The rhizosphere surrounds each root

The narrow zone of soil surrounding a root is called the rhizosphere (fig. 7.13). The rhizosphere extends up to 5 millimeters from the root's surface and is a complex and ever-changing environment. Growth and metabolism of roots modify the rhizosphere in several ways. First, roots force their way through crevices and between soil particles. Later, when the roots die and decay, they leave open channels that help aerate the soil; this aeration may improve the root growth of other plants. Roots also alter the chemical composition of the soil. For instance, respiration in roots decreases the concentration of oxygen and increases the concentration of carbon dioxide in the rhizosphere. This lowers the pH of the soil, increasing its acidity, which may change the ability of roots to absorb minerals. This is because minerals are absorbed while dissolved in water, and their ability to dissolve is affected by pH—the availability of some minerals such as iron may increase and others such as potassium may decrease in acidic soils.

Roots enrich the soil with organic matter. Some plants transport as much as 60% of the sugar not used by

Casparianstrip

FIGURE 7.10

Pathways show routes water may take as it moves from the soil, through the root, into the stele. Casparian strips in cell walls of the endodermis block off the stele of the root. As a result, water and dissolved minerals enter the stele through the cell membranes of endodermis cells (X100).

FIGURE 7.10

Pathways show routes water may take as it moves from the soil, through the root, into the stele. Casparian strips in cell walls of the endodermis block off the stele of the root. As a result, water and dissolved minerals enter the stele through the cell membranes of endodermis cells (X100).

Casparian strip

Endodermis cytoplasm

Casparian strip

Endodermis cytoplasm

Function The Casparian Strip Root

FIGURE 7.9

The Casparian strip of the endodermis blocks the movement of water and dissolved minerals in the cell walls and between cells. As a result, water and dissolved minerals are directed through the cell membrane and protoplast of endodermal cells, where subsequent use and movement are more controlled.

FIGURE 7.9

The Casparian strip of the endodermis blocks the movement of water and dissolved minerals in the cell walls and between cells. As a result, water and dissolved minerals are directed through the cell membrane and protoplast of endodermal cells, where subsequent use and movement are more controlled.

Endodermis with Casparian strip

Endodermis with Casparian strip

Primary xylem in the stele of root

FIGURE 7.11

Formation of branch roots (X40). Branch roots are formed by the pericycle, the outermost layer of the stele, just inside the endodermis. The stele is the central zone of the root, containing both xylem and phloem. The growth of branch roots eventually ruptures the epidermis of the parent root.

Primary xylem in the stele of root

FIGURE 7.11

Formation of branch roots (X40). Branch roots are formed by the pericycle, the outermost layer of the stele, just inside the endodermis. The stele is the central zone of the root, containing both xylem and phloem. The growth of branch roots eventually ruptures the epidermis of the parent root.

Mineral Nutrition Diagram

FIGURE 7.12

Cross section of a root of greenbrier (Smilax), a monocot (X50). The most obvious tissues are the epidermis, cortex, endodermis and the vascular tissues of the vascular cylinder.

FIGURE 7.13

Diagram summarizing the major functions of roots. The rhizo-sphere is the narrow zone surrounding the entire root.

the shoots to roots, and much of this sugar is deposited in the soil as mucigel and other compounds. In plants such as wheat, the amount of carbohydrate lost to the soil often exceeds that stored in the plant's fruits. Roots leave massive amounts of organic matter in the soil when they die and decay. This organic material enriches the soil by returning minerals to the soil that the living root absorbed. In addition to sugar, some plants secrete proteins from their roots that may help protect the roots and the plant against disease-causing bacteria and other organisms in the soil. Scientists are trying to make economic use of root protein secretions by growing plants hydroponically; that is, in a nutrient-rich solution rather than in soil. Investigators genetically manipulate the kind of protein the plant produces and then collect the protein secreted by the roots. In the future, this might become an economically important way of "milking" plants that serve as protein factories.

As a result of roots, the rhizosphere usually contains large amounts of energy-rich organic molecules. These molecules are often used as food for more than 1010 microorganisms (microbes) in each cubic centimeter of soil—populations that are 10 to 100 times more dense than those in the rest of the soil. Most microbes in soil live near roots. These microbes, in turn, secrete compounds that affect the growth and distribution of roots. Equally important, microbes often increase the uptake of minerals from the soil; this is why plants growing in sterile soil absorb fewer minerals than do those growing in rhizospheres that include microbes.

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Responses

  • Fre-qalsi
    What is the area that surrounds the plant roots?
    8 years ago
  • Mike
    How nutrient rich to stele?
    1 year ago
  • florian
    What element surrounds the rhizosphere?
    2 months ago

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