RHS Level 3 Question on the Rhizosphere and Mycorrhizae

RHS Level 3 Principles of Plant Growth, Heath and Applied Propagation

R3102 The Root Environment, Plant Nutrition and Growing Systems November 2012

Q.4.a. State what is meant by the ‘rhizosphere’. (1)

b. Summarise the effects of the ‘rhizosphere’ on plant growth. (3)

c. Compare endomycorrhizal and ectomycorrhizal associations. (6)

Examiners’ Comments:

a. Almost all candidates were able to define the rhizosphere correctly, but not everyone mentioned the intense biological and chemical activity that occurs in this region of the soil.

b. The examiners noted that in this part of the question, some candidates confused the rhizosphere with pore space in the soil, and for instance, the importance of achieving a balance of air and water. They note: “These are qualities that affect the soil as a whole. The distinguishing feature of the rhizosphere is that it is NOT the soil as a whole but that part of it (no more than 1 – 2 mm thick) which is in intimate contact with plant roots. No plant roots: no rhizosphere. Within the rhizosphere are water, nutrients and exudates from the plant and a complex community of bacteria, fungi and microfauna which are attracted to that nutritious mix. In the main, the micro-organisms and beneficial to the plant, mineralising nutrients, processing complex biochemicals, forming a mycorrhizal associations or undertaking a protective role.”

c. The examiners note that answers to this question needed to go beyond a definition of ectomycorrhizal and endomycorrhizal to explore key features, for instance, the range and type of plants affected, degree of protective function, physical appearance of the infected root or the fungal partner.

Sample answers:

Q.4.a. State what is meant by the ‘rhizosphere’. (1)

The rhizosphere is the zone of soil surrounding a plant root where the biology and chemistry of the soil are influenced by the root. This zone is about 1 – 2 mm wide, but has no distinct edge. It is an area of intense biological and chemical activity influenced by compounds exuded by the root, and by micro-organisms feeding on these compounds.

b. Summarise the effects of the ‘rhizosphere’ on plant growth. (3)

The ‘rhizosphere’ is believed to influence plant growth in a number of ways. The roots exude water and compounds known as exudates, which may include amino acids, organic acids, carbohydrates, sugars, vitamins, mucilage and proteins. These exudates are sometimes described as messengers which stimulate biological and physical interactions between the roots and soil organisms. They modify the biochemical and physical properties of the rhizosphere and contribute to root growth and plant survival.

The rhizosphere is believed to have several functions:

The exudates within the rhizosphere defend the root against pathogenic microorganisms: Root cells are under continual attack from harmful microorganisms and survive by secreting defence proteins and other antimicrobial chemicals.
The rhizosphere keeps the soil around the roots moist. Research has found that rhizosphere soil is significantly wetter than bulk soil, which protects roots from drying out. Exudates released from roots at night also allow expansion of roots into the soil.
The rhizosphere enables roots to take up more nutrients. Exudates within the rhizosphere help roots adsorb and store ions for plant use. For instance, flavonoids in legume roots activate Rhizobium meliloti genes responsible for root nodulation that enable the plant roots to obtain nitrogen from the air. Exudates may also be responsible for encouraging vesicular arbuscular mycorrhizae that colonise roots and send out miles of thread-like hyphae into the soil, increasing the surface area available for uptake of nutrients for the roots.
The rhizosphere changes the chemical properties of the soil around the roots to enable more effective uptake of nutrients. The rhizosphere environment has a low pH, lower oxygen and higher carbon dioxide concentrations. However, exudates can make the soil in the rhizosphere more acid or alkaline, depending on which nutrients the roots are taking from the soil.
Exudates within the rhizosphere stabilise soil aggregates around the roots. Sticky mucilage secreted from continuously growing root cap cells is believed to alter the surrounding soil.
Exudates within the rhizosphere may inhibit the growth of competing plant species. Plant roots are in continual communication with surrounding root systems and quickly recognise and prevent the presence of invading roots through chemical messengers. This is known as allelopathy.

c. Compare endomycorrhizal and ectomycorrhizal associations. (6)

The hyphae of both ectomycorrhizal and endomycorrhizal fungi enter plant roots, forming a close association between fungi and plant. However, the hyphae of Ectomycorrhizal fungi pass between root cells, they do not enter the root cells, as do endomycorrhizal fungi. Ectomycorrhizal fungi usually create a sheath, or mantle, of interwoven fungal mycelium on the surface of the finest plant roots, typically 20 – 40 μm thick, so the fungus maintains connection with the soil. The hyphae also form an internal network within the root, known as the Hartig net, which weaves between the cells but does not enter them. Ectomycorrhiza is most commonly found in subarctic and temperate trees and shrubs and typically involves a higher fungus which also forms fruiting bodies above the surface of the soil.

The hyphae of endomycorrhizal fungi enter the root cells. Within this group are a wide variety of mycorrhizae of orchids, Ericaceae and Rhododendron, and a range of herbaceous and tropical plants (including many grasses). The largest group of endomycorrhiza is the arbuscular, or vesicular-arbuscular type of mycorrhiza. The arbuscules and vesicles are structures created by the fungi with the roots of mycorrhizal plants. All arbuscular fungal species form arbuscules (branched structures inside the root cells) at some point in the colonization cycle, but not all form vesicles (oil storage organs in the roots).

In endomycorrhizae the fungal cells are NOT free living and depend totally on the plant for obtaining carbon.

Both endomycorrhizal and ectomycorrhizal fungi play a protective role. Research by the USDA has shown that beneficial soil bacteria are more abundant in soils permeated by mycorrhizal fungi, and pathogenic organisms are less abundant. Protection by endomycorrhizal fungi against pathogens seems to depend primarily on independent pathogen antagonists. Endomycorrhizal fungi, in contrast, appear to protect the root by other mechanisms, including physical shielding of the root by the fungal mantle, and perhaps by the production of antibiotics.

Note: quick diagrams can be a really useful addition to answers where you are trying to explain physical appearance, a process or some other aspect that is easily illustrated. This can add a great deal of extra depth to your answer and shows the examiner your level of understanding very clearly.

Singular: mycorrhiza; plural: mycorrhizae