From the 1985 Yearbook of the California Rare Fruit Growers

 

MYCORRHIZAE:

The Other Half of the Root System

T. V. St. John

Laboratory of Biomedical and Environmental Sciences

University of California

Los Angeles, California 90024

and

Mycorrhizal Services

P.O. Box 391

Wlldomar, California 92395

Mycorrhizae are Symbiotic Fungi

We tend to think of fungi as harmful, but in fact the fungi that infect more plant tissue than any other kind are the beneficial mycorrhizal fungi. The relationship is a symbiosis, meaning that both partners benefit. The fungus receives sugars from the plant, and the plant receives phosphorus from the fungus. The word mycorrhiza means Ďfungus-root"; its plural is mycorrhizae or mycorrhizas.

These little-appreciated and inconspicuous microorganisms live partly in the soil and partly inside the roots. Mycorrhizal fungi are allies of both wild and domesticated plants, aiding them in their struggle to extract nutrients from the competitive and sometimes hostile environment of the soil. Nutrient deficiencies are almost never found in wild plants, in large part because mycorrhizal fungi are abundant in nature. Mycorrhizae are also present, or ought to be present, in a great variety of cultivated plants. They can be found in almost every soil in the world, except where human activities have suppressed the symbiosis.

Structures that are identical to modern mycorrhizae have been found in the very earliest fossils of land plants. It has even been suggested that the formation of an association between aquatic plants and a fungus first allowed plants to move onto land. Whether this is true or not, we know that mycorrhizae have been an important component of the root system for as long as plants have been growing, and their importance is no less today than it ever was.

Mycorrhizae lie hidden below ground, and usually are invisible unless specially stained. They were known to science by the turn of the century, but have not been studied in detail until the last two decades. We classify them into several distinct types, each of which has a different kind of fungus and host plant. The most important types of mycorrhizal symbiosis are:

Vesicular-arbuscular mycorrhizae (VAM): Sometimes called "endomycorrhizae," these are by far the most widespread. VAM are the mycorrhizae of crop plants and most annual and woody natives. They do not produce visible mushroom-type reproductive structures, but form spores that are the largest of any fungi. They cannot be grown in laboratory conditions, but will grow with a wide variety of host plants. The same species of fungus may be associated with liverworts, ferns, Redwood trees, and garden vegetables.

Ectomycorrhizae (ECM): The mycorrhizae of forest trees: the Pines, Firs, Spruces, Oaks, and Willows. The Eucalypts are primarily ectomycorrhizal, as are some tropical forest trees. Among crop plants, Chestnut, Hazelnut, Pecan and possibly some tropical tree crops have ECM. The fungi of ECM form some of the mushrooms that we see in coniferous and Oak forests; truffles and the deadly Aminitas are examples. The fungi form a layer around the short side roots, and can often be seen with the unaided eye. ECM fungi are much easier to grow in laboratory conditions than VAM fungi.

Other types: The family Ericaceae has mycorrhizae which are similar in many ways to ECM. Blueberries and their relatives are examples of crop plants with the "emicoid kind of mycorrhiza. The Orchidaceae and some families of small tropical plants also have distinct kinds of mycorrhizae.

 

Mycorrhizae Aid Phosphorus Uptake

Mycorrhizae have been clearly shown to improve plant growth over that of non-mycorrhizal controls; a few of the many examples are shown in table 1. The magnitude of the mycorrhizal growth response depends heavily on such factors as the species of plant and the native fertility of the soil used in the experiment.

Both VAM and ECM can greatly improve uptake of phosphorus and micronutrients, especially zinc and copper. We also know that mycorrhizal plants are better able to withstand drought than non-mycorrhizal plants. Mycorrhizal container stock has a higher rate of survival and better performance after transplanting. Mycorrhizal plants are more resistant to some pathogens, have altered production of plant hormones, and have more highly branched root systems than non-mycorrhizal plants. Cuttings of some species have an improved rooting ability when the medium contains mycorrhizal fungi. Many of the benefits, even those which seem unrelated to phosphorus, appear to actually be side benefits of improved phosphorus nutrition.

Phosphorus occupies a special place in the processes of life. Its unique chemistry, and the various combinations it can form with other elements, make it the basis of energy transfer in biochemical reactions. Without a steady supply of phosphorus, growth stops and the leaves may show deficiency symptoms. Phosphorus is present only in low concentrations in the soil. Moreover, it easily becomes chemically bound, it is absorbed by microorganisms, and is otherwise made unavailable to plants. Unaided roots are usually not able to acquire enough phosphorus for the plantís needs. The tiny thread-like filaments of the mycorrhizal fungus spread out into soil that is in effect beyond the reach of the roots, where they can absorb phosphorus and other elements from a much larger volume of soil. The filaments of the mycorrhizal fungus, called hyphae, are especially effective at absorbing phosphorus from microsites where it is locally abundant, as near decomposing plant or insect remains.

The soil, as well as the plants, is affected by mycorrhizal fungi. The hyphae are an important component of soil structure, holding together crumbs that allow penetration of water and air, and encourage the growth of roots through the soil.

Mycorrhizae are Important to Most Fruit Trees

Table 1 shows the results of some experiments in which mycorrhizae improved the growth of fruiting species. Many crop and native plants have been tested in experiments of this type. Fruit trees have shown some of the greatest responses of any plants tested, meaning that they rely even more heavily than other plants on their mycorrhizal symbionts. The dependence demonstrated by these experiments is known as "mycotrophy.í Many additional species are thought to be mycotrophic, but the actual experiments have never been done. All we know about them is that they become mycorrhizal in the field. Table 2 shows a list of plant species that we know are mycorrhizal, but which we can only assume to be mycotrophic. Most of the information comes from my own work in Brazil, and from several older papers.

 

Tables 3 and 4 show plants about which we know even less. Those listed in table 3 are probably mycorrhizal (and mycotrophic) because other species in their families are known to be so. The species in table 4 remain a complete mystery. I will be happy to examine some specimens of plants from tables 3 or 4 if any CRFG member can contribute roots of plants grown outdoors.

 

We can see that VAM are by far the most widespread type of mycorrhiza among fruit trees, as they are among plants in general. Blueberries and other Ericads possess their own distinctive type, and ECM could still be discovered in the families Myrtaceae, Sapotaceae, or Caesalpiniaceae. However, I am not aware of direct confirmation of ECM in any fruit species. The family Proteaceae is thought to be entirely nonmycorrhizal. Thus Macadamia Nut stands as an isolated example of a species that probably does not require a fungal symbiont.

Mycorrhizae May be Lost from the Soil

It often turns out that nursery stock and potted plants have failed to become mycorrhizal. Less often, outdoor garden plants may be poorly colonized or even entirely nonmycorrhizal. The problem can usually be traced to over fertilization, lack of fungal inoculum, or anti-fungal action of pesticides.

Heavy fertilization may lead to faster growth or a larger yield over the short term, and can compensate for a lack of mycorrhizae, but creates an "addiction" from which there is no easy return. Mycorrhizae are a phenomenon of natural soils; they can only operate in the range of mineral availability found under natural conditions. A high concentration of phosphorus or nitrogen triggers a defense mechanism in the plant, and the symbionts are excluded. The potential problems arising from a loss of mycorrhizae include the so-called phosphorus-induced zinc deficiency of Citrus. The plantís phosphorus needs are met by the fertilizer, but mycorrhizae are excluded and can no longer aid in uptake of zinc. The non-nutritional benefits of mycorrhizae, such as improved soil structure, are also lost. Until mycorrhizae, and conditions allowing their survival, are reintroduced, the plant will grow only with heavy chemical fertilization.

Mycorrhizae may be absent because at some time in the past the native fungi in the soil were all killed. The fungal partners of mycorrhizae are widespread in native soils, being absent only in certain desert areas and in places devoid of plants. However, soils disturbed by human activities often lack mycorrhizal fungi. Removal of the plant cover, handling and treatment of soil, and inversion of soil profiles all reduce or eliminate natural inoculum, the spores and other fungal parts that can propagate the symbiosis. Most mycorrhizal fungi disperse slowly, especially the VAM type, and are very slow to re-establish once lost from a soil. Nursery soils and commercial potting mixes are often sterilized, and plants in such materials are never mycorrhizal until the fungi are reintroduced in some way.

Mycorrhizae may be lost through the use of pesticides. Pesticides are labeled as specific to a particular group of organisms; i.e., nematodes, insects, herbs, etc. However, all have a certain amount of activity against non-target organisms. Human beings and mycorrhizal fungi are examples of non-target organisms that can be seriously harmed by pesticides. Fortunately, not all pesticides are inhibitory, and appropriate compounds can be found to treat most pest problems without destroying the mycorrhizal fungi. Heavy and indiscriminate use of pesticides is most likely to suppress the symbiosis, as well as other beneficial soil organisms.

Mycorrhizae May be Restored to Cultivated Plants

Fortunately, once the lack of fungus has been identified as a problem, it is often possible to put it back into the soil. In the past few years a great deal of research has been directed to the problem of enriching agricultural soil with VAM fungi. The English and Australians have been particularly active in this research, because of the phosphorus-deficient soils in their countries. While there have been some encouraging results, the use of VAM in the field remains largely an experimental curiosity because of the difficulty of producing large amounts of inoculum. The inoculum usually employed is a mixture of soil, spores, fungal hyphae, and dead mycorrhizal roots that allow reestablishment of the mycorrhizal condition in new roots. It is heavy and has a limited life span, and so is difficult to transport and store. It is likely to remain impractical until increased demand encourages large-scale production. At present, commercial use of VAM is limited to certain kinds of container plants, especially those destined for use in revegetation of disturbed sites. The enhancement of transplant survival and the subsequent ability of the plants to thrive with minimal care make the introduction of VAM at the nursery worthwhile. There is also much interest in inoculating new plantings of Citrus, because it is planted into fumigated soil that lacks native fungi. In tropical and low-input agriculture, a combination of natural symbionts and rock phosphate may be preferred to expensive chemical fertilizer. It appears that rock phosphate has no fertilizer value except to mycorrhizal plants. Unfortunately, inoculum is only now becoming available. The best strategy is to encourage the spread of native fungi that may already be present.

We can encourage the natural spread of mycorrhizae by weaning our plants away from heavy fertilization. Phosphorus, in particular, is harmful. Mycorrhizal plants in most California soils can obtain all the phosphorus they need from native soil phosphorus, bone meal or rock phosphate. In moderate quantities, these substances seem to encourage rather than inhibit the mycorrhizal symbiosis. Nitrogen should be applied in organic forms where possible. Chemical nitrogen should be in the form of nitrate rather than ammonium. Organic soil amendments, such as manure, should be well-aged. Fresh organic materials may encourage large microbial populations that can be inhibitory to mycorrhizal fungi.

In container plants growing in soil-free or sterilized potting mix, mycorrhizal inoculum is entirely absent. Mycorrhizae may be introduced to containers by taking advantage of the wide host range and ubiquitous presence of the fungi. Mycorrhizal roots from field-grown plants constitute a very effective source of inoculum. Even dead roots from last yearís crop should quickly infect potted plants if placed correctly in the soil and used in mycorrhizacompatible conditions.

If you would like to give mycorrhizal inoculation a try, you can use roots from the garden as inoculum. You are not likely to be able to tell whether the roots you collect are actually mycorrhizal. To do so requires a special process of removing the cytoplasm from the root cells, then staining with a dye that colors the fungal tissue but not the plant tissue. You must also be able to distinguish mycorrhizal fungi from pathogenic and other fungi that find their way into roots. With certain species from the garden, you may take it on faith that they are mycorrhizal, unless you grew them in sterile potting mix. Garden plants that are virtually always mycorrhizal, even if they have been given a bit too much fertilizer, are Corn, Strawberry, and Citrus. Other garden and field plants that usually manage to become mycorrhizal are Dandelion, Peas and Beans, Tomatoes, and stone fruits. Plants of the Mustard and Spinach families do not become mycorrhizal, and weeds and lawn grasses are unreliable sources of inoculum. Find the smallest roots; only the young, nutrient-absorbing rootlets are mycorrhizal. Choose a donor species that is distant taxonomica]ly from your container plant. Parasites and diseases that may be included with the mycorrhizal roots are most likely to infect relatives of the plant on which they were growing.

By inoculating container plants with these methods, one accepts a few risks. Roots and soil commonly carry with them nematodes, pathogenic fungi, and other unwanted hitchhikers. The advantages of pine-sterilized potting mix will be lost. Further, not all species of mycorrhizal fungi are equally effective in aiding the host plants. You will probably introduce a grab-bag of several species, and there is no guarantee that particularly effective ones will be among them. Most experimental and nursery inoculation is done with fungi that have been proven under the local conditions. A final class of complications involve unexpected and sometimes subtle things that can go wrong. The inoculum must be placed properly in the soil, so that new roots will pass through it. Soil temperature must be neither too high or too low. Colonization of roots usually will not take place above about 35 deg. C or below 10 deg. C. The plant should be well illuminated to allow optimum photosynthesis. If it is in a dark corner it may not have any extra sugars available to give to the fungus. Inoculation attempts often fail during the winter, even in greenhouses, because the day length is too short to allow optimum photosynthesis. All of these factors vary widely with the species of fungus, of host plant, and with environmental conditions, making the use of mycorrhizae as much art as it is science.

Conclusion

The mycorrhizal symbiosis is the natural means by which most plants get the phosphorus they need from the soil. Their importance has been demonstrated in numerous experiments in which normally mycorrhizal plants have been compared to plants without symbionts. These experiments repeatedly show as much as sixty-fold differences in growth rate, with the enhancement usually attributed to improved phosphorus nutrition. Chemical fertilization of non-mycorrhizal plants can partially replace mycorrhizae, but cannot replace the soil-building and other ancillary effects of the symbionts. Because heavy fertilization prevents establishment of the symbiosis, it in effect creates a dependence on the chemicals.

Since the earliest agricultural times, mycorrhizal fungi have been taking care of themselves. In the last few decades, the widespread use of fertilizers and pesticides have created conditions unfavorable for mycorrhizae. The situation is especially acute in nurseries producing container stock. Grown in sterilized potting soil, heavily fertilized, and freely treated with pesticides, these non-mycorrhizal plants require continued intensive care after outplanting.

There has been in recent years a worldwide movement toward controlled and sustainable agriculture. In the tropics the movement is fueled primarily by the economics of importing fertilizer. In developed countries, it arises from a desire to protect and build soil resources so that high yields may be maintained far into the future. There will be an important place in this kind of agriculture for tree crops, with their ability to use soil resources conservatively, and for the mycorrhizal fungi that help them do it.

Literature Cited and Further Reading

Geddeda, Y. I., J. M. Trappe, and R. L. Stebbins. 1984. Effects of vesicular-arbuscular mycorrhizae and phosphorus on apple seedlings. Journal of the American Society of Horticultural Science. 109:24-27.

Janos, D. P. 1977. Vesicular-arbuscular mycorrhizae affect the growth of Bactris gasipaes. Principes 21:12-18.

Kleinschmidt, G. D., and J. W. Gerdemann. 1972. Stunting of Citrus seedlings in fumigated nursery soil related to the absence of endomycorrhizae. Phytopathology 62:1447-1453.

Kormanik, P. P., H. C. Schultz, and W. C. Bryan. 1982. The influence of vesicular arbuscular mycorrhizae on the growth and development of eight hardwood tree species. Forest Science 28:531-539.

Menge, J. A., J. LaRue, C. K. Labanauskas, and E.L.V. Johnson, 1980. The effect of two mycorrhizal fungi upon growth and nutrition of avocado seedlings grown with six fertilizer treatments. Journal of the American Sodety of Horticultural Science 105:400-404.

Ramirez, B. N., D. J. Mitchell, and N. C. Schenck. 1975. Establishment and growth effects of three vesicular-arbuscular mycorrhizal fungi on papaya. Mycologia 67:1039-1041.

Read, 0. J. 1983. The biology of mycorrhiza in the Ericales. Canadian Journal of Botany 61:985-1004.

St. John,T. V.1980. Uma lista de plantas Brasileiras tropicais infestadas per micorrizas do tipo vesicular-arbuscular. (A list of tropical Brazilian plant species infected with vesicular-arbuscular mycorrhizae). Acta Amazonica 10(1 ):229-234.

St. John, T. V. Mycorrhizae. Chapter 13 in G. T. Prance and T. E. Loveioy (eds.) Amazon Rain Forest. Key Environment Series, Permagon Press. To be published Summer, 1985.

St. John, T. V., and 0. C. Cdeman. 1983. The role of mycorrhizae in plant ecology. Canadian Journal of Botany 61:1005-1014.