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Part IV, continued: 

 

The microflora of the rhizosphere

  Increased accumulation of microbes in the root soilwas first observed by Hiltner in 1904. He proposed the term "rhizosphere."In investigating the root system of various plants, Hiltner came to the conclusionthat the accumulation of microbes in this area was not accidental and that it wascaused by the biological activity of the roots.

  It should be noted that some investigators before Hiltnerobserved the accumulation of certain species of bacteria in the root region, butthey looked upon this phenomenon from a narrowly specialized point of view.

  Epstein (1902) observed the development of specialbacteria on the roots of beets, which differ from those of the soil. Welich (1903),Grüber (1909), and Maassen (1905) found large numbers of Clostridium gelatinosumcells in the rhizosphere of the sugar beet.

  After Hiltner, the phenomenon of the accumulation ofmicroorganisms around roots was observed by Layon (1918). He pointed out that theroot systems of plants strongly change the natural microbial associations of thesoil. The ratios between species in the rhizosphere microflora, according to thisauthor's observations, differ from the ratios prevailing in the microbial biocoenosesof nonrhizosphere soil. Hoffman (1914). Maaasen and Behn (1923), Stoklasa (1926)and Richter and Werner (1931) observed the increased development and accumulationof fungi in the rhizosphere.

  A more detailed and systematic study of root-regionmicroflora began in the 19301's.

  Detailed studies made by Starkey (1929, 1934) showedthat the roots of plants have a considerable influence on the accumulation of microorganismsin the soil. According to his observation, the number of microorganisms in the rhizosphereis several times higher than that in ordinary soil outside the root area. For example,on the rhizosphere of beets, 427 million bacteria per gram were found, while in thecontrol soil only 8.2 million per gram were found; in the rhizosphere of clover,he found 11,320 million per g and in the control soil only 6.6 million per g; inthe rhizosphere of wheat, 653.4 million per g were found; and in the control soil,only 22.8 million bacteria per g were found. Pochenrider (1930) studied the microfloraof the soil in the root region of cruciferous plants and also observed the intensifieddevelopment of certain bacterial species (Azotobacter and others).

  The most numerous and detailed investigations wereperformed by the Soviet specialists, Isakova (1934-1940), Krasil'nikov and others,(1934, 1945), Sidorenko (1940), Obraztsova (1936), Berezova (1941, 1945), Korenyako(1942), and many others. Berezova published data on her study of the microflora ofthe rhizosphere of flax; Isakova on that of rubber plants, tangerines, tung trees,and the tea plant, and Obraztsova, on the tea plant.

  Large accumulations of microbes in the region of theroots of forest plants were found by Samtsevich and his associates (1949) and byKozlova (1953).

  Romeiko (1954) observed the development and accumulationof microflora in the rhizosphere of hops. Noting the concentration of microbes inthe root region the author emphasized the connection between this phenomenon andthe developmental phase of the plant.

  Popova (1954) obtained data on the intensified accumulationof microbes on the root systems of vines which grow in the serozem soils of Uzbekistan.

  Our investigations (1934-1945) of the soil of the rootarea have shown that plants in any soil under different climatic conditions, regardlessof the zone, possess the capacity of concentrating soil microorganisms in the vicinityof their roots, We studied the microflora of cereals (wheat, rye, oats, barley) grasses(orchard grass, rye grass, etc), leguminous plants (beans, peas, vetch, clover, lucerne,etc), commerical crops (cotton, tobacco, flax, hemp, etc), and several varietiesof fruit trees and decorative plants (acacia, poplar, lime tree, oak, apple, plum,pear, etc). Studies were performed in various parts of the Soviet Union: in the South--inCrimea, in the Caucasus, Armenia, and Georgia, on the coast of the Black Sea, inCentral Asia, and on the fields of the Tadzhik, Uzbek, and Kirgiz Republics, in Kazakhstan,in the lowlands of the Volga area and in the Astrakhan steppes; in the Ukraine, Moldavia,Siberia, on the Sakhalin; in the northern regions of Yakutiya, Igarka, and the KolaPeninsula. The microflora of plants of the moderate region was also studied: Leningrad,Moscow, Yaroslavl', Kuibyshev, and other regions, and the plants of Belorussia, Latvia,Estonia, and other places,

  According to our observations, the total number ofmicroorganisms in the zone of the plant roots exceeds the number of microbes in ordinarysoil by tens, hundreds, and thousands of times. The differences in the quantitativerelationships depend on the species of plant and the soil-climatic conditions (Table73).

Table 73
Total number of bacteria in soils
(in thousands per gram)
Soil and plants Rhizo- sphere Control Soil and plants Rhizo- sphere Control
Kola Peninsula     Central Asia, Serozem    
Potatoes 12,500 320 Cotton 3,500,000 74,000
Clover 21,000 400 Lucerne 7,100,000 60,000
Oats 7,800 270 Orchard grass (Dyctylis) 3,800,000 35,000
Mixture of grasses 19,500 350 Oat grass (Arrhentherum) 4,200,000 47,000
           
Severnaya Zemlya     Armenia, Serozem    
Saxifrage 7,600 120 Wheat 1,800,000 12,000
Poppies 5,200 210 Lucerne 3,600,000 14,000
Cereals 12,000 400      
      Chestnut Soil    
Moscow Oblast' Podsol     Wheat 2,400,000 10,000
Wheat 750,000 1,500      
Flax 500,000 1,150 Georgia, Krasnozem    
Clover 1,000,000 1,200 Tobacco 500,000 500
      Tea plant 800,000 600
Trans-Volga region, Chestnut soil          
Wheat 2,100,000 14,000 Crimea, Southern shore    
Lucerne 3,700,000 13,800 Tobacco 1,200,000 8,000
      Vineyards 1,700,000 3,000


Note: The numerical data given in the table was obtained from the study of soilsby the dilution method, by inoculation into synthetic media (Chapek, Gil'taya).

  In recent years, the microflora of the rhizospherehas gained more of the attention of many foreign specialists. In addition to theabove-mentioned studies, made by Starkey, others have been made by Lockhead and hisassociates, and by Jensen, Katznelson, Timonin, and others, on the bacteria of therhizosphere.

  All of these investigators obtained data on the considerableaccumulation of microorganisms in the root region. Katznelson (1946) found 100-1,000times more microbes in the rhizosphere of beets than were found outside the rootzone. Thom and Humfield (1932) obtained the following data: in the rhizosphere ofcorn, there are 136 million bacteria per one gram of soil, while outside the rhizosphere(control) only five million.

  According to Starkey ( 1955), in the rhizosphere ofbeans, there are 200 million bacteria per gram of soil; in beets, 427 million pergram; and in grain crops, 653 million per gram. The control samples of soil containedone to five million bacteria per gram of soil.

  The poorer the soil in organic substances, the lessfertile it is, and the weaker is the influence of the root system on the quantitativecomposition of the microflora of the rhizosphere. The quantitative relationship ofthe microflora of the rhizosphere (M.R.) to that of the control (M.C.) is most stronglyexpressed in poor soils. It is more marked in the primary, slightly fertile podsolsoils of the Kola Peninsula than in the chernozem soils of Moldavia or Kuban,.

  In the primitive noncultivated or slightly cultivatedsoils of the Kola Peninsula the total number of bacteria reaches from 50,000 to 300,000cells per gram. In the rhizosphere of clover in its first year of life, there are150-400 million per gram. The M.R. to M.C. ratio is 1000:1. In the well-cultivatedchernozem soils of the Khar'kov Oblast, the number of bacteria in the rhizosphereof Lucerne is from 2,500 to 5,000 million per one gram, and outside the rhizosphere,from 150 to 300 million per gram. The M.R. to M.C. ratio is 17:1.

  Central Asia's slightly cultivated serozem soils containcomparatively small numbers of bacteria, within the range of five to ten millionper gram. In the root region of lucerne in its first year of life, we counted upto 1,000 million bacteria per gram. This same soil, after five to eight years ofcultivation and the introduction of the corresponding fertilizers, contained 7,000million bacteria in the rhizosphere of one-year-old lucerne, and, outside the roots,60 to 100 million organisms per gram. The M.R. : M.C. ratio was 100 to 200) in thefirst case, and 70 to 100 in the second.

  Such a shift in the M.R. : M.C. ratio was observedby us in chernozems, podsols, serozems, chestnuts, and other soils, regardless ofthe geographical region, Only the quantitative expression differed.

  The quantitative ratios between the number of microbesin the rhizosphere and outside it increase, with the depth of the penetration ofthe root system. In the lower layers, the numerical indices are expressed more strongly(Table 74).

Table 74
Number of microorganisms in the rhizosphere and outside it at different soil horizons
(in thousands per 1 g of soil)

Soil

Plant

Depth, cm

Rhizosphere

Control

MR:MC ratio

Moscow Oblast' podsol  

 

 

 

 

  Rye

0-25

350,000

1,200

300

   

40-60

250,000

300

800

   

80-100

5,000

3

1,700

  Clover

0-25

950,000

1,500

630

   

50-70

300,000

300

1,000

   

90-110

10,000

5

2,000

Moldavia chernozem          
  Lucerne

0-25

5,000,000

100,000

50

   

40-60

700,000

3,500

200

   

80-100

80,000

300

270

   

120-150

10,000

20

500

  Wheat

0-25

1,500,000

75,000

20

   

40-60

300,000

2,000

150

   

80-100

30,000

100

300

  At a depth of 90-110 cm in the rhizosphere of clover,growing in podsol soil, the number of bacteria is 2,000 times higher than outsidethe rhizosphere, and in the rhizosphere of lucerne in chernozem soil at the samedepth, it os 270 times higher. Approximately the same ratios are observed in thecase of wheat.

  In the deep layers of soil there are usually very fewbacteria, while in the rhizosphere of plants, even at the depth of two to three meters,they grow abundantly.

  Under conditions of deep growth, the root systems ofplants create favorable conditions for the growth of microorganisms, not only bytheir excretion of nutrient substances, but also evidently due to such factors asthe improvement of their conditions of respiration and metabolism.

  The number of microbes growing in the root area varieswith the ago of the plant. As should be expected, the maximal number of bacteriais observed during the period of the most active growth of the plant. The more intenseare the life processes, the more organic substances are excreted by the root, andthe more intense the multiplication of microbes in the rhizosphere. Observationsshow that an abundant growth of microorganisms takes place in the early stages ofthe plant's growth; however, the most vigorous growth of microbes ensues during theperiod of flowering and in the period directly preceding it. (Figure 70). Sometimesone also observes three elevations in the growth curve of microbes: the first smallelevation is seen in the early stage; a second great elevation ensues before andduring flowering; the third elevation occurs before ripening. The last is usuallybarely noticeable.

 

Figure 70. Quantitative composition of the microflora of the root area during different phases of the plants' growth

 

  Under conditions of irrigation, small rises in thegrowth curve of microbes are observed after each irrigation (Krasil'nikov, Kriss,and Litvinov, 1936b). Under these conditions, they are caused by the increase insoil moisture. The question of whether the moisture was the direct cause of the enhancedgrowth of the microbes or whether this growth was strengthened as a result of anincreased amount of nutrient substances excreted by the roots due to the higher intensityof metabolism in the plant may probably be answered as follows: both mechanisms areoperative.

  When soils in the rhizosphere are analyzed after harvest,one can observe that the activity of microorganisms does not cease. In the presenceof moisture and higher soil temperatures, a sharp increase in the number of microbesin the root zone is observed. In this case, the dead roots are subjected to intensedecomposition. The number of microbes during this period may quite often exceed thatof the rhizosphere of living plants during rapid growth.

  In cases when there is little moisture in the soil(in arid regions), the root residues after harvest decompose slowly and there isno noticeable increase in the number of microbes. The roots remain in the soil undergoingno major changes for long periods.

  Differences in the quantitative composition of microbesin the rhizosphere of different plants are slight. In our investigations we couldnot detect regular and significant differences in the numbers of bacteria in therhizospheres of different plants of the cereal and leguminous families. In the samefield sector, under the same agrotechnical and soil-climatic conditions, the totalnumber of bacteria in the rhizospheres of wheat, oats, and barley are approximatelythe same. When comparing the microflora of the rhizospheres of cereal and of leguminousplants, one notices a slightly higher number of microbes in the leguminous type.However, this difference is not always noted.

  As to the nature of microbe distribution in the rhizosphereof vegetating plants, we noticed regular diffuse growth of microbes, as well as theexistence of more or less isolated colonies.

  In the literature, one finds indications that microbialcells are present not only on the surface of roots but also inside them, having penetratedthe tissue of the epidermis, the intercellular substance, and also the protoplasmof the cells themselves (Berezova, 1953; Rempe, 1951; Hennig and Villforth, 1940).According to the data obtained by Schanderl (1939, 1940), the cells and tissues ofplants are not sterile and always contain bacteria. In his last paper, the authorclaims that plant cells grow in symbiosis with bacteria. The latter are present inthe protoplasm in greater or smaller amounts.

  Our investigations did not confirm Schanderl's data.Neither in the tissues nor in the cells of healthy plants could we detect bacteria,fungi, or actinomycetes. Upon microbiological analysis, the plant tissues alwaysremained sterile. Even the roots of leguminous plants did not have bacteria in theirtissues outside the nodules. Other scientists also, (Burcik, 1940; Schaede, 1940)did not detect bacteria in tissues of healthy plants. Detailed studies were recentlymade by Stolp (1952). This author attempted to verify Schanderl's data. He investigatedvarious plants, leguminous, cereals, and others. in not one case did he find microbialcells in plant tissues. Bacteria can penetrate into the dying tissues and cells whenthe re sistance of the latter is weakened. For example, the root hairs, upon dying,are completely filled with bacterial cells. Obviously, Schanderl and others studiedsuch dying cells and tissues, or they accepted as bacteria the intracellular structures.

  In order to elucidate the nature of the distributionof microorganisms on and in the vicinity of roots, we employed the method of growingvegetation on glass plates mounted on special vegetative frames, set up in such away that the roots of the growing plant spread on the glass, leaving their imprintson them. For these frames we used flat boxes with glass walls, 5-6 cm wide, 20-25cm high and 50 cm long.

  The frames were filled with soil or sand and the plantswere grown in them. In order to make sure that the roots would stick to the glass,the frames were arranged at a slope of 40-50° (Figure 71). The roots, due togeotropism, grew in a downward direction and touched the glass, sticking tightlyto its surface.

 

Figure 71. Frames for growing plants:

A--frame in a sloping position, for obtaining imprins of roots and microflora on glass; B--nature of the growth of roots on the lateral side of the frame (glass).

 

  For the sake of the convenience of microscopic examinations,we placed microscopic slides on the inner side of the glass surface of the vegetationframe. After certain time intervals, the glass with the roots growing over it wastaken out and examined microscopically after having been previously stained or leftunstained,

  When the growth of microorganisms was abundant, onecould see with the naked eye a zone of film, measuring three to five mm in radius,around the root, This zone was especially noticeable on the glasses of overgrowthwhich had been immersed in sand (Figure 72). The sand was easily removed from thesurface of the glass while the microbial cells remained. One found by simple microscopythat the microorganisms grow diffusely in the zone of the roots in the form of isolatedcolonies or small foci. The colonies are located between the hairs on the surfaceof the roots, or in their vicinity (Figure 73). In cases where there was a greatdeal of moisture covering the roots and the root hairs, the bacterial cells spread.often occupying considerable segments along the roots (Figure 74).

 

Figure 72. Imprints of root branches on glass with surrounding microflora:

A--natural size of the slide; imprints of roots in the form of hairs (a) are seen; B--the same preparation magnified 1:100; branch of the root overgrown by hyphae of fungi and actinomycetes.


 

Figure 73. Colonies of bacteria and actinomycetes on the surface of roots:

a--colonies of actinomycetes (marked by +), b--bacterial colonies, c--colonies of mycobacteria (our own observations); d and e--bacterial colonies (after Linfold, 1942).


 

Figure 74. Diffuse distribution of bacteria on root surfaces:

a--nonsporiferous bacteria; b--mycobacteria; c--nonsporiferous bacteria and mycobacteria.

 

  Isolated colonies, as wen as profuse areas of microflora,mainly consist of nonsporeforming bacteria and mycobacteria. One also finds coloniesof actinomycetes and mycelia of fungi. Very seldom does one see small colonies orsingle cells of sporeforming bacteria.

  Nonsporeforming bacteria and mycobacteria form well-defined,compact colonies, located most often on the surface of the roots, between the roothairs, and also at a certain distance from them (Figure 75). Colonies of actinomycetesoften consist of proactinomycete elements, (Figure 75), or of entangled mycelia.Fungi also grow around roots in the form of single hyphae and mycelia.

 

Figure 75. Colonies of actinomycetes around roots. The zone of massive growth of bacteria, at a certain distance from the root, is clearly seen

 

  If one prepares imprints of the root area of microfloragrowing in sand, and not in soil, one does not encounter this picture. Upon the microscopicexamination of these imprints, one can observe only mycelial threads of fungi andactinomycetes, occasionally with sporangiophores on branches. One seldom observessingle colonies of actinomycetes, Around the root branches and hairs bacterial cellsare also encountered, but in limited numbers and, as a rule, they appear as singlecells or in pairs and very seldom in colonies. When such a preparation is being grown,by covering it with a thin layer of an agar medium, a considerably larger numberof bacteria and mycobacteria appear than can be observed under direct microscopy.The great majority of cells are not detected by microscopy. This in due to the factthat the cells of bacteria and mycobacteria, and possibly certain actinomycetes,exist in a fragmented state in the form of small granules hardly discernible fromsoil particles. These granular elements, stained with erythrosin, are encounteredin the soil of the root zone in great numbers. They probably form the major partof the mass of the rhizosphere microflora. These elements, when transferred to anutrient medium, grow out, giving rise to cells of normal size, which are detectableunder laboratory conditions.

  The methodof overgrowing the root zone ofplants onglass was employed by Linfold (1942). He grew seeds of corn, lettuce, pineapple,and the cowpea (Vigna sinensis) in soil in a special chamber. The imprintsobtained on glass plates were microscopically examined. The author observed an abundantgrowth of bacteria in the form of large colonies around the roots and root hairs;colonies were often located at the hair tips (Figure 76). At a certain distance fromthe roots, according to the author, amoebae, Infusoria, and nematodes grew.

 

Figure 76. Bacterial colonies on the tips of root hairs (according to Linfold, 1942)

 

  Starkey (1938) employed the method developed by Rossi-Kholodniifor the study of root-area microflora. He found that the bacteria grew on the rootsin clusters, and fungi and actinomycetes, in the form of threads. The author buriedthe slides in the soil. under the root system. Of course, he could not see the greaterpart of the bacteria on the overgrown glass. As in our experiments, the bacteriawere probably in a fragmented state and could not be detected among the numeroussoil particles without being cultured.

  Stille (1938), using this method, found an abundantgrowth of bacteria around the root hairs.

  These data show that, in the root zone, the microfloraprobably grows in the same way as it generally does in the soil, in colonies or inaggregates. Only when there is a high moisture content in the substrate do bacterialcells grow in extensive areas.

  Group composition of the microflora of the rootarea. Studies have shown that around and on the roots of vegetating plants onefinds various representatives of microorganisms--bacteria, actinomycetes, fungi,algae, yeasts, protozoa, phages, and other living organisms.

  However, the prevailing group in the rhizosphere, regardlessof the conditions of growth and the age of the plants, are the nonsporiferous bacteria,

  The second place (largest group) among rhizospheremicroflora is occupied by mycobacteria.

  The remaining forms of microbes, actinomycetes, fungi,sporiferous bacteria, etc are encountered in much smaller numbers. The quantitativeratios of these microorganisms can be found in any plating of rhizosphere soil onagar media containing protein or on synthetic media. In Table 75 data are given onthe of different groups of microbes growing in the rhizospheres of various plantsimmeditately before flowering. They were obtained by plating one drop (0, 05 ml)of a 1:1,000 dilution.

Table 75
Quantitative relationships between microorganisms in the rhizosphere of plants
(number of colonies per plate)

Soil

Plant

Total number of colonies

Nonspore- forming bacteria

Spore- forming bacteria

myco- bacteria

actono- mycetes

fungi

Trans-Volga region Wheat

1,200

950

10

200

38

2

Chestnut soil Oats

1,800

1,450

5

300

35

10

Moscow Oblast' First year clover

2,200

1,675

8

500

15

2

Podsol Rye

2,200

1,675

6

50

20

4

Central Asia Lucerne

3,200

2,700

15

450

34

1

Serozem Cotton

2,500

2,150

12

305

30

3

  Approximately the same group-composition ratio of rhizospheremicroflora is also found in other plants. Nonsporeforming bacteria are the prevailinggroup among the rhizosphere microflora of wheat, oats, clover, lucerne, cotton, andother plants either growing in the southern regions on chernozem, chestnut, and serozemsoils, or in the northern regions in podsol or other soils. In some cases, deviationsin the number of bacteria of the different groups may exist, the percentage of thisor that microbial group may change, but the general nature of the ratio between thedifferent groups of microbial biocoenoses in preserved.

  The group relation of the microflora in the root areavaries considerably with the age of plants.

  It was noted above, that upon the ripening of plants,the total number of microorganisms in the rhizosphere decreases. During this periodthe quantitative ratio between the different representatives and groups changes;the number of sporeforming bacteria, fungi, and actinomycetes increases, and neworganisms appear. At the same time, the total amount of nonsporeforming bacteriadecrease, some species disappearing altogether, etc.

  On the diagram in Figure 77, data are given on theanalysis of the rhizosphere microflora of oats during different periods of its growth,obtained in studies of the plants of the Trans- Volga region (Krasil'nikov, Rybalkina,Gabrielyan, Kondratleva, 1934).

 

Figure 77. Quantitative relationship between microorganisms at different stages of the plant's growth (oats):

A--flowering period; B--period of ripening.

 

  In later works we noted the same changes in the ratiosof the representatives of the rhizosphere microflora of many other plants, growingin different areas on differing soils (Krasil'nikov, Kriss, and Litvinov, 1936).

  The change in the quantitative composition of rhizospheremicroflora with the age of the plant was also noted by Starkey (1938), Isakova (1939),and some other investigators.

  Nonsporeforming bacteria comprise the main, and themost numerous and versitile group of soil microflora, in general. This group embracesrepresentatives of various families, genera, and species; these include such organismsas Azotobacter, rhizobia, thiobacteria, photobacteria, Azotomonas, Sulfomonas,nitrifying bacteria, denitrifying bacteria, and others.

  All these representatives are encountered in the rhizosphereof plants. Most frequently, organisms of the genera Bacterium and Pseudomonasreach the largest numbers, These organisms, comprise the main microflora of the rootarea,

  The species composition of these organisms is scarcelyknown. We have no adequate methods for the diagnosis and differentiation of thesebacteria and we are not in a position to distinguish between the nonsporeformingbacteria in the rhizosphere of one genus of plants and those of another. Only a fewgenera can be accounted for: Azotobacter, Rhizobium, and some others.

  Kostychev and others (1926) were of the opinion, thatAzotobacter adapted itself to the rhizosphere of certain plants: tobacco,rice. and others and is their essential companion.

  Later, many investigators found Azotobacterin the rhizosphere of plants. Sidorenko (1940b, c) studied its growth in the rootregion of wheat, barley, beets, lucerne, sudan grass, soybean and other plants grownon the chernozem soils of the Khar'kov Oblast'. Popova (1954) found Azotobacterin the rhizosphere of grape vines: Obraztsova (1936) and Isakova (1935) found itin the root zone of subtropical plants: lemon, tangerine, and tung tree; Daraseliya(1950) found it in the root zone of the tea plant. Raznitsyna (1947) and Tanatin(1949, 1953) observed the distribution of Azotobacter in the soils of CentralAsia under cotton, lucerne, wheat, and various vegetable crops. Pavlovich (1953)studied the distribution of Azotobacter in the rhizosphere of various plantsin the Latvian Republic; Banak (1956), in Moldavia; Gerbart (1952), in the westernUkraine districts, Petrosyan and his associates (1949) and Afrikyan (1953) in Armenia.

  According to our observations, the growth of Azotobactertakes place in the root area of various plants under differing climatic conditionsand in different geographical regions, from the northern regions to the southernmostpoints, on mountain tops and in valleys. The growth of Azotobacter is observedin the rhizosphere of forest trees (Samtsevich and others, 1952); Krasil'nikov, 1945),fruit trees, bushes, and in other plantations (Kanivets, 1951; Kulikovskaya, 1955,and others).

  The number of Azotobacter organisms in the rhizosphereof plants may reach considerable dimensions. We counted from 0 to 200 million Azotobactercells in 1 g of soil in the root area. Tanatin (1953) counted tens of thousands andhundreds of thousands in a gram of rhizosphere soil. Approximately the same numberswere found by other investigators (Pavlovich, 1953; Babak, 1956; Raznitsyna, 1947).

  No less frequent are the rhizobia. Their identificationin the rhizosphere is not only of diagnostic. value but also of practical significance,Depending on the abundance of their growth in the root area of leguminous plants,their effectiveness will differ under the same conditions of activity.

  Root-nodule bacteria, as is well known, grow abundantlyon the roots of those plants on which they can form nodules. However, they can alsogrow on roots of other plants. For instance, the nodule bacteria of lucerne growluxuriously on the roots of lucerne and also in the root zone of cotton; the nodulebacteria of clover also grow well in the rhizosphere of lucerne, peas, and certainother plants. The nodule bacteria of peas grow well in the root zone of peas, clover,wheat, etc. The total number of nodule bacteria in the rhizosphere of plants maybe considerable. Korenyako (1942) counted them in hundreds of thousands and in millionsper gram of soil. Similar data is given by Raznitsyna (1947), Petrosyan and his associates(1949), Petrosyan (1956), and others.

  Ammonia-producing bacteria, denitrifying bacteria,and certain other groups of nonsporeforming bacteria grow abundantly in the rhizosphere.Of the first two groups many millions and billions are present per gram of soil inthe rhizosphere (Krasil'nikov, Rybalkina, and others, 1934; Krasil'nikov, Kriss andLitvinov, 1936, Raznitsyna, 1947; Starkey, 1928; and others).

  Nitrifying bacteria, cellulose bacteria, nitrogen-fixinganaerobic bacteria of the genus Clostridium, and certain other groups do notgrow well, comparatively speaking, in the rhizosphere (Rempe, 1252; Krasil'nikov,1934, 1936).

  The second place, with respect to their quantitativegrowth in the rhizosphere, is occupied by mycobacteria. Their number in the rootarea reaches hundreds of thousands and millions. Their species composition was notstudied. Most often one encounters the nonpigmented forms of Mycob. album, M.mucosum, and others.

  One finds sporeforming bacteria in the rhizosphereof plants much less frequently and in smaller numbers. In general, they compriseonly a fragment of one percent of the microflora. They are especially scarce duringthe period of vigorous vegetative growth of plants. Usually these bacteria begingrowing abundantly at the end of vegetation, especially on dead, decomposed roots.

  Actinomycetes also occupy a small place among root-areamicroflora during the early stages of the development of plants. This group of organismsis very widespread and versatile in its species make-up. In the rhizosphere, thereare various representatives of actinomycetes. Toward the end of vegetative developmenttheir number increases considerably. They multiply with special intensity on semidecayeddead roots. One often sees rootlets covered completely with the mycelia of theseorganisms in the form of a fluffy or a mealy-white coating.

  Fungi are detected in the rhizosphere by the conventionalanalyses of small quantities.

  It is known that certain plants have a well-developedfungal coating on their roots, coalescing with the root tissue. This: fungal coatingis called mycorhiza. Depending on the nature of its relation to the roots one candistinguish between endotrophic and ectatrophic mycorhiza. Mycorhiza fungi are widespreadin the root systems of many plant species, both woody types and grasses. Some investigatorsare of the opinion that all plants have mycorhiza. The fungi participating in themycorhiza belong, according to their systematic positions, to different classes andorders, families and genera. It is supposed that these organisms are of great importanceto the plants. However, this problem has been only slightly studied (Lobanov, 1953;Reiner and Nelson-Jones, 1949; Kelly, 1952).

  Many plants do not grow well without mycorhiza fungi,and some do not grow at all, However, one seldom encounters roots with an abundantgrowth of these organisms in grassy field crops.

  It should be noted that upon an ordinary microbiologicalanalysis of the rootlets, one observes, as a rule, single mycelial hyphae. Fungiare detected by special studies with the use of special analytical methods.

  The question of algal growth in the root area of vegetatingplants has been only slightly studied. The research done by Katznelson (1946) andShtina (1953, 1954 b) showed that various algae live in the rhizosphere of plantsin considerable quantities., Their total number reaches tens and hundreds of thousandsin one gram of soil.

  Shtina studied the growth of algae in the rhizosphereof rye, timothy grams, clover, lupine, potatoes, barley, and oats. Among some ofthese plants, the number of algae in the root area was two to three times higherthan outside it (rye, timothy grass, clover, lupine). For example, in the root zoneof clover, 149,000 cells were found in one gram of soil, and in the control area(outside the rhizosphere), only 99,000 cells per gram were detected.

  Qualitatively, the algae composition within the rhizosphereis approximately the same as outside it. It consist mainly of diatoms, green andblue-green algae (Shtina 1954 a and b). They probably also have a certain importancein the life of root-area biocoenoses.

  In the rhizosphere of plants, one observes invertebrateanimals: protozoa, nematodes, insect larvae, etc. The number of these organisms inthe root area of healthy plants is small and does not exceed a few thousands per1 m2 under various plants. Shilova (1950) studied the soils under grasses,rye, and lupine, in long fallow, and fallow soils. According to her calculations,in 1 m of the surface horizon (0-10 cm) of soil, there are from 7,000 to 187,000invertebrates, and the number of certain members of Apterygota in this area, undergrasses, reached 760,000.

  The composition of this fauna is extremely diverse.One finds in the rhizosphere members of Acarina (mainly under forest cultures), andrepresentatives of Apterygota. Enchytraeidae, and others (Shilova, 1950; Gilyarov,1949, 1953). Katznelson (1946). Linfold (1942), and Brodskii (1935) have describedthe distribution of the protozoa, amoebae, ciliates, flagellates, and others in thesoil of the root area. These organisms are frequently encountered when studying thesoil of the root area in ordinary laboratory studies.

  Nikolyuk (1949) has established that there are twoto three times more protozoa in the root zone of lucerne than outside it.

  The greatest number of these organisms are accumulatedin the rhizosphere of cotton during its first year of cultivation (up to 100,000in 1 g soil). The author ascribes this accumulation of protozoa to the abundant growthof bacteria in the rhizosphere, which serves as nutrient material for the protozoans.

  Ressel (1955), Brodskii (1945). Nikolyuk (1949), andothers ascribe great importance to this group of organisms, as a factor affectingthe composition of microbial biocoenoses in the soil.

  Quite often one encounters worms and nematodes in theroot zone of plants. Certain nematodes, as is well known, grow well on roots and,in penetrating into plant tissues, cause diseases. Arkhipov (1954) noticed the deathof nematodes under certain plants.

  Pathogenic microbial forms--bacteria, actinomycetesand fungi--often grow and accumulate in the root area. Under certain conditions,these organisms penetrate the tissues of plants. in agricultural practice, casesof mass infections of plants, as a result of the growth of pathogenic fungi and bacteriain the root zone are not infrequently encountered. The development of microbial antagonistsinhib iting phytopathogenic organisms in the rhizosphere is also possible.

  In general, many different forms of organisms may growin the rhizosphere of plants, both useful and harmful; those which facilitate thenourishment and develop ment of plants, and, an the contrary, those which inhibitand poison them. The prevalence of these other organisms depends on soil-climaticconditions, on the manner in which the farm is handled, and on the whole agrobiologicalcomplex.



 


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