P lant Repsonses & H ormones

1. Plant Repsonses & Hormones Nancy G. Morris Volunteer State Community College Campbell, 5th edition, Chapter 39

2. Plant Hormones • Hormone = Gr. “to excite” • 1) active in small amounts • 2) produced in one part of plant & transported to another for action • 3) action is specific for that site

3. Each has a Multiplicity of Effects • Depending on site of action • Developmental stage of plant • Concentration of hormone

4. Auxins • stimulate growth but too much inhibits growth • functions: • 1) root initiation, stem elongation • 2) retard abscission (loss) of leaves & fruits (Figure 39.10) • 3) stimulates cell differentiation • 4) apical dominance (Figure 39.6)

5. Apical dominance

7. Gibberellins • discovered due to a fungus Giberella which causes Japanese “foolish” rice seedling disease • Figure 39.7 • induces flowering • stimulates growth by increasing cell size & numbers (Figure 39.8)

8. “Foolish seedling disease” in rice

9. 39.11 Effect of Gibberellin

10. Dwarf pea plant treated with gibberellin

11. Cytokinins • 1) induces cell division (cytokinesis) • 2) affects root growth & differentiation • 3) stimulates germination • 4) delays senescence (aging); the progression of irreversible change that eventually leads to death

12. Abscisic Acid • 1) stimulates abscission • 2) converts vegetative buds (active) to dormant buds • 3) inhibits growth

13. 39.16 Abscission

14. Ethylene • 1) promotes fruit ripening • 2) stimulates production of cellulase

15. Photochromes • Plant pigment • Important in processes where light is a critical factor, such as, flowering in long day short night plants • Figure 39.16

16. Plant Movements due to growth • Phototrophism – movement toward (positive) or movement away from (negative) light • Geotrophism – movement toward (positive) or away from (negative) center of the earth • Thigmotrophism – directional growth responding to contact (tendrils)

17. Photoperiodic control of Flowering

18. Plant Movements not due to Growth • Thigmonastic response – touch stimulus, e.g. mimosa leaves • Photonastic response – shamrock • Sleepresponse – caused by change in osmotic pressure

20. Sleep movements of a bean plant:

21. Movement of materials • Path from roots: • 1) epidermis of root & root hair cells • 2) roots absorb water, minerals, gases • 3) stems conduct • Xylem & phloem are vertical conductors • Parenchyma of cortex, pith, rays are lateral conductors

23. Movement • Root pressure –osmotic pressure & pressure created by active absorption & secretion by root cells; drives materials up xylem • Transpiration – evaporation of water through leaf stomata; cohesion of water molecules exerts a pull on columns of water in xylem • Guttation – release of water droplets at leaf tips; occurs when too much water is absorbed by plant & when humidity is high • Translocation – movement of material from one location to another

25. Guttation

26. Fighting Back…. • Response to herbivory(animals eating plants): • 1) physical barriers – thorns & spines • 2) chemical defenses – distasteful or toxic compounds • 3) chemical signaling – salivary enzyme from herbivore triggers a pathway that produces a volatile attractant that recruits a parasitoid wasp. The wasp lays eggs within the caterpillar, and the larvae upon hatching destroy the caterpillar by consuming its tissues. (Figure 39.22)