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Download Objectives KEY CONCEPTS Phototropism Gravitropism
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3/18/2012 Plant Growth and Development Objectives • Describe phototropism, gravitropism, and thigmotropism • Learn about plant hormones and their effects Understand how varying amounts of light/dark stimulate/inhibit flowering Chapter 37 Phototropism KEY CONCEPTS • Growth in response to the direction of light • Plants respond to external stimuli, such as light, gravity, or touch, with a directional growth (tropism) Gravitropism Thigmotropism • Growth in response to gravity • Growth in response to contact with a solid object • Tendrils of a vine Ameoplasts in root tip cells - contain starch granules - respond to gravity - involves hormone auxin 1 3/18/2012 Plant Hormones • Organic compounds (chemicals) produced by plants • Elicit responses that regulate growth and development Take Home Message • Growth and development are influenced by a plant’s internal conditions and signals from its external environment Mechanism for Plant Hormone How Plant Hormones work! • Signal Transduction: a receptor converts an extracellular signal into an intracellular signal that causes a change in the cell • Many hormones bind to enzyme-linked receptors in plasma membrane, triggering an enzymatic reax. KEY CONCEPT Repressor Protein Darwins’ Experiments (1870s) • Although there are many plant hormones, five have been well-characterized: • auxins • gibberellins • cytokinins • ethylene, • abscisic acid Conclusion: Some substance is transmitted from upper part to lower part that causes the plant to bend 2 3/18/2012 Went’s Experiments (1920s) Plant Hormones: Auxin • • • • Cause Tropisms Apical Dominance Fruit development Stimulates root development on stem cuttings Auxins promote cell elongation • Cause cells to elongate • apical meristem • explains phototropism! Fig. 36-5 Auxin and apical dominance - inhibits axillary bud growth Gibberrelins 1. Bolting in plants (mytosis & hypertrophy) 2. Flowering in plants Auxins produced by seeds promote fruit development 2,4-D 3. Seed germination 1. production of Gibberrelins in embryo 2. triggers synthesis of amylase 3. digests starch in endosperm 4. provides glucose for embryo 2,4,5,-T 3 3/18/2012 Cytokinins Auxin-Cytokinin Interactions • Promote cell division and differentiation • Delay senescence Ethylene • Ripening fruits • Apical dominance • Leaf abscission (as auxin decreases ethylene increases at abscission layer • Wound response • Thigmomorphogenesis • developmental response to mechanical stressors such as wind • Plants produce more ethylene • Inhibits stem elongation and enhances cell wall thickening • Abscisic Acid – stress hormone • Stomatal closure caused by water stress • Seed dormancy • temporary state of reduced physiological activity 4 3/18/2012 Photoperiodism • How do varying amounts of light and darkness induce flowering? response of a plant to lengths of daylight and darkness • Flowering can be a photoperiodic response - short-day plant - long day plant - day-neutral plant Short-Day Plants • Detect lengthening nights of late summer or fall and flower at that time Fig. 37-14a, p. 800 Long-Day Plants • Detect shortening nights of spring and early summer and flower at that time Fig. 37-14b, p. 800 5 3/18/2012 Intermediate-Day Plants • Flower when exposed to days and nights of intermediate length Fig. 37-14c, p. 800 Light Photoperiodic Responses Dark Flash of red light Light treatment 24 hours Short-day plant Long-day plant Fig. 37-15, p. 801 Phytochrome • The photoreceptor in photoperiodism • a family of about five blue-green pigments • Each phytochrome has 2 forms (Pr and Pfr) • named for wavelength of light absorbed Phytochrome • Pfr (the active form) triggers or inhibits physiological responses • flowering • shade avoidance • light requirement for germination 6 3/18/2012 Red light (660 nm) Circadian Rhythm Phytochrome synthesis Inactive form Pr Phytochrome degradation Short-lived intermediate forms Active form Short-lived intermediate forms Far-red light (730 nm) (Night) Pfr • A regular period in growth or activities that approximates the 24-hour day • reset by rising and setting of the sun • Examples: • opening and closing of stomata • sleep movements Physiological response (such as flowering) Fig. 37-16, p. 801 Sleep Movements Phytochrome and Cryptochrome • Both are photoreceptors • sometimes interact to regulate similar responses such as resetting biological clock 7
