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CHAPTER 1. MYCORRHIZAS AND THEIR SIGNIFICANCE IN NODULATING NITROGEN-FIXING PLANTS

CHAPTER 1. MYCORRHIZAS AND THEIR SIGNIFICANCE IN NODULATING NITROGEN-FIXING PLANTS

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J. M. BAREA AND C. AZC6N-AGUILAR



zosphere, of plants growing in soil. Actually, rhizosphere bacteria (including

actinomycetes) and fungi carry out a range of activities (e.g., the breakdown of

organic matter, nitrogen fixation, secretion of growth substances, increase of the

availability of mineral nutrients, and immobilization of those assimilable) of

great relevance to plant growth; they also cause plant disease or protect the plant

from pathogens. The extent of microbial activity depends, in most cases, on the

supply of organic substrates from the root. Hence, the abundance and activity of

soil microorganisms in general diminish with increasing distance from the root

(Newman, 1979).

From the point of view of their relationships with the plant, microorganisms

can be classified into three groups: (1) saprophytes, usually opportunists but

benefactors in some situations; (2) parasitic syrnbionts or pathogens, potentially

harmful to the plant; and (3) mutualistic symbionts, usually called symbionts in

the literature, which develop activities beneficial to plant growth (for reviews,

see Brown, 1975; Dommergues, 1978; Newman, 1979).

It is widely assumed that one of the most beneficial contributions of soil

microorganisms to plant development is the supply of nutrients essential to plant

growth, particularly those involved in nitrogen (N) and phosphorus (P) cycling.

Among these, the organisms concerned with N fixation and the enhancement of

P uptake by the plant are especially relevant. As it is well known, N and P are

two major elements in plant nutrition that commonly limit plant growth; thus,

they are usually added to soil as industrial fertilizers. However, in addition to the

energy-intensive technology, implied in the synthesis of chemical fertilizers,

most of these compounds are lost when they are added to the soil because they

are not readily used by the plant. Actually, no more than 30% of the N fertilizer

(Postgate and Hill, 1979) and only about 25% of the P fertilizer (Hayman,

1975a) are taken up by the crop in the year of its application. The rest of the N is

lost either in the soil water, causing pollution problems (Bolin and Arrhenius,

1977), or to the atmosphere as a result of denitrification; most of the P fertilizer

added is quickly fixed by some soil components and converted into forms which

are not readily available to plants.

Consequently, N fixation, which cycles N to the biosphere from the atmosphere, is an important factor in biological productivity; it is accepted that more

than 60% of the N input to the plant community through fixation has a biological

origin (Postgate and Hill, 1979; Brill, 1979). The activities of the N-fixing

bacteria either convert N into bacterial proteins (in free-living systems) or make

it directly available to plants as NH, in symbiotic associations which occur in

root nodules.

Many common soil microorganisms can release soluble phosphate from sparingly soluble inorganic and/or organic phosphates known to occur in soil. Several problems inherent with the lack of energy sources in the rhizosphere, micro-



MYCORRHIZAS IN NODULATING N-FIXING PLANTS



3



bial antangonism, difficulties in the translocation of the phosphate ions to the

absorption places at root surface, and other factors make the microbial solubilization of phosphates a minor contribution to the P nutrition of plants (Hayman,

1975a). However, mycorrhizas, mutualistic symbioses between plant roots and

certain soil fungi, play an unquestionable role in P cycling and in the uptake of

phosphate by the plant. Because the known world reserves of P could be depleted

in a few decades (Rhodes, 1980), the contribution of this symbiosis to the

reduction of fertilizer requirements is of increasing interest.

B. MYCORRHUAS

AND ROOT NODULES



All but a few vascular plants are able to form mycorrhizas. Under natural

circumstances, the mycorrhizal condition is the norm for most of the higher

plants. The mycorrhizal fungus has an ecologically protected niche inside the

plant root; the products of photosynthesis arrive here, furnishing abundant energetic substrate for the fungi which by means of their network of hyphae or

mycelial strands extend the mycelium to the surrounding soil, take up nutrients

(mainly phosphate) from the soil solution, and translocate these ions to the host

plant (Tinker, 1975; Hayman, 1978). Mycorrhizas therefore have a worldwide

recognized value for plant survival and nutrient cycling in the ecosystem. They

contribute significantly to plant productivity both in arable and in plantation

crops. Several types of mycorrhizas occur; their characteristics will be described

later.

Three different types of microorganisms (bacteria) are able to induce nodules

on roots of higher plants and to inhabit them by establishing mutualistic symbioses. As a consequence of these associations, the microsymbionts are able to

fix N. The energy requirements for these processes are satisfied by the photosynthate which is directly received by the bacteria at the plant roots (Hardy and

Havelka, 1976). The microorganism exports NH4+ to the plant, avoiding transport and dispersal problems. The bacterial genera and the corresponding host

plants involved are (1) Rhizobium, which nodulates, with one exception, on

legume roots; (2) Frunkiu, actinomycetes that fix N in nodules they form on

nonlegume, often woody, angiosperms; and (3) Nosfoc and Anubuenu,

cyanobacteria (formerly blue-green algae), which form N-fixing nodules on the

roots of plants of the family Cycadaceae (gymnosperms). Legume-Rhizobium

sp. associations are the most important for the incorporation of N into pasture

and agricultural ecosystems, whereas the nodulated angiosperms are similarly

important in forest ecosystems.

Plants bearing N-fixing nodules are usually mycorrhizal when grown in soil.

This fact has great ecological relevance because nodulation and nitrogen fixation

depend on a balanced mineral nutrition of the host plant (in particular, plants



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M.BAREA AND C. AZC6N-AGUILAR



have high phosphate requirements), and the mycorrhiza can satisfy these demands. Thus, mycorrhizal fungi not only help the plant itself but also aid the

bacterial symbiont to fix N in the nodular tissues. Nodulate and mycorrhizal

plants are therefore adapted to cope with nutrient-deficient situations (Harley,

1973).



The intent of this article is the comprehensive study of the role of mycorrhizas

in the growth and nutrition of N-fixing nodulated plants. As an introduction for a

better understanding of mycorrhizal effects, we will present a brief review of

some general, well-established principles on mycorrhizal types, morphology,

physiology, and function. Current information will be condensed to achieve an

up-to-date presentation of this sllbject and to create a conceptual background for

nonspecialist readers. This will constitute a quantitatively and qualitatively

important part of the article. Then, the interactions between nodular and mycorrhizal endophytes related to the formation and effects of these dual symbioses,

which greatly enhance the development of the common host plant, will be

discussed. This part of the article will be concerned not only with conceptual

principles but also with the rationally stated hypotheses and the current trends in

basic and applied research on this subject. Attention will be given to the ecological significance of plants bearing the two types of symbioses, with emphasis on

the possibilities of harnessing them to increase crop yield.



II. MYCORRHIZAS

A. GENERAL



The previous statements on the concept and function of mycorrhizas, although

concise, may allow us to envisage these widespread associations as the most

metabolically active parts of the absorbing organs of almost all land plants. Both

the autotrophic host plant and the heterotrophic fungal associate derive, in most

cases, physiological and ecological benefits from one another. Furthermore, the

“mycorrhiza-dependent’’ plants cannot develop adequately without their mycorrhizal partner. However, the general term mycorrhiza, broadly considered, is of

little significance. The taxonomic diversity in the fungi and plants involved and

the differences in the morphological, structural, and nutritional features of mycorrhizal associations require a subdivision to reflect the different physiological

relationships that are now recognized.



MYCORRHIZAS IN NODULATING N-FIXING PLANTS



5



I . Mycorrhizal Types and Their Structural

and Nutritional Features

Five types of mycorrhizas can be recognized. These and the main groups of

host plants on whose roots they are formed are recorded in Table I, as summarized from Smith (1980) and Azc6n-Aguilar and Barea (1980). The first type,

ectotrophic mycorrhizas (ECM),is characterized by a lack of intracellular penetration of the fungus into the cortical cells of the root. A network of fungal

mycelia, the Hartig net, is formed by hyphal growth among the host cells. This in

turn establishs a close contact between fungus and root-cell plasmalemma, which

is critical for nutrient exchange in mycorrhizal associations. In most cases the

fungus will develop a mantle or sheath of interwoven hyphae growing around the

feeder roots. The fungal mantle is extended some distance into the surrounding

soil by mycelial strands or rhizomorphs (only rarely by extramatrical hyphae)

(Harley, 1978). The fungi involved are mostly higher basidiomycetes (Boletus,

Suillus, Amanita, Lactarius, Tricholoma, Pisolithus, Scleroderma, Rhizopogon,

etc.), some ascomycetes (Tuber), and zygomycetes (Mam and Krupa, 1978).

The second group, vesicular-arbuscular mycorrhizas (VAM), is by far the

most widespread type of mycorrhiza. The nomenclature refers to the formation

of vesicles and arbuscules, typical morphological structures that will be considered later. As with ericoid, arbutoid, and orchidaceous mycorrhizas, the VA

fungus penetrates into the cortical host cells, but the invading mycelium usually

lives only a short time intracellularly (Smith, 1980); lysis of intracellular struc-



Table I

Mycorrhizal Types and the Main Groups of Host Plants Involved

Nomenclature

Traditional



Actual



Ectotrophic



Ectotrophic or

sheathing



Endotrophic



Vesicular-arbuscular



Ericoid

Arbutoid

Orchidaceous



Typical host plants

Pinaceae, Fagaceae, Betulaceae,*Eucalyptus,

Rosaceae,a Leguminosae" (woody),

Cupressaceae

Four-fifths of all land plants including agronomically important crops such as woody and

herbaceous legumes" (pasture, forage, and

grain) and Gramineae

Calluna, Vaccinium, Erica, Epacris

Arbutus, Monotropa

Orchidaceae



"Groups of plants also bearing nitrogen-fixing root nodules.



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J. M. BAREA AND



c. AZCON-AGUILAR



tures (the arbuscules in VAM) then occurs, but the host cell survives and can be

colonized again by the fungus. Vesicular-arbuscular fungi do not form sheaths

around the root, but a network of extramatrical hyphae usually develops. This

grows into the soil and can extend the mycelium several centimeters beyond the

root surface. The total hyphal length can reach more than 1 m of hyphae per

centimeter of infected root (see Smith, 1980; Hayman, 1982). These VA fungi

are members of the family Endogonaceae that are placed in the genera Glomus,

Sclerocystis, Gigaspora, and Acaulospora (Gerdemann and Trappe, 1974). Because they cannot be successfully subcultured axenically, they must be considered ecologically obligate symbionts (i.e., they do not complete their life cycle

unless they can colonize a suitable host plant) (Lewis, 1973).

An ascomycete (Pezizella ericae) has proved to be a fungal partner of the third

type of mycorrhiza, namely, the ericoid, which occurs on roots of some autotrophic shrubs in the families Ericaceae, Epacridaceae, and Empetraceae (Read

and Stribley, 1975; Read, 1983). Intracellular coils and extramatrical hyphae are

typical structures of these mycorrhizas.

The structure of arbutoid mycorrhizas, the fourth type, is characterized by the

formation of a sheath but not a Hartig net, and they also form intracellular

haustoria. Their nutritional features are not yet fully understood.

Confined to the family Orchidaceae, the fifth group of mycorrhizas shows

unique characteristics; they infect protocorms and rhizomes, but rarely the terrestrial roots. Their hosts are temporarily or permanently achlorophyllic, and the

mycorrhizal fungi (Rhizoctonia spp. and Armillaria melea), which are pathogens

for nonorchidaceous hosts, aid the heterotrophic orchid in assimilation of carbohydrates, probably from a simultaneous association with another true autotrophic

host plant (Mosse, 1978).

The major types of mycorrhizas and the groups of plants on which they occur

having been described, the discussion may now be limited to ECM and VAM,

the only mycorrhizal types formed on plant families also bearing N-fixing root

nodules (Table I). Emphasis will be placed on VA mycorrhizas because these are

the commonest type occurring on nodulated plants and also because these mycorrhizas, as deduced from their near omnipresence, play an integral role in most

crop-production systems.

2 . Occurrence and Distribution

Mycorrhizas, mainly VAM, can be found in most plant species growing in

most plant habitats under tropical, temperate, and even arctic conditions (Hayman, 1982). To understand the worldwide distribution and ecological implications of this symbiosis, it is interesting to go back 400 million years and consider

the role played by a fairly similar mutualistic association-the “ancestral mycorrhiza”-in the evolution of terrestrial plants (Pirozynski and Malloch, 1975). As



MYCORRHIZAS IN NODULATING N-FIXING PLANTS



7



pointed out by these authors and by Malloch et al. (1980), the SilurianDevonian colonization of the land by “plants” seems to have been facilitated by

the development of a mutualistic partnership between a semiaquatic ancestral

green alga and a certain aquatic fungus. The ancestral mycorrhiza probably

equipped the plants to cope with the problems of starvation and desiccation

resulting from the colonization of a nonaquatic habitat, the soil. The earliest of

the land plants preserved in a petrified form is the Rhynie fossil dated to 370

million years ago, and this possessed in its “roots” a form of mycorrhiza

remarkably similar to the modern VAM (Nicolson, 1975).

In this context it can be assumed that mycotrophy (Lewis, 1973) and mycorrhizas are as old as plants that seem to have depended on such mycorrhizas to

thrive early in their evolution. The symbiosis followed the course of evolution as

a component of the plants and, in such a way, it has been perpetuated as an

adaptation for the more efficient absorption of phosphorus. The claim of Pirozynski and Malloch (1975) is that “land plants never had any independence

(from mycorrhizal fungi), for if they had, they could never have colonized the

land.”

On these bases it can be concluded that mycorrhizas have occupied, from the

Middle Cretaceous on, a crucial role in the evolution, ecology, growth, and

nutrition of the plant cover of the surface of the Earth. They can be found in

tropical rain forests, open woodlands, grasslands, savannas, heaths, sand dunes,

and other habitats (Safii, 1980; Hayman, 1982). In spite of some descriptions

(Sondergaard and Laegaard, 1977), plants growing in aquatic habitats appear to

lack mycorrhizas, and they also seem to be rare in the families Cruciferae,

Polygonaceae, Chenopodiaceae, Cyperaceae, and others (Gerdemann, 1975).

According to Meyer (1973), only about 3% of the higher plants have sheathing

mycorrhizas (see Table I). These occur mostly in temperate timber trees (Marx

and Krupa, 1978; Fogel, 1980; Trappe, 1981). Vesicular-arbuscular mycorrhizas are formed in most angiosperms, some gymnosperms, and in pteridophytes and briophytes. These are the mycorrhizas of most of the economically

important crops [e.g., legumes, maize, wheat, barley, rice, temperate fruit trees,

many tropical timber trees, woody shrubs, tropical plantation crops (cocoa,

coffea, tea, rubber, etc.), cotton, tobacco, olive, citrus, and grapevine] (Hayman, 1982). Finally, some plants may form both sheathing and VA mycorrhizas

[e.g., apple, oak, alder, hazel, juniper, certain woody legumes, and members of

the family Populaceae (Trappe, 1977), and members of the genus Eucalyptus

(Malajczuk et al., 1981)l.

3 . The Process of Mycorrhiza Formation



The establishment of mycorrhizal status occurs in a sequence of phases involving interactions between the host, fungus, and environment. Because ECM can



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J. M. BAREA AND C. AZC6N-AGUILAR



be synthesized in vitro, the details of the process of their formation can be

accurately studied. Nylund and Unestam (1982) have well illustrated the interactions that are occurring. Taking into consideration many previous descriptions,

these authors extrapolate the findings to provide a generalizable sequence of

events. According to them, the process is controlled mainly by the host, but this

does not preclude an active participation of the fungus (i.e., different mycorrhizal structurescan be originated in the same host by different endophytes). The

process is initiated by the germination and development of propagules (spores or

hyphae) of the fungi living in proximity to the feeder roots of the host. The host

releases certain substances that produce a remote and selective stimulation of the

tentative mycosymbionts. This enhances the growth of these fungi to an extent

that is dependent on the species. Only mycorrhizal fungi have the ability to

respond, because only they recognize the host signal that is meaningless to the

other rhizosphere inhabitants. Fungal growth is stimulated, hyphae aggregate

around the root establishing a close contact between both mycorrhizal partners,

and a hyphal envelope forms, induced by host substances. This envelope structure apparently is essential for the infection. The penetration between root epidermal cells seems to be mechanical, and the host apparently does not resist

although it controls the fungal lytic enzymes.

During further development of the mycorrhiza, the fungus is more protagonistic and more interactions occur. At the labyrinthine Hartig net formation phase,

the morphogenetic changes of the fungus are again a response to host factors

although released by fungal induction. The formation of the mantle then takes

place, the fungus induces some morphological changes in the host, and the

colonization of all suitable root tissue by the fungus completes the mycorrhiza.

The mycelial strands, sclerotia, and fruiting structures are developed later (PichC

and Fortin, 1982).

Because VA fungi have not yet been successfully cultured axenically, studies

on the development of VA infections are difficult. Some hyphal growth can be

obtained in vitro from germinated spores or infected root pieces, but the growth

ceases when the hyphae are excised from the parent spore or when the root piece

dies. It is therefore said that these fungi do not grow saprophytically. Nevertheless, the assays carried out by Hepper (1979) on the germination and growth of

surface-sterilized Glomus sp. spores indicated that the protein synthesis required

for germination is programmed by stored mRNA, but that the spores also have

the ability to synthesize the new mRNA that is required for hyphal growth. The

fungi possess an Embden- Meyerhof-Pamas system, a tricarboxylic acid cycle,

and a hexose monophosphate shunt (MacDonald and Lewis, 1978). These results

suggest that the endophytes resemble saprophytic fungi more than obligate biotrophs. Moreover, a certain independent spread of VAM fungi in soil has been

reported (Warner and Mosse, 1980), suggesting some saprophytic ability of

these fungi.



MYCORRHIZAS IN NODULATING N-FIXING PLANTS



9



Studies on the development of VAM infection (Powell, 1976a) showed that

neither spore germination nor the initial direction of hyphal growth was influenced by the presence of host roots. Hyphae from spores were not attracted to the

roots until they approached them closely. First, the stimulated germ tube formed

a fanlike structure of mainly septate hyphae, from which the infective aseptate

ones developed later. Because hyphae from root segments did not form preinfection structures to infect a new root, it was suggested that these fanlike structures

have the function of absorbing nutrients or hormones from root exudates. Studies

of mycorrhizal infection in root organ cultures (Mosse and Hepper, 1975) also

indicated the lack of apparent attraction of germ tubes to the root until they grow

very close to it. Once a fungal hyphae is attached to the root surface, root

penetration may or may not occur. Young lateral roots seemed to exert a greater

stimulatory effect on the fungus.

In summarizing the previous statements we have four key facts in VAM

formation: (1) spore germination and mycelia development; (2) a stimulation of

the germ tubes when they approach the roots closely; (3) attachment of the

infective hyphae to the root surface; and (4) root penetration. With respect to the

stimulation of the hyphae in the rhizosphere, it is obvious that root exudates are

significant, and a positive correlation between VAM infection and the degree of

root exudation, which in turn is correlated with an increased permeability of root

membranes, has been found (Ratnayake et al., 1978; Azc6n and Ocampo, 1981).

However, there is another important factor that distinguishes rhizosphere from

nonrhizosphere soil, namely, the presence of active populations of microorganisms. They probably play a role in the development of VA fungi and VAM

infection. This is supported by studies on the influence of free-living rhizosphere

microorganisms on VA fungi in pure culture. The preliminary results have

shown that several fungi stimulate the “growth” of Glomus mosseae in culture.

The rate of spore germination, the length of the hyphae, and the number of

vegetative spores per resting spore were increased by the action of common

rhizosphere inhabitants (C. Azc6n-Aguilar and J. M. Barea, unpublished).

Once the infective hypha arrives at the root surface an appresorium is usually

produced on cortical cells or on root hairs, and hyphal penetration occurs into or

between these cells. When the first successful entry point is established, the root

becomes more prone to further penetration. This behavior could be because the

fungus is invigorated and/or because of changes in the root as a consequence of

the infection. The fungus then colonizes the root cortex and the hyphae multiply

both inter- and intracellularly, although they never invade the endodermis, stele,

or root meristems.

Shortly after infection, a hypha growing into a single cell may show repeated

dichotomic branching, and a treelike structure, the arbuscule, is formed. The

function of the arbuscules is the biotrophic bidirectional transfer of nutrients, the

mechanism of which requires living fungus (Cox and Tinker, 1976). Fine-struc-



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J. M.BAREA AND C.AZC6N-AGUILAR



ture studies (Cox and Sanders, 1974; Scannerini and Bonfante, 1983; Scannerini

et al., 1975; Dexheimer et al., 1979) revealed that the arbuscules are surrounded

by the intact host-cell plasmalemma. The cytological changes that occur during

arbuscule formation have been well documented by Rhodes and Gerdemann

(1980). There is an increase in host-cell cytoplasm, the starch within the invaded

cells disappears, and the nuclei become enlarged and at times divide. The cell

organelles (mitochondria, ribosomes, etc.) also increase in number. When an

individual arbuscule degenerates (they exist for 4-13 days), the cell and its

structures return to their normal stage. This cell is then ready for the formation of

a new arbuscule (Hayman, 1982). When the mycorrhiza is well established, the

fungi may form vesicles. These are oval-to-spherical structures containing oil

droplets that can develop inter- or intracellularly. They may have a temporary

storage function, after which they remain thin walled or become thick walled as

chlamydospore-like structures.

When the internal infection has been consolidated, the penetration hyphae

ramify externally. These external hyphae may grow along the root surface forming more penetration points and also grow through the surrounding soil forming

an extensive tridimensional network of mycelium. A typical feature of the VAM

is the dimorphic nature of the external hyphae: the coarse, thick-walled (20-30

pm in diameter) hyphae bearing resting spores are the permanent basis of the

mycelium, and the fine, thin-walled hyphae (2-7 pm) are more ephemeral and

have absorption functions. The density, geometry, and size of the external mycelium, and the number of entry points per unit of root length (1-25 per millimeter), are of great relevance in the functioning of VAM. When the mycorrhiza

matures the external mycelium usually produces large resting spores and smaller

secondary spores, or external vesicles. Some VA fungal species do not form

spores, and some develop sporocarps (Gerdemann, 1975).

B. PHYSIOLOGY

OF MYCORRHIZAS



Current literature on mycorrhizal research records progress toward a better

understanding of many physiological features of these symbioses, particularly of

the mechanisms that account for the mycorrhizal effects on plant growth and

nutrition. Some of these mechanisms, however, remain unexplained or poorly

understood. The review by Smith (1980) is a detailed and illustrating study on

this subject. Her qualitative model of the interactions between fungus, host, and

environment is a comprehensive summary of the available information relating

the flow of materials.and the feedback controls in mycorrhizal associations. This

review and those by Tinker (1978, 1980), Hayman (1975a, 1978, 1982, 1983),

Bowen and Bevege (1976), Mosse (1978), Rhodes and Gerdemann (1980), Safir

(1980), and Gianinazzi-Pearson and Gianinazzi (1981) thoroughly cover the

published knowledge on the nutritional relationships between the mycorrhizal

partners. Hence only the main points will be summarized here.



MYCORRHEAS IN NODULATING N-FIXING PLANTS



11



1 . Effect of Mycorrhizas on Plant Growth

There is considerable published information indicating that mycorrhizas enhance plant growth, which can be understood to be the result of an improved

mineral nutrition of the host plant, for which evidence has been provided using

isotopic tracers. An increased concentration and/or content of phosphorus in

plants is by far the response to mycorrhizas most often described. However, the

concentration and content of other nutrients can also increase (sometimes as a

consequence of a better P uptake), and there might also be some nonnutritional

effects.

Mycorrhizas not only increase plant biomass but also influence the partitioning

of this material between shoot and root. The enhanced nutrient uptake and the

subsequent translocation to the aerial part of the plant increases the utilization of

photosynthate in the shoot, hence relatively fewer photosynthesis products are

transferred to the root. Consequently, the root/shoot ratio is usually lower in

mycorrhizal plants than in the corresponding nonmycorrhizal controls (Smith,

1980). A change in the hormonal status as induced by mycorrhizal infection also

can be involved (Allen et al., 1980, 1982).

In some instances adverse effects on plant growth in response to VAM have

been found (Smith, 1980;Buwalda and Goh, 1982). In most cases the depression

is merely transitory and caused by a fungus-plant competition for available

photosynthate at the early infection stages or under suboptimal photosynthetic

conditions (e.g., shading or low temperatures). Persistent depressions take place

when supraoptimal P concentrations are reached in the plant tissues or when the

soil phosphate concentration is such that fungus maintenance becomes expensive.

2 . Eflect of Mycorrhizas on Phosphorous Nutrition

a. Source of Phosphorus for Mycorrhizas. Because approximately 95-99%

of soil P occurs in forms that are not directly available to plant roots (Bieleski,

1973), the possibility that the fungus could solubilize unavailable P was a tantalizing hypothesis to explain the mechanism for the increased P supply by

mycorrhizas. In addition, certain sparingly soluble P compounds seemed to be

utilized by VAM as a source of P. This possibility was investigated by experiments in which the labile phosphate pool was labeled with 32P (Sanders and

Tinker, 1971; Mosse and Hayman, 1971; Powell, 1975). The specific activity

(32P/31P)of P in plant tissues was similar for mycorrhizal and nonmycorrhizal

plants although the former take up more phosphate. If mycorrhizal plants did

utilize nonlabile (unlabeled) sources of P, the specific activity in these plants

would be expected to be lower than in nonmycorrhizal controls. These facts

show that the plants used the same soluble phosphate pool irrespective of

whether they were mycorrhizal. Consequently, it is widely assumed that mycor-



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J. M.BAREA AND C. AZC6N-AGUILAR



rhizal plants draw most of the phosphate from the soluble pool, although more

efficiently than nonmycorrhizal plants. This is in agreement with the finding that

VA infection does not modify the root surface-bound phosphatase activity.

Moreover, there is not an increase in the exudation of hydroxyacids that could

solubilize phosphate either by their chelating activity or merely by reducing the

pH in the mycorrhizosphere.

The apparent solubilization of poorly soluble sources of P, such as rock

phosphate, can be attributed to the greater contact between the network of external hyphae and the surfaces of phosphate particles in soil where phosphate is

being physicochemically or biologically dissociated (see Hayman 1975a, 1978).

However, further research on this subject is needed to clarify some of the points

previously discussed.

b. Phosphate Uptake by External Mycelium. The rate of nutrient absorption

by roots or mycorrhizas is known to depend on the rate of nutrient supply to the

rhizosphere, this being influenced by the mobility of the ion and its concentration

in the soil solution. These facts are of great relevance in P nutrition (Chapin,

1980). Phosphate ions, which are in low concentration in the labile pool

(Bieleski, 1973), move by diffusion very slowly because they are readily adsorbed to the soil colloids. Plants take up phosphate much faster than these ions

can diffuse to the root surface; consequently, phosphate-depleted zones normally

develop around the absorbing organs of the plant. These zones, which are 1-2

mm wide, coincide with the rhizosphere and can be visualized by autoradiography (Owusu-Bennoah and Wild, 1979). Vesicular-arbuscular mycorrhizas enhance P uptake in two different ways. One mode of fungal action is merely

physical and is based on the increased number of sites for absorption achieved by

the external mycelium. The hyphae growing through soil pore spaces are able to

effect phosphate absorption beyond the depletion zone up to 8 cm from the root

(Rhodes and Gerdemann, 1975). Thus, mycorrhizal roots explore a much greater

volume of soil to take up phosphate. A correlation has been found between the

size of the external mycelium and the flux of phosphate into mycorrhizal roots

(Sanders et al., 1977). Obviously, once inside the hyphae, phosphate ions are

protected against absorption by soil components.

On the other hand, the kinetic analyses carried out by Cress et al. (1979)

demonstrated that mycorrhizas have a lower apparent Michaelis constant (K,) of

phosphate uptake than nonmycorrhizal roots, suggesting as the second mode of

fungal action the existence of a pathway of greater affinity for P in mycorrhizal

roots. This reinforces the results of Mosse ef al. (1973) which suggest that VAM

reduce the threshold value for effective phosphate absorption from soil. In spite

of the activity of surface phosphatases in ECM, the bulk of P gained by them also

comes from the labile pool, and in this case the mycelial strands, which may

reach 12 cm in length, are responsible for the increased absorption of phosphate

(Bowen, 1973; Harley, 1978).



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CHAPTER 1. MYCORRHIZAS AND THEIR SIGNIFICANCE IN NODULATING NITROGEN-FIXING PLANTS

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