1. 25–1 Hormones and Plant Growth
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2. Patterns of Plant Growth
Biologists have discovered that plant cells send signals to one another that indicate when to divide and when not to divide, and when to develop into a new kind of cell.
One difference between growth in plants and animals is that most animals stop growing once they reach adulthood.
In contrast, plants continue to grow new needles, add new wood, and produce cones or new flowers.
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3. Patterns of Plant Growth
The secrets of plant growth are found in meristems, regions of tissue that can produce cells that later develop into specialized tissues.
Meristems can be found at places where plants grow rapidly: tips of growing stems and roots, edges of woody tissue, etc.
Plants grow in response to environmental factors such as light, moisture, temperature, and gravity.
Specific chemicals direct, control, and regulate plant growth.
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Plant Hormones
Plant Hormones
A hormone is a substance that is produced in one part of an organism and affects another part of the same individual.
Plant hormones are chemical substances that control a plant's patterns of growth and development and its responses to environmental conditions.
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Plant Hormones
Hormone producing cells
The hormone moves through the plant from where it is produced to the place where it triggers a response.
The portion of an organism affected by a particular hormone is known as its target cell or target tissue.
Movement of hormone
Target cells
Plant hormones are chemical substances that control patterns of development as well as plant responses to the environment. Hormones are produced in apical meristems, in young leaves, in roots, and in growing flowers and fruits. From their place of origin, hormones move to other parts of the plant, where target cells respond in a way that is specific to the hormone.
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Plant Hormones
To respond to a hormone, the target cell must contain a receptor, usually a protein, to which the hormone binds.
Cells without receptors are generally unaffected.
If the receptor is present, the hormone can influence the target cell by:
changing its metabolism
affecting its growth rate
activating the transcription of certain genes
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Plant Hormones
Different kinds of cells may have different receptors for the same hormone.
As a result, a single hormone may affect two different tissues in different ways.
For example, a particular hormone may stimulate growth in stem tissues but inhibit growth in root tissues.
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Auxins
Auxins
Charles Darwin and his son Francis carried out the experiment that led to the discovery of the first plant hormone and published it in the book The Power of Movement in Plants.
They described an experiment in which oat seedlings demonstrated a response known as phototropism, the tendency of a plant to grow toward a source of light.
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Auxins
In the experiment, they placed an opaque cap over the tip of one of the oat seedlings.
This plant did not bend toward the light, even though the rest of the plant was uncovered.
The effect of light on growing plants was shown in experiments by Francis and Charles Darwin and other scientists. They demonstrated that chemical substances are produced in the growing tip of a plant. Auxins stimulate cell elongation. A higher concentration of auxins accumulate in the shaded part of a stem and cause the plant to bend toward a light source.
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Auxins
However, if an opaque shield was placed a few centimeters below the tip, the plant would bend toward the light as if the shield were not there.
The effect of light on growing plants was shown in experiments by Francis and Charles Darwin and other scientists. They demonstrated that chemical substances are produced in the growing tip of a plant. Auxins stimulate cell elongation. A higher concentration of auxins accumulate in the shaded part of a stem and cause the plant to bend toward a light source.
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Auxins
The Darwins suspected that the tip of each seedling produced substances that regulated cell growth.
Forty years later, these substances were identified and named auxins.
Auxins are produced in the apical meristem and are transported downward into the rest of the plant.
They stimulate cell elongation.
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Auxins
When light hits one side of the stem, the shaded part develops a higher concentration of auxins.
This change in concentration stimulates cells on the dark side to elongate.
The effect of light on growing plants was shown in experiments by Francis and Charles Darwin and other scientists. They demonstrated that chemical substances are produced in the growing tip of a plant. Auxins stimulate cell elongation. A higher concentration of auxins accumulate in the shaded part of a stem and cause the plant to bend toward a light source.
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Auxins
As a result, the stem bends away from the shaded side and toward the light.
Recent experiments have shown that auxins migrate toward the shaded side of the stem, possibly due to changes in membrane permeability.
The effect of light on growing plants was shown in experiments by Francis and Charles Darwin and other scientists. They demonstrated that chemical substances are produced in the growing tip of a plant. Auxins stimulate cell elongation. A higher concentration of auxins accumulate in the shaded part of a stem and cause the plant to bend toward a light source.
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Auxins
Auxins and Gravitropism
Auxins are also responsible for gravitropism; the response of a plant to the force of gravity.
Auxins build up on the lower sides of roots and stems. In stems, auxins stimulate cell elongation, helping turn the trunk upright.
In roots, auxins inhibit cell growth and elongation, causing the roots to grow downward.
Auxins are responsible for the plant response called gravitropism. Auxins caused the tip of this tree stem to grow upright. Photo Credit: Dr. E.R. Degginger
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Auxins
Auxins also influence how roots grow around objects in the soil.
If a growing root is forced sideways by an obstacle, auxins accumulate on the lower side of the root.
High concentrations of auxins inhibit the elongation of root cells.
Uninhibited cells on the top elongate more than auxin-inhibited cells on the bottom and the root grows downward.
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Auxins
Auxins and Branching
Auxins also regulate cell division in meristems.
As a stem grows in length, it produces lateral buds.
A lateral bud is a meristematic area on the side of a stem that gives rise to side branches.
During normal growth , lateral buds are kept dormant because of the production of auxins in the apical meristem.
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Auxins
Most lateral buds do not start growing right away.
The reason for this delay is that growth at the lateral buds is inhibited by auxins.
Because auxins move out from the apical meristem, the closer a bud is to the stem's tip, the more it is inhibited.
This phenomenon is called apical dominance.
Apical dominance is controlled by the relative amounts of auxins and cytokinins. During normal growth, lateral buds are kept dormant because of the production of auxins in the apical meristem.
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Auxins
Apical meristem removed
When the apical meristem is removed, the concentration of auxin is reduced and the side branches begin to grow more rapidly.
Apical dominance, shown here, is controlled by the relative amounts of auxins and cytokinins. If the apical meristem is removed, the concentration of auxins drops.
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19. Auxinlike Weed Killers
Auxins
Auxinlike Weed Killers
Chemists have produced compounds that mimic the effects of auxins.
Since high concentrations of auxins inhibit growth, many of these are used as herbicides—compounds toxic to plants.
Herbicides include a chemical known as 2,4-D, which is used to kill weeds.
A mixture containing this compound was used as Agent Orange, a chemical defoliant sprayed during the Vietnam War.
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Cytokinins
Cytokinins
Cytokinins are plant hormones produced in growing roots and developing fruits and seeds.
In plants, cytokinins stimulate cell division and the growth of lateral buds, and cause dormant seeds to sprout.
Cytokinins delay the aging of leaves and play important roles in early stages of plant growth.
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Cytokinins
Cytokinins and auxins often produce opposite effects.
Auxins stimulate cell elongation.
Cytokinins inhibit cell elongation and cause cells to grow thicker.
Auxins inhibit the growth of lateral buds.
Cytokinins stimulate lateral bud growth.
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Cytokinins
Recent experiments show that the rate of cell growth in most plants is determined by the ratio of the concentration of auxins to cytokinins.
In growing plants, therefore, the relative concentrations of auxins, cytokinins and other hormones determine how the plant grows.
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Gibberellins
Gibberellins
A gibberellin is a growth-promoting substance in plants.
Gibberellins produce dramatic increases in size, particularly in stems and fruit.
Gibberellins are also produced by seed tissue and are responsible for the rapid early growth of many plants.
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Ethylene
Ethylene
In response to auxins, fruit tissues release small amounts of the hormone ethylene.
Ethylene is a hormone that causes fruits to ripen.
Commercial producers of fruit sometimes use this hormone to control the ripening process.
Many crops, like lemons and tomatoes, are picked before they ripen so they can be handled.
Just before they are delivered to market, the fruits are treated with synthetic ethylene to produce a ripe color quickly, but not always a ripe flavor.
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