Mycorrhizae

An important aspect of plant nutrition is the absorption of substances by certain fungi which pass these substances along to their associated plants. These intimate and mutually beneficial symbiotic associations between roots and fungi are called "mycorrhizae."

What are the mutual benefits of mycorrhizae? The best-documented advantage to the plant is the absorption of phosphate, but absorption of other nutrients and water is often facilitated. The plant, in turn, supplies some of the carbohydrates it made during photosynthesis to the fungi associated with its roots (fungi are heterotrophs and do not conduct photosynthesis).

Realization of the importance of mycorrhizae is as recent as the 1970s and 1980s. At first, mycorrhizae were considered isolated curiosities, explaining why European pines did not transplant well to the U.S. without their native soil containing the appropriate fungi. Now research has shown that mycorrhizae occur in perhaps 97% of plant species.

We know that mycorrhizal associations benefit plants colonizing all sorts of inhospitable soils - tropical soils (phosphorus tightly bound) and acidic soils (scarcity of available nitrogen), for instance. Mycorrhizae apparently make trees at the timberlines better able to cope with the cold, dry conditions that limit growth.

The potential exists for population of other harsh soils, such as mine-waste areas and landfills, through introduction of selected plants with appropriate mycorrhizae. Another avenue of research is the improvement of tropical and third world agriculture and forestry by careful attention to mycorrhizae, reducing plants' need for phosphate and perhaps nitrogen fertilizers and allowing better yield in arid areas. For industrialized countries, the study of mycorrhizae has important implications for the use of pesticides and herbicides, as well as fertilizers, on the soil.
A study of fossils shows that endomycorrhizal associations were just as frequent in ancient times as now. This has led to speculation that the evolution of mycorrhizal associations may have been a critical step in enabling plants to colonize land, since the land environment at that time was particularly harsh. Just as for modern slag heaps and volcanically devastated land, plants with endomycorrhizae (arbuscular mycorrhizae) had a much better chance of survival. Thus it may not have been a single organism, but rather a symbiotic association of organisms, that first invaded the land.

Mycorrhizae and Tree Nutrition

Nine-month-old seedlings of white pine (Pinus strobus) were raised for two months in a sterile nutrient solution and then transplanted to prairie soil. The seedlings on the left were transplanted directly. The seedlings on the right were grown for two weeks in forest soil containing fungi before being transplanted to the grassland soil.

Pine Seedlings: (without mycorrhizae/with mycorrhizae)

Mycorrhizae: Important for Fertility of Tropical Forest Soils

Network of roots and fungi lifted off a decomposing leaf in the Amazon forest. Such a network allows less than a thousandth of the nutrients reaching the forest floor to penetrate more than 5 centimeters into the soil. Mycorrhizal fungi transfer most of the nutrients back to the roots of the plants from which the leaves fell. When a tropical forest is cleared, this relationship is destroyed, and the soil generally becomes infertile quite rapidly.

Endomycorrhizae (Arbuscular mycorrhizae)

In endomycorrhizae (arbuscular mycorrhizae), the fungi form coils, branchings, or swellings in the cortical cells of plant roots. Two characteristic swellings are called vesicles (shown in a and b, below) and arbuscles (c, below). Endomycorrhizae are sometimes called vesicular-arbuscular mycorrhizae (or V/A mycorrhizae).

Endomycorrhizae. Glomus versiforme, a zygomycete, is shown here growing in association with the roots of leeks, Allium porrum. (a) General view of squashed leek root, showing vesicles. (b) Vesicles in leek root. (c) Arbuscles in leek root. Arbuscles predominate in young infections, with vesicles becoming common subsequently.

Ectomycorrhizae

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