Horticulture Science Lesson 13 Understanding Plant Growth Regulators

Interest Approach Start a discussion on the obvious effects of human hormones. Ask what the students know about estrogen, testosterone, adrenaline, etc. Ask students to think about differences between little boys and men and between little girls and women. Reinforce the concepts that human hormones are very potent, have a lot of individual jobs, and are found in extremely small amounts. Now ask about plant hormones. Note the silence. Tell the students that plants do indeed have hormones and that they may cause even more dramatic changes in plants than humans experience.

Student Learning Objectives Describe the work of plant growth regulators. Compare the functions of plant hormones. Examine commercial uses for plant growth regulators.

Terms abscisic acid abscission layer apical dominance auxins cytokinins ethylene etiolation gibberellins

Terms gravitropism growth retardants phototropism plant growth regulators plant hormones rooting compounds synthetic growth regulators thigmotropism tropism

What do plant growth regulators do? Plant growth regulators are chemicals that affect the plant in many complex ways. They can control such activities as cell division and differentiation, root and shoot growth, flowering, and ripening.

What do plant growth regulators do? 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. Some plant growth regulators are man-made synthetic growth regulators. These synthetic growth regulators can be applied to plants to get a myriad of changes.

What are the functions of several plant hormones? Several hormones are made by tissues of the plant. Each plant hormone has many different jobs. Auxins are growth hormones that are produced by the tip or apical meristem of the stem. Auxins cause stem cells to elongate and divide. They also flow down from the tip of the stem, preventing lateral buds from sprouting.

What are the functions of several plant hormones? Auxins set up a situation in which the middle stem grows at a greater rate, suppressing the side growth. This is called apical dominance. Apical dominance is why many conifers are pyramidal in shape. Apical dominance can be overcome by simply cutting off the dominant stem, losing the source of the auxin.

What are the functions of several plant hormones? Auxins are responsible for a response called phototropism. Phototropism is a response by a plant to grow toward a light source while tropism is a growth response to an external stimulus. Sunlight slowly breaks down auxins. When the side with more auxins grows faster, the stem starts to bend toward the sun. The earth’s gravity elicits a plant response known as gravitropism; in this situation, a plant placed on its side will still result in downward root growth and upward stem growth.

What are the functions of several plant hormones? Auxins are involved in a behavior called thigmotropism. Thigmotropism is a plant’s response to a hard object. The repeated touch of an object causes less auxin to remain on that side of the stem. When the auxin side starts to grow at a greater rate, the plant grows toward the plant. This continues to happen until the plant is actually wound around the object.

What are the functions of several plant hormones? Cytokinins are hormones that are mostly responsible for cell division and differentiation. Cytokinins are produced in the root tips and in seeds. They tend to travel up the stem.

What are the functions of several plant hormones? Ethylene is a gas that affects the plant like a hormone. Ethylene is produced by ripening fruit and dying plant materials. Ethylene stimulates flowering in some plants and causes other fruits and flowers to ripen more quickly and evenly.

What are the functions of several plant hormones? It is also associated with the abscission, or dropping, of leaves. (The abscission layer is a specialized layer of cells that extends across the petiole near its attachment to the stem.) Ethylene gas is why fruit will ripen faster in a paper bag than out on a counter. The bag concentrates the ethylene gas. Ethylene has a negative effect on cut flowers. It causes them to age more quickly, reducing their useful life.

What are the functions of several plant hormones? Gibberellins are hormones that cause internodal elongation and cell division. Gibberellins are produced in stems, roots, and young leaves. Gibberellins are commonly used on commercially grown dessert grapes to spread the fruits out and cause them to be bigger.

What are the functions of several plant hormones? However, if light levels are low, gibberellins cause the internodes to lengthen. Stretching of internodes raises leaves to a position where they can capture light. Plants grown in low light or darkness develop a severe case of stretching known as etiolation.

What are the functions of several plant hormones? Abscisic acid is a hormone that inhibits growth. It is found in seeds that are dormant and in dying leaves. It also appears to help a plant prepare its buds for the winter.

What are several commercial uses for plant growth regulators? Plant growth regulators are very useful for commercial plant crops. They can save money and time while leading to a better crop. These are some commercial uses for growth regulators.

What are several commercial uses for plant growth regulators? Growth regulators (growth retardants) are routinely sprayed on crops such as poinsettias, Easter lilies, and chrysanthemums to make shorter, bushier, and more attractive plants. Products such as A-rest, B-nine, Cycocel, and Florel are commonly used.

What are several commercial uses for plant growth regulators? Synthetic auxins (rooting compounds) are 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 several commercial uses for plant growth regulators? 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 is nontoxic over a wide range of concentrations. These materials are available in a liquid formulation or mixed with talc. Two commercially available products are Rootone and Hormodin.

What are several commercial uses for plant growth regulators? Ethylene gas is used commercially to ripen bananas once they get to market and to induce flowering in pineapple crops.

Review/Summary What do plant growth regulators do? What are the functions of several plant hormones? What are several commercial uses for plant growth regulators?