Factors Affecting Mycorrhizal Activity

Abstract

Mycorrhizal activity is affected by many factors, including biotic and abiotic factors, and these factors can affect the interaction between the plant and mycorrhizal. In short, these factors are as follows: First—abiotic factors include: 1. Soil fertility, 2. Heat, 3. The light, 4. pH, 5. Salinity. Secondly—biological factors. These factors may include some mycorrhizal effects on plant growth, as well as their effect on encouraging the absorption of nutrients, especially phosphorous and other nutrients, in addition to nonnutritive effects such as growth-stimulating effects, improving the water relationship and plant resistance to drought and plant resistance to salinity, and improving soil structure and biological control.

1. Introduction

Plant nutrition workers were interested in studying the effect of mycorrhizal fungi on plant growth, but what is the benefit of studying soil biology unless we have harnessed this biology to maximize production through symbotic relationships. At the same time, the plant host has a role that benefits the living organism so that the state of coexistence continues within the so-called microorganism plant society, which is formed from the state of coexistence between the plant host and the living organism. Some farmers in the world are directed to harness such mechanisms to increase production, such as India, New Zealand, Canada, Australia, and England, especially countries that suffer from a scarcity of energy sources, and the reason for this is to think about finding alternatives to compensate for the added fertilizers or reduce the cycle of fertilizer production, However, in this case, it is necessary to increase the pollen density, where the organisms are located within the population, which may affect the production process, but this pollen density may be weak to the extent that the effect of the organism on the plant does not appear, so the pollen density of a particular organism must be increased and then provided. This vaccine is more efficient in order to get what is called maximizing production.

2. Ecology of mycorrhiza

The mycorrhizal fungi are distinguished by their living in different environments, including tropical rainforest and sandy desert ones. Many factors affect the spread of mycorrhizal, as well as the processes of sporulation formation, and the way they coexist with mycoflora or other soil revivals, root colonization, colonization formation. We find ourselves in an urgent need to understand these stages of the presence of fungi as we approach the practical aspect of these fungi. Therefore, it is customary to express the presence of fungi with a term called dispersal. While concerning this process to such fungi, studies indicated that they were present in different continents before the flood and drought process occurred. As for the important study in mycorrhiza, the plant fossils proved that they contain a type of fungi and consist of structures similar to some extent to the mycorrhizal structures (vesicular—dendritic), which he led the researchers to say that these organisms were formed on the roots of forest plants and some natural plants. According to this, what Frank [1] made of his diagnosis of mycorrhizal fungi is a reality and a fact, but it did not attract the attention of scholars in that period or era.

2.1 Means of spreading mycorrhiza

Several ways to spread it have been suggested:

2.1.1 Active dissemination

It can appear from one area to another as a result of the growth of hyphae. It was found that the rate of mycorrhizal spread varies according to the density of these soils with these fungi, as well as the plant host that coexists with it. It has been proven that the rate of spread of these fungi reaches within 65 m per 150 years, which means 0.43 m/year, and it was clarified that the soils containing these fungi have a higher prevalence rate than the soils that do not contain these fungi. The type of plant and root density plays an important role in the spread of mycorrhizae. It was mentioned that the soils planted with subclover pasture plant in Australia have the prevalence of mycorrhizal Glomus fasciculatum up to 1 cm per week, but in another field when the vegetation cover prevails in it when one of the weeds was found. The spread of this type of mycorrhiza decreases and reaches about 0.07 cm, and from this we can show that the density of roots plays a key role in determining the spread of mycorrhizae, especially in the young stages of plant growth, where we find that the vegetation cover of weeds had a low prevalence rate, but the prevalence rate increased by changing the second type to clover because there is a wide difference in the root system of both plant hosts.

Poter [2] worked on the distribution of mycorrhizae with the type of soil when he took the mycorrhizal vaccines and added them to soil A. He noticed that the growth of hyphae as well as the infection and formation of spores differed significantly with other soil B, and at that time he attributed the reason to that there is a possibility that soil A in it encourages factors for the growth and formation of spores of this fungus compared to soil B, then he started transferring samples of mycorrhizae to soil C and noticed a strange observation than this, which is that mycorrhizae cause infection but do not form spores. As for soil B, after taking the inoculum from it and transferring it to soil C, the mycorrhizal system lost the ability to form spores, but it kept the infection, and it was found that there are three factors affecting the distribution of mycorrhizae in the soil, namely:

1. pH reaction number: it was found that the pH in soil A was within the biological pH number that enabled these fungi to grow, but the transfer of the inoculum from A to B and then to soil C became the biological reaction number eligible for infection and not eligible for spores to occur., Perhaps the process of spore formation is the reliable process in the production of the vaccine, so we can say that the soil must be chosen very carefully if I want to obtain mycorrhizal vaccines that contain significant amounts of spores per gram of soil.

2. light: the process of formation of spores in fungi is affected by a major interfacial factor, which is light. When the periods of illumination are compatible with the crop or plant host, we get the highest percentage of sporulation, but if the photoperiod is not compatible with the plant host, the process of sporulation formation may decrease or progress.

3. root density: the root density plays an important and major role in the spread of mycorrhizae. The most important studies on this subject are those of Poter [2] and Bolan et al. [3], who indicated that the plant host with a coarse root system is susceptible to infection with this type of fungus that is more than a soft root system.

2.2 Passive dissemination

It has been said that the dissemination within this method depends on the movement within the wind or water, but the surrounding environmental conditions will be a determining factor for the spread of the vesicular arbuscular mycorrhizae (VAM) fungi in this way. The plant organism, which is in the soil in many cases, has some characteristics that enabled it to spread, and one of these characteristics is that it can be the so-called spor capes structures, which are the structures of a group of spores present in one part. A spor cape is a manufacturer of large numbers of blackboards. There are indications from researchers about the free spread. It has been said that birds and insects have a role in the process of migration, through their movement within the remnants of these organisms and their spread from one area to another.

2.3 Factors affecting mycorrhizal activity

Mycorrhizal activity is affected by many factors, including biotic and abiotic factors, and these factors can affect the interaction between the plant and mycorrhizal.

In short, these factors are as follows:

2.4 First - Abiotic factors

There are many abiotic factors that affect the activity of mycorrhizal fungi, including soil fertility, temperature, pH, soil moisture and aeration, soil type, salinity, plant readiness for infection, soil organic matter, and others.

Abiotic factors include:

2.4.1 Soil fertility

Soil nutrients, especially phosphorous, are one of the most important abiotic factors that affect the mycorrhizal fungi. Both nitrogen and phosphorus significantly affect root colonies when they are present at high levels. Therefore, the state of balance for these two elements is a required condition, that is, the nutritional needs of the family. The plant and the nutritional needs of the living organism should not be within the critical sufficiency limit because this simply means entering into a kind of competition for the source of food, and this explains that some plants vaccinated with VAM may decrease their dry weight or growth rate because the VAM and the plant are included in this type from the competition so that the rate of net photosynthesis is insufficient for both the VAM and the plant, and thus the interference changes from Mutualism (+, +) to (−, +) Parasitism.

There is some information about the negative effects of nitrogen fertilizers on the formation of mycorrhizae, and it was found that nitrogen (ammonia nitrate) clearly reduces both the infection of mycorrhizae and the number of spores in wheat fields. As plants fertilized with high ratios of ammonium to nitrate have a higher phosphorous content in their tissues than plants fertilized with low ratios of (NH₄⁺) to (NO₃⁻), and these high concentrations of phosphorous in plant tissues inhibited infection. The reason for the inhibitory effect of ammonium may be attributed to the low pH in the area rhizosphere and to see the effect of fertilizers on mycorrhizae. The initial fertility of the soil must be known, because in poor soils the production of spores will be limited to the total quantity and not the percentage of infected roots derived from plant growth.

As for phosphate fertilizers, some studies have shown that these fertilizers have negative effects on the internal mycorrhizal fungi. Increasing the processed phosphorous may reduce the infection of mycorrhizae to levels that are insufficient to encourage the absorption of other elements. Phosphorus and zinc of the shoots of pollinated plants grown in low fertility soils were more than that of unpollinated plants. Increasing the level of ready phosphorus is an inhibitor of the growth of mycorrhizal, unlike insoluble forms such as rock phosphate, which is not considered an inhibitor. The results on wheat plants confirmed this, as it was found that the addition of phosphorous levels of 60, 120, and 240 kg phosphorus/ha led to a reduction in the percentage of infected roots in the fertilized treatments compared to the non-fertilized treatments, where the level led to 240 kg phosphorus/ha to the absence of infection significantly and the removal of the beneficial effect of the mycorrhizal infection.

The reason for the decrease in mycorrhizal infection as a result of the increase in phosphorous levels was shown by Cooper [4] that under conditions of phosphorus deficiency, the amount of phospholipids in the membranes of root cells decreases, leading to an increase in the permeability of these membranes, and this leads to an increase in the root secretion of reducing sugars. And amino acids lead to the formation of mycorrhizae, thus increasing the percentage of infected roots, but under conditions of availability of phosphorus, and the permeability of the membranes of the roots cells decreases due to the increase in phospholipids in them, and as a result, the secretions of the roots decrease from reducing sugars and amino acids, and this leads to a decrease in the percentage of infected roots. The decrease in the rate of infection may also be due to the increase in the concentration of phosphorus in the tissues of the plant, and the reason can be attributed to the fact that high levels of phosphorus may reduce the concentration of carbohydrates in the roots of plants, and as a result, the rate of infection is reduced. In general, high soil fertility leads to less mycorrhizal infection, so it is unlikely that we will find many mycorrhizae in densely cultivated soils. However, some crops are highly infested with fungi even in very fertile soils, as mycorrhizae are found in all poor and rich soils. Therefore, a low level of fertility is not always a condition for a significant development for mycorrhizal.

2.4.2 Temperature

Studies have shown that temperature has an effect on the formation of spores and colonies in greenhouse conditions, as the temperature usually affects the increase of colonies and thus increases the spores. The ideal temperature of mycelium on the surfaces of the roots will be 20–30°C. As for the formation of spores and the species that form the spore cyst, it will be at its strongest at 35°C. Studies have shown that the succession and decrease of temperatures increases the formation of colonies as well as the spores. Yoh-ichi Matsubara et al. [5] found that after 7 weeks of inoculation at a temperature of 20/25°C, the infection level of Gigaspora margarita in roots was 63.0 and 20% in Glomus sp. RIO, and the infected plants gave with fungi the highest values of plant height, dry weight of the vegetative part, and phosphorous concentration in the vegetative and root parts compared to unpollinated plants, as the effect was more pronounced in Gigaspora margarita than Glomus sp. RIO, but after 11 weeks from vaccination and when the temperature drops to 15°C, the infection level was recorded at 48.9% in Gigaspora margarita and 58.9% in Glomus sp. RIO. The plants infected with the fungus showed the highest values in all studied traits compared with the uninoculated ones. While after 11 weeks of insemination and at a temperature of 30°C, the infection level was 66.3% in Gigaspora margarita and 36.7% in Glomus sp. RIO.

2.4.3 Light

Light can indirectly affect soil microorganisms through its effects on plants, whose photosynthetic products are released from the roots [6]. The penetration of light through soil is important because of its effects on factors of ecological significance, such as spore germination, root growth, fungal growth, and formation of mycorrhizal and leguminous nodules. Light penetration can be affected by soil moisture content, soil type, cover material, and particle size. Phytochromes that are biliprotein photoreceptors enable some microorganisms to adapt to the light regime in the soil [7].

Fungi are unable to use light for photosynthesis; however, radiation plays a role in the biochemical and morphological responses of some fungi such as Phycomyces blakeslleanus, including their growth and differentiation. Physiologically and ecologically, a significant amount of light penetrates the soil approximately 4–5 mm from the surface, eliciting some phototrophic responses in plant roots. This information has led some VAM experts to hypothesize the function of LED on VAM formation. The induction of hyphal growth by light and chemicals, for example, the effect of blue light on hyphal branching, has been reported [8]. These authors demonstrated that blue light and some exudate components effectively stimulate hyphal branching, suggesting the involvement of a second messenger responsible for this synergism. The photo-induction caused by photo-mimetic compounds has been studied in many other fungi as well [9].

It is important to assess some environmental factors that stimulate hyphal growth and sporulation, such as root exudates, and LEDs applied individually and in combination. Lighting from red LED or red+blue LED could stimulate hyphal growth in G. margarita and Glomus spp. (R-10) in vitro [10]. Moreover, VAM colonization of corn roots was improved when the rhizosphere was exposed to light. The marginal VAM colonization of chalk false-brome [(Brachypodium pinnatum (L.) P.B.)] under shade conditions could show that when low light limits photosynthesis and thus growth of the plants, they dispense with the colonization of VAM in order to save the expenditure of organic carbon [11].

Previous studies have reported the effects of blue light on hyphae; however, its synergistic effect with root exudates on the production of new spores with minimum soil residues is still unclear. In this factor (light), two variables must be distinguished: (1) the period of illumination and (2) the intensity of illumination. The length of the illumination period is 32 hours, and the length of the illumination period is more important than its intensity in the formation of colonies. The growth of mycorrhizal onion plants was under light intensity of LUX 25,000 compared to LUX 13,000 and for 16 hours of light (23°C) with 8 hours of darkness (14°C) [12, 13].

2.4.4 The pH

Studies have shown that the reaction number pH has a significant effect on the mycorrhizal fungi, as well as the type of VAM plays a role in determining the appropriateness of the type of interaction, as it was found that G. mosseae excels in alkaline soils, most of the soils of Iraq work on this quality because it prevails in alkaline soils, and it can germinate its spores at a certain extent a broad range starts from 6 to 9, while we find that the species Gigaspora coralloida, which was isolated from acidic fluoride soils, its spores germinate at pH 6–4, and according to this we find that the G.mosseae is more suitable for alkaline media than the Gigaspora coralloida, and it was found that the type G.epigaeum has the ability to grow its spores in a wider range than the previous two species, which is from 6 to more than 8. On the other hand, it was found that both types of mushrooms G. intaradices and G.mosseae can grow in different types of soils, but they grow better in neutral and alkaline soils, while the fungus Gigaspora margarita prefers acidic soils. Therefore, it can be said that the pH affects the germination of VAM spores, and it is worth noting that there is a kind of fit between the pH appropriate for the growth of the plant host and the pH appropriate for the growth of the VAM.

To test the response of arbuscular mycorrhizal (AM) fungi to a difference in soil pH, the extraradical mycelium of Scutellospora calospora or Glomus intraradices, in association with Plantago lanceolata, was exposed to two different pH treatments, while the root substrate pH was left unchanged. Seedlings of P. lanceolata, colonized by one or other of the fungal symbionts, and nonmycorrhizal controls, were grown in mesh bags placed in pots containing pH-buffered sand (pH around 5 or 6). The systems were harvested at approximately 2-week intervals between 20 and 80 days. Both fungi formed more extra radical mycelium at the higher pH. Glomus intraradices formed almost no detectable extraradical mycelium at lower pH. The extraradical mycelium of S. calospora had higher acid phosphatase activity than that of G. intraradices. Total AM root colonization decreased for both fungi at the higher pH, and high pH also reduced arbuscule and vesicle formation in G. intraradices. In conclusion, soil pH influences AM root colonization as well as the growth and phosphatase activities of extraradical mycelium, although the two fungi responded differently [14].

2.4.5 Salinity

Several studies indicated that salinity has an important effect on the percentage of mycorrhizal infection and spore germination. These studies showed that there is a negative correlation between the incidence of infection, the number of spores, and the soil content of sodium when the Na concentration range is reached from 153 to 11,600 ppm, and it has been observed that 1 VAM fungi completely disappear when the concentration of Na increases to 3181 ppm. There are many studies that dealt with this subject, which showed that the salt concentrations are higher than 4 dSiemens. m2 in the root zone of the plant has caused a significant reduction in the infection rate for all mycorrhizal fungi, whether the addition of mycorrhizae with seeds or with seedlings, but the fungus G. mosseae recorded the highest rate of infection as an average at a higher salinity level than the rest of the species. The roots of tomato plants have caused a significant reduction in the incidence of infection with each type of mycorrhizal fungi and in the two ways of adding the inoculum (with seeds and with seedlings), area M. However, the fungus G. mosseae recorded the highest rate of infection as an average at each salinity level compared to the rest of the types and with both methods of additions to vaccines.

The process of spores of mycorrhizal fungi goes through four phases: the hydration phase, the activation phase, the emergence phase of the germination tube, and the hyphae growth phase. The failure of one or more of these phases due to high concentrations of dissolved salts in the soil solution may delay or stop the growth and development of the host plant. Three sources of salts (sodium chloride, sodium nitrate, and potassium chloride) with different concentrations were used to study their effect on spores’ germination components of five types of endophytic mycorrhizae in three separate experiments. The fungus G. mosseae was significantly superior to the rest of the spores’ germination components of other fungi, and the percentage of mycorrhizal spores germination decreased significantly when exceeding the critical concentrations of sodium and chlorine ions. Germination or inhibition of the germination process is by the toxic effect of sodium and chlorine ions. The toxic effect of sodium ion was more than the toxic effect of chlorine ion on the germination process, and in a study by Al-Khaliel [15], he used five levels of calcium carbonate (20, 30, 40, and 50%) and five levels of calcium sulfate (5, 15, 25, and 50%) to find out their effect on the components of spore germination of five types of mycorrhizal fungi. The results showed that the high levels of calcium carbonate (40 and 50%) and calcium sulfate (15, 25, and 50%) reduce the percentage of germination and other components of germination, but the effect differs according to the Mycorrhizae genus Glomus and Gigaspora, as well as between species within the genus Glomus, and the critical level of calcium carbonate is 20–30% and for calcium sulfate is 5–10% which after it is exceeded, the percentage of germination and other components of germination decrease, and they attributed the reason for the decrease in the percentage of germination to the lack of availability of nutritional needs of phosphorus and other elements due to the high percentage of calcium carbonate and sulfate, as well as due to the harmful effect of calcium and sulfate ions in the soil.

On the other hand, there are studies that showed that mycorrhizal fungi naturally appear in saline environments, despite the little affinity between mycorrhizae and halophytic plants such as hypophysis. The results of the research were that 21 of the 89 plants of the halophytic species were infected with VAM fungi, which indicates that the VAM works within saline concentrations, and 11 of the 89 plant species developed spores in the rhizosphere. This is sufficient evidence that different plants of 21% have an infection and 11% have an infection and spores. Al-Khaliel [15] found that the number of mycorrhizal spores did not decrease significantly with the increase of soil salinity, and the rate of spores was 100 per 100 g of dry soil, and that the majority of mycorrhizal species that were found in the soil of the plain whose salinity reaches 160 dSm are G.etunicanum, G.versifform, and G.intraradices, and this was attributed to stimulating the formation of spores under salt stress, meaning that the mycorrhizae is the reason for the production of spores at low levels of root infection under saline stress conditions, which inhibits the formation of mycorrhizae from spores and then the accumulation of spores in the soil.

2.4.6 Second: Biological factors

The biological interaction between mycorrhizal and other organisms took the space and thought of many researchers, and the matter of the fact is that the first to approach this topic and define the characteristics of the region in which this interference occurs is Hiltner [16] when he called the region surrounding the root hairs the rhizosphere region, which is the region affected by many factors, including the number of living organisms and growth and secretions of the roots, and he identified in an unequivocal way that the activity of organisms in this area is at its highest, and therefore it is not surprising that it is said that it is the key to microbiology to take its scope in maximizing plant production. And some are reduced, and some are free, and some are restricted. When there is an agreement between the plant and the living organism, the response is the greatest, and when there is no agreement between the plant and the living organism, the ability of the plant to grow and absorb is determined, and therefore the living organisms do not play the required role and mediate to facilitate the elements. In this area, there are mucilage, sloughed cells, and a wide quantitative production of enzymes, hormones, and growth regulators, in addition to antibiotics and multiple sugars, and all of these are diagnosed and studied, and thus the rhizosphere area has an applied importance from an agricultural point of view because it represents the true cradle of seeds and the roots in them and the increase in the activity of organisms in them. The interference between organisms, which is the effect of one organism on the activity, growth, and reproduction of another organism, and the interference is either:

1. Antagonistic: its results are inhibiting against the number of cells when the organism is bacterial and may be inhibiting the formation of spores in fungi.

2. Positive: when an organism stimulates some of the activities of an another living organism, such as mycorrhizalgia, it increases the number of spores of another fungus.

The unsatisfactory interactions that are of different types, including:

1. Simple overlap (monogamy), peaceful coexistence: it includes Micro- and Micro-plant, such as:

   1. Rhizobia and leguminous plants (symbiotic specialty).

   2. Mycorrhizae and plants (nonspecialized).

   3. Azotobacter or azospirilm and the plant (associative).

2. Unsatisfactory complex bilateral interference (Micro- and Micro-plant), such as Mycorrhizalgia + Azotobacter + Plant.

There are some organisms that are involved in the so-called Commelizan effect when two organisms are present with each other, so one of them stimulates the other. For example, the presence of mycorrhizae with Azotobacter increases the rate of cytoplasm release and thus enables plants to live in low levels of iron with no symptoms of deficiency. Many researches and studies indicated the effect of various microorganisms groups, especially bacteria, on the germination and growth of arboreal mycorrhizal fungi. Bacteria increase mycorrhizae by removing the inhibitor, such as self-inhibitors of the fungi spores or the production of chemicals that stimulate the growth of the spores, as well as an increase in plant secretions by increasing the permeability of the membranes and thus an increase in the growth of fungi and their ability to penetrate the host (we will address the relationships between mycorrhizae and other soil microbiota in other topics from this chapter). It was found that mung bean yellow mosaic virus reduced the rate of mycorrhizal infection and the production of spores by the fungi Glomus constrictum, G. Fasciculatum, and Acaulospora morrowede, but fungi Gigaspora gilmorei could not form any mycorrhizal roots in plants infected with the virus compared to healthy plants.

There have been many attempts for years that have been concerned with studying the state of competition between the mycorrhizae introduced into the growth medium (the rhizosphere) and the endemic mycorrhizae. Taking into account a number of variables, the most important of which is the rate of infection in the roots and then the calculation of the mycorrhizal roots, as well as knowledge of the formation of spores after adding VAM to the growth medium. Accordingly, the type of added vaccines will have two types of effects: either a positive effect or a negative effect (competition), the positive effect may increase the incidence of infection and spores and increase the production of spores due to the positive effect of one organism on another organism. Studies have shown that there is another type of competition that arises between the types of VAM introduced into the growth medium. And the technique that determines how this antagonism occurs between organisms is either the use of biochemistry and reliance on the metabolites secreted from living things and determining their structures, or the use of genetic engineering to determine this type of antagonism. Mycorrhizal fungi coexist with multiple plant families, and there are other families with a low tendency for this type of symbiosis, and most families have a high susceptibility to mycorrhizal infection, but some species such as cruciferous and ramiform do not infect mycorrhizal. Studies have shown that the history of vegetation cover in a region may participate in determining the density of mycorrhizal presence and in many times the supremacy of one sex over another.

3. Conclusions

The mycorrhizal fungi are distinguished by their living in different environments, including tropical rainforest and sandy desert ones. Many factors that affect the effectiveness of mycorrhiza, as well as the processes of sporulation formation, and the way they coexist with mycoflora or other soil revivals, root colonization, have been studied. We find ourselves in an urgent need to understand these stages of the presence of fungi as we approach the practical aspect of these fungi. Therefore, it is customary to express the presence of fungi with a term called dispersal. With regard to this process of such fungi, studies indicated that they were present in different continents before the flood and drought occurred. Since that time, there are many factors that limit the existence and effectiveness of mycorrhiza, and the most important of which are environmental factors and other biological factors (Figure 1).

Figure 1.

The direct and indirect effect of mycorrhiza on the activity of other microbiology in the rhizosphere [17].