Regulatory Systems in Plants (Ch 39) Plants show complex responses to environmental stimuli Problem: no nervous system (detection) & no muscular system (response) Various mechanisms for detecting stimuli Plants respond to stimulus by: cell division cell elongation cell differentiation Regulatory Systems: RESPONSE TO LIGHT Light is a major environmental stimulus seed germination shade/sun response photosynthesis flowering Plants respond: presence of light (on/off response) light quantity light quality
The Reversible phytochrome system Far-red light = rapid conversion back to P r P fr Darkness= slow conversion back to P r Red light rapid conversion to P fr P r Photoperiodic Control Monitoring daylength ratio P r :P fr Many plants respond to length of day Long-night plants (short day- flower fall/winter) Short-night plants (long day- flower spring/summer) Night-neutral plants (response is daylength independent)
Short-night plant Long-night plant Flashes of light can fool a plant! Short-night plant Long-night plant
Disco plants... Plant Hormones: Most plant responses controlled by HORMONES What are hormones? 5 major classes of plant hormones
1) Auxins Five Plant Hormones: Stem apical meristems are major site of auxin production promote cell elongation responsible for phototropic response Controlturns to light (READ pp 750-751!)
Auxin produced at growing tip. Auxin migrates away from light Auxin stimulates cells to elongation 1) Auxins Five Plant Hormones: Other effects produced in seeds --> stimulates fruit production promotes growth of adventitious roots
2) Cytokinins Five Plant Hormones: Root apical meristems are major site of cytokinin production stimulates cytokinesis (cell division) anti-aging hormone- delays scenescence Five Plant Hormones: Auxin:Cytokinin Interactions Cell differentiation in tissue culture auxin>cytokinin --> roots develop! auxin<cytokinin --> shoot bud develops
Auxin:Cytokinin Interactions Shoot and root apical dominance auxins inhibit lateral bud growth in stems, cytokinins stimulate it! auxins stimulate lateral root growth, cytokinins inhibit them Five Plant Hormones: 3) Abscisic Acid (growth inhibitor) despite name isn t really involved in leaf abscission! Prepares plant for winter dormancy maintains seed dormancy shuts down stomates under serious water stress
Five Plant Hormones: 4) Gibberellin (growth promoter) foolish rice disease promotes stem (but not root) cell division and elongation (may work w/ auxins) promotes seed germination (antagonistic effect w/ abscissic acid) promotes spring bud break (antagonistic effect w/ abscissic acid)
Spray developing grape cluster w/ gibberellin = stem elongates and grapes are farther apart= larger grapes! Five Plant Hormones: 5) Ethylene fruit ripening leaf abscission in the fall Ripe apple releases ethyleneresults in leaf drop!
Plant Control Systems: Tropism Tropism- growth response of plant to environmental stimulus Phototropism- Growth in response to light Plant Control Systems: Tropism Gravitropism - growth in response to gravity Positive gravitropismroots grow DOWN! Amyloplasts Negative gravitropismstems grow UP! auxin response
Plant Control Systems: Tropism Thigmotropism - directed growth in response to touch examples vine tendrils wind-induced shortness Plant Control Systems: Biological Clocks Plants have bio-rhythms Circadian rhythms = 24 hour internal clock usually reset daily plant jet lag under constant conditions may drift- Examples of rhythmic movements- Stomatal opening sleep movement of leaves
Noon position Midnight position Tugor Movement Movements based not on growth, but reversible changes in cell turgor pressure Examples- flowers which open/close @ day/night Rapid leaf movement in Mimosa plant