Plant Growth Regulation

1. Plant Growth Regulation Controls of growth, development and movement

2. Outline A.) Tropisms B.) Hormones C.) Nastic Behavior D.) Photoperiodism

3. A. Tropisms A tropism is a growth response in a plant initiated by an external stimulus - may be positive (grow towards stimulus) or negative (grow away from stimulus) Types of Tropisms • Phototropism - when a plant grows towards light

4. Phototropism

5. Gravitropism or geotropism is the response of a plant to the earth’s gravitational field. • present at germination & auxins (hormones) play primary role

6. Geotropism

7. Thigmotropism- growth response to touch ex. Tenderils of climbing vines (ex. Cucumber, peas) curl around a solid object

8. Electrotropism- electricity • Chemotropism - chemicals • Traumatropism – growth over a wound • Ex. Sap • Thermotropism - temperature • Aerotropism - oxygen • Hydrotropism – usually a positive response as roots try to grow towards water. ex. Maple tree roots clog up drains • Geomagnetotropism - magnetic fields

9. What causes plants to grow toward light? gravity? • Selection of an experimental system • should be easy to manipulate • should be simple • should show the response under study • For phototropism studies the oat coleoptile was chosen

10. Coleoptiles as a model system

11. Phototropism experiments with coleoptiles Darwin said some “influence” was transmitted from the tip down into the elongation region.

12. Frits Went’s experiments

13. Frits Went’s experiments

14. Auxin diffused from the coleoptile tip into the agar. Then, the auxin travelled from the agar down to the elongation zone which causes more elongation to occur. The side of the stem without the agar experiences normal growth. As a result, the stem bends. Auxin was the first plant hormone discovered.

15. Gravitropism • Increased auxin concentration on the lower side in stems causes those cells to grow more than cells on the upper side. • stem bends up against the force of gravity • negative gravitropism • Upper side of roots oriented horizontally grow more rapidly than the lower side • roots ultimately grow downward • positive gravitropism

16. B. Plant hormones • Five plant hormones known by the mid -1960s • The five hormones: • Auxins • Cytokinins • Gibberellins • Ethylene (ethene) • Abscisic acid

17. Auxin • Effect of auxin is dependent on type of tissue • Auxins tend to be unevenly distributed in the plant • Very effective at low concentrations (5 ppm)

18. Effects of Auxin • Auxins play a role in tropisms ex. phototropism • Prevents leaf abscission • Enhances fruit growth • Auxin from the developing seeds results in fruit growth • Apical dominance

19. When the apical bud is removed, the dormant lateral buds can grow

20. Fig. 28-5

21. Auxin promotes rooting

22. Auxin • Synthetic auxins • widely used in agriculture and horticulture • prevent leaf abscission • prevent fruit drop • promote flowering and fruiting • control weeds

23. Auxin as a weed killer • 2,4-D – 2,4 dichlorophenoxy acetic acid • Causes a plant to grow itself to death • More readily absorbed by broad-leaved plants • Most often the “weed” of ‘Weed and Feed’ lawn fertilizers

24. Cytokinins • Cytokinins, in combination with auxin, stimulate cell division and differentiation. • most produced in root apical meristems and transported throughout plant • Have helped in developing new ways of plant reproduction so crops have greater yields, disease resistance and temperature hardiness

25. Gibberellins • Gibberellins are named after the fungus Gibberella fujikuroi which causes rice plants to grow abnormally tall. • Cause increased cell division and elongation (growth) especially in stems and fruit • Used to increase harvest since plants grow bigger (ex. Sugar cane) • Can cause dormant seeds to germinate • Can stimulate fruit development ex. seedless grapes

27. Ethylene • Ethylene is a gas produced when auxin is transported down from the apical meristem of the stem. • plays major role in fruit development (causes fruit to ripen – soften and change colour) • Tomatoes are often picked green & ethylene is then pumped into trucks to ripen the tomatoes • used to ripen bananas and honeydew melons

28. Abscisic Acid (ABA) • Abscisic acid is produced chiefly in mature green leaves and in fruits. • Inhibits bud growth and promotes leaf senescence (ex. Nurseries spray ABA on shrubs before shipping to inhibit the growth of dormant buds to prevent injury) • Found in fleshy fruit & prevents seeds from germinating while fruit is on the tree • Wilting causes an increase in ABA which then closes the stomata

29. C. Nastic Behavior • Nastic behavior is a movement response as the result of a stimulus. However, the movement is caused by turgor pressure changes, so the movement is much faster than a growth response. • Ex. Touch stimulus – venus fly trap • Ex. Light stimulus – flower petals opening

30. D. Photoperiodism • Photoperiodism - is the idea that plant development, particularly the development of flowers, buds and seeds, is affected by the number of hours of sunlight in a day..

31. Plant types: • Short-day plants: require short time periods of light to initiate the blooming cycle - Tend to bloom in late autumn or early spring ex. Poinsettia and Chrysanthemums • Long-day: need lots of sunlight to flower (mid-summer) Ex. Azaleas • Day-neutral: grow over a wide range of photoperiods Ex. Dandelions can initiate flowers under any day length, corn, beans

32. We can control light and influence blooming or vegetative growth by: • Shorten day with blackcloth: covering the growing plant with an opaque cover to exclude light. • Lengthen day with artificial light: adding light in the evening hours. Greenhouses often use these techniques so plants bloom when needed.