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  We have already noted the importance of microorganisms in the life of plants as being among the biological factors of soil fertility. Between the two there exist special interrelationships which express themselves to some degree by the total productivity of the soils.

  Investigations show that the vegetative cover is a powerful factor in the life of microorganisms. The peculiarities of the plant species leave their imprint on the quantitative and qualitative composition of the microflora biocoenoses of soils. Plants create and form microbial societies, which effect the microbial population of the entire root system and harvest remains. During their life, plants excrete through their roots various organic and mineral substances which attract microorganisms. During the growth cycle of the plant, roots constantly form and shed root hairs, lose necrotic epidermal cells, etc. All these elements are then taken up by the microbes and become the source of their nourishment.

  The species composition of the microflora, just as the soil, has a definite and considerable influence on the growth and development of plants, and consequently on the crop yield. The study of the nature of the interaction between microorganisms and plants is one of the main and most interesting problems, not only of microbiology, but also of the study of soil and plant cultivation.

  The effect of the vegetative cover on the microflora of the soil was studied long ago by various investigators. Plants not only have an influence on living microorganisms (through their root system) but also influence them after their death and after the harvesting of the crops (in cultivated fields), In the former case, this effect is caused by root excretion and by the dying particles of the roots themselves. In the latter case, the active factors are the remnants of the roots and the aerial parts. In both cases, great changes take place in the composition of the soil microflora. In addition, the roots exert a beneficial effect on the physical and chemical properties of the soil, improving the conditions for the existence of microorganisms. One can judge the importance of plant roots in the biology of soils by the great bulk of the root mass and its extension.

  The studies made by Kachinskii (1925), Chizhov (1931), and others showed that the root system is a great mass by weight with a vast surface. This was observed by Tol'skii (1904-1911) when he studied the root system of forest plantations in the Buzuluk forest. The author gives numerical data on the development of the root system and its dispersion in woods on different soils and with differing densities of planting. At a density of 77 trees per 0.2 hectares, the length of the roots of one plant is 0.21 km, and at a density of 260 trees per 0.2 hectares, it is 0.42 km.

  L. P. and L. L. Golodkovskii (1937), on the basis of observations made on the development of the root system of lucerne under differing soil-climatic conditions of its growth, obtained the following data (Table 62).

  According to data obtained by Dittmer (1937, 1938), one plant of winter rye has a total root length (without hairs) of 623.4 km. The thinner the roots, the greater their length. The main roots have a length of 0.07 km; the secondary roots 5.4 km; the roots of the third order, 175 km, and those of the fourth order, 442.8 km. The largest mass consists of the active part of the root system.

  The area of all the roots of one plant is 237.4 m²; the area of the roots of the first order, 0.1 m² ; the root area of the second order 4.2 m²; the root area of the third order, 70.5 m²; and the area of the roots of the fourth order, 162.8 m².

  The root hairs on the roots of one plant number to 14.3 billions. They are mainly located on the roots of the third and fourth orders (14.1 billions). The length of all the hairs of one plant is more than 10,000 km. Their total area (one hair is 700-1,000 µ in length) is double the surface area of the root, i. e., more than 400 m². Consequently, the roots of one rye plant in the fourth month of growth have a total length of 11,250 km and an area of 6,388 m².

  In the textbooks written by Kursanov, Keller, and Golenkin, smaller figures are given for the length of roots. For instance, according to their data, the total length of melon roots without hairs is 25 km; in wheat, about 20 km; and in spring rye, 623 km. The surface of the root system of one rye plant is 237 m². The surface of the roots of rye is 130 times greater than the surface of the aerial organs (Kursanov, 1940).

  According to Pavlichenko (1938), the length of roots of the wild oat (Avena fatua) is 87.8 km, and of wheat, 71.2 km. In one cubic decimeter of soil, the total length of roots in as follows: in oats, 6 5 cm; in rye, 90 m; and in meadow grass, 553 m. The length of the root hairs is as follows: in oats, 11.6 km; in rye, 24.3.km; in meadow grass, 73 km.

  Detailed studies of the root system of fescue plants, conducted by Savvinov and Pankova (1942) in the Volga steppes, showed the following: in a two-meter 2 layer of soil there are 1.75 kg dry mass of roots per m², of which one third to one quarter are living. The weight of the aerial part of these plants comprises 0.48 kg, i. e., about one quarter of the total weight of the roots.

  The nature of the distribution of the roots in the soil is illustrated in Table 63.

  Shalyt and Kalmykova (1935) studied the formation of the roots of steppe vegetation in chernozem soils ("Askaniya-Nova" National Reserve). According to their data, the air-dry mass of roots of the Festuca-Stipa vegetative association is 3,002.5 g (30 tons/hectare). and in the Basaltic solonets of the same reserve 1,175.8 g per 1m³ (11.75 tons/hectare). The total surface of the Festuca roots is 225 m², and for the Stipa, 126.2 m². Savvinov and Pankova consider these numbers to be somewhat exaggerated. One can assume that the cited differences have a purely ecological cause and are not the result of a methodical error. It is obvious that under the conditions of the southern Ukraine, which has chernozem soils, the relationship a between roots and their aerial parts will differ from those of the chestnut soils near the Volga. Moreover, the differences discussed by the authors are not very striking and fundamental. As in the data submitted by Savvinov and Pankova, as wen as that of Shalyt and Kalmykova, the numerical indicators are of about the same order.

  Muntz and Girard (1888) measured the length and diameter of the roots of plants grown on the experimental fields of the Paris Agricultural Institute and obtained the following data. In 1 m³ of soil, there was a 5.18 m² total area of clover roots; in the case of meadow grasses, 7.58; oats, 10.70; winter wheat, 11.30; and poppies, 2.17m².

  According to Belyakova (1947), the root mass of the dry roots of lucerne in the soils of the Vakhsh valley is as follows: in the first year of growth, 11.12 tons, in the second year of growth, 22-24 tons, and in the third year, 30 tons and more per hectare.

  Nad"yarnyi (1939) found that mixtures, several years old, of leguminous and cereal grasses accumulate more roots than pure cultures. In the upper layer of the soil (0-20 cm), over a two-to three-year period, up to 40-75 centners* roots per hectare were found. [*One Russian centner equals 100 kg.] A greater accumulation of root mass was observed by Belyakova and Parishkura (1953) in soils having mixed crops of grasses. In other studies, many tons of dry root mass per hectare are given.

  The main mass of roots is concentrated in the surface layer (0-25 cm) or somewhat deeper, depending on soil-climatic conditions and on the type of flora. Sometimes, in the deeper layers of soil, a second maximum (less pronounced) of root concentration is observed. The nature of the distribution of the root mass along the horizons of the soil is given in Figure 69.

Figure 69. Distribution of wheat roots in the soil (according to Kachinskii, 1950)

  According to their distribution, the importance of the roots will be greatest in the upper horizons. The biological significance of the root system is determined by its activity, i e., by its ability to absorb elements of nutrition from its surroundings and excrete products of metabolism into the environment, Studies have shown that the active part of the root system is its largest part. According to some date, it comprises 50 to 75 percent of the total root mass.

  The substances which are excreted by the root system are utilized by microorganisms as nutrient sources. A great number of microbes concentrate around the roots, growing, multiplying, and excreting their metabolites, many of which are assimilated by the plant roots. The whole root system during the life of plants, as well as after their death, exert an immense influence on the growth and development of microorganisms.

  The effect of the root system on the composition of the microflora may be direct or indirect, positive or negative.

  The utilization of the nutrient elements by the plants is connected to some degree with the metabolism of microorganisms. A greater influence of plants on microorganisms is that the former enrich the soil with organic substances.

  Roots are no longer looked upon as mere suctorial organs through which plants absorb various nutrient elements from the soil. As early as the 18th century, the ability of roots to excrete certain substances, which affect the properties of the soil and determine its fertility, was noted.

  The presence of CO₂ in the excretions of roots was noted by many authors including Sossur (1804), Trevisan and Meis (1839), Pollaci (1858), and others. Sachs in 1860-1865 experimentally demonstrated that the root systems of various plants excreted CO₂ (Pryanishnikov, 1952; Konstantinov, 1950).

  Lundergardh (1924) determined the amount of CO₂ liberated by the roots of wheat grown in sterile sand and in sand containing bacteria. He obtained the following results: one gram of a dry mass of roots excretes 3.05 mg of CO₂ per hour in sterile sand and 5.57 mg in the presence of bacteria.

  The considerable excretion of CO₂ by the roots of plants was observed by Zaikovich. According to his observations, the roots of well-developed corn excreted 0.24 g per day, and, according to Knopp, 0.25 g. In the experiments carried out by Kossovich, mustard roots excreted, on the average, 27.3 mg of CO₂ per day. Barakov observed the excretion of CO₂ by the roots of different plants and concluded that the maximum amount of CO₂ excretion occurs during the period of the most active metabolism of the plant, during its flowering (according to Konstantinov, 1950).

  Chesnokov and Bazyrina (1934) grew flax in vessels with podsol soil or sand and determined that the respiration of the soil with the plants growing in it greatly exceeded the sum of the respiration of the same roots and soil taken separately.

  The more bacteria present in the rhizosphere, the more intense the formation of CO₂ by the roots of plants (Table 64).

  The intensity of CO₂ formation depends on the species of the plants, their age, the season of the year, and other factors.

  The approximate volume of the root respiration of grain cultures under conditions of growth in the field comprised 25-30 percent of the respiratory volume of the soil as a whole (Konstantinov, 1950).

  During their life, plants excrete different mineral and organic compounds via their roots. Compounds of phosphorus, potassium, calcium, sodium and other elements have been found in root excretions.

  Sabinin (1940, 1955) and his associates (Minina, 1927) have shown that the excretion by roots of elements of mineral nutrition is accomplished by exosmosis and is regulated by the concentrations of these substances in the external substrate. Tueva (1926) established that the exosmosis of calcium and potassium from roots takes place until an equilibrium state of these elements is established in the surrounding medium. Such a regularity was found by Osipova and Yuferova (1926) in relation to the absorption and excretion of sulfur and phosphorus by the roots of corn and wheat.

  Avdonin (1932) observed the loss of ash elements in cultivated oats under field conditions, These losses differ in quantity depending on the conditions of the growth of the plant.

  Akhromeiko (1936) decided that some plants excrete mineral substances via their roots, while others do not. He observed phosphoric acid in the root excretions of lupine, peas, buckwheat, mustard, and rape. The amount of phosphorus excreted attained 14-34 per cent of all the phosphoric acid taken up by the plant.

  There are some writers who deny that it is possible for roots to excrete mineral compounds. The authors of these works assume that the substances found are the decomposition products of root residues.

  Of the root excretions, the organic substances are of the greatest importance, The presence of these substances was observed for the first time at the end of the last century. Dyer (1894) established the presence of acidic compounds in the root excretion of plants of barley, wheat, oats, foxtail, and others. Acids were detected in root excretions by Lemmerman (1907), Künze, (1906), Schreiner and Reed (1907), Doyarenko (1909), and others.

  Stoklasa and Ernest (1909) found that plants excrete acetic, formic, and oxalic acids through their roots. Maze (1911) and Shulov (1913) found organic acids and sugar in root excretions. Organic substances were found by Kostychev (1926), Truffaut and Bessonoff (1925, 1927), and others.

  Mashkovtsev (1934) found that the roots of germinating seeds of rice excrete sugars, aldehydes, ethyl alcohol, and other compounds precipitating with lead acetate.

  Minina (1927) detected organic substances in root excretions of lupine, beans, corn, barley, oats, and buckwheat, upon cultivating them in Knop's nutrient solution, The excretion in most of these cultures reached its maximum during the fourth week of growth, and in buckwheat, at a somewhat earlier period. Upon the ripening and aging of the plants, the amount of root excretions decreases and toward the end of the growth period stops altogether.

  Lyon and Wilson (1928) found nitrogenous and nonnitrogenous organic compounds in the root excretions of corn. The amount of nitrogenous substances in root excretions, according to these authors, decreases with the age of the plant.

  Winter and Rümker (1952) observed phosphatides, amino acids, thiamine, biotin, meso-inositol, paraaminobenzoic acid, carbohydrates, tannins and alkaloids in the root excretions of plants. Harley (1952) found sugars, amino acids, vitamins, and other organic compounds in root excretions.

  Virtanen and his associates (Virtanen and Laine, 1937) observed in the root excretions of young sprouts of leguminous plants; peas, clover, etc, aspartic and glutamic acids, tryptophan and ß-alanine.

  Cereals, oats and barley, grown in the same vessel with leguminous plants in the complete absence of nitrogen sources in the medium, grew normally and developed at the expense of the nitrogen excreted by the leguminous plants. Similar experiments were conducted by Lipman as early as 1912.

  The possibility of transferring metabolic products from certain plant species to others was confirmed by the experiments of Preston and his associates (Preston, Mitchell and Reeve, 1954). Plants sprayed with methoxyphenylacetic acid were grown in the same vessel with plants which were not sprayed, After some time, the given substance was detected in all the tissues of the unsprayed plants, in larger or smaller quantities: the nonsprayed plants absorbed the methoxyphenylacetic acid excreted by the roots of the sprayed plants.

  Many investigators have detected a considerable amount of nitrogenous organic compounds in the roots of cereal plants when they were grown together with legumes (Scholz, 1939; Wyss and Wilson, 1938; Madhok, 1940; Nicol, 1934; Isakova and Andreeva, 1938).

  Sabinin (1940) found that the roots of pumpkins excrete from nine to eleven different amino acids. These acids were determined and differentiated by paper chromatography (Kursanov, 1953).

  Other investigators deny the presence of nitrogenous substances in the root excretions of leguminous plants (Bond, 1937, and others).

  Wilson, Wyss, and others (1937, 1938) assumed the possibilty of the excretion of nitrogenous compounds by roots. However, these compounds, according to these authors, are metabolic products of the nodular tissues and not of the roots of the leguminous plants themselves.

  Engel and Roberg (1938) in order to verify Virtanen' s data, cultivated alder which was inoculated with cultures of proactinomycetes, forming nodules, and observed in the substrate (sand) a considerable quantity of organic nitrogenous substances excreted by the roots.

  Virtanen, in reply to Wilson, Bond, and others, noted that the process of the excretion of organic substances is closely linked with external conditions--sunshine, aeration, nutrition, and the pH of the medium. Confirming earlier data by new experiments, the author stated that the detected nitrogenous compounds are products of the fixation of free nitrogen, which were not utilized in the formation of protein and plant tissues and not products of protein decomposition (Virtanen and Torniainen, 1940).

  The organic compounds excreted by the roots of various plants are not identical. In leguminous plants, one detects more nitrogenous compounds--amino acids, amide compounds, and others (Virtanen and others, 1937, 1938, 1940). In cereals. the root excretions are richer in carbon substances--sugars, organic acids, and others. According to our observations, peas, broad beans, beans, lupine, and other leguminous plants excrete substances having a neutral or weakly alkaline reaction, and cereals--corn and wheat--secrete substances having an acid reaction, According to the data of some investigators, the roots of peas excrete nucleotides and flavins (Lundegardh and Stenlid, 1944).

  West and Wilson (1939) observed biotin and thiamine in the root excretions of flax and sugar in the excretions of certain cereals, Brown and others (1949) proved the presence of pentoses or closely related compounds (alpha-ketoxylose) in the root excretions of grasses.

  Brown and Edwards (1944) found special substances which stimulate the growth of other plants in root excretions.

  Groh (1926) studied the root excretions of lupine, broad beans, wheat, oats, barley, and rye. In some plants, substances were detected which have an acid reaction, and others which have an alkaline one. On the basis of these findings, the author divides plants into two groups: the acid group, including peas, broad beans, lupine and wheat; and the alkaline group, including oats, rye, barley, and mustard. According to Pryanishnikov (1905), lupine excretes substances of an acid nature. Due to these excretions, this plant dissolves the highly soluble phosphates, transforming them into an easily assimilated form. Other plants, like mustard and buckwheat, are not able to excrete substances of an acid nature and cannot dissolve the mineral compounds essential for nourishment.

  The studies made by Fred (1918, 1919), conducted under strictly sterile conditions, clearly showed the presence of substances of an acid nature in root excretions, which dissolve marble plates. The author pointed out in this connection that, in the presence of bacteria, the process of the dissolution of marble is considerably faster.

  The roots of Italian rice excrete a substance which fluoresces with a blue light upon ultraviolet irradiation. This substance is so characteristic that it may, according to the author, serve as an indicator of the given plant (from Audus,1953)

  The difference in chemical composition between the root excretions of different varieties of the same species of plant was also noted by other investigators. Timonin (1941) established the presence of substances in a variety of flax resistant to fusariosis (Bizon var.), which activate the growth of the fungus Trichoderma viridis an antagonist of the organism causing fusariosis. In the strain which was sensitive to fusariosis (Novel var. ) substances were found in the root excretions which stimulate the development of the fungus Fusarium, the cause of fusariosis of flax.

  Chemical analysis has shown that in the root excretions of the resistant variety of flax, there is a great amount (25-30 mg per plant) of hydrocyanic acid, which possesses antimicrobial properties. In the excretions of the roots of the sensitive variety of flax, this acid is either absent or present only in traces.

  Eaton and Rigler (1946) observed an analogous situation in the root excretions of the cotton plant. In the variety resistant to root decay more carbon compounds were found than in the sensitive variety. According to the authors, the given substances attract microbe antagonists, which inhibit the development of the organism causing root rot.

  It should be noted that the problem of plant-root excretions only comparatively recently became a subject of thorough study. Therefore, we possess only scant information on the qualitative composition of root excretions. In addition, our knowledge of the quantities of the substances excreted by the roots in even more scant.

  The few studies available show that roots excrete considerable amounts of organic substances. Dyer (1894), in determining the amount of acids excreted by the roots of plants, established that 100 ml of nutrient solution from barley contained 0.38 mg of acids; from wheat, 0.58; oats 0.65, foxtail, 0,86; timothy grams, 0.80; orchard grass. 0.81; white clover, 1.28; red clover, 1.55 and from broad beans, 1.11 mg of acids.

  According to Maze (1911), one corn plant in a sterile nutrient solution excretes 57 mg of sugar and 84 mg of acids in twenty days of growth. Shulov (1913) found in the root excretions of this plant, after a two-month period of growth in a nutrient solution, 94 mg of nonreducing and 34 mg of reducing sugars and 80 mg of malic acid. The roots of peas excreted, during the same period, 140 mg of sugar. According to the observations of the author, when plants were cultivated on ammonium nitrate, there were more root excretions than when the plant was given calcium nitrate.

  Pfeifer, (1912, 1917) investigated the root excretions of wheat and buckwheat. According to this author, 0.27 g of wheat roots excreted 0.134 mg of organic acids and 0.110 g of buckwheat roots excreted 0.155 mg of organic acids, which comprises 1.3 per cent of the total weight of the plants. According to Shulov (1913), the root excretions of corn comprise 0.6 per cent of the plant's weight.

  Demidenko (1928) grew corn and tobacco in solutions which were either changed or unchanged. The corn roots of one plant, grown in a solution which was not changed excreted 486 mg of organic substances during the whole vegetation period and, when the solution was changed seven times. the roots excreted 1,136 mg of organic substances, Roots of tobacco, for the same period, excreted 158 mg in an unchanged solution, while in a changed one, it excreted 439 mg of organic substances. In summarizing these observations, the author concluded that the total root excretion comprised 27 per cent of the plant mass.

  Mashkovtsev (1934) found that seeds of rice upon germination lose 20-30 per cent of dry weight, with about one fourth of the loss consisting of root excretions of organic compounds.

  Virtanen and his associates (1933) found that the roots of peas grown in vessels along with cereals excrete 126.4 mg mineral nitrogenous compounds in 58 days, of which 77.4 per cent is comprised of the nitrogen of amino acids, 3.3 per cent amide compounds, 2.05 per cent of melanin*, and 2.73 per cent of other nitrogenous compounds. *["Melanin" appears in Russian text but it may erroneously refer to "humin."]

  When barley was grown together with peas, it grew normally and developed, although no nitrogen was introduced into the vessel with sand; in the tissues of experimental plants of barley, 32.3 mg of nitrogen were found, and in the tissues of the control plants. which were grown without peas, the nitrogen found amounted to only 0.7 mg and these plants developed very poorly.

  The barley in these experiments did not utilize all the nitrogen excreted by the peas. A considerable part of it, up to 89.0 mg, remained in the substrate in the form of these or other organic nitrogen compounds. (Virtanen, Synnöve Karström, 1933; Virtanen, 1937).

  Virtanen and Laine (1936, 1937) found that in the root excretions of clover and other leguminous plants in the period preceding flowering, mainly (75 per cent of the total bound nitrogen) aspartic acid, gluconic acid, tryptophan and ß-alanine were detected. During flowering, the major part of the nitrogenous root excretions consisted of tryptophan.

  Lyon and Wilson (1921) calculated that for the whole vegetative period, the roots of plants excrete up to 5 per cent of the total weight of the plants' organic substances.

  Engel and Roberg (1938) determined that, during a two-month period of growth, the roots of one alder plant, inoculated with proactinomycetes, excreted 27.7 mg of nitrogenous compounds and uninoculated plants excreted 23.6 mg of nitrogenous compounds (Table 65).

  Meshkov (1953) investigating the root excretions of peas and corn grown in a sterile nutrient solution, obtained the following results: during twenty days of growth, the roots of peas excreted into the solution 2.87 mg of reducing sugars in experiments performed during 1946, and 4.28 mg in the experiments carried out during 1947. The weight of the dry mass of the vegetable crop comprised 1.92 g in 1946 and 1.85 g in 1947. The roots of corn for the same period excreted into the solution 8.4 mg in 1946, and 8.17 mg in 1947. The weight of the dry mass was 3.69 and 2.35 g respectively. According to the observations of the author, the amount of root excretions depends to a considerable degree on the weight of the roots, rather than on the weight of the green parts, leaves and stems. The total weight of the latter amounts to 2 per cent in the case of peas and 1.3 per cent in the case of corn, of the total weight of the mass of the plants.

  In our investigations (1934 b) we determined the growth of microorganisms in the media to which these substances were added. For this purpose, of a great number of species tested the following cultures were chosen: two cultures of yeast; Torula rosea and Sporobolomyces philippovi, and two bacterial cultures, Pseudomonas fluorescens and Ps. denitrificans.

  These microorganisms were grown in a solution in which wheat and corn were grown, and also in a pure nutrient solution with various concentrations of glucose. After certain time intervals, the cells were counted in a Thoma counting cell and plated on liquid and solid nutrient media. The results are given in Tables 66 and 67.

  In comparing the maximum numbers of microbial calls which had grown on the rhizosphere solution with the corresponding numerical indicators of growth in glucose-containing medium, we obtained the following results: the maximum number of Torula rosea cells, which attains 1,500,000 in the rhizosphere solution, is equal to the same number in the case of a glucose concentration which slightly exceeds 20 mg. Approximately the same amount is also necessary for Sporobolomyces philippovi. In order to accumulate 150 million bacterial cells in the rhizosphere solution, we evidently require approximately 50 g of glucose or some other equivalent substance.

  Consequently, according to the data of this analysis, the roots of wheat (the vessel contained three plants) excreted in 15 days of growth about 50 mg of organic substances, utilizable by bacteria, and about 20 mg of substances utilizable by yeast.

  In experiments with corn, similar results were obtained. Substances utilizable by bacteria were excreted in a larger amount than substances suitable for the nourishment of yeast.

  It became known recently that the roots of vegetating plants excrete various enzymes into the medium. The presence of enzymes in root excretions had already been suspected when the problem of the saprophytism of higher plants and the problem of their growth and nutrition on organic media was investigated (Kamenskii, 1883; Lyubimenko, 1923, 1935; Keller, 1948).

  Eckerson (1932) has shown that plant roots are able to reduce nitrate to nitrite with the aid of excreted nitrate reductases. Thus, the roots formed up to 2 mg of nitrite nitrogen during 17 hours at 37° C. Klein and Kisser (1925), growing plants in a sterile nutrient solution, detected after some time in this solution an enzyme which reduces nitrate to, nitrite.

  Kuprevich (1949) investigated the root excretions of 23 species of plants, belonging to 16 families: oats, wheat, barley, vetch, clover, flax, heather, camomile, willow herb, dandelion, nettle, knotweed, sorrel, tea, oak, birch, poplar, willow, pine, spruce, the common brake, and others. Various enzymes were detected: catalase, tyrosinase, phenolase, asparaginase, urease, invertase, amylase, cellulase, protease, and lipase.

  The amount of excreted enzymes and their activity varies among the different species of plants. For example, the activity of amylase was expressed by indices from one to four, i. e., from barely detectable activity to the full decomposition of the substrate. Lipase was detected in traces in only four species of plants (dandelion, touch-me-not, nettle, and pine).

  We studied amylase (1952 a) in the root excretions of wheat, corn, and peas, grown under sterile conditions. It was observed that when small samples of roots were placed in a vessel with starch, the latter was comparatively quickly decomposed (Table 68). For example, 0.2 g of wheat roots decomposed 20 mg of starch in 60 minutes.

A plus stands for the presence, and a minus designates the absence of starch, plus-minus designates an undetermined reaction.

  Roots of plants grown in the field decomposed starch even more intensively. In one hour, 25 mg were decomposed by a suspension of 0.1 g of wheat roots and by a suspension of 4.5 g of corn roots.

  Starch was most quickly decomposed by roots which were not detached from the plant. Young wheat plants, extracted from the soil and washed with water, when submerged in a starch solution, decomposed 25 mg of starch in 30 minutes, while corn plants decomposed 5 mg.

  The enzyme amylase was also detected in the water in which wheat plants which were taken from the soil were immersed for some time.

  One wheat plant, two years old and grown in the field, excreted into the solution an amount of amylase which decomposed, on the average, 20-25 mg of starch in one hour at room temperature.

  It can be seen from the above that starch is most actively decomposed by the root excretions of wheat and to a lesser degree by the excretions of peas and corn.

  Ratner and Samoilova (1955) detected in the root excretions of corn and sunflower, enzymes which break down glycerophosphate and saccharose. The amount of these enzymes, according to these writers, changes with the growth phase of the plant. The maximum excretion of enzymes by corn is observed at the period preceeding flowering and during the period of the formation of the spadix (Table 69).

  The roots of these plants form enzymes which, in addition to glycerophosphate and saccharose, also split glucose phosphate and ribonucleic acid. Thus, one gram of sunflower roots splits 0.338 mg of ribonucleic acid in three hours, while one gram of corn roots splits 0.048 mg of ribonucleic acid.

  A similar picture is observed in relation to the splitting of glycerophosphate and glucose phosphate. Sunflower roots split 0.375 mg of glycerophosphate and 0.208 mg of glucose phosphate in three hours, while corn roots split 0.129 mg and 0.095 mg, respectively.

  The authors concluded that, due to the enzymatic activity of the roots, the latter can supply the plants demand for phosphorus at the expense of the organic phosphorous compounds if they are present in the medium.

  In addition to enzymes, the plants excrete into the soil a number of other biologically active compounds--various biotic substances (vitamins, auxins), toxins, etc. The amount of these substances in soils may be quite considerable.

  All these substances are sources of direct or supplementary nutrition for soil microorganisms and enhance their growth and accumulation in the soil.

  In addition to root excretion, microorganisms utilize, as sources of nutrition, dead root cells, hairs, epidermis, etc.

  The chemical composition of roots varies in different plants. The roots of some plants contain more water-soluble substances (proteins, sugars. etc), the roots of other plants contain more hemicellulose, cellulose, and lignin. Belyakova and Parishkura (1953) found the, following chemical composition of roots (Table 70).

  It is, obvious that the roots of different plants may attract different types of microflora and may be decomposed by the various species of this microflora.

  Upon the decomposition of roots of different plants by microorganisms, different products of intermediate decomposition and different products of the metabolism of the microbes themselves are formed. The latter are also sources of nutrition for other species of microorganisms and, as such, together with the decomposition of roots, attract another population of microbial biocoenoses. The shift of the microbial population continues until the organic substances of the plant, animal, and microbial residues are decomposed into their final products. These final products may be quite versatile, depending on the composition of the organic residues, on the microflora forming them, on soil and climatic conditions, and on the processes of synthesis taking place in the soil outside the cells.

  The role of roots in the life of microorganisms is not only limited to the supply of nutrient substances. Around the roots more favorable physicochemical and biological conditions for the existence of microbes, as well as for the plants themselves are created.

  In regions where there is an abundant accumulation and development of roots the physical properties of the soil improve. The soil particles have more structure in the rhizosphere of plants (see chapter on the structure of soils). With the structure of the soil particles, the respiration process of roots and microorganisms improves, moisture is better conserved, temperature is kept at a more constant level, etc.

  The soil around roots is distinguished by a higher moisture content. According to our observations, during the vegetative period, the soil in the rhizosphere of wheat under conditions of the Volga steppes had a higher (by one-two per cent) moisture content, and its moisture capacity was three to five per cent higher, than that of soils outside the root region (Krasil'nikov, 1940), This is evidently connected with the change in the structure and composition of the rhizosphere soil and with the capacity of the root systems of plants to actively change the moisture content of the surrounding soil.

  Breazeale and McGeorge (1953) have shown that when soil dries out beyond a certain threshold, the plants moisten it in the vicinity of their roots with water transported from their aerial parts. The latter utilize the atmospheric moisture.

  The authors grew tomatoes in soil which was gradually dried. When the plants began to wither, the vessels were transferred to a room with a high humidity (80-90 per cent of full humidity). The plants soon recovered, turgor was reestablished in the leaves, and the soil around the roots became more moist.

  The soil in the vicinity of roots also varies considerably with respect to acidity. Around the roots of clover, lupine, and certain other plants, the strongly acidic podsol soils became less acidic. If the control soil had a pH of 4.5. then the pH in the region of the lupine roots increased to 5-5.4. In less acidic soils, the neutralization in the zone of the roots is much more noticeable. (Table 71).

  The change which takes place in the environment of the root zone of plants was observed by Kaserer (1940) and Eklunde (1923, 1930). According to Thom and Humfield (1932), the neutralization of acidic as well as alkaline soils takes place in the root zone. For instance, acidic clay soils have a pH of 4.5 and, in the vicinity of roots--6.1. Alkaline soils of Colorado with a pH of 7.9 have a pH of 7.5 in the vicinity of the roots of cereals.

  Heller (1953) has shown that plants reduce the redox potential of the soil around the roots. This lowering of the potential, according to his data, is caused by the presence of root excretions and the microorganisms attracted by them. Intense photosynthesis of the green parts of beets lower the rH₂ value (redox potential) of the soil at a distance of one cm from the root surface. Cessation of photosynthesis is immediately accompanied by an increase of the rH₂ in the soil of the root zone. The introduction and the growth of bacteria in the zone of the roots lowers the rH₂ of the best tissues.

  The soil around the roots is richer in organic substances. As noted above, it possesses greater quantities of various products of microbial metabolism, products of the decomposition of root hairs, epidermal cells, and root excretions. In this zone, one also notices higher concentrations of enzymes, vitamins, auxins, certain amino acids and other biotic compounds,

  Nitrate nitrogen is absent from the root zone or is only present in small quantities. We analyzed the soil around roots of different plants during the whole vegetation period in the fields of the Volga area. Nitrates have only been detected in the rhizosphere during the early stages of the growth of plants and at the end of vegetation (Table 72).

  Katznelson and Richardson (1943) have found that the soil in the root area is less subject to the sterilizing effects of chemical substances. On processing soil with formalin and chloropicrin, the authors detected a much greater decrease in the number of microorganisms outside the zone of the root system. In the root region of certain plants (tomatoes and others), the microbes did not react at all to these chemicals and their number did not decrease. Living organisms, the root region and the root system of plants seem to be less accessible to chemical action.

  In our experiments, plants of corn and beans were grown under sterile and unsterile conditions, in growth containers (9 kg) filled with garden soil from the Moscow area. When the plants reached the stage of flowering or bud formation, antiseptic substances were introduced into the soil: 0.5 liter of a 30 per cent solution of formalin and two g of chloropicrin per vessel. In the sterile soil the plants perished and, in the unsterile soil, they continued to grow normally,

  It can be assumed that, in the rhizosphere of plants, a protective barrier is formed in the form of metabolic products of microbes, which are much more numerous here than outside the rhizosphere. Evidently, the chemical substances are directly decomposed in the rhizosphere by microbial organisms.

  The metabolism of microorganisms is more intense in the root region, as are many chemical and biochemical processes, as well as the transformations of various organic and mineral substances. In the rhizosphere, various minerals, rocks, limestone, marble, etc are decomposed at a faster rate. This process is not only caused by root excretions (CO₂ and other acids) but also by the microflora of the rhizosphere. The more intense the growth of microbes, the faster the decomposition process of substances. Certain compounds, for instance, tricalcium phosphate, do not dissolve in the sterile rhizosphere of plants, but when soil microbes are added to the vessel the substance becomes available to the plants (Gerretsen, 1948). One of the tasks of agricultural microbiology is the enrichment of the root region with microbes, which transform nonsoluble phosphorus compounds into the soluble compounds available to the plant.

  Under the influence of the microflora in the rhizosphere, one notices an increase in the solubility of iron and manganese compounds. According to Starkey (1955), this increase is caused by the change in the redox potential, which in quite different here than outside the rhizosphere. In the rhizosphere, iron, manganese. and other metals occur in combination with organic compounds formed by microbes. According to the author, amino acids, organic acids, and other metabolites of microorganisms form stable complex compounds, which are preserved in the soil for a long time. They are utilized by the plants and used as a source of iron, manganese, and other elements. The quantities of these organometallic compounds are greater in the rhizosphere than outside this region.

  Weinstein and others (1954) experimentally confirmed this data. They grew plants (sunflowers) in solutions both with and without the addition of microbial metabolites and they followed the absorption of the mineral salts of iron. In the presence of metabolites or ethylenediaminetetraacetic acid, the uptake of iron was faster, while in the absence of these substances and of microbes, the applied elements were not taken up by the plants. These observations showed that plants evidently take up iron, not in the form of mineral compounds, but in the form of organomineral substances formed under the influence of microorganisms.

  All the above data show that in the vicinity of the roots of vegetating plants, a special zone is formed in which more favorable conditions prevail for the existence, not only of microorganisms, but also of the plants themselves.

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