Mycorrhizae

The term "mycorrhizae" comes from the Greek "myco" or fungus and "rhiza" or root and literally means "fungusroot." Mycorrhizae have been described as perhaps the ultimate in reciprocal parasitism (symboisis) wherein the fungus supplies the higher plant partner with mineral nutrients and plant growth hormones and protects the roots against pathogens, and the higher plant supplies the fungus with energy substrates. Almost all higher plants are dependent upon one of the mycorrhizal associations outlined in Table XIX, as only crucifers, chenopods, sedges, and aquatic plants are thought to be non-mycorrhizal.

TABLE XIX Kinds of mycorrhizal associations.
I. ENDOMYCORRHIZAE A. VESICULAR-ARBUSCULAR (VA) B. ERICACEOUS C. ORCHIDACEOUS
II. ECTOMYCORRHIZAE
III. ECTENDOMYCORRHIZAE



TABLE XX Endomycorrhizae.
I. VESICULAR ARBUSCULAR. HIGHER PLANTS--GREAT MAJORITY OF THE VASCULAR PLANTS INCLUDING FERNS, MONOCOTS, DICOTS, FUNGI-ENDOGONACEAE (PHYCOMYCETES).
II. ERICACEOUS. HIGHER PLANTS-ERICALES. FUNGI--GENERALLY UNKNOWN ALTHOUGH A BASIDIMYCETE HAS BEEN FOUND IN ASSOCIATION WITH AZALEA.
III. ORCHIDACEOUS. HIGHER PLANTS--ORCHIDS. FUNGI--AGARICALES AND APHYLLOPHORALES (BASIDIOMYCETES).

TABLE XXI Ectomycorrhizae.
HIGHER PLANTS--MANY WOODY PLANTS FAMILIES INCLUDING BETULACEAE, FAGACEAE, PINACEAE, TILIACEAE, AND SALICACEAE.
FUNGI--MANY THOUSANDS OF SPECIES IN BASIDIOMYCETES, ASCOMYCETES, AND ENDOGENACEAE (PHYCOMYCETES).

Click to enlarge.

The distinction between ecto- and endomycorrhizae is based upon the morphology of the structure formed by fungus and higher plants. Ectomycorrhizal forming fungi are able to penetrate between, but not into, the cortical cells of the host root, thus forming a network of fungal strands within the cortex commonly called a "Hartig net." The fungus also normally makes a mantle which completely encloses the infected root tip and from which fungal hyphae extend into the soil. This network of hyphae greatly increase the ability of the root to take up nutrients in three ways:

1. The surface area in contact with soil particles is much greater, and the much smaller hyphae can penetrate fissures in soil particles too small for roots;

2. The fungus excretes enzymes which allow it to dissolve soil nutrients otherwise unavailable to the roots; and

3. The fungus extends the life of an absorbing root from a few days to a year or more.

Douglas-fir seedlings with and without mycorrhizae inoculation.

Inoculation of tree seedlings with appropriate fungus species to stimulate development of mycorrhiza has been shown to be critical to reforestation, especially afforestation, in a diverse range of sites and climates from timberline in the Alps to subtropical islands like Puerto Rico. The last fifteen years have seen great interest in mycorrhiza in the southern states where fungi such as Pisolithus tinctorius are believed to be responsible for increased growth and survival of loblolly pine, especially on harsh sites such as the spoils of mining operations.

Endomycorrhizae differ from the above primarily in that the fungus hyphae penetrate directly into the cortical cells of the root. Neither endo- nor ectomycorrhizal forming fungi are, however, able to penetrate beyond the cortical region -- the vascular tissues of the root are not infected.

Table XXII discusses the second major positive role in the rhizosphere--the ability of fungus to protect the root tissue from infection by parasitic organisms. The mechanisms suggested in the table are hypotheses, although there is a considerable body of data to support each. It should be noted, however, that any such protection is not absolute, i.e., mycophagous nematodes are capable of destroying mycorrhizal roots, and such protection extends only to the infected root tips; the remainder of the root systems has no such protection. What significance does this last fact have upon the possible use of mycorrhizae to protect tree roots against disease organisms such as Phellinus weirii?

TABLE XXII Possible mechanisms of reduction of infection by root pathogens by mycorrhizae.

So far we have discussed the positive contributions of the fungal partner to the higher plant as a result of the formation of mycorrhizae. Can you name a negative role of the fungus? It is a considerable drain upon the carbohydrate reserves of the higher plant. Estimates in forested areas suggest that the quantity of fungal material associated with the root system may represent 10-20% of the total root mass. Further, the fungal tissue is by no means completely recovered, since great masses ramify throughout the soil mantle. And, finally, the respiratory rate of fungi is generally significantly higher than that of higher plants and hence a given mass of fungal tissue requires relatively greater energy than a similar quantity of tissue from the host plant.


Naked Root! (No Mycorrhizal Association)

Feeder roots that have just started to grow and continue to grow actively do not have mycorrhizae. These roots have minute root hairs that push out into the soil to take up water and nutrients. In time these roots do become mycorrhizal.


Study Questions

1. Current research is concerned with identifying fungal species with particular tolerance of hot, dry sites as potential mycorrhizal components for seedlings planted in harsh sites. Do you think this approach has merit? Can you identify other extreme conditions for which mycorrhizae might be beneficial in increasing seedling survival?

2. What is the best way of minimizing loss of seedlings to disease in the nursery? Would it be effective against Botrytis? Why?

3. What is one major potential disadvantage of nursery bed fumigation? If fumigation is done primarily to control weeds, would the use of herbicides accomplish the same goal with less risk?

4. Define integrated pest management (IPM).

5. Describe the role of water in soil as a vector for disease.

6. Name an insect that can feed on agricultural crops and seedlings.


Readings

1. Sutherland, J.R. 1984. Chapter l9. Pest Management in Northwest Bareroot Nurseries. in Duryea, M.L. & T.D. Landis (eds.) The Forest Nursery Manual. p. 203-210.

2. Molina, R. and J.M. Trappe. 1984. Chapter 20. Mycorrhiza Management in Bareroot Nurseries. in The Forest Nursery Manual p. 211-223.

3. Lavender, D.P. et al., 1990. Regenerating British Columbia's Forests. Chapter 4, p. 39, Chapter 16, p. 213-214, Chapter 20, p. 269-278, Chapter 21, p. 287-301. (Briefly look this over.)


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