Root nodules. Nodule formations in non-legume plants

Nodules in non-legume plants. Root nodules or formations resembling nodules are widespread on the roots of not only leguminous plants. They are found in gymnosperms and angiosperms.[ ...]

It has been shown that the number of underground nitrogen-fixing root nodules in legumes (see Figure 4.3) is regulated by the photoperiod acting through the plant leaves. Nitrogen-fixing nodule bacteria need food energy for their functioning, and food is produced by plant leaves. The more light plants get and the more chlorophyll they contain, the more food bacteria can get. Thus, the photoperiod promotes maximum coordination between the activity of the plant and its partners - microorganisms.[ ...]

They are the main absorptive organs of the plant. Abundantly branched short roots often contain mycorrhiza. Podocarps have root nodules with bacteria resembling legume nodules, and such roots, with the exception of some genera, are equipped with root hairs. Root hairs in conifers are confined to such a narrow zone of the apex and fall off so easily when the root is washed that they are very often not noticed. The roots develop a multi-layered pericycle and a clearly expressed single-layered endoderm.[ ...]

The presence of a leguminous plant in the grass mixture provides only a small part of the high nitrogen requirement of cereals (because nitrogen is released from root nodules only very late and at too great a depth in the soil). It is believed that it is necessary to apply from 100 to 200 kg/ha of nitrogen in 3-4 doses, and the dose of 100 kg/ha is undoubtedly the minimum.[ ...]

The practically inexhaustible reservoir of atmospheric molecular nitrogen is inaccessible to the vast majority of living beings. Biological nitrogen fixation is carried out by a very specialized group of anaerobic bacteria that inhabit the root nodules of leguminous plants. With the help of a special enzyme, these soil microbes carry out a reaction that, in industrial nitrogen fixation, requires an expensive catalyst, a temperature of 500 ° and a pressure of up to 1000 atmospheres. A certain amount of molecular nitrogen is oxidized to N0 during lightning discharges and photochemical reactions in the atmosphere.[ ...]

There are only 3 genera in the sucker family and about 55 species distributed in Europe, Asia and North America. Lokhovye - trees and shrubs with characteristic pubescence of corymbose scales or stellate hairs. Their leaves are alternate or sometimes opposite, like those of Shepherdia (Sierrieria), on short petioles, whole and whole-crowned, evergreen or falling. All three genera are characterized by the presence of root nodules with nitrogen-fixing bacteria, due to which suckers can also grow on very poor soils.[ ...]

So, only prokaryotes, non-nuclear, the most primitive microorganisms, can convert biologically useless gaseous nitrogen into the forms necessary for the construction and maintenance of living protoplasm. When these microorganisms form mutually beneficial associations with higher plants, nitrogen fixation is greatly enhanced. The plant provides the bacteria with a suitable habitat (i.e. root nodules), protects the microbes from excess oxygen that interferes with fixation, and supplies them with the high quality energy they need. For this, the plant receives easily digestible fixed nitrogen.[ ...]

Nitrogen fertilizers have become very expensive due to declining fossil fuel production, and there has been increasing public and political concern about the possibility of chemical pollution. Consequently, attention is now focusing on nitrogen fixation as an alternative to nitrogen fertilization. The importance of nitrogen fixation to agriculture has led to intense research into bacteria capable of entering into a symbiotic relationship with leguminous plants. One of these bacteria are bacteria of the genus Rhizobium, which were isolated from the root nodules of various legumes, such as peas, lupins, clover, soybeans, and alfalfa.

Root nodules or formations resembling nodules are widespread on the roots of not only leguminous plants. They are found in gymnosperms and angiosperms.

There are up to 200 species of various plants that bind nitrogen in symbiosis with microorganisms that form nodules on their roots (or leaves).

Rice. 168. Alder nodule tissue (according to J. Becking).

Nodules of gymnosperms (orders Cycadales - cycads, Ginkgoales - ginkgo, Coniferales - conifers) have a branching coral-like, spherical or bead-like shape. They are thickened, modified lateral roots. The nature of the pathogen causing their formation has not yet been elucidated. Endophytes of gymnosperms are classified as fungi (phycomycetes), actinomycetes, bacteria, and algae. Some researchers suggest the existence of multiple symbiosis. For example, it is believed that Azotobacter, nodule bacteria and algae take part in symbiosis in cycads. Also, the question of the function of nodules in gymnosperms has not been resolved. A number of scientists are trying, first of all, to substantiate the role of nodules as nitrogen fixers. Some researchers consider podocarp nodules as reservoirs of water, and cycad nodules are often credited with the functions of aerial roots.

Rice. 169. Nodules on tribulus roots (according to E. and O. Allen).

In a number of representatives of angiosperms, dicotyledonous plants, nodules on the roots were discovered over 100 years ago.

First, let us dwell on the characteristics of the nodules of trees, shrubs and semishrubs (families Coriariaceae, Myricaceae, Betulaceae, Casuarinaceae, Elaeagnaceae, Rhamnaceae) included in this group. The nodules of most representatives of this group are coral-like clusters of pink-red color, acquiring a brown color with age. There is evidence of the presence of hemoglobin in them. In species of the genus Elaeagnus (loch) nodules are white.

Rice. 170. Nodules on the roots of the forest weisht (according to I.L. Klevenskaya).

Often nodules are large. In casuarina (Casuarina) they reach a length of 15 cm. They function for several years.

Plants with nodules are common in different climatic zones or confined to a specific area. So, Shepherdia and Ceanothus are found only in North America, Casuarina - mainly in Australia. Lokhovy and sea buckthorn are much more widespread.

Many plants of the group under consideration grow on nutrient-poor soils - sands, dunes, rocks, swamps.

The nodules of alder (Alnus), in particular A. glutinosa, discovered in the 70s of the last century by M. S. Voronin, have been studied in the most detail (Fig. 167). There is an assumption that nodules are characteristic not only of modern, but also of extinct species of alder, since they were found on the roots of fossil alder in the Tertiary deposits of the Aldana river valley - in Yakutia.

Rice. 171. Scheme of the structure of the nodule of the forest reed grass: 1 - bark, 2 - bark parenchyma; h - pericyclic parenchyma; 4 - vascular bundle; 5 - endoderm; 6 - bacteria (according to I. L. Klevenskaya).

Endophyte in nodules is polymorphic. It usually occurs as hyphae, vesicles, and bacteroids (Fig. 168). The taxonomic position of the endophyte has not yet been established, since numerous attempts to isolate it into a pure culture turned out to be fruitless, and if it was possible to isolate the cultures, they turned out to be non-virulent.

The main significance of this entire group of plants, apparently, lies in the ability to fix molecular nitrogen in symbiosis with the endophyte. Growing in areas where the cultivation of agricultural plants is not economically rational, they play the role of pioneers in the development of the land. Thus, the annual increase in nitrogen in the soil of the dunes of Ireland (Cape Verde) under plantings of Casuarina equi-setifolia reaches 140 kg/ha. The content of nitrogen in the soil under alder is 30-50% higher than under birch, pine, and willow. In the dried leaves of alder, nitrogen is twice as much as in the leaves of other woody plants. According to the calculations of A. Virtanen (1962), an alder grove (an average of 5 plants per 1 m 2) gives an increase in nitrogen of 700 kg/ha in 7 years.

Nodules are much less common in representatives of the Zygophyllaceae family (parnophyllous). They were first discovered by B. L. Isachenko (1913) on the root system of Tribulus terrestris. Later nodules were also found in other anchor species.

Most members of the Zygophyllaceae family are xerophytic shrubs or perennial herbs. They are common in the deserts of tropical and subtropical regions, and grow on sand dunes, wastelands and temperate swamps.

It is interesting to note that tropical plants such as the bright red parophyllum form nodules only at high temperatures and low soil moisture. Soil moisture up to 80% of the total moisture capacity prevents the formation of nodules. As is known, the reverse phenomenon is observed in leguminous plants of a temperate climate. With insufficient moisture, they do not form nodules.

The nodules in plants of the Parnolistaceae family differ in size and location on the root system. Large nodules usually develop on the main root and close to the soil surface. Smaller ones are found on lateral roots and at greater depths. Sometimes nodules form on stems if they lie on the soil surface.

The nodules of terrestrial tribulus on the sands along the Southern Bug look like small white, slightly pointed or round warts.

They are usually covered with a plexus of fungal hyphae penetrating into the root bark.

In the bright red parnolistnik, the nodules are the terminal thickenings of the lateral roots of plants. Bacteroids are found in nodules; bacteria are very similar to root nodules.

Nodules of tropical plants Tribulus cistoides are hard, rounded, about 1 mm in diameter, connected to the roots by a wide base, often whorled on old roots. More often located on the roots, alternating, on one or both sides (Fig. 169). Nodules are characterized by the absence of a meristem zone. A similar phenomenon is noted during the formation of nodules in coniferous plants. The nodule therefore arises due to cell division of the pericycle of the stele.

Histological study of nodules of Tribulus cistoides at different stages of development showed that they lack microorganisms. Based on this, as well as the accumulation of large amounts of starch in the nodules, they are considered formations that perform the function of providing plants with reserve nutrients.

The nodules of the forest reedweed are spherical or somewhat elongated formations up to 4 mm in diameter, tightly seated on the roots of plants (Fig. 170). The color of young nodules is most often white, occasionally pinkish, old -yellow and brown. The nodule is connected with the central cylinder by a wide vascular bundle. Just like in Tribulus cistoides, reed nodules have bark, bark parenchyma, endoderm, pericyclic parenchyma and vascular bundle (Fig. 171).

Bacteria in nodules of wood reedweed are very reminiscent of root nodule bacteria of leguminous plants.

Nodules are found on the roots of cabbage and radish - representatives of the cruciferous family. It is assumed that they are formed by bacteria that have the ability to bind molecular nitrogen.

Among plants of the madder family, nodules are found in coffee plants Coffea robusta and Goffea klainii. They branch dichotomously, sometimes flattened and look like a fan. Bacteria and bacteroid cells are found in the tissues of the nodule. Bacteria, according to Steyart (1932), belong to Rhizobium, but they named them Bacillus coffeicola.

Nodules in plants of the rose family were found on the dryad (partridge grass). Two other members of this family, Purshia tridentata and Cercocarpus betuloides, have described typical coral nodules. However, there are no data on the structure of these nodules and the nature of their pathogen in the literature.

Of the heather family, only one plant can be mentioned - the bear's ear (or bearberry), which has nodules on the root system. Many authors believe that these are coral-like ectotrophic mycorrhiza.

In angiosperms monocotyledonous plants, nodules are common among representatives of the cereal family: meadow foxtail, meadow bluegrass, Siberian hairweed and saline hairweed. Nodules are formed at the ends of the roots; are oblong, rounded, fusiform. In the foxtail, young nodules are light, transparent or translucent, becoming brown or black with age. Data on the presence of bacteria in nodule cells are contradictory.

Leaf nodules. Over 400 species of various plants are known to form nodules on leaves. The nodules of Pavetta and Psychotria have been studied the most. They are located on the lower surface of the leaves along the main vein or scattered between the lateral veins, have an intense green color. Chloroplasts and tannin are concentrated in nodules. With aging, cracks often appear on the nodules.

The formed nodule is filled with bacteria that infect the leaves of the plant, apparently at the time of seed germination. When growing sterile seeds, nodules do not appear and the plants develop chlorotic. Bacteria isolated from the leaf nodules of Psychotria bacteriophyla turned out to belong to the genus Klebsiella (K. rubiacearum). Bacteria fix nitrogen not only in symbiosis, but also in pure culture - up to 25 mg of nitrogen per 1 g of sugar used. It must be assumed that they play an important role in the nitrogen nutrition of plants on infertile soils. There is reason to believe that they supply plants not only with nitrogen, but also with biologically active substances.

Sometimes glossy films or multi-colored spots can be seen on the surface of the leaves. They are formed by microorganisms of the phyllosphere - a special kind of epiphytic microorganisms, which are also involved in the nitrogen nutrition of plants. The bacteria of the phyllosphere are predominantly oligonitrophils (they live on negligible impurities of nitrogen-containing compounds in the medium and, as a rule, fix small amounts of molecular nitrogen), which are in close contact with the plant.

Free-living nitrogen-fixing microorganisms. Azotobacter (AZOTOBACTER)

In 1901, Beijerinck isolated an aerobic, non-spore-forming gram-negative bacterium that fixes molecular nitrogen from the soil and named it Azotobacter chroococcum (the generic name reflects the ability of the bacterium to fix nitrogen, while the species name reflects the ability to synthesize a brown pigment - chroo and form coccoid cells - coccum). Azotobacter is a typical representative of free-living microorganisms. Free-living - these are all those microorganisms that live in the soil, regardless of whether the plant develops near or not.

Rice. 173. Dividing cells of Azotobacter (A. agilis), peritrichial flagella are visible (7), in A. macrocyto-genes polar flagella (?) are visible (according to A. Bayle et al.) -

Azotobacter cultures in the laboratory are characterized by polymorphism. Cells of different types of Azotobacter at a young age are shown in Figure 172. Young cells of Azotobacter are mobile; they have numerous or single flagella (Fig. 173). Azotobacter has pili-like outgrowths (Fig. 174). In old cultures, Azotobacter cells are covered with a dense membrane, forming cysts. They can germinate, giving rise to young cells (Fig. 175).

Azotobacter polymorphism depends to a large extent on the composition of the medium on which it is grown. On a medium with ethyl alcohol (as the sole source of carbon), Azotobacter retains its mobility and rod shape for a long time. At the same time, polymorphism manifests itself very sharply in many other environments.

Rice. 174. Fimbra-like formations in Azotobacter cells. Increased X 24,000. (According to E. V. Boltyanskaya.)

On dense nutrient media that do not contain nitrogen, Azotobacter forms large slimy, sometimes wrinkled colonies (Fig. 176), which turn yellowish-greenish, pink or brown-black with aging. Colonies of different species of Azotobacter have their own specific pigmentation.

To date, a number of species of Azotobacter are known: Azotobacter chroococcum, Az. beijerinckii, Az. vinelandii, Az. agilis, Az. nigricans, Az. galophilum.

Rice. 175. Azotobacter cysts (according to I. Chan and others). A mature cyst filled with fat granules and surrounded by a thick hard shell (right) and a germinating cyst (a growing young cell ruptures the cyst shell - left). Increased X 35,000.

A variety of mineral (ammonium salts, nitric and nitrous acids) and organic (urea, various amino acids) compounds can serve as a source of nitrogen for Azotobacter. However, if Azotobacter develops only at the expense of nitrogen bound in the environment, it does not fulfill its main function - the fixation of molecular nitrogen. Azotobacter usually fixes up to 10-15 mg of molecular nitrogen per 1 g of carbon source used (eg glucose, sucrose). This value varies greatly depending on the growing conditions of the culture, the composition of the nutrient medium, its acidity, temperature, and aeration.

In relation to carbon sources, VL Omelyansky (1923) called Azotobacter a polyphage (“omnivore”).

Rice. 176. Development of A. chroococcum colonies around soil lumps in a nitrogen-free environment.

Azotobacter assimilates a variety of carbohydrates (mono- and disaccharides, some polysaccharides), organic acids, polyhydric alcohols (glycerol, mannitol) and other substances well.

Many researchers have been able to grow Azotobacter in cups with a nutrient medium without nitrogen and carbon, if the cups were placed in a chamber containing vapors of acetone, ethyl alcohol, or some other organic compounds. In the presence of readily available forms of carbon-containing compounds, Azotobacter can partially use carbon dioxide from the atmosphere. Increasing the concentration of carbon dioxide to 0.5% in the air somewhat stimulates the development of Azotobacter. But easily accessible forms of carbon-containing organic compounds are better absorbed by Azotobacter. The stock of mobile organic matter in the soil is small; therefore, it is the lack of readily available carbon compounds that primarily limits the development of azotobacter in natural conditions.

Rice. 177. Beyerinkia colonies of different types (smooth and folded variants): 1-4, 6-8, 10 - according to N. I. Gogorikidze; 5, 9, 11 - according to J. Becking.

What organic compounds can Azotobacter use in the soil? The humus substances of the soil are practically not absorbed by Azotobacter. Therefore, in soils, even very rich in humus, in the absence of fresh organic residues, intensive reproduction of Azotobacter does not occur.

However, if there are organic compounds and decay products of plant and animal cells in the soil, Azotobacter develops well. In particular, it multiplies intensively in soils fertilized with straw and straw manure, as well as in various composts containing cellulose. Azotobacter well assimilates substances formed during the breakdown of cellulose.

The development of Azotobacter and its fixation of nitrogen largely depend on the presence of phosphorus in the environment. Both organic and mineral phosphorus-containing compounds can serve as a source of phosphorus. The high sensitivity of Azotobacter to phosphorus made it possible to develop a microbiological method for determining the need for soils in phosphate fertilizers.

Azotobacter is used as a test organism in this method. Microbiological methods for determining the need for soil fertilizer have a number of advantages over chemical analyzes, although, of course, they are inferior in accuracy.

Calcium plays an important role in the metabolism of Azotobacter. This element is necessary for Azotobacter when fed with both molecular and ammonium nitrogen (G.N. Zaitseva, 1965). The lack of calcium in the medium leads to strong vacuolization of cells and their swelling.

The high sensitivity of Azotobacter to calcium, as well as to phosphorus, is used to determine the need for liming in soils.

Trace elements (molybdenum, boron, vanadium, iron, manganese) are necessary for Azotobacter primarily for the implementation of the process of nitrogen fixation. The need for trace elements is determined to a large extent by the geochemical conditions for the existence of Azotobacter in soils. Microorganism strains isolated from soils with a high natural content of one or another microelement require, as a rule, higher concentrations of these elements.

Interestingly, radioactive elements (radium, thorium, uranium) have a stimulating effect on the development of Azotobacter and the process of nitrogen fixation.

Azotobacter is extremely sensitive to environmental reactions. The optimal pH range for its development is 7.2-8.2. However, Azotobacter is able to develop on media with a pH of 4.5 to 9.0; the acid reaction of the environment adversely affects its development. From acidic soils, inactive forms of Azotobacter are isolated, which have lost the ability to bind molecular nitrogen.

Soil moisture has a great influence on the development of Azotobacter. Azotobacter cells have a lower osmotic pressure than the cells of fungi and actinomycetes; the need for moisture is similar to the need for higher plants. Azotobacter is common in fresh water bodies, silts, flooded rice fields, sewage, highly moistened soils, on aquatic plants in ponds and reservoirs. This indicates its high degree of hydrophilicity. Based on the high moisture requirement of soil forms of Azotobacter, it is assumed that the ancestors of some marine and soil species of Azotobacter could be common.

With regard to temperature, Azotobacter is a typical mesophilic organism, with an optimum development of about 25-30 °C. Azotobacter tolerates a decrease in temperature well, therefore, even in northern latitudes, the number of its cells in the soil does not noticeably decrease in winter.

Of the biological factors influencing the development of Azotobacter, soil microorganisms should be noted first of all. They can influence the vital activity of Azotobacter in the soil indirectly by changing, for example, pH or redox conditions, and directly by producing nutrients and biologically active substances. Thus, the activating effect of cellulose-destroying and butyric microorganisms on the development of Azotobacter and its antagonistic relationship with representatives of the soil microflora was noted by many Soviet and foreign researchers. The biocenosis of microorganisms, which is formed under the conditions of a particular soil, changes to a large extent under the influence of the vegetation cover. And Azotobacter as a member of the biocenosis also depends on this factor. It has been established using the method of autoradiography that when phosphorus-labeled Azotobacter cells are applied to seeds of cereal crops, the cells usually concentrate around the growing root system of seedlings.

There is, however, evidence that there are very few Azotobacter cells in the plant rhizosphere. In the best case (with the complete absence of antagonists and favorable environmental conditions), their number does not exceed 1% of the total number of rhizosphere microflora.

Azotobacter cultures, as a rule, form a significant amount of biologically active substances: B vitamins, nicotinic and pantothenic acids, biotin, heteroauxin and gibberellin. However, despite the fact that cultures of Azotobacter produce a whole series of biologically active substances, the addition of vitamins, gibberellin and heteroauxin to the medium accelerates the growth of Azotobacter. The reaction to the additional introduction of vitamins into the medium is an individual feature of the strains.

Azotobacter can produce growth substances such as auxins. This is confirmed by experiments in which the formation of additional roots in bean cuttings under the influence of auxins produced by Azotobacter was established. A biological test - a dwarf form of Pioneer peas - makes it possible to determine gibberellin-like compounds in azotobacter culture.

All these compounds together are capable of stimulating the germination of plant seeds and accelerating their growth in those cases, of course, when there are a sufficient number of Azotobacter cells on the plant root system.

In addition, the antagonistic activity of Azotobacter against pathogens of bacterial plant diseases was found. Azotobacter synthesizes a fungistatic (delaying the development of fungi) antibiotic of the anisomycin group. A number of fungal organisms found on seeds and in soil (species from the genera Fusarium, Alternaria, Penicillium) can inhibit the development of many plant species, especially in cold weather. Azotobacter, by producing antifungal antibiotic substances, helps plants grow and develop, which is especially important in the early phases of development.

Unfortunately, the ability of azotobacteria to propagate in the soil and to show multifaceted qualities is very limited due to the lack of readily available organic substances in the soil and the high demands of the world organism on environmental conditions. Therefore, the stimulating effect of Azotobacter is manifested only on fertile soils.

The distribution of Azotobacter in the soils of the Soviet Union has certain regularities. In virgin podzols and soddy-podzolic soils characterized by an acidic reaction, the conditions for the development of azotobacter are unfavorable. Only the cultivation of such soils creates opportunities for its development. In soils with increased moisture and a predominance of meadow vegetation (floodplain soils), Azotobacter is usually found in large quantities during the entire growing season. In peatlands Azotobacter is either absent or develops very weakly. Azotobacter develops well in the zone of sufficiently moist northern thick chernozems, and in the zone of ordinary and southern chernozems in the absence of irrigation, as well as in virgin and non-irrigated cultivated chestnut soils, only as a spring ephemeral. The maximum development of Azotobacter in the spring period is observed both in virgin and rainfed soils of the serozem zone. Predominantly salt-resistant races of Azotobacter are common in solonetzes and solonchaks. Basically, Az dominates in the soils of our country. chroococcum.

Beyerinkia (BEIJERINCKIA)

For the first time, aerobic bacteria of the genus Beijerinckia were isolated from the acidic soils of rice fields in India (in 1939). G. Derks (1950), having discovered this bacterium in the soil of the Botanical Garden in Bogor (Java), proposed to name it after M. Beijerinck, one of the first researchers of nitrogen fixers.

Cells of bacteria of the genus Beijerinckia are round, oval or rod-shaped; sticks are sometimes curved. The size of young cells is 0.5-2.0 X 1.0-4.5 microns. There are mobile and fixed forms. Cysts and spores do not form. Cultures are characterized by slow growth. Typical colonies usually form after 3 weeks at 30°C. Most cultures of Beijerinckia form convex, often folded, shiny mucous colonies of a very viscous consistency on nitrogen-free agar with glucose (Fig. 177). As cultures age, they tend to form a dark-colored pigment.

Organisms of the genus Beijerinckia fix 16-20 mg of molecular nitrogen per 1 g of used energy material. The range of carbon-containing compounds available to Beyerinkia is much narrower than that of Azotobacter. Mono- and disaccharides are well used, worse - starch, organic acids, aromatic substances are not absorbed. Bacteria of the genus Beijerinckia prefer mineral nitrogen and many amino acids to molecular nitrogen.

The main differences between Beyerinkia and Azotobacter are high acid resistance (they can grow even at pH 3.0), calcephobicity (negligible doses of calcium inhibit growth), and resistance to high concentrations of iron and aluminum.

Bacteria of the genus Beijerinckia are widespread in the soils of the southern and tropical zones, and are less common in the temperate zone. Beijerinckia is often found on the leaf surface of tropical plants in Indonesia.

Previously, it was believed that bacteria of the genus Beijerinckia could only exist in acidic soils. It has now been established that they develop well in neutral and alkaline soils. Nevertheless, it should be assumed that Beijerinckia play a significant role in the nitrogen balance of mainly acidic soils (laterites, krasnozems), having no significant agronomic significance for neutral soils.

Clostridium (CLOSTRIDIUM)

The first anaerobic microorganism that absorbs molecular nitrogen was isolated and described by S. N. Vinogradsky in 1893. It turned out to be a spore-forming bacterium, which was given the name Clostridium pasteurianum (the generic name comes from the Latin word clostrum - spindle; specific - pasteurianum - given in honor Louis Pasteur).

Cells Cl. pasteurianum are large, their length is 2.5-7.5 microns, their width is 0.7-1.3 microns. They are located singly, in pairs or form short chains. Young cells are motile, have peritrichous flagella, their plasma is homogeneous. As the cell ages, the plasma becomes granular and accumulates granulosa (a starch-like substance). In the center of the cell or closer to its end, a spore is formed, which is much wider in diameter than the vegetative cell, and therefore the cell takes the form of a spindle during this period. The spore size is 1.3 x 1.6 µm. Figure 178 shows the cells of Cl. pasteurianum with spores. The spore morphology and the behavior of the nuclear substance during spore formation in Clostridium are described in detail on p. 228 by V. I. Duda.

Rice. 178. Cells of Clostridium pasteurianum with spores. Increased X 3500 (according to V.I. Duda).

The nitrogen-fixing function was found in many representatives of the genus Clostridium: Cl. pasteurianum, Cl. butyricum, Cl. butylicum, Cl. beijerinckia, Cl. pectinovorum, Cl. acetobutyli-cum and other species. The most energetic nitrogen accumulator is Cl. pasteurianum - fixes 5-10 mg of nitrogen per 1 g of carbon source consumed.

Along with molecular nitrogen, bacteria of the genus Clostridium well assimilate mineral and organic nitrogen-containing compounds. Bacteria of the genus Clostridium use various compounds as a source of carbon nutrition, which usually serve as an energy source for them at the same time. They are much less sensitive to phosphorus, potassium and calcium than Azotobacter. However, soil fertilization with phosphorus-potassium salts, liming of soils or composts always leads to an increase in abundance.

Clostridia are relatively resistant to acid and alkaline environments. The pH range at which their development proceeds normally is quite wide; the minimum pH value is below 4.5, the maximum is above 8.5.

The influence of the air-water regime on the development of bacteria of the genus Clostridium has been studied quite fully. Being anaerobic, they tolerate high soil moisture saturation well. However, the optimal degree of moisture for them is determined by the type of soil and the availability of organic matter. Clostridia develops best when the soil moisture is 60-80% of the total moisture capacity.

Table 46. Bacteriosis of oats and Sudan grass: 1 - brown (red) bacteriosis of oats; 2 - bacteriosis of Sudan grass.

Most bacteria of the genus Clostridium are in the upper layers of the soil, which are rich in organic matter.

Bacteria of the genus Clostridium but differently related to temperature, are found as mesophilic. and thermophilic bacteria. Molecular nitrogen is fixed only by mesophiles.

In mesophilic forms, the optimal development temperature is most often in the range of 25-30 °C. The maximum temperature limit is 37-45 °C.

Table 47. Diseases of cotton, tobacco and beets: 1 - cotton blight; 2 - bacterial grouse tobacco; 3 - silver beet disease (on the right - spots under magnification).

Clostridial spores are very resistant to high temperatures. They withstand heating at 75 °C for 5 hours and for 1 hour heating at 80 °C. The spores of thermophilic clostridia die when boiled after 30 mcn. Higher temperatures (110°C) quickly kill them.

With many microorganisms in the soil, Clostridium is in a metabolic relationship, in which the exchange of metabolic products is expected. Thus, Azotobacter improves the living conditions of Clostridium by absorbing oxygen, and Clostridium produces organic acids from organic compounds that are inaccessible to Azotobacter, which Azotobacter can assimilate.

It would be difficult to answer the question: what soils do not have Clostridium? The "omnivorous" Clostridium, low exactingness to environmental conditions, as well as the ability to go into a state of spores under adverse conditions explain their wide, almost ubiquitous distribution.

The accumulation of nitrogen in soils due to the activity of Clostridium, however, is small and, as a rule, does not exceed a few kilograms per hectare of soil.

Nodules and mycorrhiza on roots

In the roots of plants of the legume family, special bacteria settle, penetrating there from the soil through root hairs. They cause increased reproduction of parenchymal cells and an increase in the size of the peripheral part of the root. As a result of this, growths are formed on the root - tumors, or nodules, which are clearly visible to the naked eye. Bacteria live in their cells. They have the ability to assimilate free nitrogen from the air located in the intercellular spaces of nodules and penetrating there from the soil. Other plants do not have this ability to assimilate free nitrogen, and all the vast reserves of atmospheric nitrogen are inaccessible to them. Part of the bacteria dies and is absorbed by leguminous plants, therefore, legumes indirectly feed on atmospheric nitrogen, which for others

plants are not available. In addition, from the roots of leguminous plants, even during the growing season, part of the nitrogenous compounds is released into the soil, where they are absorbed by other plants growing together with them. After harvesting, part of the atmospheric nitrogen bound by nodule bacteria remains in the soil in the root system of legumes; after decay, its nitrogen in the form of mineral salts remains in the soil, which is thus enriched with nitrogen compounds available to other plants. Cereals sown after legumes give a significant increase in yield compared to those sown on soil that was not under legumes; sometimes this increase reaches 100% or even more. Therefore, legumes must be introduced into properly constructed crop rotations.

1 - without mycorrhiza; 2 - with mycorrhiza.

Nodule bacteria belong to the same species Bacterium radicicola. It breaks up into several races, each of which has adapted to a certain group of leguminous plants. Therefore, when a leguminous plant new to a given area is introduced into the culture and the necessary race of nodule bacteria is absent in the soil, it is recommended to introduce into the soil along with the seeds a specially prepared bacterial preparation nitragin of this race of nodule bacteria. A similar introduction of nitragin into the soil was carried out in our USSR when soybean was introduced into the culture in areas where it had not been bred before.

Fungi settle on the roots of many woody and herbaceous plants, forming the so-called mycorrhiza. There are two types of mycorrhiza: endotrophic and ectotrophic.

With endotrophic mycorrhiza, the vegetative body of the fungus, consisting of microscopically small filaments, the so-called hyphae, is located mainly inside the cells of the parenchymal tissue of the root, and only a few hyphae emerge from the root out into the soil. At the same time, no noticeable changes were observed in the external structure of the roots. Inside the root cells, a gradual destruction of part of the fungal hyphae and the assimilation of their contents ("digestion") by the root cells are usually observed.

Endotrophic mycorrhiza occurs, for example, in all representatives of the heather and orchid families, as well as in many other plants from various families.

In plants with ectotrophic mycorrhiza, the hyphae of the fungus wrap around part of the short young lateral roots on the outside, forming a rather dense sheath around them. The longer roots, from which these lateral ones depart, are not entwined with fungal hyphae, they continue to grow in length and provide an increasing penetration of the root system into the soil. The lateral roots, on which mycorrhiza has formed, stop growing in length and begin to branch, sometimes more or less forked, forming characteristic coral-like clustered branches (Fig. 204-205). In these mycorrhizal roots, the root cap is absent or very poorly developed; there are no root hairs either, and their functions are performed by the hyphae of the fungus, extending from the mushroom cap around the roots and penetrating the soil. On the other hand, part of the fungal hyphae extends from the fungal cap into the root; partially dissolving intercellular pectin plates, hyphae penetrate between the outer cells of the primary root cortex, forming here a characteristic mesh arrangement; from them, in turn, thin ramifications penetrate into the parenchymal cells of the cortex and are subsequently partially dissolved and "digested" by them. Thus, ectotrophic mycorrhiza is not entirely external, and is often called ectoendotrophic.

at the end - growth ending, from the sides - roots with mycorrhiza and mycelial strands.

The anatomical structure of roots with ectoendotrophic mycorrhiza also differs from nonmycorrhizal roots. In addition to the already indicated absence of root hairs, the absence or very weak development of the root cap, they do not have a secondary thickening, the primary cortex is not shed, and its cells increase somewhat in size (Fig. 206, 207).

a- mycorrhizal cover; b- collapsing outer root cells with tannins; in- apical meristem; G- exoderm.

Ectoendotrophic mycorrhizae develop in most of our woody plants - conifers and deciduous. The mushrooms that form them belong to ordinary forest cap mushrooms and are very diverse - boletus, boletus, mushrooms, boletus, butter, russula, fly agaric, etc. Great specialization of certain types of mushrooms, i.e., their confinement to one particular type of tree, in most fungi do not seem to exist. Outside the forest, fungi that form mycorrhiza do not grow, and, therefore, cohabitation with roots is necessary for them. Apparently, they obtain nitrogen-free organic compounds from the roots - carbohydrates, which are abundant in the form of easily digestible compounds in photosynthetic green plants. On the other hand, numerous observations indicate that woody plants also die or grow poorly if they do not form mycorrhiza on their roots. It can be assumed that the significance of mycorrhiza fungi for higher plants is versatile. They supply them with water and mineral salts, and the absorption capacity of the roots is significantly increased due to the branching of mycorrhizal roots, as well as the strong branching of fungal hyphae in the soil. The use of labeled atoms proved the entry of phosphorus and nitrogen into the roots through mycorrhiza. In addition, fungi assimilate complex organic nitrogenous compounds of the soil, which are directly inaccessible to the higher plant; dissolving then partially in the cells of the root and assimilated by the latter, the fungi enable the higher plant to assimilate the otherwise inaccessible organic compounds of the soil. Finally, it is quite possible that mycorrhiza fungi supply the higher plants with vitamins and some growth-stimulating substances. The method of nutrition of higher plants with the participation of mycorrhiza fungi is called mycotrophic.

The relationship between the fungus and the higher plant in mycorrhiza is unlikely to be in the nature of harmonic symbiosis, which, in general, probably does not exist in nature. In the first stages of mycorrhiza formation in the past and at the beginning of their ontogeny in the present, the fungus, perhaps, first assimilates some root secretions, then, penetrating into the root, behaves like

An in-depth study of the mycorrhiza of tree species is of great importance here in the USSR in creating field-protective afforestations where forests did not grow before.

Root nodules or formations resembling nodules are widespread on the roots of not only leguminous plants. They are found in gymnosperms and angiosperms. There are up to 200 species of various plants that bind nitrogen in symbiosis with microorganisms that form nodules on their roots (or leaves). Nodules of gymnosperms (orders Cycadales - cycads, Ginkgoales - ginkgoes, Coniferales - conifers) have a branching coral-like, spherical or bead-like shape. They are thickened, modified lateral roots. The nature of the pathogen causing their formation has not yet been elucidated. Endophytes of gymnosperms are classified as fungi (phycomycetes), actinomycetes, bacteria, and algae. Some researchers suggest the existence of multiple symbiosis. For example, it is believed that Azotobacter, nodule bacteria and algae take part in symbiosis in cycads. Also, the question of the function of nodules in gymnosperms has not been resolved. A number of scientists are trying, first of all, to substantiate the role of nodules as nitrogen fixers. Some researchers consider podocarp nodules as reservoirs of water, and cycad nodules are often credited with the functions of aerial roots. In a number of representatives of angiosperms, dicotyledonous plants, nodules on the roots were discovered over 100 years ago.

In the literature, there is a characteristic of nodules of trees, shrubs and subshrubs (families Coriariaceae, Myricaceae, Betulaceae, Casuarinaceae, Elaeagnaceae, Rhamnaceae) included in this group. The nodules of most representatives of this group are coral-like clusters of pink-red color, acquiring a brown color with age. There is evidence of the presence of hemoglobin in them. In species of the genus Elaeagnus (loch) nodules are white. Often nodules are large. In casuarina (Casuarina) they reach a length of 15 cm. They function for several years. Plants with nodules are common in different climatic zones or confined to a specific area. So, Shepherdia and Ceanothus are found only in North America, Casuarina - mainly in Australia. Lochaceae and sea buckthorn are much more widespread.

Many plants of the group under consideration grow on nutrient-poor soils - sands, dunes, rocks, swamps. Nodules of alder (Alnus), in particular A. glutinosa, discovered in the 70s of the last century by M. S. Voronin, have been studied in the most detail. There is an assumption that nodules are characteristic not only of modern, but also of extinct species of alder, since they were found on the roots of fossil alder in the Tertiary deposits of the Aldana river valley - in Yakutia.

Endophyte in nodules is polymorphic. It is commonly found as hyphae, vesicles, and bacteroids. The taxonomic position of the endophyte has not yet been established, since numerous attempts to isolate it into a pure culture have been fruitless, and if it has been possible to isolate cultures, they have turned out to be non-virulent.

The main significance of this entire group of plants, apparently, lies in the ability to fix molecular nitrogen in symbiosis with the endophyte. Growing in areas where the cultivation of agricultural plants is not economically rational, they play the role of pioneers in the development of the land. Thus, the annual increase in nitrogen in the soil of the dunes of Ireland (Cape Verde) under plantings of Casuarina equisetifolia reaches 140 kg/ha. The content of nitrogen in the soil under alder is 30-50% higher than under birch, pine, and willow. In the dried leaves of alder, nitrogen is twice as much as in the leaves of other woody plants. According to the calculations of scientists, an alder grove (an average of 5 plants per 1 m 2) gives an increase in nitrogen of 700 kg / ha in 7 years.

Nodules are much less common in representatives of the Zygophyllaceae family (parnophyllous). They were first found on the root system of Tribulus terrestris. Later, nodules were found in other species of Tribulus.

Most members of the Zygophyllaceae family are xerophytic shrubs or perennial herbs. They are common in the deserts of tropical and subtropical regions, and grow on sand dunes, wastelands and temperate swamps.

It is interesting to note that tropical plants such as the bright red parophyllum form nodules only at high temperatures and low soil moisture. Soil moisture up to 80% of the total moisture capacity prevents the formation of nodules. As is known, the reverse phenomenon is observed in leguminous plants of a temperate climate. With insufficient moisture, they do not form nodules. Nodules in plants of the parnophyllous family differ in size and location on the root system. Large nodules usually develop on the main root and close to the soil surface. Smaller ones are found on lateral roots and at greater depths. Sometimes nodules form on stems if they lie on the soil surface.

The nodules of terrestrial tribulus on the sands along the Southern Bug look like small white, slightly pointed or round warts. They are usually covered with a plexus of fungal hyphae penetrating into the root bark.

In the bright red parnolistnik, the nodules are the terminal thickenings of the lateral roots of plants. Bacteroids are found in nodules; bacteria are very similar to root nodules.

Nodules of tropical plants Tribulus cistoides are hard, rounded, about 1 mm in diameter, connected to the roots by a wide base, often whorled on old roots. More often located on the roots, alternating, on one or both sides. Nodules are characterized by the absence of a meristem zone. A similar phenomenon is observed during the formation of nodules in coniferous plants. The nodule therefore arises due to cell division of the pericycle of the stele.

Histological study of nodules of Tribulus cistoides at different stages of development showed that they lack microorganisms. Based on this, as well as the accumulation of large amounts of starch in the nodules, they are considered formations that perform the function of providing plants with reserve nutrients.

Reed nodules are spherical or somewhat elongated formations up to 4 mm in diameter, tightly seated on the roots of plants. The color of young nodules is most often white, occasionally pinkish, old - yellow and brown. The nodule is connected with the central cylinder of the root by a wide vascular bundle. Like Tribulus cistoides, reed nodules have bark, core parenchyma, endoderm, pericyclic parenchyma, and vascular bundle. Bacteria in nodules of wood reedweed are very reminiscent of root nodule bacteria of leguminous plants. Nodules are found on the roots of cabbage and radish - representatives of the cruciferous family. It is assumed that they are formed by bacteria that have the ability to bind molecular nitrogen.

Among plants of the madder family, nodules are found in coffee Coffea robusta and Coffea klainii. They branch dichotomously, sometimes flattened and look like a fan. Bacteria and bacteroid cells are found in the tissues of the nodule. Bacteria, according to Steyart, belong to Rhizobium, but he named them Bacillus coffeicola.

Nodules in plants of the rose family were found on the dryad (partridge grass). Two other members of this family, Purshia tridentata and Cercocarpus betuloides, have described typical coral nodules. However, there are no data on the structure of these nodules and the nature of their pathogen in the literature.

Of the heather family, only one plant can be mentioned - the bear's ear (or bearberry), which has nodules on the root system. Many authors believe that these are coral-like ectotrophic mycorrhiza. In angiosperms monocotyledonous plants, nodules are common among representatives of the cereal family: meadow foxtail, meadow bluegrass, Siberian hairweed and saline hairweed. Nodules are formed at the ends of the roots; are oblong, rounded, fusiform. In the foxtail, young nodules are light, transparent or translucent, becoming brown or black with age. Data on the presence of bacteria in nodule cells are contradictory.

Leaf nodules. Over 400 species of various plants are known to form nodules on leaves. The nodules of Pavetta and Psychotria have been studied the most. They are located on the lower surface of the leaves along the main vein or scattered between the lateral veins, have an intense green color. Chloroplasts and tannin are concentrated in nodules. With aging, cracks often appear on the nodules. The formed nodule is filled with bacteria that infect the leaves of the plant, apparently at the time of seed germination. When sterile seeds are grown, nodules do not appear and the plants develop chlorotic. Bacteria isolated from the leaf nodules of Psychotria bacteriophyla turned out to belong to the genus Klebsiella (K. rubiacearum). Bacteria fix nitrogen not only in symbiosis, but also in pure culture - up to 25 mg of nitrogen per 1 g of sugar used. It must be assumed that they play an important role in the nitrogen nutrition of plants on infertile soils. There is reason to believe that they supply plants not only with nitrogen, but also with biologically active substances.

Sometimes glossy films or multi-colored spots can be seen on the surface of the leaves. They are formed by microorganisms of the phyllosphere - a special kind of epiphytic microorganisms, which are also involved in the nitrogen nutrition of plants. The bacteria of the phyllosphere are predominantly oligonitrophils (they live on negligible impurities of nitrogen-containing compounds in the medium and, as a rule, fix small amounts of molecular nitrogen), which are in close contact with the plant.

"Types of roots and root systems" - Types of roots. Solving cognitive problems. The root is the vegetative organ of a plant. Chicory. Generalization of the studied material. Living specimens of plants with different root systems. The first page of the Oral Journal. main root. Laboratory work. During the classes. What other plant organs are vegetative. Answer the questions. What is the function of the root. Root. Houseplant in a flower pot.

"Organ of plants root" - Root. variety of roots. Root structure. root system. Human influence on root systems. Root tubers (root cones). Functions. root pressure. Mycorrhiza. Root crop. Root respiration. bacterial nodules. Types of roots. Content. root zones. Root growth. Mineral nutrition.

"The structure and functions of the root" - Functions of the root. Abode. Development of the root system. Root cap. Deposition and accumulation of spare nutrients. Mineral nutrition of plants. Spine. Anchoring and holding the plant in the soil. Types of root systems. Rod and fibrous root systems. Root modifications. Root. Types of roots. Root idea. The main organ of the plant. root zones. Development of the germinal root.

"Types of root systems" - Types of roots. Study. Type of root systems. The structure of seeds. root zones. Root cap. One of the important vegetative organs. The study of the structure. adventitious roots. An excerpt from the fable of I. Krylov.

"Root and root system" - Nourishing. Rod root system. Lesson topic: Types of roots. Beans and dandelions? Beans. Support. Fibrous root system. We will find out what kind of roots a plant has, get acquainted with various root systems. The direction of the roots to the power source. Reserve. Types of root systems. Geotropism at the roots. What type of root system do chicory and oats have? Root functions. Let's take a look at the flower pot. Root growth.

"The structure and functions of the plant root" - Types of root systems. Types of roots. Root functions. Respiratory roots. Conduct area. Root growth. Root. Roots are supports. Root modifications. The role of root hairs. Serpentine roots. Stilted roots.