1. Plant Responses to Internal and External Signals Plant Hormones Plant Movements Control of Daily and Seasonal Responses Phytochromes

3. Plant Hormones Hormone-compound produced by one part of an organism that is transported to other parts where it triggers a response in target cells  Sends signals between different plant parts  Tracks time of day and year  Sensing and responding to gravity, direction of light, etc  Adjusts growth patterns and development  Phototropism-responsible for many of the discoveries about plant hormones

4. Experiments  Darwin: experiment-removed coleoptiles; result-no phototropism; conclusion- tip responsible for sensing light  Peter Boysen-Jensen: experiment-separated tip w/ block of gelatin; result-normal behavior; conclusion-signal was a mobile substance  F.W.Went: experiment-removed tip, placed it on agar block, placed block back on plant; result-normal behavior;conclusion-chemical in block from tip was responsible. AUXIN

7. Functions of Plant Hormones  Coordinate growth and development by affecting division, elongation and differentiation of cells  Regulate responses to environmental stimuli  Difficult to identify-varying concentrations affect target cells differently; and different target cells are affected differently

8. Classes of Plant Hormones  Auxins (IAA)  Cytokinins  Gibberellins (GA)  Abscisic Acid (ABA)  Ethylene

10. Auxins  Produced by apical meristem  Stimulate cell growth  Induces vascular cambium cell division and differentiation of secondary xylem  Promotes formation of adventitious root  Promotes fruit growth  2,4 D-selective herbicide-dicots

12. Cytokinins  Modified adenine  Stimulates cytokinesis  Controls cell division and differentiation (in conjunction with auxins) by stimulating RNA and protein synthesis  Controls apical dominance  Anti-aging hormone

13. Gibberellins  More than 80 gibberellins have been identified  Primarily produced in roots and young leaves  Stimulate growth of leave and stems, but not roots  Work with auxins to stimulate cell elongation  Control fruit development along with auxins  Causes seeds to break dormancy, stimulated by imbibing water

15. Abscisic Acid  Produced by terminal bud-prepares plant for winter (suspends primary and secondary growth)  Stress hormone-closes stomata as a result of excess transpiration

16. Ethylene  Growth inhibitor  Produced by high auxin concentrations  Gas  Promotes sinescence (aging) examples: xylem, leaf fall, withering of flowers, death of annuals after flowering  Fruit ripening  Leaf abscission

18. Plant Movements  Tropisms-growth responses (+ or -), one time events, not repeatable  Phototropism-light, differential distribution of auxins, blue light  Gravitropism-gravity, roots positive, stems negative; process involve statoliths (starch grains), calcium and auxin concentration changes  Thigmotropism-touch, twining of a tendril

20. Turgor Movements (Nastic Movements)  Reversible movements caused by changes in turgor pressure  Rapid leaf movements (Mimosa)  Sleep movements  K+ movements changes osmotic conditions and leads to turgor changes

22. Control of Daily and Seasonal Responses  Circadian rhythm-physiological cycle with a frequency of about 24 hours  Photoperiodism-physiological response to day length—flowering,  Phytochromes play a critical role in seasonal cycles.  Two photoreversible forms: P fr and P r