Animal, Plant & Soil Science E4-11 Plant Growth Regulators

Interest Approach Several days before the introduction of this lesson prepare two identical plants for an interest approach. Select an upright growing potted herbaceous plant such as a coleus or tomato. Lay one of the two plants on its side. The plant on its side will curve upward in response to gravity. Present the two plants to the class and ask a series of questions. Ask the students why the one plant is curved. Is there an advantage to the plant for responding in this way? Is there a term used to describe the plant response? What caused it to curve?

Objectives 1 Define plant growth regulator, identify the classes of regulators and describe their functions. 2 Define tropism and examine the types of tropisms. 3 List examples of synthetic growth regulators and discuss how and why they are used. 4 Describe commercial uses of plant growth regulators.  

Terms _ abscisic acid _ apical dominance _ auxins _ cytokinins _ ethylene _ etiolation _ gibberellins

Terms _ gravitropism _ growth retardants _ indoleacetic acid (IAA) _ indolebutyric acid (IBA) _ naphthaleneacetic acid (NAA) _ phototropism

Terms _ plant growth regulators _ plant hormones _ synthetic growth regulators _ thigmotropism _ tropism

What are plant growth regulators and what are their functions? I. Plant growth regulators (PGR) are chemicals which affect the plant in many complex ways. They promote growth, inhibit growth, and modify growth and development. They can control such activities as cell division and differentiation, root and shoot growth, flowering, and ripening. Plant growth regulators may be naturally occurring and manufactured in a plant or they may be synthetic, man-made chemicals.

What are plant growth regulators and what are their functions? A. Naturally occurring chemicals that regulate all growth and development of plants are called plant hormones. Hormones are moved around the plant in extremely low concentrations. Five different hormones are auxins, gibberellins, cytokinins, ethylene, and abscisic acid.

What are plant growth regulators and what are their functions? 1. Auxins consist of a group of hormones that have a primary role in promoting cell elongation. The primary auxin is indoleacetic acid (IAA). Plants produce auxins in the apical meristem. They migrate down the stem moving through one cell to the next rather than being transported through the phloem or xylem.

What are plant growth regulators and what are their functions? Auxins have the greatest influence on cells closest to the apical meristem. The higher concentrations of auxins give the apical meristem dominance over growth below. Apical dominance is a term used to describe the apical bud’s dominance over the lateral buds.

What are plant growth regulators and what are their functions? 2. Gibberellins induce cell elongation and cell division. They are produced in stem and root apical meristems, seed embryos, and young leaves. Gibberellins play a key role in the development of flowers and in the production of enzymes during seed germination. 3. Cytokinins are hormones responsible for cell division and differentiation. They are produced in the roots and transported throughout the plant via the xylem. Cytokinins encourage the growth of lateral shoots. They inhibit the branching of the roots. As an ingredient in tissue culture medium, cytokinins are an important promoter of cell division.

What are plant growth regulators and what are their functions? 4. Ethylene is a colorless gas that speeds the aging of plant parts, particularly fruit. It is produced in stems, ripening fruit, and dying leaves. 5. Abscisic acid is a growth-inhibiting hormone largely responsible for seed dormancy. It also causes stomata to close in times of drought in an effort to conserve water.

What are plant growth regulators and what are their functions? B. Some plant growth regulators are man-made synthetic growth regulators. These synthetic growth regulators can be applied to plants to get a number of changes. These changes often make the plant better or more saleable.  

What are plant tropisms? II. External stimuli and environmental conditions cause plants to respond in certain ways. A plant response to an external stimuli is known as tropism. Three significant tropisms are phototropism, gravitropism, and thigmotropism.

What are plant tropisms? A. Phototropism is a plant’s response to the source of light. Auxins move down the shaded side of the plant stem. The presence of auxins causes the cells on the shady side of the stem to elongate more than cells than the bright side of the stem. The result is a stem that bends towards the light. Under low light or dark conditions cells elongate on all sides of the stem. The result is an appearance of the stem stretching. Stretching due to low light is known as etiolation.

What are plant tropisms? B. Gravitropism also referred to as geotropism is a plant response to gravity. The stems of plants laid on their side curve upward. In this scenario auxins settle to the bottom side of the stem and cause cells to elongate.

What are plant tropisms? C. Thigmotropism is a response to mechanical stimuli. A good example is the tendrils of a cucumber plant. When the tendrils touch an object, the response is to curl around that object.

What are synthetic growth regulators and their uses? III. Synthetic growth regulators are used in agriculture to regulate plant growth. A. Synthetic auxins are also used in the horticulture industry to promote rooting of cuttings. Cuttings are treated with synthetic growth regulators to increase the number of cuttings that form roots, to speed rooting, to increase the number and quality of roots, and to increase the uniformity of the roots.

What are synthetic growth regulators and their uses? 1. Some materials are used extensively in the horticulture industry to promote the rooting of cuttings. The two synthetic root-promoting materials most widely used are naphthaleneacetic acid (NAA) and indolebutyric acid (IBA). IBA is regarded as the best material for general use. It can be used with a wide variety of plants and it is nontoxic over a wide range of concentrations. These materials are available in a liquid formulation or mixed with talc.

What are synthetic growth regulators and their uses? B. Growth retardants are widely used in the greenhouse industry. These chemicals inhibit the action of gibberellins on stem elongation. As a result, plants are more compact. Compact plants are more attractive, often greener, and easier to transport. Although growth is slowed, flowers tend not to be affected.

What are synthetic growth regulators and their uses? C. A number of herbicides are classified as growth regulators due to their mode of action. Growth regulator herbicides disrupt hormone balance and protein synthesis. Symptoms of damage include abnormal growth. Leaves may cup, crinkle, become brittle, and form parallel veins. Stems may display a twisting or epinasty. Callus tissue may also form. The herbicides are absorbed primarily through the leaves and are translocated through both the xylem and phloem. Growth regulator herbicides are effective with broadleaf weeds. Banvel, Clarity, 2, 4-D, and Stinger are growth regulator herbicides.

How are plant growth regulators used commercially? IV. Plant growth regulators are used in agricultural practices to improve the quality and quantity of yields. A. Auxins in the forms of indoleacetic acid (IAA) and indolebutyric acid (IBA) are widely used to speed the rooting of cuttings in the horticulture industry. Naphthaleneacetic acid (NAA) is sprayed on apples to prevent pre-harvest drop of the fruit.

How are plant growth regulators used commercially? B. Gibberellins are used to induce flowering. Some plants that respond by flowering are carrots, endive, cabbage, turnips, and chrysanthemums. Gibberellins serve as growth stimulants making plants, including sugar cane, grow larger. Gibberellins are used with some plant species, such as grapes, to produce larger fruit. C. Cytokinins have been shown to extend the shelf life of lettuce. Cytokinins are also an important ingredient of tissue culture medium, as they promote cell division.

How are plant growth regulators used commercially? D. Ethylene is used in the ripening of fruits before being placed on grocery shelves. E. Many synthetic growth regulators are used to regulate plant growth. Some promote growth. Others inhibit growth. Growth retardants are widely used in the horticulture industry to keep plants compact.

REVIEW 1. What are plant growth regulators and what are their functions? 2. What are plant tropisms? 3. What are synthetic growth regulators and their uses? 4. How are plant growth regulators used commercially?