Unit

1. Unit Plant Science

2. Problem Area Managing Plant Growth

3. Lesson Regulating Plant Growth

4. Student Learning Objectives • 1. Understand plant growth regulators and plant hormones. • 2. Identify different types of plant hormones. • 3. Identify the uses of growth regulators on plants. • 4. Define allelopathy.

5. Terms • Cuticle • Cytokinins • Daminozide • Ethylene • Exudation • Gibberellins • Internode • Leaching • Abscissic acid • Abscission • Allelopathy • Alleurone layer • Apical dominance • Auxins • Chlormequat • Creosote

6. Terms cont. • Plant Growth Regulators • Plant hormones • Senescence • Tannic acid • Tensile strength • Volatilization

7. What are plant growth regulators and plant hormones? • Plant growth regulatorsare organic compounds, either natural, or synthetic, that modify or control one or more specific physiological processes with a plant. • A. Natural plant growth regulators are produced by plants are plant hormones. Plant hormonesare naturally occurring compounds produced by the plant to accelerate or retard the rate of growth or maturation.

8. 1. Plant hormones are produced in minute quantities in one part of a plant and then translocated to another part of the plant where growth and development are modified. • 2. Plant hormones have many different effects on plant growth and development. • B. Scientists simulate the naturally occurring plant hormones to artificially produce synthetic plant growth regulators.

10. What are the different types and effects of plant hormones? • A. Auxinsare produced in the apical meristem of a plant’s stem and migrate down stem. • 1. Auxins cause cells to elongate. • 2. Auxins are responsible for phototropic responses or the ability of a plant to bend toward a light source. • 3. Auxin plays a role in apical dominance. Auxins will move down the stem and inhibit the growth of side shoots. Pinching off the apical meristem stops the flow of auxins and side shoots are free to develop.

11. 4. Auxins attach to cell walls and activate a series of processes that lower the tensile strength, resistance to lengthwise stress, of the cell membrane. This allows the cell to enlarge. Auxins, being acidic in nature, increase the acidity in the cell membranes making the cell wall less resistant to stretching from osmotic forces. • 5. Auxins are commonly used to promote root growth. The faster a cutting develops roots, the better chance for survival. Plant propagators will apply auxins to the base of cuttings to promote root growth. • 6. Auxins delay abscission(shedding) of leaves and maturing fruits. Auxins will work to breakdown the thin layer of cells in the abscission zone.

12. B. Gibberellins(gibberellic acid) are produced in stem and root apical meristems, in seed embryos, and in young leaves. • 1. Gibberellins stimulate stem growth. They induce stem cell elongation, cell division, and control enzyme release. • 2. Gibberellins produced in the embryo during the germination of cereal seeds moves to the aleurone layer, the outer layer of the endosperm, activating the synthesis of enzymes. • 3. Seventy different gibberellins have been discovered to exist. A plant species will respond to only certain types of gibberellin.

13. C. Cytokininsare responsible for cell division and differentiation. Roots supply cytokinins upward toward the shoots. • 1. Cytokinins cause cell enlargement, tissue differentiation, dormancy, and retardation of leaf senescence. • 2. Cytokinins are most abundant in seeds, fruits, and roots. • 3. The balances of auxins and cytokinins dictate whether cells will develop shoots, roots, or remain undifferentiated. • 4. Cytokinins slow the process of senescence, or aging, by preventing the breakdown of chlorophyll in leaves.

14. D. Abscissic Acidis a growth inhibitor within the category of plant growth regulators. • 1. Abscissic acid promotes dormancy in seeds and buds, and flowering in some short-day plants. • 2. Abscissic acid prevents seeds from germinating in fruit. • 3. Abscissic acid assists in maintaining water supplies within the plant. When a plant becomes stressed, greater amounts of abscissic acid are produced. Abscissic acid interferes with the availability of potassium in the guard cells, causing the stomata to close. Increased water supplies will breakdown the abscissic acid causing the stomata reopen. • 4. Abscissic acid will slow or shutdown the metabolism in plants. When abscissic acid is reduced, metabolic processes will resume.

15. E. Ethyleneis a water-soluable gas that moves readily throughout the plant. The cuticle, a thin, waxy layer of cutin that covers the epidermis, prevents loss of ethylene from plant tissue. • 1. Ethylene is produced in ripening fruits, senescent flowers, plant meristems and at sites where plant or fruit injury occurs. • 2. Ethylene is commonly used to promote fruit ripening and flower initiation.

17. What are the uses of growth regulators on plant tissues? • Each hormone promotes many different responses, and each is effective in very low concentrations. • A. There are many practical applications of auxins in the agricultural industry. • 1. Auxins are used in the horticultural industry to promote rooting of cuttings. • 2. Auxins are used to thin fruit blossoms, resulting in larger fruit, and in growing seed-less grapes.

18. B. There are numerous applications of gibberellins. • 1. Gibberellins play an important role in the expression of hybrid vigor in corn plants. Larger growing hybrids contain higher concentrations of gibberellins than inbred plants. • 2. Gibberellins stimulate the development of flowers. • 3. Gibberellins are applied to grape vineyards increasing the berry size by thinning clusters and increasing internode, stem region between nodes, length, allowing for more space for the fruit to grow. • 4. Gibberellins used on citrus crops assist in keeping the peels tough and resistant to molds. Gibberellins keep the peel green, and growers could use the ripening hormone ethylene to bring fruit to its normal color.

19. C. Cytokinins have numerous applications in today’s agricultural industry. • 1. Cytokinins are added to media for cell division to occur. • 2. Cut flowers that lose their source of cytokinins are sprayed with cytokinins to extend their vase life. • D. Ethylene has many applications for use in the agriculture industry. • 1. Bananas are picked green for shipment to prevent bruising. They are treated with ethylene to promote ripening. • 2. Cut flowers are never stored with ripening fruit or decaying leaves that might give off ethylene and shorten the life of the flowers.

20. E. Abscissic Acid has many uses in the agricultural industry. • 1. Numerous synthetic growth inhibitors have been produced and used commercially in the horticulture industry. These synthetic regulators were developed from abscissic acid or the way it functions from the plant. • 2. Daminozideretards growth and stimulates flowering in some plants, including chrysanthemums, bedding plants and azaleas. • 3. Chlormequat(Cyclocel, CCC) is used to reduce the height of poinsettias and prevent lodging in wheat.

21. What is allelopathy? • Allelopathyis the release of chemicals by certain plants that inhibit the growth of competing plants. • A. Plants release chemicals into the soil, water or air that have specific effects on neighboring plants (i.e. inhibiting growth or germination). Two examples of familiar chemicals that may be used for allelopathy by plants include tannic acid and creosote.

22. 1. Tannic acidis found in the leaves and bark of oak and sumac trees and is used to tan leather. • 2. Creosoteis the black tar-like substance put on telephone poles, railroad crossties and pier pilings to slow decay and rot. Creosote bushes are the dominant plant in the deserts of Texas. Scientists around the world are studying allelopathy as a natural way to inhibit weeds in crop fields and reduce chemical herbicide use.

23. B. There are several ways in which an allelopathic plant can release its protective chemicals: • 1. Volatilization: Allelopathic trees release a chemical in the form of a gas through small openings in their leaves. Other plants absorb the toxic chemical and die. • 2. Leaching: All plants lose leaves. Some plants store protective chemicals in the leaves they drop. When the leaves fall to the ground, they decompose. As this happens, the leaves give off chemicals that protect the plant. • 3. Exudation: Some plants release defensive chemicals into the soil through their roots. Those chemicals are absorbed by the roots of other trees near the allelopathic one. As a result, the non-allelopathic tree is damaged.

25. Review/Summary • What are plant growth regulators and plant hormones? • What are the different types and effects of plant hormones? • What are the uses of growth regulators on plant tissues? • What is allelopathy?