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Plant Responses to Internal and External Signals

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

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Plant Responses to Internal and External Signals

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

  2. Plant Hormones Hormone-compound produced by one part of an organism that is transported to other parts where it triggers a response in targetcells • 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

  3. 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

  4. 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

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

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

  13. 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: Pfr and Pr

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