Plant Biology and Production

Unit 3 Seed Germination, Growth, and Development

Lesson 5 Examining the Roles of 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.

Interest Approach 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?

Terms Abscisic acid Apical dominance Auxins Cytokinins Ethylene Etiolation Indoleacetic acid (IAA) Indolebutyric acid (IBA) Gibberellins Gravitropism Phototropism Plant growth regulators Synthetic growth regulators Thigmotropism Tropisms

Student Learning Objectives Identify plant growth regulators (PGR) and their functions. Explain plant tropisms. Discuss synthetic growth regulators. Describe commercial uses of plant growth regulators.

What are Plant Growth Regulators and What are Their Functions? I. Natural occurring or synthetic chemicals that regulate plant growth and development are plant growth regulators.

What are Plant Growth Regulators and What are Their Functions? A. Plant growth regulators promote growth, inhibit growth, and modify growth and development. Plants produce compounds to regulate growth. These natural chemicals are hormones. B. Plants produce five different hormones.

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.

What are Plant Growth Regulators and What are Their Functions? 3. Cytokinins are hormones responsible for cell division and differentiation. They are produced in the roots and transported throughout the plant via the xylem.

What are Plant Growth Regulators and What are Their Functions? 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.

What are Plant Growth Regulators and What are Their Functions? 5. Abscisic acid is a growth-inhibiting hormone largely responsible for seed dormancy. It also causes stomates to close in times of drought in an effort to conserve water.

What are Plant Tropisms? II. External stimuli and environmental conditions cause plants to respond in certain ways. Plant responses to the external stimuli are known as tropisms.

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.

What are Plant Tropisms? 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? III. Other human-made chemicals that regulate plant growth are referred to as synthetic growth regulators. A. Many synthetic growth regulators have been developed to regulate plant growth.

What are Synthetic Growth Regulators? 1. Two common root-promoting materials used in the propagation of plants are naphthaleneacetic acid (NAA) and indolebutyric acid (IBA). 2. Growth retardant chemicals inhibit cell elongation and keep plants compact. Many growth retardants are available.

What are Synthetic Growth Regulators? 3. Some synthetic growth regulators serve as herbicides by altering growth and metabolic processes of weeds.

How are Plant Growth Regulators Used Commercially? IV. Plant growth regulators are used in agricultural practices to improve the quality and quantity of yields.

How are Plant Growth Regulators Used Commercially? 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.

How are Plant Growth Regulators Used Commercially? 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.

How are Plant Growth Regulators Used Commercially? 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.

How are Plant Growth Regulators Used Commercially? 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 / Summary 1. What are plant growth regulators and what are their functions? 2. What are plant tropisms? 3. What are synthetic growth regulators? 4. How are plant growth regulators used commercially?