Horticulture Unit 5

1. HorticultureUnit 5 GROWTH STIMULANTS, RETARDANTS AND ROOTING HORMONES

2. Growth Regulating Substances • Called hormones: organic chemicals which act and interact to affect growth rate • Auxins: accelerate growth by stimulating cell enlargement • Gibberellins: stimulate growth in stem and leaf by cell elongation • Cytokinins: stimulate cell division (only works with auxins present) • Inhibitors: inhibit seed germination, stem elongation and hasten ripening of fruit.

3. Growth Hormones • Organic chemicals principally produced by actively growing plant tissue such as short tips and young leaves. • They move throughout the plant and can be found in many parts. • They react with one another.

4. Apical Dominance • The terminal bud secrets chemicals which inhibit or prevent growth of lateral buds. • This causes the plant to grow tall and not send out side branches. • Seems to be a genetic program directing the plant to grow above competing plants. • Once the plant reaches flowering age, the terminal bud becomes a flower bud and the chemicals are no longer secreted. • Pinching the terminal bud has the same effect and side branching occurs sooner.

5. Major Discoveries • Chemical and natural stimulants that cause plants to grow taller or faster • Chemical retardants that cause plants to grow slower • Hormones that cause plants to root faster • Dwarfing rootstock for fruit trees

6. Chemical Stimulants • Enables plants to grow taller. • Most common is gibberellic acid • Gibberellic acid causes the stems of plants to stretch out • The nodes (the joints at which bulbs, leaves and branches) are further apart

7. Natural Stimulants • Chemical known as alcohol Triacontanol • Found in alfalfa and has been known to nursery men for 30 years. • Stimulation is brought about by mulching with alfalfa hay or by watering plants with an alfalfa tea

8. Chemical Retardants • Used to retard the growth of plants making them shorter and more compact and therefore more attractive. • One of the newest applications for growth retardants is on lawns, a chemical called “Limit” is used new and it restricts grass growth for 6 to 8 weeks. • A new plant regulator called “Sumagic” reduces the height of plants by inhibiting production of the hormone Gibberellic acid. (which cause stems to elongate)

9. Rooting Hormones • When propagating plants from cuttings, it is very important that they root as quickly as possible • Indoleacetic acid (IAA) is a natural plant hormone that causes roots to form • Chemical hormones, indolebutyric acid (IBA) or naphaleneacetic acid (NAA) are the most common used today.

10. Rooting Hormones • Rooting hormones are either mixed with talc and used as powders or mixed with liquid and used as a wet dip • All rooting hormones should contain a fungicide.

11. Dwarfing Rootstock • As the cost of labor has increased, growers are looking for smaller trees. • Discovered trees from certain types of roots, do not grow as large and bear fruit earlier. • A complete series of rootstock known as malling rootstock was developed.

12. Commercially Used Plant Growth Regulators

13. Student Activities • Complete self-evaluation at the end of this unit. • Using nursery catalogs and books, make a list of available dwarf fruit trees and details.