Mycorrhizae are mutualistic relationships between a plant and a fungus

Many relationships have evolved between roots and beneficial fungi. Such associations between a root and a fungus are called mycorrhizae. In mycorrhizal roots of trees such as pine and oak, the fungus produces a mass of filaments on the surface of the root. These filaments invade the root and form an extensive netlike structure between the cells of the cortex. A mature tree may have thousands of mycorrhizae.

Mycorrhizae are a type of mutualism, meaning that both the plant and the fungus benefit from the association. The fungus absorbs nutrients from the soil that the tree uses; in return, the host plant provides the fungus with sugar, amino acids, and other organic substances. Mycor-rhizal roots often lack root hairs, suggesting that the fungi replace the absorptive functions of root hairs. Mycorrhizae increase mineral absorption, especially for phosphorus. Roots with mycorrhizae absorb as much as four times more phosphorus than do roots that lack them. Two factors contribute to this efficient absorption: (1) the fungal body,

Phosphorus Lack

FIGURE 7.22

Mycorrhizae stimulate the growth of plants. This photo shows how mycorrhizae affect the growth of lemon trees (Citrus limon): the plants on the right were grown with mycorrhizae, whereas those on the left were grown without mycorrhizae. These trees are all 4.5 months old.

FIGURE 7.22

Mycorrhizae stimulate the growth of plants. This photo shows how mycorrhizae affect the growth of lemon trees (Citrus limon): the plants on the right were grown with mycorrhizae, whereas those on the left were grown without mycorrhizae. These trees are all 4.5 months old.

made up of hairlike filaments called hyphae, has a much greater surface area than roots, and (2) the fungal hyphae permeate a greater soil volume than do the roots (fig. 7.21).

As a result of this increased absorption, mycorrhizae often dramatically increase plant growth; they can increase the growth of wheat by more than 200%, corn by 100%, and onions by more than 3,000%. Mycorrhizae also improve a plant's endurance of drought, disease, extreme temperatures, and high salinity and are important for reclaiming strip-mined land and establishing nursery stock plants. Many plants require mycorrhizae for vigorous growth; orchid seeds germinate only in the presence of a mycorrhizal fungus, and many citrus trees and gymnosperms are difficult to grow unless mycorrhizae are present (fig. 7.22). Mycorrhizae occur in more than 80% of all plants studied.

Mycorrhizae play critical roles in determining whether tree species can invade a disturbed area and are especially important for plants growing in nutrient-poor soils. Pioneer plants—plants such as alder (Alnus) that colonize bare or disturbed soils—growing on nutrient-deficient soil invariably have mycorrhizae, as did the first plants that invaded the land millions of years ago.

Was this article helpful?

+1 0
Growing Soilless

Growing Soilless

This is an easy-to-follow, step-by-step guide to growing organic, healthy vegetable, herbs and house plants without soil. Clearly illustrated with black and white line drawings, the book covers every aspect of home hydroponic gardening.

Get My Free Ebook


Responses

Post a comment