Mycorrhizal Associations Practice Questions

Correct answer: Fungi and roots

Mycorrhiza is a mutualistic association between a fungus and the roots of a higher plant. The fungus assists in nutrient absorption while receiving energy in the form of carbohydrates from the plant.

Correct answer: Pinus

Pinus seeds have an obligate association with mycorrhizae and cannot germinate or establish into a healthy plant without the fungal partner. This relationship is essential for the tree's survival in nutrient-poor soils.

Correct answer: Phosphorus

Mycorrhizal fungi are particularly efficient at absorbing phosphorus from the soil and passing it to the plant. This is especially important as phosphorus is often immobile and difficult for plant roots to reach directly.

Correct answer: Ectomycorrhiza

Ectomycorrhizae form an external sheath or mantle around the root and grow into the intercellular spaces of the cortex (Hartig net). They do not penetrate the cell walls of the root cortex.

Correct answer: Endomycorrhiza

VAM is the most common form of endomycorrhiza where the fungi penetrate the cortical cells to form specialized structures like vesicles and arbuscules. These structures facilitate the exchange of nutrients between the fungus and the plant.

Correct answer: Root-borne pathogens

Mycorrhizal associations provide plants with enhanced resistance to root-borne pathogens and increased tolerance to salinity and drought. This significantly improves the overall growth and survival of the plant.

Correct answer: Increasing the surface area for absorption

The extensive network of fungal hyphae spreads far beyond the reach of the root hairs, dramatically increasing the surface area available for the absorption of water and minerals. This allows the plant to tap into a much larger volume of soil.

Correct answer: Sugar and nitrogen-containing compounds

The plant provides the fungus with organic nutrients, primarily sugars and nitrogen-containing compounds produced through photosynthesis. This represents the 'cost' to the plant for the minerals and water it receives.

Correct answer: Brassicaceae

Members of the family Brassicaceae (like mustard) and Chenopodiaceae are typically non-mycorrhizal. They have evolved different strategies for nutrient acquisition that do not rely on fungal symbiosis.

Correct answer: Nutrient exchange between fungus and plant

Arbuscules are highly branched structures within the cortical cells that provide a massive surface area for the transfer of nutrients. They are short-lived and are the primary site of metabolic exchange.

Correct answer: Hartig net

The Hartig net is a complex of fungal hyphae that grows between the epidermal and cortical cells of the root. It is the primary site of nutrient exchange in ectomycorrhizal associations.

Correct answer: Glomeromycetes

Most arbuscular mycorrhizal fungi belong to the phylum Glomeromycota. These fungi are obligate symbionts and cannot complete their life cycle without a plant host.

Correct answer: Nutrient-deficient

In nutrient-poor soils, plants struggle to acquire enough minerals (especially phosphorus). Mycorrhizae provide a critical advantage by scavenging these limited resources more effectively than the plant roots could alone.

Correct answer: By improving water uptake and regulating ion balance

Mycorrhizae help plants cope with salt stress by maintaining a favorable K+/Na+ ratio and enhancing water absorption. This helps mitigate the physiological drought and ion toxicity caused by salinity.

Correct answer: Heterotroph

Fungi are heterotrophic organisms that cannot synthesize their own food. In mycorrhizae, they depend on the host plant's photosynthates to meet their energy requirements.

Correct answer: A dense sheath of fungal hyphae covering the root tip

The mantle is a thick layer of fungal tissue that completely covers the outer surface of the root. It effectively replaces the root hairs in terms of function and protects the root tip.

Correct answer: Frequent branching and a 'coralloid' or 'stubby' appearance

Mycorrhizal infection often induces the root to branch more frequently and suppresses the development of root hairs. This results in a distinctive, clustered appearance of the root system.

Correct answer: Have virtually no food reserves (starch/fat)

Orchid seeds are 'dust-like' and contain very little stored energy. They rely entirely on a mycorrhizal fungus to provide them with carbon and nutrients during the early stages of germination.

Correct answer: Mutualism

Mutualism is an interaction where both participating species benefit. The fungus gains carbohydrates, while the plant gains improved access to water and essential minerals.

Correct answer: Secreting organic acids

Mycorrhizal fungi can secrete organic acids that help dissolve minerals from rock surfaces and soil particles. This biochemical weathering makes nutrients like phosphorus and potassium more available to the plant.