis isolated from the turfy podsol soils.

  Inhibitors among nonsporiferous bacteria are encounteredmuch less frequently than among sporiferous bacteria, According to our calculations,their number can be expressed in a tenth part of one per cent. Some species of thegenus Bacterium and Pseudomonas possess, however, strongly expressedtoxic properties in relation to plants and microorganisms.

  It should be noted that certain microorganisms amongbacteria and fungi react to toxic substances in the same way as do higher plants,which enables us to use them as test organisms in the screening for and the studyof phytotoxins. Microbial tests have a number of advantages, With them one can morequickly determine and solve a number of problems related to the toxicosis of thesoil and the poisoning of plants, In mass studies we often use both tests; the microbiologicaland the plant test.

  We carried out the quantitative evaluation of microbialinhibitors in different soils, but went into greater detail in the turfy podsol soilsof the Moscow Oblast', the Kola Peninsula, and in other regions of the USSR. Virginand cultivated soils, forest and swampy soils, meadows, ate were investigated,

  We counted from 5,000 to 450,000 inhibitors in onegram of soil depending on the properties of the latter (Table 100). In slightly cultivatedsoils, the absolute number of inhibitors is smaller, but its percentage may be higherthan in wellcultivated soils.

Table 100
Number of inhibitors in podsol soils of Moscow area
(Number of cells in 1 g of soil)

Soil

Bacteria inhibiting azotobacter

Actino- mycetes inhibiting azotobacter

Fungi inhibiting azotobacter

Bacteria inhibiting beet seedlings

Actino- mycetes inhibiting beet seedlings

Fungi inhibiting beet seedlings

Dolgoprodnoe

 

 

 

 

 

 

Virgin soil

15,000

23,000

1,300

8,000

3,000

500

Plowed fields

45,000

17,000

2,000

15,000

7,000

1,000

 

 

 

 

 

 

 

Agricultural Academy Timiryazev

 

 

 

 

 

 

Forest

40,000

80,000

17,000

25,000

10,000

4,000

Virgin soil

10,000

32,000

1,500

10,000

3,000

500

Plowed fields

120,000

150,000

2,300

50,000

35,000

3,000

 

 

 

 

 

 

 

Chashnikovo

 

 

 

 

 

 

Forest

120,000

82,000

12,000

20,000

12,000

7,000

Virgin soil

40,000

16,000

1,600

10,000

5,000

1,000

Plowed fields

450,000

160,000

1,400

150,000

60,000

500

  Inhibitors which suppress the growth of Azotobacterin podsol soils are much more numerous than microbes which suppress plant growth.Mirchink (1958) studied the fungal flora of soils of the experimental station Chashnikovo(Moscow Oblast') and found that 11-38% were inhibitors which suppress plant growth.They were distributed in the following manner: in forest soil--13%, in glades--11%,in cultivated soils, 15-38% of the total microflora, detected by by existing methods(Table 101).

Table 101
Number of fungi in podsol soils
(thousands in 1 g of soil)

Soils

Total

Inhibitors, %

Control soils without fertilizers

60

32

Fertilized with mineral nitrogen

138

38

Calcium-containing fertilizers + manure

36

24

Calcium-containing fertilizers + manure + P.K.

18

15

  As can be seen from the data given, the greatest numberof inhibitors was found in soils cultivated to a limited extent. Mineral fertilizersdo not diminish but, on the contrary, they noticeably increase the content of inhibitors.

  It was experimentally established that microbial inhibitorsform toxic substances directly in the soil in which they grow.

  If these organisms are introduced into nontoxic orinactivated soil and the soil is incubated under certain conditions of humidity andtemperature, then after a certain time it will become toxic for these or other plantsor for certain species of microorganisms, depending on the peculiarities of the inhibitor.

  Rybalkina (1938 a) observed the appearance of toxicosisin flax-exhausted soil upon growth of the fungus (Fusarium lini).

  Mirchink (1956) incubated soil (podsol) with fungi-inhibitorsand she observed the appearance of toxicosis. In soils in which the fungus Penicilliumcyclopium grew abundantly if artificially introduced, seeds of wheat did notgerminate at all or germinated in small numbers (Figure 91). Other species of fungiisolated from podsol soils also poisoned the soil but to a lesser degree. On suchsoils germinating wheat seedlings constituted 15-60% of the number of seedlings innormal control soil.

 

Figure 91. Poisoning of soil by cultures of fungi upon artificial infection. Germination of wheat seeds:

a--in control (noninfected) soil; b--in soil in which Penicillium nigricans grew; c--in soil infected with Penicillium cyclopium.

 

  Clover-exhausted soil inactivated by heating regainsits toxicity by growing the appropriate microbial inhibitors in it. In such soilwith regenerated toxicity, clover and root nodule bacteria grew much more poorlythan in normal soil. There was either no nodule formation on the root of clover orit was considerably suppressed (Table 102). Root-nodule bacteria in such soil becameavirulent and lost the capacity to penetrate the roots and form nodules in them.Cultures of some fungi of the genera Penicillium, Fusarium, Trichoderma andsome sporeforming bacteria, when growing abundantly in inactivated forest or fieldsoil restored the soil's original toxicity in relation to wheat and Azotobacter(Table 103).

Table 102
Growth of clover on soil with restored toxicity

Expermiental conditions

Number of sproutings

Number of sprouts on the 30th day

Number of nodules per plant (average)

Inactivated soil not infected with inhibitors (control)

46

46

23

Inactivated soil, infected with inhibitors:

 

 

 

Ps. pyocyanea

32

26

0.05

Ps. tumefaciens

39

22

0.0

Fusarium sp.

41

31

0.5

Mixture of all bacteria

30

19

0.0

Note: Each vessel contained 50 seeds. Inoculation was performed with active cultures of Rhizobium trifolii.

 

Table 103
Accumulation of toxic substances in podsol soil an a result of growth of inhibitors

Name of inhibitor

Length of wheat rootlets, cm

Length of wheat sprouts, cm

Survival of Azotobacter cells, hours

Control: inactivated soil (not infected)

12

8

240

Infected with:

 

 

 

Bacillus strain 12

3

5

6

Bacillus strain 23

6

2

12

Bacillus strain 8

4

4

16

  Toxins produced by inhibitors may, under certain conditions,accumulate in considerable quantities and endow the soil with toxic properties. Theextent of accumulation of toxic substances depends upon the intensity of their formationby microorganisms, the rate of destruction and leaching, and also upon the degreeof adsorption.

 


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