Regulatory Systems in Plants (Ch 39)
• Plants show complex responses to environmentalstimuli
• Problem: no nervous system (detection) & nomuscular 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
Pr
Pfr
Darkness=slow conversionback to Pr
Far-red light =rapid conversionback to Pr
Red lightrapid conversionto Pfr
Photoperiodic Control
• Monitoring daylength– ratio Pr:Pfr
• 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
Five Plant Hormones:
• 1) Auxins– Stem apical meristems are major site of auxin production
• promote cell elongation• responsible for phototropic response
Control- turns to light
(READ pp 750-751!)
Auxin produced at growing tip.
Auxin migrates away from light
Auxin stimulates cells to elongation
Five Plant Hormones:
• 1) Auxins– Other effects
• produced in seeds --> stimulates fruit production• promotes growth of adventitious roots
Five Plant Hormones:
• 2) Cytokinins– Root apical meristems are major
site of cytokinin production• stimulates cytokinesis (cell
division)• “anti-aging” hormone- delays
scenescence
Five Plant Hormones:
• Auxin:CytokininInteractions– Cell differentiation in
tissue culture• auxin>cytokinin --> roots
develop!
• auxin<cytokinin --> shootbud develops
• Auxin:CytokininInteractions– Shoot and root apical
dominance• auxins inhibit lateral bud
growth in stems, cytokininsstimulate it!
• auxins stimulate lateral rootgrowth, cytokinins inhibitthem
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 clusterw/ gibberellin = stem elongatesand grapes are farther apart= larger grapes!
Five Plant Hormones:• 5) Ethylene
– fruit ripening– leaf abscission in the fall Ripe apple releases ethylene-
results in leaf drop!
Plant Control Systems: Tropism
• Tropism- growth responseof plant to environmentalstimulus
• Phototropism- Growth inresponse to light
Plant Control Systems: Tropism• Gravitropism - growth in
response to gravity• Positive gravitropism-
roots grow DOWN!– Amyloplasts
• Negative gravitropism-stems grow UP!– auxin response…
Plant Control Systems: Tropism• Thigmotropism - directed
growth in response totouch
• 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