plant response. tropisms plant growth toward or away from a stimulus gravitropism gravity is “+”...
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PLANT RESPONSE
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Tropisms• Plant growth toward or away from a stimulus• Gravitropism gravity is “+” in roots and “-” in shoots
– Plastids containing starch grains (statoliths) settle to the low point of the cell.
• Thigmotropism – touching stunts growth or affects shape– Difference in turgor pressure
• Phototropism light is “+” shoots
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Discovery of Plant Hormones
• Classic experiments– Darwin– Boysen-Jensen– Went
• Phototropism– grasses bend toward the light
because the cells on the dark side grow faster that the lighted side.
– Darwin concluded that a plant hormone made in the coleoptile tip could somehow move down and induce other cells to elongate.
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Auxin• Produced in the tip of the shoot and transported down
the stem.• Promotes cell elongation
– causes cells to pump H+ into their cell walls. – The higher pH activates enzymes to break cross linkages in the
wall allowing cell elongation.
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Cytokinins• Produced in the root• stimulate cytokinesis
– Anti-aging effect• Continues protein synthesis after
leaves have been picked
• usually coupled with auxin. – When added to undifferentiated
cell mass (callus) • Higher auxin conc. root• Higher cytokinin conc. shoot
– Apical dominance• Higher auxin conc. terminal bud• Higher cytokinin conc. axillary bud
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Gibberellin• Originally gibberellin found in a fungus, causing rice seedlings
to grow tall and spindly. – Stimulate cell elongation & division
• Stimulates the synthesis of hydrolytic enzymes during germination of seedlings in order to insure release of stored nutrients.– What are the practical uses?
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Ethylene
• a gas and is produced by fruit• Response to Stress
– Growth around obstacles• Triple response
– Drought• Leaf Abscission & Apoptosis
• Ripening of fruits– Why do organic fruits ripen slower?– Why are tomatoes picked green?
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Abscisic Acid
• Slows growth• induces winter dormancy by suppressing mRNA production.
– auxin and gibberellins are no longer produced.
• enters guard cells during periods of water stress– outward transport of potassium ions (K+). – How will this help prevent the plant’s water loss?
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Photoperiodism• The response to changing lengths of light (day) and dark
(night)• Flowering is usually controlled by length of nights.
– Chrysanthemums bloom in the fall when the nights are longer and the days are shorter. Leaves detect the longer nights and signal the flower buds . Mums are “short day” plants
– Confirmed by interrupting the period of darkness with flashes of light• Long Day-Short Night plants will begin to flower during winter• Short Days-Long Night may be prevented from flowering
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Phytochromes
• Pigments that control plant responses to light– responds reversibly to red (660nm ) and far red (730nm) light
• Pr is sensitive to red light (daytime)– necessary to reset the clock which counts the hours of darkness
slowly accumulates spontaneously during the night. • Pfr is sensitive to far-red light (nighttime)
– Switches on physiological and developmental responses (germination, flowering, etc.)
– Slowly converts to Pr
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Circadian Rhythms• ~24 hr. physiological signals that are innate
but are set/ entrained by night and day.– Present in plants and animals
• a plant’s biological clock is reset by light through phytochromes– in dark all phytochrome are Pr– dawn light restores the balance of Pr : Pfr clock is reset