Introduction

AM fungi utilise host organic carbon for growth and respiration. In the absence of a supply of energy from a host, any fungal growth uses reserves and is limited. Reliance on the plant is important for several reasons. Host photosynthates are diverted to supporting the mycelium. Thus the requirement for carbon by the fungus will reduce the rate of plant growth in some circumstances. The fungus may also increase rates of photosynthesis, primarily through improved host nutrition, but also possibly because of alterations to host signalling. In addition, carbon from one host is moved around the mycelium, and may become important in a plant coincidentally linked into the mycelium.

Removal of Carbon From the Host

In experimental conditions between 6 and 20% of host photosynthate is removed by AM fungi attached to the root. The quantity of carbon removed is largely determined by how much carbon is fixed and the quantity of fungal mycelium being supported. Photosynthesis is maximised under high irradiation. As the photosynthetically active radiation (PAR) decreases, the amount of carbon fixed declines. At compensation point, the quantity of energy lost through respiration equals that fixed in photosynthesis. Below compensation point, the loss of energy is greater than energy gained from PAR.

AM fungi improve rates of growth of plants under high PAR, in most circumstances. However, as the PAR declines, the carbon cost of the fungus may exceed any benefit provided by increased uptake of minerals. Experimental evidence using single plants in single pots indicate that just above compensation point, the carbon cost of the fungus is detrimental to the plant.

This experimental result may not apply to plants in the field. This is due to two factors. The plant regulates the quantity of AM in roots. Under high PAR, the net gain from photosynthesis is more than adequate to support the attached mycelium under most circumstances. As PAR declines, however, the proportion of the root colonised declines. This indicates that the host regulates colonisation, and that reallocation of resources to shoot growth overrides mineral uptake from the roots.

In the field, plants are interconnected by mycelia of several fungi. The fungi acquire energy from several hosts, and move the carbon around within the mycelia. Some of the carbon can be shifted from fungus to host, especially where the host is either shaded, or of a species that is less dependent on the fungi for acquisition of minerals. In the case of achlorophyllous plants, the mycorrhizal association is the only source of carbon. While the importance of the movement of carbon to the fungus is still being debated, it is clear that experiments with single plants in experimental pots provide only part of the story.

A satisfactory analysis of carbon efficiency remains undeveloped. Efficiency of symbiosis and cost-benefit analyses have been suggested but not yet widely adopted. The problematic “dependency” approach, which is a measure of plant responsiveness under specific circumstances, is still being used despite being highly qualified. One general trend however, appears to hold. At equivalent P status, mycorrhizal plants have a lower efficiency of carbon production than nonmycorrhizal plants. In soils where available P is less than adequate, this may be unimportant. LINK In plants where the rate of growth is normally slow, the cost of the fungus may be undetectable. However, in fast-growing crop plants (eg well fertilised tobacco) the reduction in plant growth rates may be noticeable and economically important.

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Improved Nutrition

The increased exploration of soil leads to increased plant uptake of immobile minerals in unfertilised soils. The improved mineral nutrition of the host increases rates of photosynthesis. Thus more energy becomes available for both partners. Some evidence also supports the suggestion that the mycorrhizal symbiosis influences plant growth rates due to some factor or factors other than mineral nutrition. While the measure of plant growth is gain in carbon fixed, it is possible that the increase is due to changes in water relations or the hormonal balance within the host plant which is directly, or more probably, indirectly related to the presence of the fungi in the roots. The issues remain to be explored thoroughly.

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Distribution of Carbon in Soil

The plant normally exudes significant quantities of organic carbon. The presence of AM in roots reduces the loss of carbon through the root surface. This reduction in exudation and change in quality of exudates influences the population of microbes found in the rhizosphere. The population of microbes associated with roots is smaller and the structure of the microbial community differs bewteen mycorrhizal and non-mycorrhizal roots. Changes in the populations of Mn bacteria, plant-growth-promoting bacteria, N fixing bacteria and pathogens have been noticed.

Further, the turnover in hyphae and exudation from hyphae, especially at the hyphal tips, suggest that the distribution of microbial activity in soil will be more widespread in the presence of mycorrhizal fungi. The release metabolites alters the abundance and diversity of microbes away from the root.

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Distribution of Organic Carbon Between Plants

That organic carbon is transferred from plant to fungus is not disputed. However, that significant carbon may be reallocated underground such that secondary plant effects are significant is debated. The hypothesis is as follows:

1. Donor plants transfer carbon to the attached mycelium
2. Carbon moves through the mycelium, and into any plants attached to the mycelium
3. Carbon is transferred from fungus to the attached recipient plants
4. The transferred carbon increases growth of the recipient plant.

The growth of achlorophyllous plants is entirely due to acquisition of carbon from their mycorrhizal fungus. The mechanism for significant transfer of carbon exists in plants. In experimental microcosms, changes in relative rates of growth between grasses and dicot hosts have been demonstrated. These changes are associated with the movement of radioactive carbon from the grass to the dicot. However, carbon also moves in the other direction, and the total amount appears to be small, in terms of the total carbon delivered to the mycelium, and the carbon required to noticably increase plant growth.

The movement of carbon has been suggested to be important for seedlings in shade. Shade plants however, usually allocate few resources to their mycorrhizal symbiont in experimental pots. Field measurements of colonisation indicate that for some plants colonisation is greater in the field grown seedlings. The relative allocation of carbon from the mycelium necessary to maintain respiration in a seedling is minor. Thus in a forest, seedlings may be maintained in the absence of adequate irradiance. If patches of light appear, then rapid growth of the connected seedling in the patch is possible. As seedlings usually appear adjacent to their parent in a forest, this means a plant population may be sustained. While this scenario is largely speculative, survival of seedlings of some plant species in deep shade for years remains to be explained.

A fungus attached to one host may function more aggressively. Interesting information on the process of colonisation is now emerging. When mycorrhizal fungi are attached to a mycorrhizal host, roots of other plants species that are not normally mycorrhizal may become colonised by the fungi. The process of colonisation is more aggressive when the plants are in low P soils. More importantly, the results indicate the importance of carbon to mycorrhizal fungi, whose colonising potential is enhanced. Carbon is essential to mycorrhizal fungi, both for maintenance and extension. In the absence of carbon, the capacity of the fungi to function is severely reduced.

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Conclusion

Host-derived organic carbon is essential to arbuscular mycorrhizal fungi. The use of photosynthates by mycorrhizal fungi also influences both the host plant and the associated microflora. Mycorrhizal fungi may drain their host, but different hosts are drained to different extents, and probably in different ways by each fungus. The consequence is that carbon is moved from the host into the connected mycelium. Carbon moves around soil within the mycelium. The carbon from one host may influence the growth of an interconnected plant, and it certainly enables mycorrhizal fungi to elongate and colonise other plants.

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References

Smith SE & Read DJ 1997 Mycorrhizal Symbiosis. Academic press.

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