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Part IV, continued: 

  In discussing the problems of interaction between microorganisms and higher plants one cannot ignore the epiphytic microflora. Microbial epiphytes are the organisms which concentrate on the surface of the green parts of vegetating plants and are nourished by the excretions of the latter.

  The epiphytic microflora has been but little studied, especially its quantitative and qualitative composition on various plants.

  On the surface of the aerial parts of plants one finds different microorganisms--bacteria, actinomycetes, fungi, yeasts, algae and protozoa. Their number may by very high. Duggeli (1904) counted many thousands of microorganisms on the surface of cereal seeds. From 80,000 to 25 million bacterial cells and from 4,000 to 7,200 fungi in one gram of wheat seeds have been detected by Morgentaller (1918). The author points out that on healthy seeds there are almost no fungi.

  In germinating wheat seeds there are 60,000 bacterial cells per gram of grains, and in nongerminating seeds--13 millions. Mack (1936), Kent-Jones and Amos (1930), inspected 21 samples of wheat seeds from different countries, and found from 8,000 to eight million bacterial cells on the surface of one gram of seeds. Gustafson and Parfeitt (1933) in a similar study counted from 46,000 to 3,260,000 bacterial cells in one gram of wheat seeds.

  Rautenshtein (1939) studied the microflora of wheat seeds in the various stages of ripening: milky, waxy, and full maturity stage. The results of his observations are given in Table 136.

  Bacteria are the most numerous among the microorganism groups which were found. Yeasts are not always encountered.

  With ripening of the seeds the number of microbes on their surface increases. James, Wilson and Stark (1946) counted from 280,000 to 164 million microorganisms in one gram of wheat seeds. The numbers of microbes found on the surface of the green parts of plants are not smaller. Many investigators counted from 49,000 to 6,300,000 epiphytes in one gram of tissue (Khudiakov, 1953; Thomas and Hendricks, 1950; Stirling, 1951; James, 1955, and others). According to Kroulik, Burkey and Wiseman (1955). the number of epiphytes in one gram of tissue of green plants of corn, oats, clover, lucerne, garden grass, and other plants varies from 1,540,000 to 99,200,000. These numbers vary from species to species, in relation to the age of the plants, and also with the soil-climatic conditions. As a rule, the number of epiphytes on the surface of young plants is larger than the number on ripening ones. In relation to seeds, the opposite picture was observed. Bacteria form the greatest part of the epiphytic microflora. The species composition of the bacteria is quite diverse, but the dominating part of it is considerably small. Almost all the investigators noted the predominance of bacteria with a yellow pigment, classified as Ps. herbicola on plants. This bacterial species was described by Duggeli (1904), He found that these bacteria were the dominant species. Their total number reached 380,000 and more in one gram of tissue.

  According to Weller (1929), Ps. herbicola comprises 90-100% of the total bacterial flora on seeds of wheat and rye. Upon germination of seeds in the soil, these bacteria soon disappear, reappearing toward the end of ripening. On growing plants, according to the author, there is abundant growth of lactic-acid bacteria. On seeds of barley and oats Weller found sporeforming bacteria. Rautenshtein (1939, a, b) found 75-98% of Ps. herbicola among the bacteria populating the surface of wheat seeds. Cocci and Sarcina are encountered in a few cases. He also found a great number of lactic-acid bacteria. Among the fungal flora, Rautenshtein found the fungi Cladosporium herbarum, Trichoderma koninglii and less often Dematium, Asperigillus, Penicillium, Oospora and also the species A. globisporus and A. griseus and the yeast species of the genus Torulopsis. Many heat-resistant and thermophilic bacterial forms were found. The majority of these forms belonged to the sporeforming species of the type Bac. mesentericus. They also grow at a temperature of 17-20° C.

  James, Wilson and Stark (1946) distinguish between two types of epiphytic bacteria--type A and type B. Type-A bacteria form yellow colonies and are all considered to be cultures of the same species--Ps. herbicola. The other type belongs to the colorless Pseudomonas species. Of the fungal flora these authors found the following on plants: Acrostalagmus, Alternaria, Penicillium, Aspergillus, Botrytis, Cephalosporium, Fusarium, Torula, Monilia and other fungi. There were also phytopathogenic species among them Helminthosporium sativum, Hormodendron pallidum, H. viride, Alternaria tennis, Fusarium culmorum, Cladosporium herbarum, Septoria nodorum, etc.

  James (1955) gives the following data on the extent of distribution and accumulation of Ps. herbicola. Out of 200 plants of oats, barley, and flax, growing in different regions of Canada which were studied, more than half contained this microbe in amounts of 100,000 and more, about 15 % of the plants contained from 10,000 to 100,000 bacteria and some samples were free of this bacterial species altogether.

  Clark (1947) and others observed the yellow bacteria in great numbers on the green parts of the cotton plant.

  The predominance of these bacteria on other plants was noticed by many investigators (Burri, 1903; Mack, 1936; Thomas and Hendriks, 1950; and others).

  Wallace and Lochhead (1951) point out the connection between the epiphytic and rhizosphere microflora. The latter, according to these authors, is intermediate between the microflora of the soil and the epiphytic microflora.

  Khudiakov studied a great number of plants (1953a). He investigated the migration of bacterial epiphytes from the surface of the seeds during the germination of the latter, to the seedlings and later to all the plant organs including fully formed and mature seeds. More than 20 microbial species were isolated from different wheat varieties by this author. Among these microbes there were three species of yeasts, two species of fungi and the remaining were bacteria, among which three,. cultures produced a yellow pigment, three others orange or red, one green, and all the others colorless. According to the author's observations, each of these species is the predominant form on some seed variety.

  Upon analysis of preharvest seeds of wheat (Moscow variety, 2411) grown in the Moscow Oblast', 97% of the bacteria found were Ps. herbicola; no yeasts or colorless bacteria of the Ps. fluorescens group were observed. On another wheat variety which was grown alongside the former and under the same conditions, there were 60% yeasts and no Ps. herbicola bacteria were observed.

  Reciprocal cross infection of the wheat varieties by the isolated cultures has shown that the latter were not specific. Epiphytes from one wheat variety, when transmitted to another variety, grew as well as they did on seedlings of their own host plant. Khudiakov has shown that epiphytic microflora can be changed at will by treating the seeds before sowing with the corresponding microflora. He treated sterilized oat seeds with cultures of epiphytic yeasts and bacteria which he isolated, and sowed them in open ground.

  Those microbes that had been artificially introduced (Table 137) were found on these plants. On the control plants the bacteria which usually concentrate on this species predominated.

  Kvasnikov and Sumnevich (1953) investigated woody plants in Central Asia--poplar, apple, pear, cherry, maple, Greek nut, etc and grassy cultures: corn, lucerne, cotton, Sorghum cernium, milo, potato, sugar beet, cabbage, etc. In all cases, lactic-acid bacteria were found on all the plants in great quantities. According to the authors, on wild plants the number of these bacteria is considerably smaller. The nearer the plants are to places of human habitation, the more lactic-acid bacteria one finds on the surface of plants.

  Kroulik, Burkey and Viseman (1955) divide the bacteria into chromogenic and colorless groups. Among the former, the Ps. herbicola type predominates and among the latter--lactic-acid bacteria Lactobacterium plantarum.

  In our studies we investigated various species of grassy and woody plants growing in the central belt of the USSR. The number of bacteria and fungi on the surface of leaves and branches has been determined. Similarly to other investigators, we also detected hundreds of thousands and millions of bacterial cells per gram of tissue. In the different plants the predominant epiphytic microflora varies in its species composition. In some plants Ps. herbicola predominates and in others--other species of the genus Pseudomonas and Bacterium, and sometimes--lactobacilli. Quite often one finds large numbers of yeasts of the genus Torula (T. rosea, or T. alba) and of the genera Sporobolomyces and Mycotorula.

  Large numbers of microorganisms are found on the surface of fruits. Studies show that on berries and fruits there are bacteria, fungi and yeasts, actinomycetes and even protozoa. Epiphytes are encountered on wild as well as on cultivated fruits.

  On berries as on other parts of the plants the most numerous group of microbes are the bacteria with fungi and yeasts following. Often there are as many as hundreds of thousands or even millions of them on 1 g of berries. The number of microbes varies with the variety and species of the berries, with the degree of naturity, and with climatic and other external conditions. As a rule, their number increases with the ripening of the berries.

  The quantitative ratio between bacteria, yeasts and fungi also change.

  The microflora of the vine grapes has been the most thoroughly studied. According to the data of Akhinyan (1952) the total number of microorganisms on the surface of vine grapes of the "Kakhet" variety ranges between 3,000 and 4,000,000 per 1 g, depending on the region and where the vine was grown (Table 138).

  The presence of such a large microflora on the surface of plants cannot be explained by their being carried over mechanically from the air. The accumulation of certain specific species speaks against it. The latter evidently grow and multiply on the plant surface. Consequently, they must find there sufficient quantities of food substances necessary for mass reproduction.

  Plants, as has been pointed out above, excrete various volatile and nonvolatile substances--with the aid of special glands or by guttation. In the drops formed by the guttation of rye grass glutamine was found (Chibnall, 1939), Genkell (1946) observed the excretion of mineral salts together with the fluid excreted by plants of salty marshes. Vigorov (1954) found in the guttation drops of 7-to 9-day-old wheat seedlings 1.8 mg/ml of dry substance, containing 5-10 mg/ml ammonium-nitrogen and 40-45 mg/ml phosphorous compounds. The author observed, that the intensity of guttation depends on illumination, soil humidity, on the presence of nitrogen and other nutrient elements. Introduction of ammonium salts into the soil elevates the excretion of nitrogen compounds in the guttation drop. One of the tests for the presence of organic substances in the fluids is the growth of microorganisms in them. According to the author, fungi grow abundantly in guttation drops.

  According to the data of Kholodny (1944 a,b,c) all or many of the organic substances excreted by plants are used by microbes as sources of nutrition.

  The significance of the epiphytic microflora in the life of the plant is many-faceted. Among the epiphytic microflora there are many activators (Ps. herbicola, yeasts, etc), which form biotic substances--vitamins, auxins, folic acid, thiamine, riboflavin and other compounds, and also organisms forming antibiotic substances with strong antimicrobial properties.

  On the surface of leaves and stems of plants there are microorganisms forming toxic substances. In the epiphytic group there are also parasitic and phytopathogenic forms.

  One should assume that the metabolic products of the epiphytic microflora behave in a certain manner in the plant tissue, having a definite effect on them. The ability of leaves to absorb various substances was known for a long time. On this basis methods of extrarhizal feeding have been elaborated and also methods of introduction of substances with the aim of changing certain physiological functions shedding of leaves, arresting of flowering, etc.

  It was also established that plants can absorb various microbial metabolites, vitamins, antibiotics and other compounds through the leaf surface. As was indicated above, these substances not only enter the plant through the leaves but they can be introduced by this route in large quantities for the purpose of feeding as well as for fighting bacterial and fungal infections. Among the epiphytic microflora there are numerous antagonists which produce antibiotic substances which suppress their competitors, and among them also phytopathogenic microbes. Growing abundantly on plants, such organisms may fulfill a protective role removing or suppressing infectious agents originating from without. If we were to change the composition of the epiphytic microflora on the surface of the green parts of plants at will, and form certain coenoses of antagonists there, this would prove to be of great value to plant and fruit growing.

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