Several Factors Control the Growth and Distribution of Roots

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Much of what we know about roots comes from studies of potted plants or seeds germinated in artificial environments such as sterile dishes or moist paper towels. However, there is little evidence that roots grow similarly in the ground. Understanding how roots grow in their natural environment requires some ingenious as well as tedious work and involves underground cameras and painstaking excavations (see fig. 2.12). The most elaborate methods for studying roots involve laboratories called rhizotrons, which are underground walkways with glass walls (fig. 7.14). As they grow, many roots press against these glass walls and can be easily studied.

Observations in rhizotrons have been supplemented with other studies to reveal that the growth and distribution of roots are primarily controlled by four factors:

  1. Gravity. Different roots respond differently to gravity. For example, primary roots grow down; that is, they are positively gravitropic (see figs. 6.4 and 7.15). In plants such as castor bean (Ricinus communis), branch roots grow laterally for several centimeters, after which they become responsive to gravity and grow downward. As a result of this differential responsiveness to gravity, roots permeate the soil at various angles and efficiently absorb water and minerals.
  2. Genetic differences. The growth and distribution of roots are also controlled genetically. For instance, plants such as locoweed (Astragalus mollissimus) grow deep
Gravitropism Corn

FIGURE 7.15

Primary roots of corn (Zea mays) are positively gravitropic; that is, they grow downward. They do so whether the grain germinates in a vertical, horizontal, or inverted position in this petri dish standing up on its edge. What do you predict would happen if a light was placed directly above the petri dish? What if the petri dish were placed in the dark?

FIGURE 7.15

Primary roots of corn (Zea mays) are positively gravitropic; that is, they grow downward. They do so whether the grain germinates in a vertical, horizontal, or inverted position in this petri dish standing up on its edge. What do you predict would happen if a light was placed directly above the petri dish? What if the petri dish were placed in the dark?

taproots, whereas most grasses produce shallow, fibrous root systems. Similarly, the root systems of corn are denser near the soil surface than those of soybeans, regardless of the type of soil in which they grow.

  1. Stage of plant development. Just before and during fruit formation, most of a plant's resources are used for shoot rather than root growth. Thus, root growth typically slows during flowering and fruit formation. This shift in the allocation of resources is most obvious in some monocot crops such as corn, in which much of the root system dies before harvest.
  2. Soil properties. Roots of many species grow best in loosely packed soil. For example, roots of wheat grow four times faster in loose sand than in tightly packed clay. Roots that grow in tightly packed soil are usually shorter and thicker than those that grow in loosely packed soil. Similarly, roots usually grow deeper in moist, aerated soil than in soaked, poorly aerated soil.

If you water your lawn shallowly each day, the root system will be restricted to the upper levels of the soil. This means that if you go on vacation during the summer, your lawn grasses may be threatened because the top layers of soil dry out first and, because your lawn's roots are limited to this dry region, the entire lawn is in jeopardy of dying. However, if you water infrequently, but deeply, the roots of your lawn grasses can grow deeply into the soil. Thus, even if the top layers of soil dry out, the grass plants have roots deep enough in the soil to absorb the water necessary to keep them alive.

Competition with other plants is minimized by growing into different areas of the soil. Consider the root systems of mesquite (Prosopis) and a saguaro cactus (Carnegiea gigantea), two plants that grow in dry environments. Mesquite produces long taproots that obtain water from deep underground. Saguaro cacti (see fig. 2.25) survive in the same environment by producing an extensive mass of shallow roots that spreads as far as 30 meters and that maximizes water absorption after infrequent rains in the desert.

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Responses

  • Risto
    What is an example corn root positive gravitropism?
    8 years ago
  • zufan
    How plant absorb water from roots?
    8 years ago

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