Roots

This topic has 4 questions:

  1. What is the rhizosphere?
  2. What are the main characteristics of the rhizosphere?
  3. What is the rhizoplane and what are its main characteristics?
  4. What are some examples of how organisms in the rhizosphere influence plant roots?

Question 1. What is the rhizosphere?

The rhizosphere is the region of soil that is immediately adjacent to and affected by plant roots. The rhizosphere is a very dynamic environment where plants, soil, microorganisms, nutrients and water meet and interact. The rhizosphere differs from the bulk soil because of the activities of plant roots and their effect on soil organisms.

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Question 2. What are the main characteristics of the rhizosphere?

A major characteristic of the rhizosphere is the release of organic compounds into the soil by plant roots. These compounds, called exudates, make the environment of the rhizosphere very different from the environment in the bulk soil. The exudes can be used to increase the availability of nutrients in the rhizosphere and they also provide a food source for microorganisms. This causes the number of microorganisms to be far larger in the rhizosphere than in the bulk soil. Their presence attracts larger soil organisms that feed on microorganisms and the concentration of organisms in the rhizosphere can be up to 500 times higher than in the bulk soil.

Another characteristic of the rhizosphere is the uptake of water and nutrients by plants. Plants take up water and nutrients into their roots. This draws water from the surrounding soil towards the roots and rhizosphere. The balance between the movement of water and nutrients towards the roots and their removal from the soil by roots means that their concentration in the rhizosphere is usually very different from what it is in the bulk soil.

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Question 3. What is the rhizoplane and what are its main characteristics?

The rhizoplane is the surface of plant roots in the soil. Like the rhizosphere, the rhizoplane is a constantly changing environment. As roots grow they cast off dead cells and must navigate around soil particles, making the rhizoplane highly irregular. The rhizoplane is the site of water and nutrient uptake and the release of exudates into the soil.

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Question 4. What are some examples of how organisms in the rhizosphere influence plant roots?

  •  Organisms in the rhizosphere can affect the plant roots by altering the movement of carbon compounds from roots to shoots.
  • The burrows of earthworms in soil provide an easy route for plants roots to take as they grow through the soil.
  • Various associations between roots and microorganism can increase nutrient uptake by plants in nutrient poor environments, such as associations with mycorrhiza, rhizobia and Azospirillum.
  • Some soil organisms are pathogenic and attack plant roots.
  • Azospirillum is a bacterium that commonly lives in the rhizosphere of grasses. Some strains of this organism produce hormones that stimulate plant growth and other strains fix atmospheric nitrogen and make it available to the grass .

More on mycorrhizal fungi

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