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Plant Responses

Plants respond to stimuli, as all other living organisms do A tropism is plant growth toward or away from a unidirectional stimulus positive = growth toward negative = growth away These are due to differential growth One side of a cell is elongating faster than the other.

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Plant Responses

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  1. Plants respond to stimuli, as all other living organisms do • A tropism is plant growth toward or away from a unidirectional stimulus • positive = growth toward • negative = growth away • These are due to differential growth • One side of a cell is elongating faster than the other Plant Responses

  2. There are 3 well known tropisms in plants: • Phototropism • response to a light stimulus • usually positive • Gravitropism • response to gravity • positive in roots; negative in stems • Thigmotropism • response to touch Tropisms

  3. Response to a stimulus that is independent of the direction of the stimulus • Result from touch, shaking, or thermal stimulation • Example - Mimosa pudica touching leaf, leaflets fold because petiole droops, a thigmonastic movement (bioelectrical signals?) • Example - Venus flytrap has 3 sensitive hairs at base; if touched, stimulus causes closing of trap (turgor pressure changes) Nastic Movements

  4. Sleep movements are nastic responses daily in light & dark • Example - Prayer plant (Maranta leuconeura) leaves fold upward at night • Plants display circadian rhythms (biological rhythm with 24 hour cycle) Sleep Movements • Biological clock = internal mechanism by which biological rhythm is maintained in absence of appropriate environmental stimuli • Clock set by photoperiod (length of daylight compared to length of darkness)

  5. Many physiological changes are due to seasonal change in day length • seed germination (breaking of bud dormancy) • senescence • Photoperiodism is the physiological response prompted by changes in length of day or night • May influence flowering Photoperiodism

  6. Plants can be divided into 3 groups: • short-dayplants • Flower when day length is shorter than critical length • Example - poinsettia, cocklebur • long-dayplants • Flower when day length is longer than critical length • Example - wheat, barley, spinach Photoperiod plants

  7. day-neutral plants • Not dependent on day length for flowering • Example - tomato & cucumber • Plants have a way to detect these changes, a substance called phytochrome Photoperiod plants

  8. Phytochrome is a blue-green leaf pigment existing in 2 forms: • Pr (phytochrome red) • Absorbs red light & converts to Pfr • Pfr (phytochrome far-red) • Absorbs far red light • Direct sunlight has more red than far-red • More Pfr during day & Pr during night (allows plants to detect changes in photoperiod) Phytochrome

  9. Phytochrome also promotes: • seed germination • leaf expansion • stem branching • Stem elongates and grows toward sunlight when Pr dominates) Phytochrome Functions

  10. Almost all communication in a plant is done by hormones • They travel within phloem or cell to cell in response to stimulus • 5 basic classes: • Auxins • Gibberellins • Cytokinins • Abscisic acid (ABA) • Ethylene Plant Hormones

  11. Responsible for plant growth & development • Most common = Indoleacetic acid (IAA) produced by shoot apical meristem • Apically produced auxin prevents growth of axillary buds • Removing apical bud causes axillary buds to produce lateral branches Auxins

  12. Auxin production by seeds promotes growth of fruit • As long as auxin is concentrated in leaves and fruits (rather than stem), they do not fall to ground • Auxin involved in gravitropism & phototropism • Moves to lower surface of roots & stems with gravity • Causes root growth downward and stem growth upward Auxins

  13. Growth promoting hormones that bring about elongation of resulting cells • Gibberellic acid (GA3) is most common of the 70 • Applied externally stimulates stem elongation • Dormancy of seeds and buds can be broken by applying gibberellins Gibberellins

  14. Cytokinins • Class of plant hormones that promote cell division • Derivatives of adenine • Initiate growth • Prevent senescence (ex. leaf losing its color)

  15. Initiates and maintains seed and bud dormancy (stopping growth because of adverse conditions) • Brings about closure of stomata • Called stress hormone • Causes K+ ions to leave guard cells (unknown how) and stomata close • Produced by any “green” tissue Abscisic Acid (ABA)

  16. Involved in abscission (dropping of leaves, fruits, & flowers from a plant) • Stimulates enzymes that promote abscission • Ripens fruit • Released at site of plant wound due to damage or infection • Example: rotten apple spoils bunch; ripened fruit stimulates ripening of unripened fruit Ethylene

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