Lesson 2: The Rhizosphere
"Rhizo"= root • "sphere"=zone
the Root Zone

When we think of plants we generally think only of what we see above ground level: leaves, stems, flowers, etc. However, just beneath the soil surface lies a stupendous factory buzzing with life-essential biological processes of incredible complexity. This zone of intensive activity is called the rhizosphere ("root-zone").

Most of us know that roots of plants are vital to their existence, but do we understand how vital? It is simple to outline their basic functions:

• Roots anchor the plant to the soil.
• Roots extract water and minerals.

Point 1 is rather easy to understand: tough, fibrous tissues containing cellulose, hemicellulose, and lignin branch into the soil mass, anchoring it firmly. Point 2, however, is not as simple as it looks. A typical root is illustrated above.

The root itself makes up part of the rhizosphere (the endorhizosphere), while the root hairs, mucigel, and root cells that have sloughed off constitute the ectorhizosphere.

Root-Microbe Symbiosis

Some rhizosphere microbes: From top: bacteria, fungi, actinomyceters, algea, protozoa and cyanobacteria.

The plant actually grows its own garden of microbes, as it were, along the root surfaces. To do this, the sunlight energy captured from photosynthesis in the leaves is channeled down the stem through the phloem vessels and out through epidermal cells to the external root surface. Incredibly, up to 80% of the total plant energy--but usually 12 to 40%--is exuded as mucigel into the ectorhizosphere as carbohydrates, amino acids, and other energy-rich compounds. This food doesn't stay around long. Billions of bacteria, fungi, algae, actinomycetes, protozoa, and other microbes feed upon this exudate.

Those Phenomenal Mycorrhizae

Especially important are the mycorrhizal fungi which extend their thread-like hyphae from inside cortex cells out into the soil for several millimeters. They extend the feeding volume of the root by 10 to 1,000 times or more for most plant species (the cabbage family being a notable exception), and extract and carry nutrients back to the root. So important are they that scientists sometimes call the root zone the mycorrhizosphere. Pine trees will hardly grow without these fungi.

Feeding On Exudate

All of these microbes consume exudate, and depending on the species may produce acids to dissolve and chelate minerals, fix nitrogen, generate vitamins, or synthesize hormones or regulators that can be absorbed by the roots to stimulate additional growth and disease resistance. It is a beautiful symbiosis! Each helps the other. Some scientists have even speculated that the plant might adjust the quality of its exudate to encourage the growth of specific microbes that will most benefit the plant under prevailing conditions. Plants are indeed "intelligent"!

The interrelationship of the root and its microbes may be pictured as a "rhizosphere symbiotic cycle" (see the figure at the left below).

The rhizosphere is always functioning for the plant whether it is growing in a field, in a pot, in a hydroponic media, or even in a lake or ocean. The details of function may differ somewhat, but the principles are the same in order for the plant to survive.

We can promote a plant's rhizosphere capability by insuring it has the proper soil minerals, adequate porosity for air and water movement, organic amendments like compost or manure, and biostimulant triggers that will encourage the proliferation of rhizosphere microbes.

See How Much You Learned

1. What does rhizosphere mean?
2. What is the term used to describe the mutually beneficial interaction of the plant roots and its rhizosphere microorganisms?
3. Which of these organism groups are found in the rhizosphere?
   a. Fungi
   b. Bacteria
   c. Algae
   d. Protozoa
   e. All of these
4. Because mycorrhizae are so critical for most plants to function properly, some have renamed the rhizosphere the __________
5. The energy moved into the rhizosphere from the leaves and stem originates from the __________
6. How much of a plant's total energy captured in carbon compounds is generally moved out of the root. into the mucigel layer?
   a. 5 to 10 %
   b. None
   c. 12 to 40%
   d. 75 to 90%
7. Do plants depend on an active rhizosphere to survive?

Answers: 1. Root-zone; 2. Symbiosis; 3. e; 4. Mycorrhisosphere; 5. Sun; 6. c; 7. Yes.

The Vital Earth News  /  Spring 1996